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A critical study of the accuracy of various weighing forms of calcium

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A critical study of the accuracy of various weighing forms of calcium

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Content A CRITICAL STUDY OF THE ACCURACY
OF VARIOUS WEIGHING FORMS OF CALCIUM
A T h e s is
P re s e n te d to
th e F a c u lty o f th e D epartm ent o f C h em istry
U n iv e r s ity o f S o u th e rn C a lif o r n i a
In P a r t i a l F u lf illm e n t
o f th e R eq u irem en ts f o r th e D egree
M aster o f A rts
by
Sigmund H. H uth
A ugust 1941
UMI Number: EP41537
All rights reserved
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I'M-
This thesis, written by
....................Sig?aund..H.,.„.Hut^........... ...........7 „„.
under the direction of ht&. Faculty Committee,
and a p p ro ved by all its members, has been
presented to and accepted by the Council on
Graduate Study and Research in partial fulfill
ment of the requirements for the degree of
Magte^oX.Art.
Dean
Secretary
Faculty Com m ittee
Chairman
....
TABLE OF CONTENTS
CHAPTER PAGE
I . INTRODUCTION................................................................................ 1
P u rp o se o f s tu d y and i n v e s t i g a t i o n ....................... . . 1
I I . DISCUSSION OF MATERIALS AND APPARATUS....................... 6
I I I . PREPARATION OF THE SAMPLE............................ 10
P ro c ed u re fo llo w e d ................................................... 10
IV. CALCIUM OXIDE AS-TIE. WEIGHING FO RM OF CALCIUM. . IE
D is c u s s io n o f th e problem ................................. 12
E x p e rim e n ta l p ro c e d u re fo llo w e d .................................. 15
E x p e rim e n ta l r e s u l t s . ...................................... 17
D is c u s s io n o f r e s u l t s ......................................................... 18
V. CALCIUM CARBONATE AS THE NEIGHING FORM OF
CALCIUM.......................................................................................... 19
D is c u s s io n o f th e p r o b l e m ................................................ 19
E x p e rim e n ta l p ro c e d u re fo llo w e d . * .......................... 23
E x p e rim e n ta l r e s u l t s .............................................................. 25
D is c u s s io n o f r e s u l t s ............................................... 26
V I. CALCIUM SULFATE AS THE WEIGHING FORM . OF CALCIUM. 27
D is c u s s io n o f th e p r o b le m ................................................ 27
E x p e rim e n ta l p ro c e d u re fo llo w e d .................................. 29
E x p e rim e n ta l r e s u l t s .............................................................. 30
D is c u s s io n o f r e s u l t s ......................................................... 31
V II. SU M M A RY.............................................................................................. 32
BIBLIOGRAPHY............................................................................................... 35
CHAPTER I
INTRODUCTION
PURPOSE OF THE STUDY AND INVESTIGATION
T h is s tu d y and la b o r a to r y i n v e s t i g a t i o n was made to
d e te rm in e th e r e l a t i v e v a lu e s o f th e q u a n t i t a ti v e g r a v im e tr ic
m ethods o f c a lc iu m d e te r m in a tio n a s th e o x id e , c a rb o n a te ,
and s u l f a t e .
The number o f d e te r m in a tio n s o f c a lc iu m made e v e ry day
in i n d u s t r i a l l a b o r a t o r i e s , a s w e ll a s in e d u c a tio n a l and
r e s e a r c h l a b o r a t o r i e s , i s so la r g e t h a t any s a v in g in tim e ,
m a te r i a l s o r a p p a r a tu s , o r any im provem ent in th e a c c u ra c y o f
th e m ethods u sed seems d e f i n i t e l y w o rth w h ile .
P r a c t i c a l l y a l l th e d e te rm in a tio n s o f c a lc iu m a re
b a se d upon th e s e p a r a tio n o f ca lc iu m from o th e r c a tio n s by
p r e c i p i t a t i o n a s c a lc iu m o x a la te by means o f a s o lu t io n o f
ammonium o x a l a te . The o x a la te i s p r e f e r r e d b ecau se i t i s
th e m ost in s o lu b le s a l t o f c a lc iu m and b ecau se i t s c r y s t a l l i n e
c h a r a c te r makes i t e a s y to f i l t e r and w ash. C alcium o x a la te
i s o n ly s p a r in g ly s o lu b le in c o ld w a te r. A cco rd in g to th e
I n t e r n a t i o n a l C r i t i c a l T a b le s : 100 m l. o f w a te r w i l l d is s o lv e
0.0 0 0 5 5 4 g . o f c a lc iu m o x a la te m o n o h y d rate, a t S0° C. In a
more r e c e n t r e p o r t g iv e n by F a le s and K enny,^ 100 m l. o f w a te r
^H. A. F a le s and F . Kenny, In o rg a n ic Q u a n tita tiv e
A n a ly s is . p . 305.
2
d is s o lv e s 0 .6 7 mg. o f c a lc iu m o x a la te m onohydrate a t 25° C .,
and 1 .4 mg. a t 95° C. C alcium o x a la te m onohydrate i s s l i g h t l y
s o lu b le in a c i d s , th e d eg ree o f s o l u b i l i t y d ep en d in g upon th e
c o n c e n tr a tio n o f th e a c i d s . In q u a n t i t a t iv e w ork, an e x c e s s
o f ammonium o x a la te i s added to th e c a lc iu m s a l t s o l u t i o n ,
fo llo w e d by a sm a ll amount o f c h e m ic a lly p u re ammonium h y d ro
x id e , e lim in a tin g th e d an g er o f any a p p r e c ia b le s o l u b i l i t y .
P r e c i p i t a t i o n o f th e c a lc iu m io n by c o ld s o lu tio n s
o f ammonium o x a la te p ro d u c e s a f i n e l y d iv id e d p r e c i p i t a t e
w hich i s v e ry d i f f i c u l t to f i l t e r * I n f a c t , when f i l t e r e d
th ro u g h f i l t e r p a p e r and w ashed, th e o x a la te p r e c i p i t a t e h as
a te n d e n c y to c re e p up th e s id e s o f th e f i l t e r p a p e r, o v e r
th e to p and i s th e n c a r r i e d dovra on th e u n d e rs id e o f th e
p a p e r, f i n a l l y p a s s in g in to th e f i l t r a t e . A cco rd in g to
T . W. R ic h a r d s ,2 i t i s b e t t e r to p r e c i p i t a t e c a lc iu m o x a la te
by a d d in g ammonium o x a la te to a h o t a c id s o lu t io n o f th e
c a lc iu m s a l t and f i n a l l y n e u t r a l i z i n g i t w ith ammonium h y
d r o x id e . I t was found by i n v e s t i g a t i o n t h a t t h i s m ethod was
m ost s a t i s f a c t o r y when th e c a lc iu m s a l t was h e a te d to i n c i
p ie n t b o ili n g and th e n c o n s ta n tl y s t i r r i n g th e s o lu t io n
