How to solve the determination of sulphate without a reference electrode or ion-selective electrode (ISE), by thermometric instead of potentiometric titration
Sulphate is an important anion in a variety of industrial solutions, as well as in some saline solutions of environmental origin.
The classical wet chemistry determination of sulphate is the gravimetric procedure, where sulphate is precipitated from solution by the addition of barium ions.
While very accurate in skilled hands, it is a very slow procedure and not well suited to the demands of modern process control.
The classical approach has been adapted to a titrimetric determination, where a standard solution of barium chloride is titrated against the sulphate analyte.
The endpoint may be determined potentiometrically or thermometrically.
In the potentiometric determination, a barium ion selective electrode (ISE) is used to indicate the endpoint. Disadvantages of the potentiometric approach are: Barium ISEs have a slow response time and are subject to interference, most importantly by sodium, potassium and strontium.
ISEs have a limited service life, and may be subject to fouling.
ISEs require a reference electrode, whose liquid junction may also be subject to fouling.
In contrast, the thermometric approach uses a single, tiny thermistor sensor with a very rapid response time, which is not prone to fouling, not subject to interference and has a long service life.
The procedure is rapid and highly precise, with analytical precisions of less than 0.01g/l (one standard deviation) being obtained routinely.
The lower limit for practical determinations is approximately 0.2g/l, so it is best suited to the analysis of industrial samples as well as environmental samples where sulphate is abundant.
Where samples are not too acidic, barium acetate may give slightly better results than barium chloride as a titrant.
