AO method for SO2 measurement - impact of oxygen on final measurement

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owner, winemaker
Jun 13, 2017
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So this is somewhat of a technical and nitpicking question on the AO procedure. The way it was taught to me, and the way I've been doing it, was to bubble air through the acidified wine solution. However, some of the sulfites in the wine that I'm trying to measure likely end up binding with the oxygen from the air bubbling through rather than being outgassed into the collection beaker.

The question is, how much does this lower the effective sulfite measurement in the end? I suppose I could use nitrogen as a control and find out. Anyone know offhand what the measurement offset is? 5ppm? 10ppm?


Veteran Winemaker
Jun 16, 2014
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The reaction with oxygen directly is not that fast, in fact it is very slow compared to other reactions going on. Normally the oxygen reacts with wine components first, usually chemical oxidation of diphenols, which ultimately produces hydrogen peroxide which then reacts with the bisulfite present. During the test, when you add the phosphoric acid to the sample and move the pH way down, the bisulfite is pushed to the molecular form which is SO2 gas and it is quickly swept into the peroxide trap.