d u rin g th e a d d itio n o f ammonium o x a l a te . I t was found t h a t
th e p r e c i p i t a t e form ed in t h i s way c o u ld be f i l t e r e d and
w ashed w ith o u t d i f f i c u l t y .
2R ic h a rd s , M cC affrey , and B is b e e , Z. A norg. Chem.
2 7 ,7 1 , (1 9 1 1 ).
3
The use o f ammonium o x a la te in p r e c i p i t a t i n g ca lciu m
h a s p ro v en v e ry s u c c e s s f u l, even in th e p re se n c e o f m agnesium
and o th e r m e ta ls .^ A cco rd in g to F re s e n iu s ,^ 1 more o x a la te i s
n eed ed when m agnesium i s p r e s e n t th a n i s n e c e s s a r y in a s o lu
t i o n o f pure c a lc iu m c h l o r id e . A cco rd in g to B l a s d a l e ,5 i f
th e p r e c i p i t a t i o n i s a c co m p lish ed a s recom m ended, magnesium
o x a la te w i l l n o t be p r e s e n t . By p r o p e r ly r e g u la tin g th e
e x c e s s ammonium o x a la te i t i s p o s s ib le to p r e c i p i t a t e p u re
c a lc iu m o x a l a te .
Once th e c a lc iu m h a s been s e p a r a te d a s o x a la te th e r e
a re a number o f p ro c e s s e s t h a t can be fo llo w e d in d e te rm in in g
th e c a lc iu m q u a n t i t a t i v e l y . The v o lu m e tric m ethod, b ased on
th e t i t r a t i o n o f th e o x a la te io n , b ro u g h t in to s o lu tio n w ith
d i l u t e s u l f u r i c a c id , g iv e s r e s u l t s s u f f i c i e n t l y a c c u r a te
f o r m ost p u rp o s e s . The g r a v im e tr ic m ethods in c lu d e w eig h in g
th e c a lc iu m o x a la te m onohydrate a f t e r b r in g in g i t to c o n s ta n t
w eig h t a t 110 d e g re e s C. T h is m ethod i s n o t v e ry h ig h ly
r e g a r d e d , a lth o u g h i t i s recommended f o r some a n a ly s e s by
th e A m erican S o c ie ty f o r T e s tin g M a te r ia l s . A cco rd in g to
S a n d e ll and K o lth o f f ,6 th e c a lc iu m o x a la te m onohydrate m ethod
*%. T. H a ll, J o u r . Amer. Chem. S oc. 5 0 , 2704, (1 9 2 8 ).
4K. R. F r e s e n iu s , Q u a n tita tiv e C hem ical A n a ly s is .
V o l. I , p . 269.
5J . A. B la s d a le , J o u r . Amer. Chem. S o c . . 31, 917, (1 9 0 9 ).
6S a n d e ll and K d tlth o ff, I n d . E ng. Chem. . 11, 90, (1 9 3 9 ).
4
w ould p o s s e s s d i s t i n c t a d v a n ta g e s i f th e a c c u ra c y w ere s u f
f i c i e n t l y h ig h . The p o s s ib le so u rces o f e r r o r in v o lv e d
in c lu d e r e t e n t i o n o f f o r e i g n w a te r a f t e r d ry in g , l o s s o f
h y d ra te w a te r on d ry in g , and c o - p r e c i p i t a t i o n o f ammonium
s a l t s o r o x a lic a c id . The id e a l p ro c e d u re from th e s ta n d
p o in t o f s i m p l i c it y w ould in v o lv e d ry in g th e washed o x a la te
a t room te m p e ra tu re a f t e r tr e a tm e n t w ith a lc o h o l and e t h e r
o r a c e to n e , b u t f o r f in e q u a n t i t a t i v e r e s u l t s th e m ethod i s
n o t recom m ended.
The com plete i g n i t i o n o f th e o x a la te to th e oxide i s
u s u a lly c o n s id e re d th e s im p le s t g r a v im e tr ic m ethod, b u t c e r
t a i n p o i n t s , to be d is c u s s e d l a t e r , w i l l be b ro u g h t up a s
e v id e n c e t h a t th e a c c u ra c y o f t h i s m ethod i s f r e q u e n tly o v e r
e s tim a te d .
W eighing th e c a lc iu m a s c a rb o n a te a f t e r i g n i t i o n a t a
te m p e ra tu re s u f f i c i e n t to in s u r e th e com plete d e c o m p o sitio n
o f th e o x a l a te , and s t i l l n o t h ig h enough to cause d i s s o c i a
t i o n o f th e r e s u l t i n g c a rb o n a te i s a m ethod h ig h ly recommended
by some i n v e s t i g a t o r s , b u t i t w i l l be shown t h a t c o n d itio n s
m ust be c l o s e ly c o n tr o ll e d i f t h i s m ethod i s to be u s e d , and
th e a p p a ra tu s n eed ed f o r t h i s p ro c e d u re i s n o t alw ay s found
in th e a v e ra g e l a b o r a t o r i e s .
C o n v ersio n o f th e ca lc iu m o x a la te to c a lc iu m s u l f a t e
a s th e g r a v im e tr ic w eig h in g form i s a m ethod t h a t h a s n o t
won wide s u p p o r t, and y e t i t seems t o have much t o recommend
i t . The m ethod seem s t o be c o n v e n ie n t, r a p id , and a c c u r a te .
5
The v o lu m e tric d e te rm in a tio n o f c a lc iu m f a l l s o u ts id e
th e f i e l d o f t h i s p r e s e n t i n v e s t i g a t i o n .
The g r a v im e tr ic m ethods seem t o w a rra n t a co m p arativ e
i n v e s t i g a t i o n , and th e r e s u l t s found by u s in g th e m ost com
m only u se d w eig h in g fo rm s; th e o x id e , c a rb o n a te , and s u l f a t e ,
c o n s t i t u t e th e m ain to p i c o f d is c u s s io n in t h i s in v e s t i g a
t i o n .
CHAPTER I I
MATERIALS AND APPARATUS
In t h i s i n v e s t i g a t i o n , th e p ro c e d u re s u se d were o f
su c h a n a tu re t h a t th e y r e q u ir e d o n ly su ch m a te r ia l s and
a p p a ra tu s a s a re a v a ila b le in m ost l a b o r a t o r i e s . P a r t l y f o r
t h i s r e a s o n b o th th e p o r c e la in and p la tin u m c r u c i b le s w ere
u sed s in c e th e expense o f th e p la tin u m w ould p r o h i b i t i t s
use in some c a s e s . However, i t was found t h a t p o r c e la in
c r u c i b le s were l e s s s a t i s f a c t o r y th a n th e p la tin u m s in c e th e
p la tin u m c o u ld be h e a te d t o a much h ig h e r te m p e ra tu re in a
s h o r t e r tim e by th e same k in d o f a fla m e . I t i s to be remem
b e re d , to o , t h a t p o r c e la in r e q u ir e s a lo n g e r p r e h e a tin g
p e rio d b e fo re th e c o n s ta n t w eig h t o f th e c r u c ib le can be o b
ta i n e d . W ith p la tin u m c r u c i b l e s F is h e r b u rn e rs p ro d u ced a
s u f f i c i e n t l y h ig h te m p e ra tu re f o r i g n i t i o n , b u t i t was n e c e s
s a ry to use F is h e r b l a s t b u rn e rs c o v e re d w ith c la y c y lin d e r s
f o r i g n i t i o n s in p o r c e la in c r u c i b l e s , when w eig h in g th e c a l
cium a s th e o x id e . I t was found t h a t 20 g . f i r e c la y a s s a y
c r u c i b le s w ith th r e e h o le s on th e s id e w a ll w ere v e ry s a t i s
f a c t o r y f o r t h i s i g n i t i o n , f o r th e h e a t i s th e n d ir e c te d
down upon th e to p s o f th e c r u c ib le and p r e c i p i t a t e , a s w e ll
a s r e c e iv in g h e a t d i r e c t l y fro m th e b u rn e r u n d e rn e a th .
P la tin u m c r u c ib le s a re v e ry s u i t a b l e f o r th e i g n i t i o n
