Sulfites (Sulfur Dioxide) in wine are present in two forms – free and bound. The Sulfite test strips only detect free sulfite. The buffer agents on the test pad convert the free sulfur dioxide to sulfite ions, which then react with the dye indicator developing color proportional to the amount of free sulfur dioxide (sulfite) present. Unfortunately, the majority of sulfur dioxide in wine is in the bound form. The test strips, as is, will not detect this form. It is necessary to adjust the pH of the wine to above 7, preferably to about 12, to liberate all of the bound sulfur dioxide. Once liberated, the sulfur dioxide will convert to sulfite ions and can be detected by the test pad. In adjusting the pH, care should be taken. In addition, the end result will need to compensate for any dilutions made.
The color of the wine poses a challenge in that the color will absorb into the test pad affecting the interpretation of the results. An alternative method for testing red wine involves liberating the free sulfite present by adding approximately 1 teaspoon of citric acid to a 1 ounce sample of red wine in a small cup. Wet the pad of the Sulfite test strip with bottled water. Holding the Sulfite test strip above the solution for 5 minutes will result in the test strip reading the free sulfite level. The citric acid will cause free sulfites in the wine to convert to sulfur dioxide gas. The gas will react with the test strip. We have tested some sulfite-spiked red wine and found that the amount of sulfite added to spike the sample when reacted with acid correlates with the color chart as a measure of the sulfite in the wine sample. Other acids will work as well, but citric acid is probably the least hazardous choice (hydrochloric acid works well but it is not easily handled). This technique will help with testing for free sulfites but will not address the issue of combined sulfites.