THE TEMPERATURE AND IONIC-STRENGTH DEPENDENCE OF THE SOLUBILITY PRODUCT CONSTANT OF FERROUS PHOSPHONATE

A new compound with implications in scale and corrosion control has be en isolated and its solubility measured under varying conditions of te mperature and ionic strength from 25 to 75 degrees C and from 1 to 3 M ionic strength. Ferrous phosphonate was formed using the phosphonate nitrilotris(methylene phosphonic acid) (NTMP) and was found to have th e stoichiometry Fe2.5HNTMP. Using a complexation and speciation model, the stability constants for the complexation of iron(II) with phospho nate were calculated, and the solubility product constant was derived for each temperature and ionic strength; at 25 degrees C and 1 M ionic strength, K-sp 10(-31.2). The temperature (T, K) and ionic strength ( I, M) dependence of the negative logarithm of the ferrous phosphonate solubility product constant (pK(sp)) was determined to be: pK(sp) = 39 .54 - 6.14I(1/2) + 2.8I - 1315/T. In simulated calculations using actu al field data to compare iron and calcium phosphonates independently, ferrous salts were predicted to form in all instances and matured calc ium salts were predicted to form occasionally. Furthermore, a relation ship was established whereby the concentration of free iron in a calci um-iron-phosphonate system can be predicted for a given system.