The solubility of metal formates has been measured in water, organic and organic-water solvents, but most of the solubility data for metal formates has been obtained from systems in which water is the solvent. Of the organic solvents, formic acid is especially suited to serve as a solvent for the metal formates. However, the formates are less soluble in formic acid than in water. There are three exceptions to this statement: ammonium formate, lithium formate and sodium formate. The behavior of ammonium formate will here be considered separately from that of the other metal formates

        (1) The NH₄CHO₂-HCHO₂ system. Three publications^1-3 present solubility data for ammonium formate in formic acid. The solubility data were obtained over a temperature range of 193.0 to 390 K. The values werre determined by the freezing point method. The data in two of the articles^1,2 agree with each other to within 0 to 7%. The agreement improves as the temperature increases. There is agreement also with respect to the identity of the equilibrium solid phase. The difference in the solubility values is likely due to the fact that in one report¹ the starting salt was NH₄CHO₂·HCHO₂ but the solubility was calculated as due to NH₄CHO₂. Becker and Davidson³ studied the concentration-temperature equilibrium of the formic acid-ammonia system over a wide range of component concentrations. The data are presented only in graphical form in Becker and Davidson³ but the numerical data are also available.⁴ Because the data were obtained for a wide range of concentrations, they may be considered as representing two systems: NH₄CHO₂-HCHO₂ and NH₄CHO₂-NH₃. The data for the NH₄CHO₂-HCHO₂ system is consistent with the data of Kendall and Adler.² In our opinion, The data of these two studies are more reliable than the data presented by Groschuff.¹ The reason for this estimate is that the data obtained in Kendall and Adler² were obtained using NH₄CHO₂ as the starting material and these data have a smaller experimental error associated with them. As a result, the recommended value for the solubility of ammonium formate in formic acid at 298.2 K is 4.19 mol%. The equilibrium solid phase is NH₄CHO₂·HCHO₂.

        (2) The NH₄CHO₂-NH₃ system. The solubility of ammonium formate in ammonia has been reported by Lindenberg⁵ over a temperature range of 195 to 243 K and a pressure range of 0 to 1 atm. Only a graphical representation of the data is given. However, the concentration-temperature data of the system agree within about 4% of the data for this system by others. All the data are in agreement at temperatures below 213 K. The solubility of ammonium formate in ammonia at 237 K and 1 atm is given as 133.1 g NH₄CHO₂ in 100 g NH₃ (Becker and Davidson³) and 123.1 g salt in 100 g NH₃ (Lindenberg⁵). An equilibrium solid phase is NH₄CHO₂·3NH₃. However, these data can only be classified as tentative because there are no independent data to confirm them.

        Groschuff¹ has reported solubility data for the NH₄CHO₂-HCHO₂-H₂O system at 266.7 to 312.2 K. These values show that the solubility of ammonium formate decreases as the water content of the system increases. The values are classified as tentative because there is no other report of a solubility study of this system.

Experimental Data:   (Notes on the Nomenclature)