o f th e c a lc iu m t o th e o x id e . P la tin u m i s a v e ry good c o n d u c to r
7
o f h e a t and a good r a d i a t o r . I t seem s n o t t o a b s o rb w a te r
v a p o r t o any a p p r e c ia b le e x te n t a s com pared w ith p o r c e l a in ,
e s p e c i a l l y i f th e p o r c e l a in h as l o s t some o f i t s g la z e .
I t was fo u n d t h a t th e Y/hatman Wo. 40 f i l t e r p a p e r
c o u ld be u se d i f c a re was ta k e n in th e p r e c i p i t a t i o n o f th e
c a lc iu m o x a l a te . O th e rw ise , i f a v e ry f in e c r y s t a l l i n e
p r e c i p i t a t e was fo rm ed , i t had a te n d e n c y t o c re e p th ro u g h .
The r e g u la r ty p e d e s ic c a to r was u s e d , b u t i t was found
more s u c c e s s f u l to u se a v e ry c o n c e n tra te d s u l f u r i c a c id a s
th e d ry in g a g e n t .7 The s u l f u r i c a c id was c o n c e n tra te d by
p la c in g th e n e c e s s a ry am ount in a one l i t e r K je ld a h l f l a s k .
T h is was h e a te d in th e hood u n t i l c o n s id e r a b le w a te r had
been d r iv e n o f f . D u rin g th e c o o lin g p r o c e s s , i f a f r e s h l y
p re p a re d c a lc iu m c h lo r id e tu b e i s f i t t e d i n t o th e mouth o f
th e f l a s k , i t w i l l p re v e n t th e a c id from r e a b s o rb in g some of
th e m o is tu r e . S h o rt g la s s tu b e s were p la c e d in th e low er
com partm ent o f th e d e s ic c a to r to e lim in a te th e p o s s i b i l i t y
o f s p la s h in g when i t was c a r r i e d .
The t r i a n g l e s u se d f o r th e p la tin u m c r u c ib le s were o f
q u a r tz c o n s tr u c tio n and th e to n g s w ere o f n i c k e l . 8
The a n a l y t i c a l b a la n c e o f th e r i d e r ty p e was u sed f o r
a l l w e ig h in g p u rp o s e s . The s e n s i t i v i t y o f t h i s b a la n c e was
7J . H. Bower, B u r, o f S td s . I . o f R e s e a rc h . I S , 24,
(1 9 3 4 ).
8K o lth o ff and S a n d e ll, Q u a n tita tiv e I n o rg a n ic A n a ly s is .
p . 166.
8
d e te rm in e d f o r v a r io u s lo a d s , w hich v a r ie d from 0 .2 2 mg. p e r
s c a le d i v is i o n , w ith an em pty b a la n c e , to 0 .4 mg. p e r s c a le
d i v i s i o n w ith a lo a d o f 5 0 .0 g .
I n t r a n s f e r i n g c o o le d c r u c i b le s from th e d e s ic c a to r
to th e b a la n c e pan i t i s cu sto m ary to use c r u c ib le to n g s ,
b u t in th e case o f c a lc iu m o x id e , th e te n d e n c y to a b so rb
m o is tu re and carb o n d io x id e i s so pronounced t h a t much b e t t e r
r e s u l t s a re o b ta in e d i f th e t r a n s f e r can be made w ith o u t u n
c o v e rin g th e c r u c i b le s . U n less s p e c i a l l y shaped to n g s a re
a v a i l a b l e , th e b e s t m ethod seem s to be to g ra s p th e c r u c ib le
in th e f i n g e r s . W hile f i n g e r s w hich a r e n o rm a lly f r e e from
e x c e s s iv e p e r s p i r a t i o n seem to in tro d u c e an e r r o r w hich n e v e r
ex c ee d s 0 .1 m g., i t seem s s a f e r , and v e ry c o n v e n ie n t, to use
ru b b e r f in g e r t i p s , o b ta in e d from o f f i c e su p p ly s t o r e s , o r
c u t from an o ld ru b b e r g lo v e .
Much i s to be s a id in f a v o r o f th e e l e c t r i c f u r n a c e .
C alcium and s i l i c o n w e ig h in g s a re a f f e c t e d by th e h ig h e s t
te m p e ra tu re o f th e Meker b u rn e rs and b l a s t b u r n e r s . P la tin u m
c u r c i b le s undergo a l o s s o f w e ig h t when ig n it e d in an open
f la m e .9 The q u e s tio n o f th e p ro p e r te m p e ra tu re i s o f m ost
im p o rtan ce i n i g n i t i o n o f th e c a lc iu m t o th e c a rb o n a te . The
e l e c t r i c fu rn a c e sh o u ld be eq u ip p ed w ith an a c c u r a te th erm o
m e te r. The a c c u ra c y o fu th e th erm o m eter was checked w ith
9R. C. B en n er, I n d . E ng. Chem. 4 3 , (1 9 1 2 ).
9
c h e m ic a lly p u re s i l v e r s h lo r id e w hich h a s a m e ltin g p o in t o f
455° G. and vsrith c h e m ic a lly p u re le a d c h lo r id e w hich h a s a
m e ltin g p o in t o f 501° C ., s in c e th e s e p o in ts f a l l in th e
ran g e o f c h ie f im p o rtan ce f o r th e c o n v e rs io n o f c a lc iu m
o x a la te to c a lc iu m c a rb o n a te .
CHAPTER I I I
PREPARATION OF THE SAMPLES
PROCEDURE FOLLOW ED
Two sam ples o f th e c a lc iu m s a l t w ere p re p a re d f o r t h i s
i n v e s t i g a t i o n . These sam p les f u r n is h e d th e d a ta f o r th e p r e
lim in a r y and f i n a l r e s u l t s .
Sample No. 1 was p re p a re d by d is s o lv in g a s u f f i c i e n t
am ount o f c h e m ic a lly p ure c a lc iu m c a rb o n a te in d i l u t e h y d ro
c h l o r i c a c id to form two l i t e r s o f c a lc iu m c h lo r id e s o lu t io n
in w hich 25 m l. o f th e s o lu t io n c o n ta in e d a p p ro x im a te ly 0 .2 5 g .
o f c a lc iu m o x id e . The c a lc iu m c h lo r id e s o lu t io n was th e n
p la c e d in a g round s to p p e re d b o t t l e f o r l a t e r use*
Sample No. 2 was p re p a re d by ta k in g 60 g . o f c h e m ic a lly
p u re c a lc iu m c a rb o n a te from th e same re a g e n t b o t t l e and d i s
s o lv in g i t w ith 6 N. h y d r o c h lo r ic a c i d . The c a lc iu m c h lo rid e
was th e n h e a te d to i n c i p i e n t b o i l i n g , and a s o lu t io n co n
t a i n i n g 120 g . o f ammonium o x a la te was added slo w ly and w ith
c o n s ta n t s t i r r i n g . T h is p ro c e d u re i s recommended by W illa rd
and F urm an,10 and r e p r e s e n ts a p p ro x im a te ly a 50 p e r c e n t e x
c e s s o v e r t h a t r e q u ir e d f o r p u re c a lc iu m c a rb o n a te . Chemi
c a l l y p u re 6 N. ammonium h y d ro x id e was th e n added c a r e f u l l y
to th e h o t s o lu tio n u n t i l i t gave a f a i n t a l k a li n e r e a c t io n
l° W illa r d and Furm an, E le m e n ta ry Q u a n tita tiv e A n a ly s is .
2nd. e d i t i o n , p . 317.
11
w ith m eth y l o ra n g e . The s o lu t io n was a llo w e d to s ta n d in
p y re x b e a k e rs o v e r n ig h t, and was th e n f i l t e r e d w ith th e a id
o f a B uohner f u n n e l. The p r e c i p i t a t e was w ashed w ith c o ld ,
v e ry d i l u t e ammonium o x a la te s o lu t io n u n t i l th e w ash in g s
gave no t e s t f o r th e c h lo r id e io n . In m aking t h i s t e s t ,
d i l u t e n i t r i c a c id was added to th e f i l t r a t e b e fo re a d d in g
