14-22: pH Impact on Inhibitor Performance

Water treatment chemists have long observed that some scale inhibitors work better at high pH rather than low pH and that some inhibitors have little, if any activity at very low pH.  Examples would be the effectiveness of polyacrylic acid at high pH as a calcium carbonate inhibitor, as in ash sluice and some mining applications, mediocre performance near a neutral pH, as in cooling water applications, and very low activity in an acid pH range, as in gypsum control in the pH range from 2 to 4.  This paper provides a framework for evaluating relative inhibitor activity using dissociation profiles for common inhibitors and calculating the distribution of inhibitor species versus pH.  Minimum effective dosages and inhibitor species concentrations are used to calculate the relative efficacy of different dissociation states for common technical grade scale inhibitors.