We have some new information regarding the use of the Vinmetrica SO2 test on the SC-100A or the SC-300. This information is also referenced in the latest versions of the manuals, which you can always access here: Manuals On-line
- Running SO2 standards to check accuracy:
Sometimes we are asked if there is a way to check or calibrate the Vinmetrica free SO2 test, to make sure it’s accurate. The short answer is yes, but it’s tricky, and you generally don’t need to do so. “Calibration” is built into the SO2 reagents in the form of the SO2 Titrant, whose concentration is carefully controlled (at 0.0156 N, so that each mL used equals 20 ppm of SO2). The reagents are quite stable up to and even beyond their stated expiration date.
If you want to try checking the accuracy of your SO2 titration, you can prepare “standards” of known concentration. The following link will lead you to the methods that we recommend for preparing SO2 standards: SO2 Standardization Check
This describes how to use potassium metabisulfite (KMBS) or preferably, sodium sulfite as a standard. It also describes an alternative method that uses vitamin C (ascorbic acid) as a substitute for SO2.
As mentioned, the methods are a bit tricky to get good results. We won’t go into details for now, but if you look it up and have questions, we’re always here to help!
- Running Total SO2 tests:
Total SO2 tests are usually not necessary for most purposes of controlling wine oxidation and stability. Sometimes commercial export requirements call for a total SO2 test, and sometimes winemakers want to know if total SO2 is creeping up in the background; for health and safety, the TTB has a limit of 350 ppm for total SO2.
The Vinmetrica method for total SO2 has a simple variation on the free SO2 method. You add 10 mL 1N sodium hydroxide (NaOH) to a 25 mL wine sample, cover the vessel to restrict air access, and let react 20 minutes with occasional or constant stirring. At the end of this time, you add 8 mL (instead of the usual 2 mL) of your Acid Solution, 2 mL of the Reactant, then proceed with the normal SO2 titration.
What’s new here is that in the past we recommended a 10 minute reaction time with the sodium hydroxide. We have found that in some cases, that resulted in a slightly lower value compared with a 20 minute time period. Using reaction times longer than 20 minutes does not seem to make any difference.