We get this question a lot so we thought we would discuss it here. [Note: Most of this information is available on our troubleshooting guide under SO2 Problems]
There’s two situations that may cause concern about the reading.
- Your instrument indicates an endpoint (beeps) right away, either before adding any SO2 Titrant, or after adding just 1 or 2 drops.
- Your instrument apparently never indicates an endpoint, even after titration with a large amount of the SO2 Titrant.
Situation (1.) happens a lot. Most of the time, this is just the normal endpoint response, telling you that your wine’s ppm of free SO2 is zero. To verify that the endpoint indication is valid, add 1 drop of 10% KMBS and stir. The signal should drop to below 50 right away, and the endpoint indicators should stop signaling. If the endpoint signaling does not stop, you may have a problem with the instrument.
Situation (2.) also crops up from time to time. Let’s say you are running an SO2 test on your wine and you use the ENTIRE 5mL syringe of SO2 Titrant, but your instrument stays on 0.00 or a low number, indicating that you still haven’t reached the endpoint. This would normally mean you have over 100 ppm free SO2 in your wine. But you’re pretty sure there isn’t that much SO2 present. So your instrument must not be working, right?
Actually, it might be working just fine!
There are a few considerations and tests to help you determine what might be happening. The easiest and first test we recommend is the SO2 Reagent Test. This ensures that your reagents and instrument are working appropriately in a mock endpoint condition:
- Place 20mL DI water into a small beaker.
- Add 1mL Acid solution and 1mL Reactant solution. Mix well.
- Turn your instrument on (SC-100, SC-100A or SC-300). If necessary, navigate to SO2 mode and press Enter if prompted to do so.
- Attach your SO2 electrode. Make sure there is a good connection between the connector and the electrode plug. Your instrument should read 0.0 or a number well below 50 with the electrode not immersed in solution.
- With the solution stirring, place the electrode into the beaker. Your instrument should still read 0.0 or a number well below 50.
- Add 1-2 drops of your SO2 Titrant to the beaker. The solution should turn slightly yellow. Mix well. With the solution stirring, the response should rise pretty quickly (within 3-5 seconds) to a value above 200 (over 4 on the original SC-100), and endpoint indicators (beeping and red STOP light flashing) should activate. This is the mock endpoint you’re looking for.
- Now add 1 drop of a 10% KMBS solution to the beaker. Make sure the solution is still mixing. The solution should turn clear and the unit should drop back below 50 (ideally down to 0.0).
If your unit passes this test, then your SO2 electrode, SC unit and SO2 Reagents are all apparently functioning properly.
If your electrode hasn’t been used in a while, or if the two platinum wires at the end of the electrode look dirty, it may need to be cleaned. (If you run the above test and the value on the screen only goes up slowly, and to a signal less than 200, then your electrode probably needs to be cleaned.) To do this, soak your SO2 electrode in your SO2 Acid solution for about 10 minutes. Remove and rinse with DI water. Then, using something like a small spatula or flat edge of a knife, very gently scrape the two platinum wires at the end of the electrode. (Be careful not to bend or break them!) Some small deposits may come off. The wires should appear bright and shiny. Rinse well with DI water and try the above SO2 Reagent Test again.
If your unit does NOT respond to the SO2 Reagent test, there could be something wrong internally in either your instrument or your electrode. Contact us for further instructions.
Okay…. so my equipment passes the SO2 Reagent Test. Now what?
First, is it possible you are using fruit or juice that has a large amount of vitamin C (ascorbic acid) in it? Most grape varietals don’t, but if you are making a wine from citrus, some kinds of berries, persimmons, or other sources, there could be significant amounts of ascorbic acid present. Molecule for molecule, ascorbic acid reacts just as free SO2 does in the Ripper titration that is the basis of the Vinmetrica test. In that case you may need to use a different method. Contact us for advice. Also see here for more information.
If ascorbic acid is not likely, then it is possible (we see it all the time) that your wine actually has a lot more sulfite in it than you think. Yes, I know you haven’t added any or if you did, you made sure to measure it accurately. It’s okay! It happens! You are not alone (we’ve been there…)! Let’s run that SO2 test again, but this time, we are going to get a smaller volume of wine, 10mL, and top it off with 15mL of DI water (adding DI water will not negatively affect your results, we promise!). Run the SO2 test as you normally would, by adding in your Reactant and Acid solutions and titrating in the normal way. When you reach your end point, take the volume of SO2 Titrant that you used and multiply that value by 20 (this is the normal calculation) but then to account for the different volume of wine that you used, you will then need to multiply that result by 2.5. This will give you the ppm of SO2 in your wine.
If you are still using the whole 5mL syringe then try taking a smaller wine sample (5.0 mL), adding 20 mL of DI water, and performing the test in the normal way. Do the final calculation in the normal way (multiply your value by 20) but then multiply that value by 5.
If you still are not getting an endpoint reading on your SC device then something might be wrong. Please give us a call and we can help troubleshoot.
Last but not least, if you want to send your unit and electrodes in for general maintenance and testing we would be happy to take a look at them for you. Additional fees will apply.
2 thoughts on “Not Getting An SO2 Reading.. Why?”
Should I calibrate my Vinmetrica 300 every time I turn it on?
Right now I only calibrate when testing pH. My friend thinks the calibration function is for the machine not the probe. She calibrates before testing pH, SO2, and TA. Is this correct?
Thanks for reaching out to us.
There is no calibration function for the SO2 electrode. Simply attach the SO2 probe, navigate to SO2 mode, and perform the SO2 test.
The unit needs to be calibrated with the pH electrode once per day of use. You need to calibrate the unit with the pH electrode when you need to test your pH or your TA only. (If you are NOT testing for pH or TA (only for SO2) you do not need to calibrate the unit.) You can then attach the pH electrode, put the unit in CAL mode and calibrate as instructed in the manual. After a successful calibration you can then navigate to pH mode and read your pH or TA mode and perform the TA test. You do not need to recalibrate in between tests.
Does that make sense?
Let us know if you have any other questions.