SOLCBILITY O F CALCIUM HYDROXIDE BY L. B. hIILLER A S D J . C. WITT
Introduction The solubility of calcium hydroxide has been repeatedly determined by numerous workers with greatly varying degrees of precision. One of the first facts discovered was that the solubility of this compound decreases with rise in temperature. In summarizing the results of some of the more exact data, 34ellor’ [from the composit,e results of Dalton2, P h i l l i p ~ ,Wittstein4 ~ and Bineauj] gives the solubilit,y of calcium hydroxide expressed in grams of calcium oxide per 1000 grams of water as approximately 1.18 at 30 degrees C. Seidel1,e from the average curve obtained by plotting the results of Lamy, Mobeu, Herzfeld and Guthrie, gives a solubility for this substance of 1.16 expressed as grams calcium oxide per 1000 grms of saturated solution at 2 j°C. A. Lamy’ found t,hat calcium hydroxide varies in solubility according to the source from which it is obtained. He gives the following values for the solubility expressed as grams calcium oxide per liter of solution at 30 degrees C . (‘alcium hydroxide obtained by hydration of calcium oxide I . 142 from the calcination of calcium nitrate Calcium hydroxide obtained by hydration of calcium oxide from I . 162 the calcination of marble Precipitated calcium hydroxide 1.19j With the increased interest in solutions during the last quarter century, the solubility of calcium hydroxide has been redetermined under carefully controlled conditions. Soyes and Chapin8 obtained a value of 1.13 grams of calcium oxide per liter of saturated solution at 25°C. Bassett and Taylorg in their study of the three-component system calcium nit,rate-calcium oxidewater, obtained a solubility of 1.150 grams of calcium oxide per 1000 grams of saturated solution at 2 5°C. They used calcium carbonate precipitated from calcium nitmte by ammonium carbonate as their source of calcium oxide. The solubility of the compound was det,ermined analytically after shaking for six months in cerasin-coated bottles at the indicated temperature. Cameron and Robinsonlo obtained a value for the solubility of 1.18grams of calcium oxide per 1000grams of water at z j”C. I n their determination calcium oxide was shaken in small glass bottles for ninety days. “ A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” 3, 677 (1923). “ A S e w System of Chemical Philosophy,” 2, 331 (1810). Annals of Philosophr, (21, 17, 107 (1821). Report. Pharm., 1, i8z (181j). 5Ann. Chim. Phrs. (3), 51, 290 (1857). ;“The Solubilities of Inorganic and Organic Compounds,” (1919). ‘.inn. Chim. Phys. 1 5 ) , 14, 145 (1875). 8 No es and Chapin: 2. physik. Chem., 28, 518 (1899). J. $hem. Sac., 105, 1926 (1914). ’“J.Phys. Chem., 11, 273 (1906). 3
286
L. B. MILLER A S D J. C. TWlT
Purpose of the Investigation In connection with certain researches upon a related subject, it was fount1 desirable to determine the solubility of calcium hydroxide in water at ,io"C. It was of especial interest and value to us to know the solubility exhibited when freshly hydrated calcium oxide is kept in contact with water for n relatively short period of time. Apparatus and Method of Procedure The solubility and conductivity of calcium hydroxide in water was tlr. o o j was obtermined for a temperatme of 30°C. h temperature of 30' tained with a De Khotinsky constant,-temperature ivater-bat,h. Conductivity was measured by means of the Leeds and Yorthrup equipment for precision work. Well-steamed Pyrex bottles of one liter capacity were used as containers. The cell constant was determined at 3oOC. with solutions made from conductivity water and three times recrystallized and fused potassium chloride. Calcium oxide was obtained from t,wo sources. Iceland spar was ignited t o constant weight in plat,inum, and constit,uted the first source. For the other a c.p. grade of precipitated calcium carbonate was dissolved in c.p. nitric acid, and precipitated with a similar grade of ammonium carbonate. The precipitated calcium carbonate was filtered, washed, dried, and ignited t o constant weight in platinum. Conductiilty water was obtained by distillation from alkaline permanganate solution. Since, from the nature of the materials the solubility of which was being studied (fine-grained solids), it was difficult t o obtain samples of the solution for analysis free from the solid phase, except by allowing the solutions to stand a t constant temperature over periods of several hours, the conductivity of the solution was used t o indicate when equilibrium had been established. This method has the advantage of being simple and rapid, and of being unaffected by the presence of the undissolved solid materials. Determinations of conductivit,y may be made after successive small increments of time and the approach to equilibrium may thus be closely followed. Two complete and separate determinations of the conductivity and solubility of calcium hydroxyde were made. The procedure followed in t,he first solubility determination was as follows: Two separate portions of Iceland spar were ignited to constant weight in platinum containers. Each of these portions was then divided into fractions A and B. Samples I-A and 2-A were slaked with a small quantity of conductivity water and allowed to st,and over night in stoppered containers. These were then placed in well-steamed one-liter Pyrex flasks containing conductivity water previously brought to temperature, immersed in the water bath a t 3 0 O C . and shaken in a mechanical shaker until equilibrium was reached. At the same time samples I-B and a-B were placed in one-liter Pyrex bottles nearly filled with conductivity mater. allowed to slake, and likewise shaken until equilibrium was reached. In all cases sufficient calcium oxide had been used so that an excess of the solid phase was present. The
287
SOLCBILITY O F CALCIUM HYDROXIDE
conductivity of the solutions was determined from time to time until three successive readings at two-hour intervals of shaking showed no further change in conductivity By this procedure, solubility equilibrium was approached from both sides. In the case of the samples 1--4and 2-A. which had been previously slaked with water the conductivity was found to increase with time until a constant value was reached Sample I-Band 2-B formed supersaturated solutions of calcium hydroxide, the conductivity first rising to a maximum and then decreasing to a constant value. The solid phase was then allowed to settle and aliquot portions of the supernatant solution drawn off for analysis. The calcium oxide content was determined by precipitation as the oxalate followed by ignition to constant weight. The results are given in Table I.
