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The present technique employed by Bio-Lab to test for Chlorine Demand is based on the reduction of iodide (the Chlorine Demand Iodide Reagent) to iodine after acidifying the sample with Chlorine Demand Acid Reagent:

ClO- + 2 I- + 2 H+ = Cl- + I2 + H2O

then titration of the resultant yellow-brown iodine solution with thiosulfate (Chlorine Demand Titrating Reagent) to a colourless endpoint:

I2 + 2 S2O32- = 2 I- + S4O62-

The test solution becomes a pale yellow colour several drops of Titrating Reagent before the endpoint of the titration. Unfortunately, this can make the endpoint difficult to detect. Some operators even mistake the pale yellow for colourless and, hence, underestimate the endpoint.


The addition of a milligram (the small scoop in the Chlorine Demand Test Station is more than adequate) of sodium starch glycollate indicator to the test solution, however, gives rise to an intensely blue-coloured complex. (1) At the endpoint of the titration, the solution still goes colourless, but the transition is much easier to detect. Marketed as Iotect®, (2) the indicator has no effect on the value obtained for the endpoint, nor does it suffer interference from other components in the sample.


Iotect is available in Australia through Selby-Biolab. At the quoted 1998 price, use of the indicator adds about 3 cents to the cost of each Chlorine Demand test undertaken.



1. J. Bassett, R.C. Denney, G.H. Jeffery and J. Mendham, "Vogel's Textbook of Quantitative Inorganic Analysis", 4th edition, Longman Scientific and Technical, Harlow, Essex, 1986, pp. 281-288.

2. Iotect is a registered trademark of May and Baker, Dagenham, England.


The above information is supplied by Bio-Lab and represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers. It is not to be taken out of context nor used as support for any other claim not made herein.