The determination of the solubilities of these formates in water began in the early years of this century. Groschuff¹ published the results of a determination of the solubility of the formates of lithium, sodium and ammonium. Some time later others² reported an investigation of the polytherms of rubidium and cesium formates, and, in a more limited temperature range, of lithium, sodium and potassium formates. Most of the reported solubility studies of these systems deal with formates of sodium ^1,2,5-9, lithium^1-4 and potassium^1,2,10,11. Solubility values have also been reported in connection with the study of some three-component systems. Those values can be found on data sheets in Sec. 2 of this volume. The solubility data values are presented here as temperature-composition diagrams. On the diagrams, the solid line represents the most probable solubility values. Exact evaluation of the polytherms is difficult because of the limited temperature ranges studied experimentally and because no data are given for the eutonic points. Each binary system in this Section is presented and discussed separately.

Lithium formate.
Figure 1contains the published solubility data for the LiCHO₂-H₂O system. The data from the different studies agree rather well with each other, and the data of Groschuff¹ are tentatively recommended.

Sodium formate.
Of all the metal formates that are considered in this section, more solubility data has been measured and reported for the solubility of sodium formate than for any of the other metal formates. The reported values are shown in Fig. 2. The data agree fairly well with each other. But there are differences. According to Groschuff,¹ the trihydrate, dihydrate and the anhydrous salt all exist at 18 ºC, while others⁵ maintain that the trihydrate and dihydrate both exist at 15.3 ºC and the dihydrate is in equilibrium with the anhydrous form at 27.9 ºC. Because of these differences the data can only be accepted tentatively.

Potassium formate.
There are relatively few solubility data for the potassium formate-water system ( Fig. 3). Besides the polytherms of Groschuff¹ and Sidgwick and Gentle,² there are only two other data points^4,10 and a few values obtained from the study of three-component systems. The latter values vary significantly from the measurements made by others.^1,2 The polytherms of Groschuff¹ and of Sidgwick and Gentle² appear to be consistent with each other, but can only be considered as tentative values because of the lack of other solubility data for the system.

Rubidium formate.
There are only two reports dealing with the solubility of rubidium formate in water.^2,12 Sidgwick and Gentle² reported a complete polytherm. The other report¹² contributes just one data point and that was obtained in the study of a three-component system. The value does not agree with those of Sidgwick and Gentle ( Fig. 4). The data are presented here for information but cannot be evaluated.

Cesium formate.
Only one complete polytherm has been reported for the cesium formate-water system.² In addition to this there are two values reported from a study of three-component systems. The values cannot be evaluated until further solubility data for this system are available (see Fig. 5.)

Thallium formate.
The only report on the solubility of thallium formate in water has been given by Boinon et al.¹³ The data are shown in Fig. 6. Two other values have been reported in connection with studies of three-component systems, see the data sheets in Sec. 2 of this volume. The solubility values that have been reported cannot be evaluated because of the lack of additional experimental data.

Ammonium formate.
There is only one published report about the solubility of ammonium formate in water.¹ The data are presented on Fig. 7. These data cannot be evaluated because they are the only ones available.

Experimental Data:   (Notes on the Nomenclature)