TABLE I Sample
I-A I-B 2 -A 2-B Average
Solubility expressed as grams CaO per 1000 cc of saturated solution at 3oT
Conductivity expressed as mhos X 1 0 3
1.19j
8.937 8,937 I . 198 8.918 I .2 0 2 8.946 1 I99 8.933 Before the second series of experiments was made. some time had elapsed. The cell constant was therefore redetermined.. Sample 3 was prepared by igniting a sample of Iceland spar t o constant weight in platinum. It was then divided into portions 3-A and 3-B. A sample of precipitated calcium carbonate (prepared as previously described) was similarly ignited and divided into portions 4-A and 4-B. Samples 3-.A and 4-A were first hydrated in a small quantity of conductivity water for twenty-four hours. They were then placed in one liter quantities of conductivity water at 30'C. and shaken until no further change in conductivity occurred with three readings taken at two hour intervals. At the same time, samples 3-B and 4-B were thrown into one liter quantities of conductivity water brought t o a temperature of 3 0 T . and similarly observed. The solubility was determined as in the previous series. The values obtained are given in Table 11. The samples were then shaken for seventy-two hours more, but no further change in conductivity occurred. TABLE I1 Sample
I .200
'
Solubility expressed as grams CaO per 1000 cc. of saturated solution at 30°C
3-A 4-A 3-B
I.
4--B
I.
Average
196
1 . I94
I .2 0 1
196
I . 197
Conductivity expressed as mhos X 101
8.856 8.856 8.929 8.911 8.888
288
L. R . MILLER A S D J. C. WITT
I n Fig. I are plotted the time-conductivity curves for samples I-h and I-B. These are typical of the type of curves obtained in this work.
Discussion
h comparison of the values for the solubility of calcium hydroxide obtained in this work with the best values reported in the literature show them to be somewhat higher than those previously obtained. I t is a well-known fact that a very finely divided crystalline substance will show a higher solu-
cy
z .?SO
k
52 aoo T
s 2 2 0
110
Y
z b I 100 h
t
?
Y
50
z
0
e
E
TIWE
IN
ao HOURS
FIG.I Rate of Solution of Calcium Hydroxide as indicated by Conductivity.
bility t,han the same substance in a more coarsely crystalline state. The solution of higher solubility will gradually assume the same solubility as that of t,he more coarsely crystalline substance if the solid phase is left in contact with the solution for a sufficient time. The calcium hydroxide used in this work, prepared by the rapid hydration of calcium oxide, was very finely crystalline as was readily demonstrated with a microscope. I t is possible, therefore, that, the difference in solubility of calcium hydroxide as determined here and in previous researches, may be accounted for by a difference in the size of particles constituting the solid phases. I t must be emphasized, hoxever, that, if such is t,he case, the true solubility value is represented by an equilibrium which is attained very slowly, since the equilibrium value we obtained was maintained for several days without appreciable change, as was evidence by the constancy of the electrical conductivity. S o attempt was made to connect these results with those of previous workers by continuing the solubility experiments over an extended period of time.
SOLUBILITY O F C A L C I L X HYDROXIDE
289
Summary I . The solubility of calcium hydroxide in water as exhibited by freshly hydrated calcium oxide has been made the subject of a detailed study. The progress towards equilibrium of the system consisting of an excess 2. of freshly hydrated calcium oxide and water was followed closely by means of the electrical conductivity of the solution. 3 . Values for the solubility of calcium hydroxide and for the conductivity of the saturated solution are given. Chicago, Illtnois.
