166
R. SHEHYN AND D. B. PALL
T H E SOLUBILITY OF CALCIUM OXALATE I N VARIOUS SALT SOLUTIONS H. SHEHYN
AND
D. B. PALL
Analytical Laboratories, Aluminum Company of Canada, Arvida, Quebec, Canada Received November 94, 1938
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INTRODUCTION
Complementary to some analytical investigations, the solubility of calcium oxalate in various salt solutions has been established over a range of temperatures. The accuracy of these results appears to be greater than that of any previously reported work, making them of sufficient general interest to warrant publication. MATERIALS
Calcium oxalate monohydrate was made from Baker’s C.P. analyzed calcium chloride and oxalic acid. The material first obtained by direct addition of the reagents in hot solution, was recrystallized after dissolving in hot hydrochloric acid, by addition of ammonia solution. Finally it was filtered and washed with hot water and dried a t 105OC. The soft cake so obtained was reduced to a powder by grinding in a mortar. Sodium sulfate was prepared from sodium hydroxide and sulfuric acid used in equivalent proportions, crystallized by evaporation, and dried a t 105OC. Ammonium sulfate was prepared from the acid and base and crystallized by evaporation, keeping the mother liquor alkaline to methyl red by small additions of ammonia solution. The crystals were dried a t room temperature. When results obtained with this material were compared with those using ammonium sulfate of Baker’s C.P. analyzed grade, excellent agreement was found. All other salts and all starting materials were of Baker’s C.P. analyzed grade. EXPERIMENTAL METHOD
At room temperature: To 250 ml. of salt solution of the required concentration, 125 mg. or more of CaC204.HzO was added. The mixture was then stirred mechanically for a t least 10 min. After filtration, and without washing, the filtrate was acidified with 10 ml. of 1:1 sulfuric acid and titrated with 0.02 N potassium permanganate. Blanks were run in all cases. Temperatures were measured just before filtration and are good to about 0.5OC. The error due to hold-up in the filter paper is about 0.5 per cent; this was neglected. The highest concentration of chloride ion used did not affect the permanganate titration.
SOLUBILITY O F CALCIUM OXALATE
167
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At 50°C.: The mixture, made up as before, was placed in a covered beaker and mechanically stirred in a water bath maintained at 50OC.; it was then filtered through a funnel in a bath kept at 55°C.; in this manner the temperature of the mixture was maintained at 50°C. i'1 throughout the whole of the manipulation. At 95°C.:The solution was first made up with about 20 ml. of water in excess and heated to boiling with an excess of calcium oxalate. The mixture w a then poured into a specially constructed flask (A in figure 1).
[ y-. . _j ..
.. .
Fro. I . Set-up for determining solubilities at 95°C.
After a temperature of 99OC. had been reached, as indicated by the thermometer in A, filtration was begun by opening the stopcock. When the lower bath w&smaintained a t lOO"C., the material in the filter was at 95OC. This was then assumed to be the temperature to which the measured solubility corresponded. This assumption seems justified, because the filtration was relatively slow and because while the solution, which had been previously saturated at a higher temperature, was in the funnel it wm in intimate contact with crystalline monohydrate. The volume and temperature of the filtrate were now measured, and the volume calculated to mom temperature. In order to obtain the results calculated to the
168
H. SHEHYN AND D. B. PALL
volume at 95"C., 4 per cent should be subtracted from the values listed in the tables.
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TIME REQUIRED FOR EQUILIBRIUM
This point is dealt with rather fully, because it is believed that some of the data appearing in the literature are low, owing to lack of equilibrium conditions. In figure 2 curves are given in which the amount of calcium oxalate monohydrate dissolved is plotted against the time during which the mixture was rapidly stirred. If stirring were "occasionally by hand,"
FIG.2. Plot of calcium oxalate monbhydrate dissolved against the time during which the mixture was rapidly stirred. 0, 200 mg. of CaCzOc added per liter of solution; 0 , 400 mg. of CaC204added per liter of solution; 0 , 2000 mg. of CaCZO, added per liter of solution.
as is reported by at least one investigator (3), the curves would be considerably flatter, and especially if insufficient excess oxalate were added, low results might easily be obtained. From an examination of figure 2, it was easy to decide on a minimum excess of calcium oxalate and a stirring time which would still make certain the establishment of equilibrium. SOLUBILITIES
The solubilities are summarized in table 1. They are plotted against the concentrations at the three temperatures in figures 3 and 4. The solubility of calcium oxalate in water was determined by the same method and found to be 8.27 mg. per liter at 30"C., and 14.5 mg. per liter at 95°C. The value a t 50°C. was obtained by interpolation, using a
169
BOLUBILITY O F CALCIUM OXALATE
straight line on a log solubility-reciprocal temperature plot, and wm found to be 10.0 mg. of CaCzOd per liter. The units of solubility are milligrams of anhydrous salt per liter of solution. TABLE 1 Solubilitw_of .calcium oxalate in various salt solutions Downloaded by UNIV OF SHEFFIELD on September 11, 2015 | http://pubs.acs.org Publication Date: February 1, 1940 | doi: 10.1021/j150398a003
YILLIQRAYB OB
CaC2O4 PER LITER OF SOLUTION AT
SALT SOLUTION
27.5'C.