s i l v e r n i t r a t e , in o r d e r t o p re v e n t th e fo rm a tio n o f any
s i l v e r o x a l a t e T h e p r e c i p i t a t e was rem oved from th e
p a p e r, c a re b e in g ta k e n n o t to g e t any p a r t i c l e s o f p a p e r
f i b e r m ixed in w ith th e o x a l a te . I t was p la c e d in a b e a k e r
and d r ie d in an e l e c t r i c oven a t a te m p e ra tu re below 110° C.
A f te r th e o x a la te was c o m p le te ly d ry , i t was gro u n d t o a f in e
powder in a p o r c e la in m o rta r and th e n p la c e d in w e ig h in g
b o t t l e s f o r f u r t h e r u s e .
^■ J. B la s d a le , J . Amer. Chem. S o c. 3 1 , 917, (1 9 0 9 ).
CHAPTER IV
CALCIUM OXIDE AS THE WEIGHING FORM OF CALCIUM
DISCUSSION OF THE PROBLEM
C alcium o x a la te i s n o t w eighed a s su ch b ecau se i t s
co m p o sitio n a c c o rd in g to many i n v e s t i g a t o r s i s c o n s id e re d
to be n o t s u f f i c i e n t l y u n ifo rm . I t i s m a in ly c a lc iu m o x a la te
m o n o h y d rate. T h is compound on d ry in g lo s e s w a te r in c o m p le te
l y , and a t 200° C. i t b e g in s to decompose i n t o c a lc iu m c a r
b o n a te and ca rb o n m o n o x id e.12 j n a d d i tio n to b e in g u n s ta b le
a t te m p e ra tu re s above 100° C ., t h i s d r ie d p r e c i p i t a t e i s
s tr o n g ly h y g r o s c o p ic .13 Due to th e s e c o n d itio n s i t i s e x
p e c te d t h a t w eig h in g th e c a lc iu m a s th e o x a la te w ould n o t
g iv e s a t i s f a c t o r y q u a n t i ta t i v e r e s u l t s .
On i n v e s t i g a ti o n W i n k l e r , w h o had t e s t e d a number
o f q u a n t i t a t i v e m eth o d s, recommended a s th e m ost r e l i a b l e ,
th e p r e c i p i t a t i o n o f th e c a lc iu m a s o x a l a te , fo llo w e d by th e
i g n i t i o n t o th e o x id e . B ru n c k ,15 a f t e r m aking a s e r i e s o f
i n v e s t i g a t i o n s , found t h a t th e g r a v im e tr ic m ethods f o r th e
o x id e d e te r m in a tio n s were more v a r i a b l e .
12F a le s and Kenny, In o rg a n ic Q u a n tita tiv e A n a ly s is .
(1939) p . 306.
13
K o lth o ff and S a n d e ll, Q u a n tita tiv e C hem ical A n a ly s is .
(1936) p . 328.
14C. W in k le r, Z. Angew. Chem. 31, (1), 187, 203, (1 9 1 8 ).
1 5 0 . B runck, Z. A n a l. Chem. 4 5 , 7 7 , (1 9 0 6 ).
I g n i t i o n o f th e o x a la te lias some s e r io u s d is a d v a n ta g e s
U n less th e p r e c i p i t a t e i s c o m p le te ly b ro k en up i t may be d i f
f i c u l t to o b ta in com plete c o n v e rs io n to th e o x id e . I g n i t i o n
to th e o x id e b e g in s a t a b o u t 600° C ., b u t i t i s p r e f e r a b le
to c a r r y o u t t h i s i g n i t i o n above 850° C. On h e a tin g ca lciu m
o x a la te i t decom poses t o c a lc iu m c a rb o n a te w hich decom poses
g r a d u a lly u n d er some c irc u m s ta n c e s to th e o x id e a t a te m p e ra
t u r e below 600° C. The r e a c t i o n i s a r e v e r s i b l e o n e, and
f o r e v e ry te m p e ra tu re th e r e i s a c o rre s p o n d in g e q u ilib r iu m
p r e s s u r e o f carb o n d io x id e . T h is b e h a v io r i s to be e x p e c te d
i f th e sy stem c o n s i s t s o f two s o l i d p h a s e s , c a lc iu m o x id e and
c a lc iu m c a r b o n a te , and a v a p o r phase o f carb o n d io x id e . At
low te m p e ra tu re s th e phenomena, of. r e a b s o r p tio n p ro c e e d slo w ly
and depend v e ry much on th e c o n d itio n o f th e s o l i d p h a s e .
The te m p e ra tu re o f d i s s o c i a t i o n o f ca lciu m c a rb o n a te a t
a tm o sp h e ric p r e s s u r e , 760 mm., a c c o rd in g to R i e s e n f e ld ,16 i s
908^5° C. H e d v a ll1? found th e te m p e ra tu re f o r th e d i s s o c i a
t i o n to be 913-923° C ., and J o h n sto n 18 found i t to be 898° C.
I g n i t i o n o f th e c a lc iu m t o th e o x id e a t a h ig h te m p e ra tu re
rem oves any t r a c e s o f carb o n w hich m ig h t be form ed a t a lo w er
te m p e r a tu r e . I g n i t i o n t o th e o x id e sh o u ld alw ays be done
l s R ie s e n f e ld , J . Chem. P h y s. . 7 , 561, (1 9 0 9 ).
■ ^ H ed v all, Z e it s c h . A norg. Chem. . 4 7 , 98, (1 9 1 6 ).
18J o h n s to n , J . Amer. Chem. S o c ., 32, 938, (1 9 1 0 ).
1 4
w ith o u t th e l i d s . The d e c o m p o sitio n o f th e c a rb o n a te ta k e s
• •
p la c e more r a p i d l y i f th e carb o n d io x id e i s a llo w e d to e sc a p e
f r e e l y . I g n i t i o n t o th e o x id e w ith th e use o f p la tin u m
c r u c i b le s g iv e s s a t i s f a c t o r y q u a n t i t a t i v e r e s u l t s . W ith
p o r c e la in c r u c i b le s a much h ig h e r te m p e ra tu re i s n eed ed and
more tim e i s r e q u ir e d . I t was found t h a t th e F is h e r b l a s t
b u rn e r was n e c e s s a r y and th e c r u c i b le s had to be co v e re d w ith
c la y c y lin d e r s b e fo re a c o n s ta n t w e ig h t o f o x id e co u ld be
o b ta in e d . The d i f f i c u l t y a r i s e s in th e f a c t t h a t th e i n t e r i o r
p o r tio n o f th e p r e c i p i t a t e d o es n o t r e a c h th e r e q u ir e d te m p e ra
tu r e w ith o r d in a r y b u r n e r s . Many i n v e s t i g a t o r s d i f f e r on
th e i g n i t i o n p ro c e d u re , and a c c o rd in g to Scott,-*-9 th e o x a la te
decom poses c o m p le te ly and he s u g g e s ts t h a t 15 m in u te s o f
h e a tin g w ith a Meker b l a s t lamp i s s u f f i c i e n t to o b ta in co n
s t a n t w e ig h t when u s in g p o r c e la in c r u c i b l e s .
The o b je c tio n to w eig h in g th e p r e c i p i t a t e a s th e o x id e
i s t h a t th e o x id e a b s o rb s w a te r and ca rb o n d io x id e d u rin g
c o o lin g and w e ig h in g . In o rd e r to red u ce t h i s e r r o r t o a
minimum, th e c a lc iu m o x id e i s co v e re d d u rin g t h i s tim e , and
th e p ro p e r w e ig h ts a re p la c e d on th e b a la n c e pan b e fo re th e
c r u c ib le i s rem oved from th e d e s i c c a t o r .
19S c o t t , S ta n d a rd M ethods o f C hem ical A n a ly s is . V o l. I ,
p . 107.
EXPERIMENTAL PROCEDURE
U sing c a l i b r a t e d 50 m l. p i p e t t e s , 50 m l. sam ples were
drawn from s o lu t io n n o . 1 . The c a lciu m c h lo rid e s o l u t i o n
was h e a te d to i n c i p i e n t b o i l i n g , and 50 m l. o f ammonium
o x a la te c o n ta in in g 1 g . o f ammonium o x a la te in s o lu t io n w ere
