Inhibition Of Calcium Sulfate Dihydrate Crystal Growth By Phosphonic Acids - Influence Of Inhibitor Structure And Solution Ph
Authors: Griffiths David W. , Roberts Stephen D. , Liu Sung-Tsuen 
Published: December 2007

Abstract
Seeded crystal growth techniques have been applied to study the influence of pH on the performance of several phosphonic acids as calcium sulfate dihydrate crystal growth inhibitors. An increase in the pH of the crystal growth medium over a pH 4 to 9 range pH of the crystal growth medium over a pH 4 to 9 range brings about an improvement in inhibitor performance that runs parallel to the phosphonic acid titration curve. Accordingly, full inhibitor activity is realized only at pH values that approach the pKa value associated with the loss of the final phosphonic acid proton. proton

Introduction
It has been recognized for several centuries that the crystallization of a sparingly water soluble salt is influenced by impurities dissolved in the aqueous medium. Although an impurity can be a problem when the objective is to produce pure and perfect crystals, it can be beneficial when the goal is to control an unstable situation. For the latter purpose yesterday's "impurities" have evolved into a class of structurally well defined "scale inhibitors" that exert at substoichiometric concentrations (with reference to the unstable salt concentration) a profound influence on both the rate of crystal growth and, usually, the morphology of the ultimately deposited crystal.
For the control of industrially troublesome scales, today's most effective inhibitors are polymeric carboxylic acids, polyphosphoric acids, esters of polymeric carboxylic acids, polyphosphoric acids, esters of orthophosphoric acid, and/or any one of a variety of phosphonic acids. As polyfunctional acids, these phosphonic acids. As polyfunctional acids, these materials share in common the ability to undergo ionnation by the loss of protons to the aqueous medium, a process that is controlled largely by the pH of the medium in which they are used. As a consequence an inhibitor can exist as an acid and in as many alternative forms as there are ionizable protons.
Given a structure that is dependent on pH, it seemed likely that performance should also be PH-dependent. Indeed, McCartney and Alexander, and PH-dependent. Indeed, McCartney and Alexander, and later Smith and Alexander, have already shown that the performance of a polyacrylic acid as a calcium sulfate dehydrate inhibitor improves with increasing pH, and have attributed the improvement to an increase pH, and have attributed the improvement to an increase in the degree of polyacrylic acid deportonation. Here, we are concerned with the influence of pH on the performance of each of several structurally diverse performance of each of several structurally diverse phosphonic acids, again as calcium sulfate dehydrate phosphonic acids, again as calcium sulfate dehydrate inhibitors. The calcium sulfate dehydrate system is well suited to an investigation of the influence of pH on performance since, in the absence of an additive, pH on performance since, in the absence of an additive, the rate of calcium sulfate dehydrate crystal growth is relatively insensitive to pH, at least over a pH 4 to 10 range.
In this paper, our approach to an understanding of the influence of pH on performance will be to discuss the influence of pH first on structure, then on performance, and to conclude by exploring structure/ performance, and to conclude by exploring structure/ activity relationships. In the accompanying paper, we will discuss the influence of pH on phosphonic acid adsorption.

EXPERIMENTAL TECHNIQUES
A representative cross-section of industrially useful phosphonic acids was acquired. They are, first in a generally accepted, then an abbreviated system of nomenclature:nitrilotri (methylenephos-phonic acid) - NP3;ethylenediamine-N, N, N', N'-tertra-(methylenephosphonic acid) - EDP4;hexamethylene diamine- N, N, N', N'-tetra (methylenephosphonic acid) HMDP4;1-hydroxyethylidene-1,1-diphosphonic acid HEP2; and2-phosphonobutane-1,2,4-tricarboxyliacid - PB (CO2)3.
Two of the materials (EDP4 and HMDP4) were prepared by known methods and isolated as analytically pure solids. The remaining three materials (NP3, HEP2, and PB(CO2)3) were acquired from commercial sources and used without further purification. Other materials were reagent grade while purification. Other materials were reagent grade while the water, in all cases, was Millipore deionized. Buffers employed for pH control were acetic acid/ sodium acetate (pH=4.0), trishydroxymethylaminomethane/ trishydroxymethylaminomethane hydrochloride (pH=7.0), and ammonia/ammonium chloride (pH=9.0). pH was measured at 25 degrees C. using a Radiometer pHM 64 meter equipped with a Radiometer GK2401C electrode.