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Solubility Product of Iron Phosphate
Author(s) -
Chang S. C.,
Jackson M. L.
Publication year - 1957
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1957.03615995002100030005x
Subject(s) - solubility equilibrium , chemistry , solubility , phosphate , dissolution , precipitation , iron phosphate , isoelectric point , inorganic chemistry , ionic strength , salt (chemistry) , reagent , nuclear chemistry , aqueous solution , organic chemistry , physics , meteorology , enzyme
The solubility product of iron phosphate was determined by both dissolution and precipitation methods in dilute salt solutions. Solution pH values slightly below the determined isoelectric pH of 2.75 for iron phosphate were employed to get determinable amounts of iron in the solution. The pK sp value was calculated from the ionic activities of Fe 3+ and H 2 PO 4 ‐ in the equilibrium solution by the following equation, pK sp = pFe 3+ + pH 2 PO 4 ‐ + 2pOH ‐ which was found to coordinate the data most satisfactorily. The pK sp value of iron phosphate increased with a 1000‐fold decrease of the solid‐to‐solution ratio from 33.6 to 35.1. It increased from 33.0 to 33.2 over a 50‐fold decrease of the concentration of the precipitation reagents. A medial value of 33.5 was approached by the two procedures; however, for low iron phosphate‐solution ratios, such as those commonly employed for soil extraction, the applicable pK sp value agrees with the most dilute solid‐solution value of 35 or more. Similarly for aluminum phosphate, the applicable pK sp value extends as high as 32 for dilute suspensions. The variation in apparent solubility product with amount of solid per unit volume of solution is negligible compared to the magnitude of the pK sp which is the reciprocal of a billion trillion trillion. The calculations from solubility products show that aluminum phosphate and iron phosphate should precipitate and accumulate even in neutral soils, as observed experimentally.