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 OC. 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 I:type hypothetical reactions. That is, HA <----> H+ + A(-) with pK(H) = pH - log(theta (u)/theta (H)) and Ca(A(...)A ) A <----> Ca2+ + (A(...)A)(2-) with pK(Ca) = pCa - log(theta (u)/theta (Ca)) where Bu stands for the dissociated fraction of HA, BH stands for the protonated fraction, theta (Ca) stands for the calcium-complexed fraction, (A(...)A)(2-) is a unit of arbitrary combinations of two dissociated A- units, and pCa is the negative logarithm of the free calcium concentration. The corresponding constants for these two reactions, KH and Kc,, are determined from the acid/base titrations. These constants are then fitted with a linear electrostatic model: pK(H) = pK(H,int) + b(elec)theta (u) and pK(Ca) = pK(Ca,int) + 2b(elec)theta (u) where K-H,K-int and K-Ca,K-int are intrinsic constants at ionic strength (I), and refer to the condition of zero dissociation, and belec is an electrostatic factor determined from polyelectrolyte theory. Both pKH and pKCa are expressed as a function of ionic strength and temperature through pK(H,int), PKCa,int, and b(elec): pK(H,int) = 4.856 -0.984I(1/2)+0.253I - 198.7l/T for proton dissociation, pK(Ca,int) = 3.968 - 2.671I(1/2) + 0.750I - 1102.3/T for Ca-PPCA dissociation, and b(elec) = 2.778 - 1.081I(1/2) + 0.226I, where I stands for ionic strength in molality and T stands for temperature in Kelvin. The fitting results show that the electrostatic factor b(elec) is not significantly influenced by temperature, which is reasonable according to electrostatic theory. These results can be used to analyze the equilibria of PPCA in a solution, given pH, ionic strength, temperature, and total metal (Ca2+ here) concentration.