3.2 Bi-positive Metal Formates Experimental investigation of the solubility of these formates in water began toward the end of the nineteenth century with studies of the alkaline earth metal formates.1-4 Following these reports there were only occasional, somewhat isolated, published reports of other solubility studies. Then, about two decades ago Balarew and his group began to measure water solubilities of the formates of the metals in the first transition series of the Periodic Table (Mn, Fe, Co, Ni and Cu )5-9. These data were obtained by studies of binary systems. The data have been supplemented by values obtained during the course of studies of three-component systems and are presented in the data sheets of Sec. 2 of this volume. The values for the binary systems are presented visually on Figs. 8-18.
It is evident from a review of these data that the published values are in fairly good agreement with each other, but for some formates there is relatively little solubility information available. In fact, for the manganese, iron, cobalt, nickel and cadmium formates there are only two sources of information. For some formates, there are differences of opinion as to which solid phase(s) are present at equilibrium. These factors make it difficult to provide firm recommendations as to the validity of the solubility data that have been published. The following comments are based on a review of the data shown on Figs. 8-18. The lines on those figures connect the most probable, but not necessarily recommended solubility values.
Alkaline earth metal formates. These formates are not very soluble and the solubility varies only slightly with temperature ( Fig. 8, Fig. 9, Fig. 10, Fig. 11). Magnesium, calcium and barium formates show only one crystalline branch in the 0-100 ºC temperature range whereas strontium formate crystallizes in two polymorphic forms in the same temperature range. This matter will be discussed a little later.
The solubility data for magnesium formate ( Fig. 8) agree fairly well with each other, and the solubility data of Ashton et al.10 and can be tentatively recommended pending further solubility studies of this system.
There is also fairly good agreement among the reported water-solubility data for calcium formate ( Fig. 9), although there is some scatter in the values obtained from studies of three-component systems. More solubility studies should be available before a recommendation can be made.
Only Stanley4 and Ashton et al.10 have published polytherms for the solubility of strontium formate in water ( Fig. 10). The solubility values of these two studies agree with each other in the 0-70 ºC temperature range but vary significantly from each other at higher temperatures. This disagreement must be resolved before any solubility recommendation can be made.
The water-solubilities of the formates of manganese, iron, nickel and cobalt are rather small and show only a slight temperature dependence ( Fig. 12, Fig. 13, Fig. 14, Fig. 15). On each of these figures, the data are provided by only one polytherm,6,11,5,8 and just a few other values. Because of this scarcity of experimental data, no solubility recommendations can be made for any of these formates.
Formates of copper, zinc, cadmium and lead. Of these formates copper formate has received the most attention and lead formate the least. There are four reports of experimental investigations of the solubility polytherm of copper formate.5,12-14 These, except for Ref. 14, are shown on Fig. 16. There are significant differences in the reported solubility values at temperatures above 30 ºC. One report13 states that the Cu(CHO2)2·2H2O is converted to Cu(CHO2)2 at 61.1 ºC, while another report12 shows that the dihydrate still crystallizes from solution at 64 ºC. (The data from Ostanni et al.13 could not be presented on Fig. 16.) There also is evidence9 that the dihydrate to anhydrous form conversion occurs between 55 and 60 ºC. Until additional experimental work is reported for this system to resolve these differences, no water-solubility values can be recommended for copper formate.
The water solubilities for zinc formate ( Fig. 17) are based, essentially, on the data of two reports.5,10 The results are consistent with each other, but can only be considered tentative because of lack of more data for this system.
The solubility of cadmium formate in water is presented visually in Fig. 18. The data consist of two experimentally determined polytherms.5,10 However, because of the absence of additional experimentally determined values, the values in Fig. 18 can only be considered as tentative.
The only water-soluble values for lead formate come from studies of three-component systems. There is no report of a solubility polytherm for this system. No evaluation of the few solubility values that are available is possible until more experimental work is reported for this system.