Free Sulfite in Wine

There are two methods in common use for determining Free sulfite in wines and musts, the Rosanaline method and the DTNB method. Both of these methods can be automated using the Astoria 2 analyzer. Unfortunately we do not have a reliable method for this test for the rAPID-T instrument at this time.

DTNB method. The applicable range for free sulfite is 5.0 to 100 mg/L. The upper range can be extended by sample dilution.

Summary

The sample is acidified to liberate free SO2 gas. The SO2 gas diffuses through a Teflon membrane where it is reacted with buffered 5-5’–Dithiobis 2-nitrobenzoic acid (DTNB). The absorbance of this product is measured at 410 nm.

Rosaniline method. The applicable range for free sulfite is 5.0 to 70 mg/L. The range for total sulfite is 5.0 to 200 mg/L. The upper range can be extended by sample dilution. The lower range for free sulfite may be extended to 2.0 mg/L by adding a lower calibrant.

Summary

The sample is acidified to liberate SO2 gas from free sulfite and labile bound sulfite. For total sulfite, it is then heated to 90°C to liberate the SO2 from the aldehyde bound sulfites. The SO2 gas diffuses through a Teflon membrane where it is reacted with formaldehyde and p-Rosaniline to form p-Rosaniline methylsulfonic acid. The absorbance of this product is measured at 570 nm.