Factors influencing the Knight and Allen EDTA invert sugar test method for white sugars

Monitoring sugar quality is crucial to many industries. Rapid and accurate methods are needed which are not expensive and complex, and which can be carried out by relatively inexperienced technical staff anywhere in the world. This paper is concerned with aspects of the physical experimental conditions of the Knight and Allen test for the determination of trace mass fractions, up to 0.05 g/100 g, of ‘invert sugar’ (dextrose and fructose) in purified sugar (i.e. sucrose). It is an ICUMSA^® standard Method, GS2-5 (2011), that is used across the whole sugar producing industry many times daily.

The test is a titration method using a calibrated alkaline cupric sulphate, Fehling’s solution, which is a deep blue complex solution of salts. If invert sugar is present and the solution heated, the invert sugars react with some of the cupric solution and the copper is thrown down as a brick-red precipitate. Sucrose has no effect on Fehling’s solution. Any excess unreacted cupric ions are then titrated with standardised EDTA solution with murexide as indicator and the amount of invert sugar is reliably deduced. In this paper the essential science of the method and then effects of various variables of boiling time, size of tubes, weight of sample, titration volume and the addition of the indicator are examined. In the future automated potentiometric end point determinations may become possible, but in the meantime this paper will be useful for laboratory operators to gain procedural confidence and accuracy with this test and hone their experimental technique.