Acid−Base and Metal Complexation Chemistry of Phosphino-polycarboxylic Acid under High Ionic Strength and High Temperature

Phosphino-polycarboxylic acid (PPCA), a phosphorus-labeled linear polymer, has been widely used for controlling scale formation in the oil/gas field and water treatment processes because of its strong scale-inhibiting capability, high thermal stability, and environmental acceptability. PPCA is similar to poly(acrylic acid) with about 1% (w/w) phosphorus added to facilitate analytical measurement. In this study, the thermodynamic properties of PPCA and Ca−PPCA complexes in aqueous solution have been studied by combining electrostatic theory with potentiometric titrations. The acid−base and calcium complex solution chemistry of PPCA has been determined from 0.01 to 5 m ionic strength and from 25 to 90 °C. For simplicity, PPCA is reduced to a hypothetical, averaged monoacid, HA, with the same concentration of the PPCA monomer. Both proton and calcium dissociation reactions are defined as 1:1 type hypothetical reactions. That is, HA ↔ H+ + A- with pKH = pH − log(θu/θH) and Ca(A···A) ↔ Ca2+ + (A···A)2- with pK...