908
R. D. EDDP, P. E. MACHEMS3 AND A. W . C. MENZIES
RETROFLEX AQUEOUS SOLUBILITY CURVES, AND T H E RESPECTIVE SOLUNlLITIES OF T H E MONOHYDRATE AND MONODEUTERATE OF MAVJGANOUS SULFATE I N ORDIVARY AND I N DEUTERITJh/I WATER ROBEItT D. EDDY Pearson Memorial Loboratory, Tufts College, Medford, Massachusetts AND
PAUL E. MBCHEMER AND ALAN W. C. MENZIES Frick ChemrcaE Laboratory, Princeton Uniiiersity, Princeton, N e w Jersey Ileceived September I, 1040
Retroflex solubility curves are t’hose of subst,ances whose solubility decreases with rise of temperature. As an extreme case of such behavior, the manganous salts named above are here reported upon, because of the interest attaching to the extent of difference of solubility in the two available varieties of water. In that portion of the International Critical Tables devoted t80 the solubility of strong electrolytes in water (44, over six hundred substarma are included, of which about one-sixth show solubility curves that we retroflex a t some portion of their course. Excluding salts of organic acids, a majority of these electrolytes have an anion of the type XOd. In order to make clear the rate of change of solubility with temperature we have, in figure 1, graphed the percentage decrease in molal solubility per degree of temperature risc against the molal conc,entration, for eleven conspicuous cases, through the temperature ranges noted on the key to the graph numerals. The solubility data are for ordinary water, and are extracted from the In.&rruztional Critical Tables except in the case of manganous sulfate monohydrate, where we have used the data given later in this article for concentrations below 2.5 molal. From this graph we have excluded cases of salts exhibiting a maximum of solubility on the concentration-temperature graph for, in such cases, the change of solubility obviously becomes infinite for a small change of temperature a t the position of the maximum. Inspection of this graph reveals that manganous sulfate monohydrate appears to be unique, both in the manyfold range of concentration available for studv and in the high values of rate of decrease of solubility with rise of tt . d u e . Only the metastable Cez(SO&. 5H20shows a higher value. In this way we were led to the study of manganous sulfate. MATERIALS AND METHODS
Manganous sulfate. Crystals of the “tetrahydrate,” of Baker and Adamson’s reagent grade, were tested for impurity chiefly by the pro-
909
RETROFLEX SOLUBILITY CURVES
cedures of Rosin (Y), with results as follows: insoluble matter,
