The alkali metal formates are readily soluble in formic acid. The available data for alkali metal formate solubilities in formic acid are summarized in Table 1.

In the remainder of this section the solubility data will be discussed and evaluated for each of the alkali metal formates.

Lithium formate

        (1) The LiCHO₂-HCHO₂ system. There are four publications that present data for the solubility of lithium formate in formic acid. In two of these,^1,2 the data were obtained by the freezing point method over a temperature range of 273.2 to 325 K (Groschuff¹) and 265 to 437 K (Kendall and Adler²). Paul et al.³ obtained solubility data at 298.2 K by the isothermal saturation method. The value of the solubility of lithium formate in formic acid at 298.2 K presented in these three papers agree within experimental error (refer to Table 1). The three publications also agree that the equilibrium solid phase is LiCHO₂. Taking into account the experimental information provided (or lack of it) and the method used to obtain the results, the recommended value for the solubility of lithium formate in formic acid at 298.2 K is 23.72 ± 0.1 mol %, and the equilibrium solid phase is LiCHO₂.

        A fourth source of solubility data for the LiCHO₂-HCHO₂ system is the work of Ivanova et al.⁴ These investigators used the freezing point method to study the system in the 258.2 to 295.2 K temperature range. They also reported the presence of solid phases of the type LiCHO₂·nHCHO₂ where n = 3, 3.5 and 4. However, the composition of these salts was not confirmed by analysis. The solubility data were presented only in graphical form. The data do agree within experimental error with other solubility data for the system. However, the composition of the equilibrium solid phase is controversial. Consequently, the data of Refs. (1-3) is preferred to those of Ivanova et al.⁴

        (1) The LiCHO₂-HCHO₂-H₂O system. There are two articles that report a study of this system.^5,6 In both studies the experimental method used was the isothermal saturation method. One of the studies⁵ was made at 298.2 K while the other⁶ was made at 298.2 and at 323.2 K. A direct comparison of the two studies cannot be made because, where the temperature was the same in both studies, the composition ranges are different. However, there are similarities that should be noted. Both studies show that at 298.2 K the solubility of lithium formate increases as the formic acid content increases from 12.13 to 23.18 mol %. A similar trend prevails at 323.2 K. The equilibrium solid phase is LiCHO₂·H₂O in systems having a formic acid concentration of 0 to 50 mol %. When the formic acid concentration exceeds 55 mol % the equilibrium solid phase is LiCHO₂. Thus, both studies are consistent with each other, but, since there is no independent experimental confirmation for either set of data, both the studies can only be classified as tentative.
        (2) The LiCHO₂-C₂H₆O-H₂O system. This system has been studied over the 298 to 338 K temperature range.⁷ The experimental values were obtained by the Schlieren method. The ethanol concentration varied from 0 to 40 vol %. The solubility of lithium formate decreased as the ethanol content of the system increased. Equation (1) is an empirical equation obtained by a curve-fitting procedure. S_II is the solubility of lithium formate in alcohol-water solvents while S_I is the solubility of the salt in water.

The graphical representation of the data shows that the solubility of lithium formate decreases from about 9.6 to 6.7mol % when the ethanol concentration increases from 0 to 40 mol % (these numerical values were calculated from the solubility curves by the evaluators). The lithium formate solubility in this system increased with increasing temperature. However, there is no other reported study of this system , so the values can be classified as tentative at best.

Sodium formate

        (1) The NaCHO₂-HCHO₂ system. Five papers report solubility data for this system. Two of these used the freezing point method to measure the solubilities in the 273.2 to 358.2 K temperature range¹ and the 255.8 to 410.9 K temperature range.² Paul et al. ³ and Elöd and Tremmel⁸ used the isothermal saturation method to determine solubility values at 298.2, 296.6 and 318.2 K but there is no indication as to the extent of error in these values. Solubility values at 298.2 K are given in Table 1. Some of the discrepancy in those data is due to the fact that in one study¹ the starting salt was NaCHO₂·HCHO₂ and the results were calculated in terms of NaCHO₂. The data of Paul et al.³ are considered to be more reliable than the values of others because the experimental precision is greater. A possible reason for the lower values⁸ is the fact the formic acid had a higher moisture content than that used in the other studies. Another difference in the reported results is the nature of the equilibrium solid phase at 298.2 K. Two studies^1,8 report it to be NaCHO₂·HCHO₂ while Kendall and Adler² report it to be NaCHO₂·2HCHO₂. Ivanova et al.⁴ used the freezing point method to measure the solubility of sodium formate in formic acid over a temperature range of 257 to 280 K. The data are presented only in graphical form. The existence of NaCHO₂·5HCHO₂ as an equilibrium solid phase is also reported but has not been confirmed in any other report. The recommended value for the solubility of sodium formate in formic acid at 298.2 K is 21.5 ± 1.2 mol % with NaCHO₂·HCHO₂ as the equilibrium solid phase. There are isolated reports of solubility data at other temperatures.^2,3,6 Solubility data at the system boiling point have also been reported,⁹ but there is no independent confirmation of any of these values. Hence, they can be considered as no better than tentative values.

        (2) Solubility in other solvents. The solubility of sodium formate in other non-aqueous solvents has been reported ,e.g., in acetic acid,¹⁰ in methanol,¹¹ in ethanol,^12,13 and in acetone.¹⁴ The values are summarized in Table 2. The two reports^12,13 on the solubility of sodium formate in ethanol at 298.2 K are in agreement with each other and this is the recommended value: 0.14 mol % with NaCHO₂ as the equilibrium solid phase.

