Chemical composition determination of some polyphosphonic acids and their corresponding salts by acid-base titration coupled with a computer simulation. Acidity constants evaluation.

Due to their great resistance to the hydrolyse at high temperuture, their stability in a wide range of pH and their very low active concentration, phosphonic acids and their corresponding salts are commonly used in water treatment. Nevertheless, one big difficulty for their use is the composition imprecision of their formulation. We have developed an acid-base titration method coupled with a computer simulation. This method allows not only to determine the composition of acids and their corresponding salts in practice, but also their theoretic acidity constant (pKa). For example, for the determination of an industrial sample, HEDP-S, which has a formula of Na4(HEDP) according to the manufacturer, we obtained, however, a composition of a mixture of Na4(HEDP) (63,70%), Na3H(HEDP) (9,43%) and H2O (26,87%). The obtained acidity constants are respectively pKa,4 = 1,00 ; pKa,3 = 2,20 ; pKa,2 = 6,40 ; pKa,1 = 9,95 which are very close to the values found in literature