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The p H of phosphate and citrate buffers is markedly reduced by the Dissociation constants in water a t 25' are reported for the species addition of calcium ions, owing to the formation of association products.CaH,PO,+, CaHPO,, CaH,Cit+, and CaHCit (Cit = citrate).IT is to be expected by analogy with other multivalent electrolytes that fairly extensive ion-pair formation will occur when calcium ions are introduced into buffer solutions containing multivalent anions (Davies, Endeavour, 1945, 4, 15). We have confirmed this by pH measurements. The pHs of Sorensen's hydrogen phosphate buffers are appreciably lowered by saturating them with the sparingly soluble calcium iodate; and with acid citrate buffers the effect is so great that the pH may be reduced by more than a unit. This increase in the hydrogen-ion concentration is due to the preferential association of the calcium ion with the most highly charged anions, leading to the displacement of equilibrium by reactions such as : Ca2+ + H,Cit-+ CaHCit + H+. A further effect of the interaction is the enhancement of the solubility of the calcium salt, and we have studied the interaction quantitatively by means of solubility measurements. EXPERIMENTALCalcium iodate was prepared, and its solubility measured, according t o the methods usedThe buffer solutions were made up from " AnalaR " p H measurements were made colonmetrically by in our previous work ( J . , 1951, 233). reagents and carbon dioxide-free water.

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