        (1) The NaCHO₂-HCHO₂-H₂O system. Four publications present solubility data at the boiling poiont of the system: at 298.2 K ^8,15; at 293.2 K ¹⁶; and at 296.6 and 318.2 K 8. However, all the data are for different temperatures except for those at 298.2 K. The two sources for values at 298.2 K are for different compositions, but appear to be consistent with each other. Consequently, the values cannot be compared directly with each other , and so must be considered tentative.

        (2) The NaCHO₂-C₂H₆O-H₂O system. The three publications presenting data for this system^11,12,16 supplement each other. Solubility values were measured for the 292.7 to 313.2 K temperature range. The solubility of sodium formate in the water-ethanol solvent decreases by two orders of magnitude as the ethanol concentration increases from 0 to 100 mol %. The solubility diagrams for the system indicate the following equilibrium solid phases: NaCHO₂·2H₂O, NaCHO₂·1.75H₂O and NaCHO₂. There are two zones of solution exfoliation. The degree of exfoliation increases as the temperature increases. All this data is classified as tentative because of the lack of sufficient data for adequate comparison to be made.

         (3) Ternary systems containing acetone. There is one report of solubility data for sodium formate in water-acetone solvents.¹³ In this solvent here are two exfoliation zones: one in water saturated with acetone; the other in acetone saturated with water. At the one-phase critical point the system has the following composition: NaCHO₂, 3.83 mol %; C₃H₆O, 15.24 mol %; H₂O, 80.93 mol %. Solubility values for sodium formate in the entire concentration range of acetone-formic acid solvents at 298.2 K are also available.¹⁴ In such solvents the solubility of sodium formate changes by several orders of magnitude as the formic acid concentration changes from 0 to 100 mol %. The equilibrium solid phases in this system are reported to be: NaCHO₂·HCHO₂, 3NaCHO₂·HCHO₂ and NaCHO₂. All the data for this system is classified as tentative because there are no other similar data available for comparison.

        (4) The NaCHO₂-C₅H₅N-H₂O system. Data for solubility values in this system is limited to one publication.¹⁵ The experimental values were obtained by the isothermal saturation method, but the results are presented only in graphical form. Addition of sodium formate to a homogeneous pyridine-water solvent caused exfoliation of the solution. The composition of the limiting systems is: NaCHO₂, 19.19 mol %; C₅H₅N, 1.1 mol %; H₂O, 79.0 mol % and NaCHO₂, 0.4 mol %; C₅H₅N, 61.8 mol %; H₂O, 37.8 mol %. The composition at the critical point is: NaCHO₂, 3.4 mol %; C₅H₅N, 61.8 mol %; H₂O, 85.4 mol % ^a. The only stable equilibrium solid phase observed was NaCHO₂·H₂O. All these values are classified as tentative because of the lack of corroborating information. [^a These numbers do not add up to 100 %.]

Potassium formate

        (1) The KCHO₂-HCHO₂ system. Solubility data for this system are available in four separate reports.^1-4 In three of these studies,^1,2,4 the values were obtained by the freezing point method. In the other study³ the isothermal saturation method was used. All the data are summarized in Table 1. The values obtained by the freezing point method are in good agreement with each other while the values obtained by the isothermal saturation method are somewhat lower. The discrepancy is probably due to the fact that about 24 hours are needed to attain equilibrium. Therefore, the values obtained by the isothermal saturation method are probably more accurate. The equilibrium solid phase^1,2 is KCHO₂·HCHO₂. Ivanova et al.⁴ also report KCHO₂·2HCHO₂ and KCHO₂·3HCHO₂ as equilibrium solid phases. However, there are no other reports of these salts as equilibrium solid phases, so these conclusions are classified as doubtful. From the data in Table 1 the recommended value for the solubility of potassium formate in formic acid at 298.2 K is 25.1 ± 1.1 mol % with KCHO₂·HCHO₂ as the equilibrium solid phase.

        (1) The KCHO₂-HCHO₂-H₂O system. Only two reports present values for the solubility of potassium formate in this system at 298.2 K (6,15). Sas et al.⁶ also report data for the solubility at 323.2 K but only the solubility curve is presented. In both studies the same experimental method was used to obtain the solubility values. However, the estimated experimental error was lower in one report6 than in the other.15 No direct comparison can be made in these two sets of data at 298.2 K because the concentration range is different in the two studies. Sas et al.⁶ reported KCHO₂·HCHO₂, KCHO₂·HCHO₂·H₂O and KCHO₂ as equilibrium solid phases whereas Biancoet al.¹⁵ reported only KCHO₂·HCHO₂ and KCHO₂ as the equilibrium solid phases. It is impossible to tell which of these two conclusions is the more accurate because no other work has been reported for this system. Both sets of data agree within 0 to 10 %. The data are classified as tentative.

Cesium formate

        (1) The CsCHO₂-HCHO₂ system. Only the report by Ivanova et al.⁴ gives data for the solubility of cesium formate in formic acid, but the data are presented only in graphical form. A value obtained from the solubility curve is included in Table 3.

The values in Table 3 indicate that the alkali metal formates are only slightly soluble in formic acid at 273.2 K. It was impossible to estimate the solubility values at other temperatures. Ivanova et al.⁴ report that CsCHO₂·7HCHO₂ is the equilibrium solid phase. However, because this is the only reported work on this system the data presented in it can only be classified as tentative, at best.

Experimental Data:   (Notes on the Nomenclature)