Solving the analysis of acetic anhydride

Water becomes a titrant rather than a solvent, helped by a judicious dose of catalyst, in this example of lateral thinking

Acetic anhydride is a very useful industrial reagent (if somewhat unpleasant to handle).

About 75% of production goes into manufacturing cellulose acetate fibre, which is used in textiles as well as cigarette filters.

Much smaller amounts are used in the production of aspirin and paracetamol, and unfortunately some finds its way into illegal drug manufacture.

In the production of cellulose acetate, acetic anhydride is used in a mixture with acetic acid, and it was in this matrix that Stirling was asked to analyse it.

There is a visual indicator titration method, but this is reportedly subject to error, particularly as the reaction mixture darkens with use and the endpoint becomes difficult to see.

Could Multitrator provide a solution? With a little lateral thinking, the answer was disarmingly simple: water.

Water reacts with acetic anhydride exothermically, but by itself it reacts too slowly to be of benefit in a thermometric titration.

However, if the reaction is catalysed, then that's a different matter.

Multitrator added a couple of drops of a catalyst and the problem was beaten.

Large temperature rise, razor-sharp endpoints, great precision.

We like the concept of using water as a titrant.

Normally, we think of water as a solvent, not as a reagent used in accurate chemical analysis.

When you think about water as a titrant instead of a solvent, it has some great things going for it.

It's available in extremely high purity and it is exceedingly cheap.

Moreover, as 100% H2O it's very strong, approximately 55.7 moles per litre.

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