Sodium sulfate: 3 g. NazSO4 per liter. . . . . . . . . . . 15 g. NazSO4 per liter. . . . . . . . . . 40 g. iVasS0, per liter. . . . . . . . . . Ammonium chloride : 3 g. NH,Cl per liter.. . . . . . . . . . . 16.40 15 g. NH&l per l i t e r . ,, . , . . , , . , 28.63 40 g. NH,Cl per liter.. . . . . . . . . . 41.22 Ammonium nitrate: 3 g. ",NOs per l i t e r . . . . . . . . . . 14.66 40 g. ",NOS per l i t e r . . , . , , , . . 35.771 80 g. ",NO3 per l i t e r . . . . . . . . . 46.49 Ammonium sulfate: 3 g. (NHJ2S04per l i t e r . . . . . . . . 15 g. (NHJ~SOIper l i t e r . . . . . . . 25 g. ("I)ISOI per lite,r.. . . . . . 27.5 g. (NHa)zSO4§ per l i t e r . , . . 40.0 g. (NHI)~SO,$per l i t e r . . . . 40 g. (rU"4)ZSOI per liter.. . . . . . 90 g. (xH1)2S04 per l i t e r . . . . . . .
28'C.
30°C.
17.18 34.08 55.19
-
50°C. 9YC. --
24.97 49.75 84.30
39.9 83.5' 144.0
21.62 41.42 61.14
34.2 66.9 104.7
21.5 42.4 57.0 70.2 72.1 109.4
* Average of two results (83.1 and 83.9).
t Average of
two results (35.72 and 35.82).
$ Average of two results (133.0 and 135.0).
5 Baker's C.P. analyzed. 7 Average of two results (313.5 and 315.5). CORRELATION
While the results with ammonium chloride are in rough agreement with those of Maljaroff and Gluschakoff (3), those with ammonium sulfate and ammonium nitrate differ by factors of four and three, respectively. This serious discrepancy made some sort of absolute check desirable. This is available in the rule which states that the eaect of the addition of a second salt on the solubility of a given slightly soluble salt is the same for all salts and depends only on the ionic strength (2). Thus if we read from the curves of figures 3 and 4, the solubilities of
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CONCENTRATION OF SALT,
GRAMS PER LITER
FIG.3. Plot of solubility of calcium oxalate against the concentration of the salt solution. Curve 1, ammonium chloride a t 27.5"C.; curve 2, ammonium chloride a t 50°C.; curve 3, ammonium chloride a t 95°C.; curve 4, sodium sulfate a t W C . ; curve 5 , sodium sulfate at, 50°C.; curve 6, sodium sulfate a t 95°C.
CONCENTRATION OF
SALT, GRAMS PER LITER
FIQ.4. Plot of solubility of calcium oxalate against the concentration of the salt solution. Curve 1, ammonium nitrate a t 27.6"C.; curve 2, ammonium sulfate a t W C . ; curve 8, ammonium sulfate a t 60°C.; curve 4, ammonium sulfate a t 95'C. 170
171
SOLUBILITY OF CALCIUM OXALATE
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calcium oxalate should be the same a t the same "ionic strength concentration" of the second salt, regardless of the specific salt chosen. The ionic strength is defined as:
where m, is the molarity of the pth ion, and e, is the charge of the pth ion. For sodium sulfate and ammonium sulfate, the ionic strength is three times the molar salt concentration. For ammonium chloride and ammonium nitrate the ionic strength equals the molar salt concentration. In table 2 the solubilities of calcium oxalate are compared for various ionic strengths of the second salt. The small temperature extrapolations required were easily made using a plot of the logarithm of the solubility against the whole range of the reciprocal of the temperature, which should TABLE 2 Solubility of calcium oxalate i n various salt solutions, compared at the same ionic strengths at 38OC. YILLI(IRhYB O F
IONIC 6TRENOTH
-
0.01 0.02 0.05 0.10 0.20
NH~NOI
10.4 12.1 16.1 21.6 28.0
1
1
NH&I
10.0 11.6 15.9 20.2 25.9
CaCrOl PER
LITER O F SOLUTION
'
(NHOzSOt
i
10.3 12.0 16.6
~
I
NszS01
i
'
10.0
!
1
11.6 15.8
i ~
,
26.5
i
NaC1
11.3 11.6 13.3 18.6 23.0
give a straight-line relationship. The agreement is fairly good. It s e e m probable that the deviations a t higher concentration are not experimental, and simply show that at these concentrations the rule of ionic strength no longer holds better +h-n the experimental accuracy. The data for sodium chloride are from the work of Hammarsten (1). SUMMARY
The solubility of calcium oxalate in ammonium chloride, ammonium sulfate, and sodium sulfate sulutions has been determined at room temperature, 50°C., and 95"C., and over a range of salt concentrations. The solubility in ammonium nitrate solutions is given a t room temperature only. The solubilities a t room temperatures are correlated using the rule of ionic strength, and found to be consistent among themsclves, and also with outside data for sodium chloride solutions. REFERENCES (1) HAMMARSTEN, G . : Compt. rend. trav. lab. Carlsberg 17, S o . 11 (1929). (2) LEWIS,G. N., ASD RAKDALL, M.: Thermodynamics, p. 371. McGraw-Hill Book Company, Inc., New York (1923). (3) MALJAROFF, K . L., AXD GLUSCHAHOFF, A. J.: Z. anal. Chem. 93, 266 (1933).
