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THE SOLUBILITIES OF LEAD PHOSPHATES 997 the theory of Bronsted of velocity of reactions catalyzed by strong electrolytes. It seems to be very difficult to find a thoroughly satisfactory explanation of the changes of the decomposition velocity of diacetone alcohol in various solutions of strong electrolytes. Philadelphia, Pennsylvania

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Vol. 5ishow that Ypbtt is given fairly accurately by the Debye-Hiickel expression in solutions of alkali chlorides up to 0.16 M. SummarySolubilities of Pb3(P04)2 and PbHPOa in aqueous solutions have been measured, lead-ion activities being determined with the lead electrode. The values of the solubility products deduced are as follows: Sz = a p b t + X aHpo4--= 10-9.90 at 25"; 10-9.62 at 37.5". Sa = up,,++ x a2p04---= 10-42.10 a t 2 j 0 , 10-42.00 a t 37.5", when K3 (the third ionization constant of phosphoric acid) is taken to have the values 10-12.30 at 25' and at 37.5". and the biological bearing of the work is mentioned. more stable salt under body conditions. 3The conditions for equilibrium between the two salts are discussed Pbs(PO4)z is the LIVERPOOL, EXGLANDIn the course of an investigation2 on the solubilities of the secondary and tertiary phosphates of lead, it became evident that the values available in the literature for the ionization constants of phosphoric acid were insufficient for the purpose of that work. We therefore have made measurements to determine the first ionization constant a t 25 and 37.5". -4 few measurements bearing on the second ionization constant are also reported. Relevant data of other observers are also considered and a value is derived for the third ionization constant. The activity concept is made use of throughout. Me tho dsMeasurements of hydrogen-ion activity were made by means of cells in which a quinhydrone electrode was connected with a normal calomel electrode by a bridge of saturated potassium chloride solution. This bridge is assumed to eliminate liquid-liquid P. D's.At 25" the PH is obtained by means of the relation3

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