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

Abstract

Strict control of the water chemistry in modern high-pressure boilers often requires that the measured pH of boiler feedwater is referred to a standard temperature of 25 °C. At present most of the pH meters installed in power stations cannot meet this requirement unless the sample temperature is controlled to 25 °C, because their temperature compensation circuits cannot correct for temperature-induced changes in the pH of alkaline feedwater. These changes can be considerable because the temperature coefficient of feedwater is of the order of –0.033 pH °C^–1.

Experimental glass electrodes have been developed which, when used in conjunction with a specified reference electrode, can provide over-all temperature compensation in ammonia-dosed feedwater over the range 15–35 °C. The chemistry of these electrodes has been arranged such that the cell e.m.f. only responds to changes in pH brought about by changes in alkalinity. At constant ammonia concentrations equivalent to pH values in the range 9–9.3, the temperature-induced variations in the cell e.m.f. over the temperature range 15–35 °C were equivalent to less than 0.05 pH for a pH electrode containing N-glycylglycine in its internal reference solution and using silver-silver chloride/0.1 mol l^–1 potassium chloride or calomel/3 mol l^–1 potassium chloride reference electrodes.