Improved pH cells for over-all temperature compensation in the measurement of the pH of boiler feedwater

Abstract

The pH of boiler feedwater is raised by the addition of ammonia to a level at which corrosion of the boiler is limited and consequently the measurement of pH is of primary operational interest. It is considered necessary to refer this measurement to a standard temperature of 25 °C because under conditions of varying sample temperatures, some pH meters will record changes in pH that are solely due to the effects of temperature on the chemical equilibrium of the ammonia-dosed water. At present, readings at a reference temperature can be made by controlling the sample temperature or by inclusion of additional compensation circuits in the pH meter. An attractive alternative to either of these methods is the use of self-temperature-compensating electrodes whose physico-chemical properties compensate for the temperature-induced changes in the pH of the sample.

When the experimental glass and reference electrodes were tested in simulated ammonia-dosed boiler feedwater over the temperature range 15–35 °C the temperature compensation gave an accuracy of ca.±0.05 pH about the value predicted at 25 °C. Use of these electrodes gives an improvement over those previously reported, in that glass electrodes are more stable and their thermal capacities match more closely those of the reference electrodes.