added slo w ly and w ith c o n s ta n t s t i r r i n g to th e h o t sam ple
s o l u t i o n . The sam ples w ere a llo w e d to s ta n d o v e r n ig h t. A f te r
f i l t e r i n g , th e f i l t r a t e was h e a te d and t r e a t e d w ith 10 m l.
o f 0 .2 5 M. ammonium o x a l a te , and i f more p r e c i p i t a t e was
form ed, th e s o lu t io n was f i l t e r e d a g a in . Whatman No. 40
f i l t e r p a p e r was u s e d . The p r e c i p i t a t e and p a p e r was t r a n s
f e r r e d to th e c r u c i b le s w hich had been p r e v io u s ly ig n ite d
and w eig h ed . The m a te r ia l was c h a rre d a t th e lo w e s t p o s s ib le
te m p e ra tu re . A f te r th e ca rb o n was b u rn ed off, th e sam p les
in th e p la tin u m c r u c i b l e s , on q u a r tz t r i a n g l e s , w ere h e a te d
o v e r F is h e r b u rn e rs .. The f i r s t h e a tin g was f o r one h o u r,
fo llo w e d by tw e n ty m inute h e a tin g s u n t i l a c o n s ta n t w eig h t
was r e a c h e d . The sam ples in th e p o r c e la in c r u c ib le s w ere
h e a te d f o r th e same le n g th o f tim e b u t o v e r F is h e r b l a s t
b u rn e rs a n d .th e c r u c i b l e s w ere co v ered w ith 20 g . f i r e c la y
a s s a y c r u c i b l e s . A ll i g n i t i o n s to th e o x id e w ere made w ith
th e c r u c ib le l i d s off. When th e c r u c i b le s w ere p a r t l y c o o le d ,
th e y w ere p la c e d in a d e s ic c a to r c o n ta in in g c o n c e n tra te d s u l
f u r i c a c id a s th e d ry in g a g e n t. The p o r c e l a in c r u c i b le s were
allo w e d to c o o l 30 m in u te s and th e p la tin u m c r u c i b le s f o r 20
16
m in u te s. The w eighings were perform ed a s q u ic k ly a s p o s s ib le ,
and a f t e r th e f i r s t w eig h in g , the p ro p e r w e ig h ts were alw ays
p la c e d on th e pan and th e r i d e r a d ju s te d to th e p ro p e r p o s i
t i o n b e fo re th e c r u c ib le m s removed from th e d e s ic c a to r , so
t h a t th e w eighings could be made more q u ic k ly and a c c u r a te ly .
The c r u c ib le s were alw ays han d led w ea rin g ru b b e r f in g e r t i p s
in o rd e r to a v o id th e e r r o r o f w eighing th e m o istu re d e p o s ite d
on the c ru c ib le from th e f i n g e r s .
The second group of oxide d e te rm in a tio n s were made
u sin g sample no. 2 which was th e p re p a re d calcium o x a la te
t h a t had been p la c e d in w eighing b o t t l e s . I n d iv id u a l sam ples
w eighing from 0 .3 g . t o 1 .0 g . were u sed . Both p o r c e la in
and p la tin u m c r u c i b le s were u sed a g a in . The same method a s
e x p la in e d above was fo llo w ed in th e i g n i t i o n , c o o lin g and
w eighing p ro c e d u re .
17
EXPERIMENTAL RESULTS
Table I
G ra v im e tric d e te rm in a tio n o f th e calciu m as th e oxide
As d eterm in ed from 50 m l. p o r tio n s c o n ta in in g 0 .5 g . sam ples
In p la tin u m c r u c ib le s
Wt. o f CaO j o 5a found
.5176
.5181
.5171
.5190
.5194
.5200
.5201
.5197
.5200
A v ..5186
.3699
.3702
.3697
.3710
.3712
.3716
.3717
.3715
.3716
Av. .3710
In P o r c e la in cru cib le©
Y/t. CaO
.5294
.5305
Av .~T5299
% Ca found
.3782
.3791
Av. .3787
TAble I I
G ra v im e tric d e te rm in a tio n o f th e calcium a s th e oxide
As d eterm in ed from calcium o x a la te (Sample no. 2)
In p la tin u m c r u c ib le s
Wt. sample Wt. CaO % Ca
.5645 .2166 .2743
.5401 .2077 .2749
.6469 .2476 .2736
Av. .2739
In p o r c e la in c r u c ib le s
Wt. sample Wt. OaO % Oa
.4851 .1890 .2785
.6582 .2534 .2752
.7890 .3058 .2770
.6469 .2476 .2739
Av*.2769
DISCUSSION OF RESULTS
From th e r e s u l t s o b ta in e d in the d e te rm in a tio n o f
calciu m a s o x id e , both in th e p re lim in a r y a n a ly s is o f th e
s o lu tio n and th e f i n a l check on th e s o l i d sample o f th e
o x a l a te , i t seems e v id e n t t h a t much g r e a t e r c o n s is te n c y i s
found by th e use of p la tin u m c r u c i b l e s . M oreover, th e r e s u l t s
o b ta in e d in p o r c e la in a re d i s t i n c t l y h ig h e r. Since i t i s
m a n if e s tly im p o ssib le to reduce calciu m oxide to a low er
v alen ce form , o r to a low er m o le cu lar w eight compound under
a n a l y t i c a l c o n d itio n s , i t seems e v id e n t t h a t th e r e s u l t s
o b ta in e d in p la tin u m a re more a c c u r a te .
I t i s custom ary in many e d u c a tio n a l l a b o r a t o r i e s to
c a r r y o u t th e i g n i t i o n s to calciu m oxide in p o r c e la in c r u
c i b l e s , m ain ly f o r re a s o n s o f economy, b u t th e r e s u l t s here
found in d ic a te t h a t th e p e rc e n ta g e s o f calcium found in t h i s
way must alw ays be to o h ig h .
CHAPTER V
CALCIUM CARBONATE AS THE WEIGHING FORM OF CALCIUM
DISCUSSION OF THE PROBLEM
W illa rd and B o ld y re ff 20 in v e s tig a te d th e g ra v im e tr ic
method o f calcium c a rb o n a te as th e w eighing form o f calciu m .
T h e ir in v e s t i g a t i o n le d them to b e lie v e t h a t th e calcium could
b e s t be weighed a s a c a rb o n a te . They found t h a t a t a te m p era
tu r e a s low a s 400° C ., a th re e hour i g n i t i o n gave p r a c t i
c a l l y q u a n t i t a t i v e r e s u l t s , i f th e sample was n o t to o l a r g e .
They a ls o observed t h a t a t 450° C. and 500° C ., one hour
i g n i t i o n s gave good q u a n t ita tiv e r e s u l t s b u t a t 550° C. and
above th e r e s u l t s were low er due to the d i s s o c i a t i o n o f th e
ca rb o n a te in to calcium oxide and carbon d io x id e . The r e s u l t s
o f ? / i l l a r d and B o ld y re ff were d eterm in ed from i g n i t i o n s made
in an e l e c t r i c c r u c ib le fu rn a c e in which th e te m p e ra tu re was
c o n t r o ll e d w ith a v a r i a t i o n o f 5 d e g ree s C. As a com parison
to th e above in fo rm a tio n , F r e s e n i u s ^ was o f th e o p in io n t h a t
ca lciu m ca rb o n a te rem ained u n a lte r e d when exposed to th e a i r
a t 100° C. and even a t a low re d h e a t, b u t upon a p p ly in g
s tr o n g e r h e a t i t would d i s s o c ia te t o the o x id e . U nless one
were e x p e rie n c e d w ith t h i s method o f p ro c e d u re , r e l y i n g on
SO w iH ard and B o ld y re ff. J . Amer. Chem. Soc. 52. 1888.
(1930).
21jpr e s e n i u s , Q u a n tita tiv e Chemical A n a ly s is . V ol. I,
p . 174.
20
th e c o lo r o f th e c r u c ib le w hile h e a tin g would n o t w arra n t
