How to test for so2 in a tannic red wine?

I have a box of Titrets to test for so2 but after reading further both my book (Techniques in Home Winemaking) and the instructions both state "This test kit is not recommended for use with red wines or white wines containing ascorbic acid or tannin. These wines often give false high test results. As a rule, a test result of greater than 40 ppm free sulfite for any wine should be considered suspect and an alternative sulfite determination method should be employed."

The book goes on to say that all kits have the same interference problem from ascorbic acid, but it doesn't mention about tannins.

Is that alternative method they are referring to the Aeration-Oxidation (AO) method?

I just want a fool proof method (if there is one) for testing for so2, and the easier the better. But if the AO method is the only way than so be it.

Thoughts/info?

Sorry, I don't have any info as to testing for SO2. Ascorbic acid is also vitamin C, and not the same thing as tannins.

So why are you testing for sulfur dioxide? If you're testing for sulfites because of an allergy, people that are allergic to sulfites are also allergic to many other foods as well. Red Wine Headache happens because of something else in the wine. The jury is out on what causes it though. Here is an interesting article on Red Wine Headaches, which I get if I have several glasses of red wine.

Yeah, AO is the only really reliable way to go. However, it's expensive and not really convenient for the home winemaker. I'd use the titrets as an approximation and just use logic on your additions. One of my books from the American Wine Society says that if you add 20 ppm for each operation, you won't end up oversulfiting. Hope that helps!

Yeah, your problem is that with ascorbic acid and tannins around the test will read a false-positive because SO2, ascorbic acid (vitamin C), and tannins all react with the color creating compound (iodine + starch) and reduce the iodine to iodide.

Another method is called the "method of standard additions." You could conceivably do standard additions with your Titrets kit. What you would do here is take x mL's of your wine (whatever the Titrets call for) and add 1 mL (or some other volume you calculate to work) of bisulfite solution of concentration X. (You should be able to find graduated syringes for animal use or a baby medicine dropper to accurately measure out small volumes.) Then you do another titration adding 1 mL of bisulfite solution with concentration Y. Finally you titrate the wine + 1 mL distilled water. If you plot response (amount of titrant added) vs. the concentration of the standard addition then you can back out a value for SO2. (Do a search for the method of standard additions on your favorite search engine.)