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.
The ammonia 0-100ppm test strip technically has two test pads put together. One pad absorbs the dissolved ammonia and converts it to a gas, which the second pad registers and then changes color when dipped into an Ammonia solution.
We ran a test to see if it would work to detect ammonia gas. The short answer is yes, however, the color chart is not calibrated to the correct colors for this application.
The test was performed using 4 test strips:
- Strip 1 was left as is, dry.
- Strip 2 was dipped in DI water.
- Strip 3 was left dry and held over ammonia fumes from a bottle of cleaner for just a few seconds.
- Strip 4 was dipped in DI water, then held over ammonia fumes from a bottle of cleaner for just a few seconds.
The results of the test are shown below:
The Lead Acetate paper detects the presence of sulfur. The strip works best when wet (when testing for sulfur gas – the gas absorbs into the wet paper) or dipped into a test solution. The sulfur will form a black precipitate with the lead, lead sulfide. In dilute solutions, the paper may turn grey, and not black. The detection limit in solution is 5 ppm.
When testing gas, there are several keys…
- The paper must be moist to facilitate absorption of the Sulfide gas and formation of sulfide ions that would then react with the lead on the paper.
- The paper cannot be “too wet”. The Lead Acetate impregnated in the paper is not “bound” to the paper and can be washed away if the paper “wetted” with too much water.
- The intensity of the reaction of the moist lead acetate paper and sulfide gas will depend on how much gas is getting exposed to the paper and how long the moist test paper is exposed to the gas. The more intense the gas and the longer the exposure, the better the detection.
Various types of sulfites can be used – sulfite, bisulfite, and metabisulfite. Our original color chart was developed to measure sulfite ions in ppm (from sodium sulfite standards), however, we have checked standards and found that bisulfite and metabisulfite also match the same color chart.