fa v o ra b le q u a n t ita tiv e r e s u l t s .
From Jo h n sto n * s m easurem ents of e q u ilib r iu m p r e s s u r e s
o f th e th e rm a l d i s s o c i a t i o n s o f calciu m carb o n ate and from
th e p a r t i a l p re s s u re o f carbon d io x id e in a i r i t was found
t h a t calciu m c a rb o n a te was s ta b le in a i r up t o 550° C .22
A ccording to A ndrussow ,2^ d i s s o c i a t i o n p r e s s u r e s of
calciu m c a rb o n a te depend to a c e r t a i n e x te n t on such f a c t o r s
a s p a r t i c l e s i z e , th e degree o f p e r f e c t io n o f th e c r y s t a l s ,
a n d -th e p resen ce o f t r a c e s o f im p u r itie s . At 500° C.
R ie s e n fe ld 24 o b serv ed t h a t th e d i s s o c i a t i o n p re s s u r e o f c a l
cium ca rb o n a te was 0 .1 1 mm. A cc o rd in g ly , i f calciu m c a rb o n a te
were h e a te d a t t h i s te m p e ra tu re in such a manner as to a llo w
th e f r e e escape o f carbon d io ± id e , and i f th e p r e c i p i t a t e
were h e a te d in an atm osphere f r e e from carbon d io x id e , th e n
th e p r e c i p i t a t e could be q u a n t i t a t i v e l y decomposed to ca lciu m
o x id e . The atm osphere o r d i n a r i l y c o n ta in s about 0 .0 3 p e r
ce n t carbon d io x id e which i s e q u iv a le n t to 0.0003 x 760 =
0 .2 2 8 mm. p re s s u re o f carbon d io x id e a t o rd in a ry a tm o sp h eric
c o n d itio n s . At 500° C. th e d i s s o c i a t i o n p re s s u re i s l e s s
th a n t h i s , and a t such a te m p e ra tu re th e re would be no decom
p o s it io n o f calciu m c a rb o n a te . In th e l a b o r a t o r i e s th e carbon
2 2 jo h n sto n , J . Amer. Chem. S oc. . 32, 938, (1910).
23Andrussow, J . P h y s ik . Chem. . 110, 95, (1925).
2 4 R i e s e n f e l d , j # Chim. P h y s. . 7, 561, (1909).
21
d io x id e p re s s u re w i l l be two o r th r e e tim e s th e amount shown
by th e atm osphere o u ts id e , and t h i s c o n d itio n w i l l make i t
p o s s ib le to o p e ra te a t a h ig h e r te m p e ra tu re . At 550° C. th e
d i s s o c i a t i o n p re s s u re o f calcium ca rb o n ate i s 0 .5 7 mm. I f
th e p o r c e la in and p la tin u m c r u c ib le te m p e ra tu re i s judged by
h e a tin g to a d u ll re d C olor a s su g g e sted by F re s e n iu s , and
under o rd in a r y c o n d itio n s i f a te m p eratu re o f 500° C. o r
more i s re a c h e d unknow ingly, th e calcium c a rb o n a te w i l l have
gone in to a p a r t i a l d i s s o c i a t i o n . For t h i s re a s o n i t was
su g g e sted t h a t th e p r e c i p i t a t e should b e o tr e a te d w ith a few
drops o f ammonium ca rb o n a te s o lu t io n to co n v e rt any oxide
formed in tc r ca rb o n ate and a g a in ig n ite d a t a low te m p e ra tu re
f o r a few m in u te s, and th e n w eig h e d .25 T h is p ro ced u re w ith
ammonium ca rb o n a te sh o u ld be re p e a te d between h e a tin g s u n t i l
th e w eight i s c o n s ta n t. W ith th e s e p r e c a u tio n s in mind i t
i s q u ite obvious t h a t i t i s f a r more co n v e n ien t to use an
e l e c t r i c c r u c ib le fu rn a c e a d ju s te d to a te m p e ra tu re o f 475° C.
to 525° C. f o r th e purpose o f d eterm in in g th e calciu m o x a la te
a s ca lciu m c a rb o n a te .
When th e e l e c t r i c fu rn a c e i s used fo r c o n v e rtin g th e
ca lciu m o x a la te in to th e c a rb o n a te , i t should be p ro v id e d
w ith a pyrom eter o r a s u it a b le therm om eter. The fu rn a c e
sh o u ld be c a l i b r a t e d by means o f a therm ocouple o r by making
use of th e m e ltin g p o in ts o f c e r t a i n s u it a b le s a l t s .
25F re s e n iu s , Q u a n tita tiv e Chemical A n a ly s is , p . 269.
22
I t has been recommended t h a t th e p r e c i p i t a t e sh o u ld
be c o l le c te d in Gooch o r p o r c e la in f i l t e r c r u c i b l e s , and n o t
in paper,* a s t h i s g iv e s an. ig n it e d r e s id u e c o n ta in in g ca rb o n .
The c o lo r of th e p r e c i p i t a t e i s g re y . The i g n i t i o n o f chem i
c a l l y pure calcium o x a la te a s p u rch ased lik e w is e r e s u l t s in
a p ro d u ct which i s n o t p e r f e c t l y w hite in c o l o r .
The stu d y and th e in v e s t i g a t i o n o f th e i g n i t i o n o f
th e v a rio u s forms o f calcium w i l l have i l l u s t r a t e d s e v e r a l
re a s o n s why d is c r e p a n c ie s a re l i a b l e to ap p e ar in recommenda
t i o n s c o n c ern in g th e r ig h t, c o n d itio n s f o r i g n i t i o n s . A ccord
ing to Foote and B r a d le y ,26, who had in v e s tig a te d many o f th e
m ethods o f c a rb o n a te d e te rm in a tio n , th e d ecom position o f
ca lciu m o x a la te to carb o n ate a t a low er te m p e ra tu re , w ith o r
w ith o u t the a d d itio n o f ammonium c a rb o n a te , g iv e s in a c c u r a te
r e s u l t s because a t th e te m p eratu re where th e o x a la te decom
p o ses r a p id ly , th e d i s s o c i a t i o n o f th e c a rb o n a te i s a p p re
c i a b l e . They recommend th e i g n i t i o n o f th e calcium o x a la te
to th e b arb o n ate by h e a tin g th e o x a la te a t a te m p e ra tu re b e
tw een 675° C. and 800° C. in a c u r r e n t o f d ry carbon d io x id e .
They found t h a t th e o x a la te decomposed r a p i d l y and c o m p le te ly
to th e c a rb o n a te , g iv in g good r e s u l t s . However, t h i s method
i s n o t a v e ry co n v e n ien t p ro c e d u re .
26Foote and B ra d le y , J X Amer. Chem. S a c ., 48, 676,
(1926).
EXPERIMENTAL PROCEDURE FOLLOW ED
Sample n o . 2, c o n ta in in g th e calciu m o x a la te in w eigh
in g b o t t l e s , was used f o r th e ca rb o n a te d e te rm in a tio n . W eighing
by d if fe re n c e was th e method used in w eighing th e sam ples f o r
t h i s d e te r m in a tio n , s in c e t h i s a v o id s tr o u b le from th e some
what h y g ro sc o p ic n a tu re o f th e o x a la te . The sam ples were
p la c e d in p o r c e la in c r u c i b l e s , t h a t had been p r e - i g n i t e d ,
co o led in a d e s ic c a to r , and weighed a f t e r c o o lin g w ith th e
c r u c ib le l i d s . The w eighing b o t t l e and c r u c ib le s were o n ly
h an d led a f t e r th e two f i n g e r s and thumb had been covered w ith
th e ru b b e r f in g e r t i p s .
The i g n i t i o n s to th e c a rb o n a te were made in an e l e c t r i c
c r u c ib le fu rn a c e a t a te m p e ra tu re between 475° C. and 525° C.
