Practical investigations
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I'm investigating sulphites in wine using ripper methods and have read other questions related to this on here, but can’t seem to find an answer to my question(s)? To calculate the amount of sulphur dioxide, total SO2 = moles of iodine x cm3 of iodine used x 1280 (1280 is a constant - but do you know where this comes from)? Also I understand that NaOH is added to dislodge bound SO2, but why is it acidified again with H2SO4 (don’t the SO42- ions in the acid also contribute to increased reported levels of SO2?). I would be most grateful for your help.
Igloo writes ...
The formula you have given me is not entirely correct. It should read:
Total SO2 (ppm) = concentration of iodine solution x volume of iodine solution x 1280 [ppm = parts per million]
Here’s how the equation is derived. Assume that a 50 cm3 wine sample is being titrated with iodine solution of concentration c mol dm-3, and that the titration reading is V cm3.
Moles of iodine used = (V/1000) x c
Since SO2 + I2 + 2H2O ---> H2SO4 + 2HI, 1 mole of iodine reacts with 1 mole of SO2.
So moles of SO2 present = moles of iodine used = (V/1000) x c
But this is the SO2 in 50 cm3,
So, in 1000000 (1 million) cm3, the moles of SO2 = (V/1000) x c x (1000000/50) = 20Vc
In mass units, since Mr[SO2] = 64 g mol-1, 20 Vc moles = 20Vc x 64 = 1280 Vc grams
So in grams per 1000000 cm3 (ppm), the sulphur dioxide content of the wine is 1280 Vc, which is effectively the formula given at the start of my answer.
If you decide to carry out your own investigation, I strongly recommend that you work through your calculation step-by-step using numerical values, and do not simply substitute numbers into a formula. In any case my formula only works if you use a 50 cm3 wine sample, whereas you are more likely to choose a 25 cm3 or 10 cm3 sample.
Many websites are American and designed for wine enthusiasts, including those who actually produce wine. Concentrations are often expressed using “equivalent weights” and refer to “normal” solutions, using the N symbol. Chemists these days always use the concept of molarity using molar masses and molarities (M) instead. A normal solution (1N) is not always the same as a molar solution (1M). Do not get confused - work in molarities, as I have above.
In order to dislodge bound SO2 from certain types of molecule e.g. carbonyl compounds such as aldehydes and ketones within the wine, sodium hydroxide is used. However, the iodine titration itself must be carried out in neutral or acidic conditions, since iodine reacts with alkali to form ions such as IO- and IO3-. Sulphuric acid is a perfectly acceptable choice for the acidification necessary, and the SO42- ions do not interfere at all since they contain sulphur in oxidation state +6. SO2 and its derivatives within the wine contain sulphur in oxidation state +4. In fact, the titration involves an oxidation of S(IV) to S(VI) – see the equation in the calculation above.
This should help you understand what is going on.
Look at the following web article by Ben Rotter. It is excellent and covers most of the theory you need for an an investigation based on SO2 in wine.
Investigating SO2 in wine Click on Articles and then on Sulphur dioxide.
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updated: 20 December 2006
