ACID-BASE AND METAL-COMPLEX SOLUTION CHEMISTRY OF THE POLYPHOSPHONATEDTPMP VERSUS TEMPERATURE AND IONIC-STRENGTH

Phosphonates are important compounds in water treatment, research, and medicine. The acid-base and alkaline-earth metal ion complex solution chemistry of the polyphosphonate diethylenetriaminepentakis(methylene phosphonic acid) (DTPMP) has been determined up to 2 m ionic strength and from 25 to 90 90-degrees-C. The phosphonate acid-base and metal co mplex stability constants were modeled with a simple polymer-type mode l permitting the solution chemistry to be viewed either as a polymer o r as discrete steps. This facilitates interpretation and comparison wi th previously published discrete constants. Two of the three sp3-hybri dized nitrogen atoms are protonated first with stability constants K1N = 10(12.58 +/- 0.05) and K2N = 10(11.18 +/- 0.03); the third nitrogen is too weak of a base to be protonated under most solution conditions . The stepwise proton association constants of the phosphonate groups were modeled with the following one parameter model: log(K(i)) = b(H+) \q(i-1)\, where K(i) is the ith proton association constant (molality) , \q(i-1)\ is the value of the charge on the (i - 1)th species, and b( H+) was found to be about 1.04 with slight ionic strength and temperat ure dependence. A one-parameter model has been used to describe multip le metal complexation stability constants: log(K(ij)) = b(M2+\q(ij-1)\ , where \q(ij-1)\ is the value of the charge on the phosphonate specie s, H(i)M(j-1)Phn and b(M2+) was found to be about 0.63, again with sma ll dependence on ionic strength and temperature. Results are shown to be consistent with the electrostatic theory of complex formation. Thes e results have been used to establish the existence of a new and highl y insoluble calcium phosphonate phase. Applications of these results t o other metals and phosphonates are discussed along with several pract ical examples.