THE IONIZATION CONSTANTS OF PHOSPHORIC ACID¹

Abstract

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

Polyphosphoric Acids As A Reagent In Organic Chemistry

“…Neutralization of this hydrogen is achieved at pH 3.8 to 4.2. A variety of ionization constants have been reported for orthosphosphoric acid and pyrophosphoric acid (1,44,147,196,206,207,234,235,243,345,360). Apparently reliable values for orthophosphoric acid are Kx = 7.516 X 10~3 (147), K2 = 6.226 X 10-8 (147), and K2 = 2 X 10-13 (196), all at 25°C.…”

“…A variety of ionization constants have been reported for orthosphosphoric acid and pyrophosphoric acid (1,44,147,196,206,207,234,235,243,345,360). Apparently reliable values for orthophosphoric acid are Kx = 7.516 X 10~3 (147), K2 = 6.226 X 10-8 (147), and K2 = 2 X 10-13 (196), all at 25°C. ; for pyrophosphoric acid, Kx = 1.4 X 10"1, ft = 1.1 X 10~2, K2 = 2.9 X 10~7, and K4 = 3.6 X 10-9 at 18°C.…”

The Chemistry of Sodium Phosphates.

“…For H3P04 the values of Earned and Owen (51) are 7.516 X 10~3 and 6.226 X 10-8 for the first and second hydrogens, respectively, at 25°C. The value for the third hydrogen is probably near 2 X 10-13 (63). For H4P2O7 at 18°C.…”

A titrimetric method for total phosphoric anhydride

“…Although "quite a variety of dissociation constants can be found for H3PO~" the only ones cited are those of Harned and Owen (7), viz., "7.516 X ]0 -z and 6.226 X 10 -s for the first and second hydrogens, respectively, at 25°C. The value for the third hydrogen is probably near 2 }< 10 -13 (8)." Kolthoff and Stenger (9), in 1942, using dissociation or ionization constants of unidentified source, calculated the pit values corresponding to neutralization of the first two acid hydrogens as 4.4 and 9.6, respectively.…”