The therm om eter on t h i s fu rn a c e had been p r e v io u s ly c a l i b r a
te d by ch e m ic a lly pure s a l t s which had s u it a b le m e ltin g p o i n t s .
The f i r s t h e a tin g was c o n tin u e d f o r two h o u rs , fo llo w ed by
30 m inute i g n i t i o n s u n t i l a c o n s ta n t w eight was re a c h e d . The
i g n i t i o n s were made w ith th e l i d s o f f th e c r u c i b l e s . A f te r
th e i g n i t i o n , th e c r u c i b le s were covered and cooled in th e
d e s ic c a to r f o r 30 m in u te s. The c r u c ib le s were weighed w ith
th e l i d s on, a s r a p i d l y a s p o s s i b le . A fte r the f i r s t w eig h in g ,
th e p ro p e r w e ig h ts were alw ays p la c e d on th e b alan ce pan and
th e r i d e r a d ju s te d b e fo re th e c r u c ib le s and co v ers were ta k e n
from th e d e s i c c a t o r . The c a rb o n a te was n o t n e a r ly a s hy g ro
sc o p ic a s th e o x id e , but s t i l l i t does g a in w eight more
r a p i d l y on th e b a la n c e pan th a n one m ight e x p e c t from pre
v io u s l i t e r a t u r e on th e s u b je c t. The p re c a u tio n s about
m a n ip u la tio n and w eighing here su g g e sted seem j u s t i f i e d .
25
EXPERIMENTAL RESULTS
Table I I I
G ra v im e tric d e te rm in a tio n s of calciu m a s tlie c a rb o n a te
As d eterm in ed from ca lciu m o x a la te
P re lim in a ry r e s u l t s
Wt. o f sample
..5329
.8391
1.0503
.8393
.7151
.8091
.7684
Wt. o f CaCO
.3576
.5730
.7179
.5732
.4889
.5526
.5251
j o Ca found
.2738
.2734
.2737
.2735
.2737
.2735
.2736
Av. .2735
Table IV
G r a v -M e tr ie I d e te rm in a tio n !o f' ca lciu m a s th e ^ c d rb o n a te
As d eterm in ed from calciu m o x a la te
F in a l r e s u l t s
Wt. o f OaCO^
.3059
.3736
.2973
.3802
.3042
.4592
.3920
Wt. o f sample
.4495
.5493
.4364
.5572
.4477
.6754
.5764
% Ca found
.2726
.2724
.2728
.2730
.2722
.2721
.2724
Av. .2725
DISCUSSION OF RESULTS
26
The r e s u l t s in T able IIX and Table IV were o b ta in e d
from d i f f e r e n t l o t s o f calcium c a rb o n a te , and th e s l i g h t
d is c re p a n c y can be a t t r i b u t e d t o a l i t t l e v a r i a t i o n in th e
m o istu re in th e two l o t s .
In each ca se th e concordance in r e s u l t s i s h ig h ly
s a t i s f a c t o r y and seems s l i g h t l y b e t t e r th a n t h a t o b ta in e d
by i g n i t i o n to th e o x id e , even when th e i g n i t i o n was c a r r ie d
out, in p la tin u m c r u c i b l e s .
The averag e o f 0.2725 f o r w eighing a s c a rb o n a te i s
to be compared w ith t h a t o f 0.2739 f o r w eighing a s th e o x id e ,
ig n it e d in p la tin u m . I t w i l l be seen t h a t th e r e s u l t s a s
oxide come out a l i t t l e h ig h e r , and t h i s m ight be e x p e c te d
when one c o n s id e rs th e re a d y a b s o rp tio n o f carbon d io x id e
and m o istu re by th e o x id e , and th e d i f f i c u l t y o f co m p letely
rem oving e v e ry tr a c e of ca rb o n a te from th e i n t e r i o r o f th e
p a r t i c l e s o f th e o x id e . The f ig u r e o f 0.2769 f o r th e oxide
ig n i t e d in p o r c e la in comes much h ig h e r th a n th e r e s u l t s a s
c a rb o n a te , a s i t i s to be e x p e c te d from th e .p r e v io u s d is c u s
s io n .
CHAPTER VI
CALCIUM SULFATE AS THE WEIGHING FORM OF CALCIUM
DISCUSSION OF THE PROBLEM
The g ra v im e tr ic d e te rm in a tio n o f calcium a s ca lciu m
s u l f a t e i s a method w hich w a rra n ts fa v o ra b le c o n s id e r a tio n .
I n v e s ti g a to r s ag ree t h a t the s u l f a t e method i s a good w eigh
in g form b u t c o n s id e r th e co n v e rsio n o f th e p r e c i p i t a t e in to
th e s u l f a t e a s th e p o s s ib le source o f e r r o r . I f th e p r e c i
p i t a t e i s c o l le c te d on f i l t e r p a p e r, th e f i l t e r p ap er must
be burned b efo re th e co n v e rsio n to th e s u l f a t e i s made. I f
t h i s i s n o t done, calciu m ca rb o n a te i s form ed d u rin g th e
i g n i t i o n . When th e s u l f u r i c a c id i s added, th e re i s an e v o
l u t i o n o f carbon d io x id e which u s u a lly ca u ses a lo s s o f
m a t e r i a l , even i f th e c r u c ib le i s co v e re d . For t h i s re a s o n
i t i s b e t t e r to i g n i t e th e calcium p r e c i p i t a t e and co n v e rt
i t to th e o x id e , th e n a f t e r c o o lin g , add w a te r r a p i d l y to
s la k e th e lim e . D ilu te s u l f u r i c a c id i s th e n added to co n v e rt
th e re s id u e to th e s u l f a t e . The e x c e ss s u l f u r i c a c id i s th e n
removed by slow e v a p o ra tio n , o v er a d i r e c t flame or by means
o f an a i r b a th a t ab o u t 300° C. The f i n a l i g n i t i o n i s made
in a p o r c e la in c r u c ib le w ith o u t th e l i d o ver a low flam e
w hich produces a d u l l re d h e a t.
Anhydrous calcium s u l f a t e i s a lo o se w h ite powder
when a r t i f i c i a l l y p re p a re d . When i t i s exposed to th e a i r ,
28
i t slo w ly a b so rb s m o is tu r e .26 I t rem ains u n a lte r e d a t a d u ll
re d h e a t, but i f i t i s h e a te d to a b r ig h t re d n e s s , i t f u s e s ,
l o s i n g w eight from th e l o s s o f s u l f u r i c a c i d . ^ 7 A ccording
to T re a d w e ll-H a ll,28 h e a tin g ca lciu m s u l f a t e to a b r ig h t re d
w i l l cause c o n s id e ra b le l o s s in w eig h t due to th e d i s s o c i a
t i o n o f th e calciu m s u l f a t e and lo s s of s u l f u r t r i o x i d e .
pQ
M its o h e rlic h r e p o r t s t h a t th e r e i s no lo s s of s u l f u r t r i
oxide a t a d u l l re d h e a t, b u t fu s io n w ith a l o s s o f s u l f u r
t r i o x i d e o c c u rs w ith i g n i t i o n to th e b r i g h t re d h e a t. In
a d d i tio n to th e lo s s o f w eig h t a s a r e s u l t o f the d i s s o c i a
t i o n of th e s u l f a t e because of a h ig h te m p e ra tu re , th e re
o c c u rs a l o s s of w eight due to th e re d u c tio n of s u l f a t e to
a s u l f i d e , a s induced by th e sm all amount of o rg a n ic m a tte r
w hich i s c o n ta in e d in c h e m ic a lly pure s u l f u r i c a c i d . 30 A cids
o th e r th a n s u l f u r i c a c id must n o t be p r e s e n t because of t h e i r
te n d en cy to in c re a s e th e s o l u b i l i t y o f ca lciu m s u l f a t e . 31
26F re s e n iu s , Q u a n tita tiv e Chem ical A n a ly s is ^ p . 175.
