The traditional way of analysing thiosulphate/sulphite mixtures is to mask the sulphite with formaldehyde, and titrate the thiosulphite with iodine in the usual way. The titration is then repeated without the use of formaldehyde to give sulphite + thiosulphate. This method is to be found in many textbooks and works well. The problem we have is that we're now banned from using formaldehyde because of its toxicity, even if we have very effective fume extraction in use. This affects two of our routine analyses: the titration I've mentioned, and the use of iodine/amylose colorimetry for determining trace amounts of thiosulphate.
It has occurred to me that ion chromatography might be applicable, but this is not a technique to which we have access; it also occurs to me that the instability of sulphite to aerial oxidation could be a significant issue, particularly in the dilute solution you'd presumably need to use.
Can anyone help solve this problem?
Kindly check https://www.jstage.jst.go.jp/article/analsci1985/3/6/3_6_543/_pdf
Also check https://link.springer.com/article/10.1007/s13738-017-1053-9
Many thanks: the Anal Sci paper described a spectrophotometric technique which is fundamentally the same as one we already use (i.e. negative-going spectrophotometry of the iodine-starch blue colour); this requires formaldehyde to block the sulphite (it certainly mentions this; if it suggests an alternative, I've missed it!). The J Iranian Chem Soc paper, on the other hand, suggests another way round the problem, which looks as if it might be possible. I'll have to have a closer look at this when I'm back in work after the Christmas/New Year break, and see what we might make of it. It doesn't appear to be very toxic - always a relief to find that for a chemical I've not used before!
If anyone else has any good ideas on this, please add further answers to this question.
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