6.3. Solubility of Alkaline-Earth Metal Formates There are relatively few reports on the solubility of alkaline-earth metal formates in non-aqueous solvents and most of these are limited to formic acid and formic acid-water solvents. The solubility of alkaline-earth metal formates in formic acid is considerably less than the solubility of alkali metal formates in the same solvent. A summary of the solubility of alkaline-earth metal formates at 25 ºC is given in Table 1.
Binary Systems Two publications provide data for the solubility of alkaline-earth metal formates in formic acid.1,2 Kendall and Adler1 used the freezing point method to measure the solubilities of calcium formate in the 280 to 402 K temperature range and of barium formate in the 260 to 305 K temperature range. Paul, et al.2 used the isothermal saturation method to measure the solubility of calcium formate in formic acid at 25 ºC. In neither work is there any mention of the precision of the solubility measurements. The work of Paul, et al.2 does give the uncertainty (± 0.5 K) of the temperature measurements. The values for the solubility of calcium formate by Kendall and Adler1 is about half that reported by Paul, et al.2. This is true also for the solubility values for barium formate. A possible reason for these discrepancies is the difference in moisture content of the formic acid and of the salt. The work of Kendall and Adler1 fails to give essential details of the experimental procedure or an estimate of the uncertainty of the temperature and solubility measurements. Therefore, those data are rejected. The data of Paul, et al.2 are considered to be more reliable because of the greater precision of the results.
The solubility of strontium formate in formic acid was determined by Balashova, et al.3 at 298.2, 308.2 and 323.2 K. The solubility was also measured at 298.2 K by the isothermal saturation method2. The values for the solubility at 298.2 K given in these two reports agree within about 6%, but all these data are classified as tentative because of a lack of independently determined corroborating data.
There are no other reports for the solubility alkaline-earth metal formates in non-aqueous solvents except for the work of Henstock4 who reported the solubility of calcium formate in methanol at 298.2 and 339.2 K. The values, however, are classified as tentative because there is no other similar report available.
Ternary Systems Dunn and Philip5 measured the solubility of calcium formate, barium formate, magnesium formate and strontium formate in formic acid-water solvents at 25 ºC. A wide range of solvent concentrations was used. The precision of the temperature measurements is given but nothing is said about the precision of the analytical work. Balashova, et al.3 measured the solubility of strontium formate in formic acid-water solvents at 298.2, 308.2 and 323.2 K by the isothermal saturation method. This article is also one of the very few that also reports an estimate of the precision of the solubility values obtained. The two values given for the solubility at 298.2 K agree well with each other. The experimental method and the purity of the materials were the same for both these studies.
The solubility of alkaline-earth metal formates either increase or decrease with changing water content of the solvent. As the water content of the solvent increases, the solubility of Mg(CHO2)2 and Ca(CHO2)2 also increase, but the solubility of Ba(CHO2)2 and Sr(CHO2)2 decrease. It has often been said that an increase in the solubility of a salt on the addition of an acid indicates a change in the solid phase. So far as the alkaline-earth metals are concerned, it appears that the tendency to form crystallosolvates decreases in the following order: barium, strontium, calcium. A solvate of Ba(CHO2)2 with formic acid is stable in a solvent containing at least 22 mol % formic acid. Sr(CHO2)2·HCHO2 is unstable in solvents containing less than 74 mol % acid. No crystallosolvate of Ca(CHO2)2 with formic acid is formed even in a solvent containing 77 mol % formic acid. All solubility values reported for these systems are classified as tentative because no other data are available for comparison.