27M its c h e r lic h , £ . o f P r a k t . Chem. . LXXXIII, 485.
28T re a d w e ll-H a ll, A n a ly tic a l C hem istry, V ol. I I , p . 216.
29M its c h e r lic h , A n a l. Chem. 1 s t e d . , 2, 66, (1904).
30W i l l i s and M a c ln tire , I n d . Eng. Chem. . 9, (1917).
31C aley and E lv in g , I n d . E ng. Chem. . 10, 264, (1938).
EXPERIMENTAL PROCEDURE FOLLOW ED
29
Sample no. 2 furn ish ed , the m a te r ia l f o r th e i g n i t i o n
and w eig h in g . W eighing hy d if f e r e n c e was th e method employed,
in w eighing th e sam ples f o r a n a l y s i s . The m a te r ia l was.
p la c e d in p o r c e la in c r u c ib le s and i g n i t i o n to th e oxide was
made by h e a tin g th e calcium o x a la te f o r 30 m in u te s w ith F is h e r
b u r n e r s . The oxide was allo w ed to c o o l, and th e n from 2 -3 m l.
o f w a te r was added r a p i d l y to each c r u c ib le to sla k e th e '
o x id e . 0 .9 m l. o f 6 N. s u l f u r i c a c id was added f o r each
0 .1 g. of a sam ple, so t h a t a 0 .6 g . sample would r e q u ir e
5 .4 m l. o f a 6 N. s u l f u r i c a c id . The s u l f u r i c a c id was added
v e ry c a u tio u s ly to a v o id any l o s s , in case th e re sh o u ld be
t r a c e s of carbon d io x id e g iv en o f f . The e x c e ss s u l f u r i c a c id
was removed by p la c in g th e c r u c ib le s on an a s b e s to s w ire
gauze and h e a tin g v e ry c a u tio u s ly . The i g n i t i o n s were made
by h e a tin g th e c r u c i b le s to a d u l l r e d h e a t f o r 10 m inute
i n t e r v a l s . The c r u c ib le s were h e a te d w ith th e c r u c ib le l i d s
o f f . The c r u c ib le s were co o led f o r 30 m in u tes in th e d e s ic
c a to r b e fo re w eig h in g . The c o r r e c t w e ig h ts were p la c e d on
th e b alan ce pan and t h e . r i d e r a d ju s te d to th e p ro p e r p o s it io n
b e fo re th e c r u c ib le s were ta k e n from th e d e s ic c a to r sin c e
th e s u lf a t e a ls o seems to be r a t h e r h y g ro sc o p ic . The c r u
c i b l e s were weighed w ith th e co v e rs on. The w eighing b o t t l e
and c r u c ib le s were h an d led o n ly a f t e r th e f in g e r s and thumb
had been covered w ith ru b b e r f in g e r t i p s .
30
EXPERIMENTAL RESULTS
Table V
G ra v im e tric d e te rm in a tio n o f calciu m a s th e s u l f a t e
As d eterm in ed from calciu m o x a la te
P re lim in a ry r e s u l t s
Wt. o f sample W t. o f CaSO^ % Ca found
.7776 .7231 .2733
.5632 .5219 .2727
.5850 . 5412 .2723
.6095 .5673. .2740
.3839 .3555 .2724
.4660 .4314 .2725
.4035 .3745
T able VI
.2732
Av. .2729
G ra v im e tric d e te rm in a tio n o f calciu m a s th e s u l f a t e
As d eterm in ed from calcium
F in a l r e s u l t s
o x a la te
Wt. o f sample Wt. o f CaSOA % Ca four
.5004 .4627 .2721
.5339 .4944 .2727
.5531 .5118 .2723
.7611 .7042 .2721
.8480 .7877 .2723
.4020 .3717 .2721
.4711 .4357 .2722
.4461 .4124 .2720
Av. .2722
DISCUSSION OF RESULTS
31
The d e te rm in a tio n o f calciu m by w eighing a s a s u l f a t e
has much to recommend i t . While c a r e f u l m a n ip u la tio n i s
needed to av o id any l o s s from s p l a t t e r i n g , t h i s m a n ip u la tio n
i s n o t p r o h i b i t i v e l y d i f f i c u l t . A gain, th e r e a re s l i g h t
v a r i a t i o n s between T able V and Table VT, due to th e two l o t s
o f calciu m o x a la te a s d is c u s s e d under the c a rb o n a te method
o f d e te rm in a tio n .
On com paring th e r e s u l t s shown in Table VI w ith th o se
f o r th e d e te rm in a tio n a s c a rb o n a te , shown in T able IV, we
f i n d 0.2722 a g a in s t 0 .2 7 2 5 . T h is v a r i a t i o n f a l l s p r a c t i c a l l y
w ith in th e l i m i t s o f e x p e rim e n ta l -e rro r, so th e s u l f a t e and
c a rb o n a te m ethods seem to s ta n d sid e by s id e in p o in t o f
a c c u ra c y .
CHAPTER V I I
SU M M A RY
A stu d y and i n v e s t i g a t i o n h as te e n c a r r ie d o u t in a
com parative way of th e th r e e p r i n c i p a l g ra v im e tric m ethods
f o r th e d e te rm in a tio n s o f calciu m .
C e rta in la b o r a to r y te c h n iq u e s have been d is c u s s e d and
have been shown n e c e s s a ry i f r e l i a b l e r e s u l t s a re t o be ob
ta in e d .
By d i r e c t com parison th e r e s u l t s of th e th r e e methods
o b ta in e d were a s fo llo w s :
W eighing as oxide.,, in p la tin u m : 0.B739 g . Ca.
?/eighing as o x id e , in p o r c e la in : 0.2769 g . Ca.
W eighing a s c a rb o n a te : 0.2725 g . Ca.
Yifeighing a s s u l f a t e : 0.2722 g . Ca.
From th e s e r e s u l t s i t ap p e ars t h a t th e l e a s t a c c u ra te
method i s t o i g n i t e th e oxide in a p o r c e la in c r u c i b l e , and
t h i s method i s n o t to be recommended, even f o r e d u c a tio n a l
l a b o r a t o r i e s . I f th e i g n i t i o n to oxide i s c a r r i e d out in a
p la tin u m c r u c ib le th e r e s u l t s a re j u s t a l i t t l e h ig h , but
a re s a t i s f a c t o r y f o r t e c h n ic a l w ork. The b e s t w eighing form s
seem t o be th e c a rb o n a te and th e s u l f a t e , and th e re seems
l i t t l e to choose between them in p o in t o f a c c u ra c y .
33
To c o n v e rt th e o x a la te to c a rb o n a te c o n v e n ie n tly , one
sh o u ld r e a l l y have an a c c u r a te ly a d ju s ta b le e l e c t r i c m u ffle
c r u c ib le f u r n a c e , and sin c e t h i s i s n o t a v a o ia b le i n many
l a b o r a t o r i e s , th e s u l f a t e method i s w orthy o f f a r more re c o g
n i t i o n th a n i t h as so f a r a t t a i n e d .
B I B L I O G R A P H Y
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University of Southern California Dissertations and Theses

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Asset Metadata

Creator Huth, S. H (author)

Core Title A critical study of the accuracy of various weighing forms of calcium

Degree Master of Arts

Degree Program Chemistry

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag chemistry, inorganic,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor [illegible] (committee chair), Copeland, Charles S. (committee member), Harrison, Bruce M. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c17-791160

Unique identifier UC11347768

Identifier EP41537.pdf (filename),usctheses-c17-791160 (legacy record id)

Legacy Identifier EP41537.pdf

Dmrecord 791160

Document Type Thesis

Rights Huth, S. H.

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA