pH control

Figure: pH control system.


Systems for pH control are characterized by extreme rangeability and sensitivity, and are also subject to difficulties arising from contact between measuring electrodes and hostile fluids. Case histories of representative installations show that success in implementing these control systems depends not only on assessing the complexity of the loop and selecting a control strategy, but also on recognizing and avoiding pitfalls while specifying and installing instrumentation, equipment, and piping.

One basic source of difficulty is that the pH scale corresponds to hydrogen ion concentrations from 100 to 10 -14 moles per liter. No other common measurement covers such a tremendous range. Another intrinsic constraint is that measuring electrodes can respond to changes as small as 0.001 pH, so instruments can track hydrogen ion concentration changes as small as 5x10-10 moles per liter at 7 pH. No other common measurement has such tremendous sensitivity. The implications of such great rangeability and sensitivity can be illustrated by considering a continuous feedback neutralization system for a strong acid and a strong base.

The reagent flow should essentially be proportional to the difference between the hydrogen ion concentration of the process fluid and the set point. A reagent control valve must therefore have a rangeability greater than 10,000,000:1 for a set point of 7 pH when the incoming stream fluctuates between 0 and 7 pH. Moreover, uncertainties in the control valve stroke translate directly into pH errors, such that hysteresis of only 0.00005 percent can cause an offset of 1 pH for a 7 pH set point.