Ask the Produce Docs (Commodity)
We measured residual SO2 in table grapes using Ripper method. We determined total, free and bound SO2 in ppm by this method. Which parameter (total, free or bound SO2) gives residual SO2? As you know the maximum tolerance limit is 10 ppm for table grapes. Is it total, free or bound SO2? (E.E.C.)
The Ripper method is an iodimetric reaction (oxidation/reduction) generally associated with free sulfites.
The official method for sulfite determinations is the Monier-Williams method (acid distillation) which is not fully ideal, but it is one officially on use. They are certified laboratories where samples can be sent for analysis.
My last reference on this is in 1978 (work done at Davis) “SO2 Determination by Aeration-Oxidation: