1. Starch Indicator.
Add 3.0 g starch to 500 mL saturated sodium chloride solution (saturated at room temperature to give a solution that doesn’t grow bugs) and boil until starch has dissolved. Filter to remove any residue and add 20 mL water plus 80 mL of acetic acid (also helps to stabilize solution).
2. Sulphuric acid.
Add (CAREFULLY) 100 mL conc. sulphuric acid to 300 mL water, resultant solution is 5 M. Store in a plastic bottle with a plastic cap that seals well. VERY CORROSIVE. Clean up any drops right away as they get more concentrated on drying out.
3. Oxidant.
Weigh, carefully, 27.8 mg of potassium iodate and dissolve in 500 mL water. This solution reacts with iodide in acid solution to produce iodine equivalent to 5 ppm SO2 per mL of periodate solution.
Determination of Free Sulphur Dioxide.
Take 10 mL wine + 1 mL starch indicator solution + 1 mL sulphuric acid. Add a pinch of NaHCO3 (sodium bicarbonate) to dispel air (not essential) + a trace of potassium iodide (essential as catalyst). Titrate with oxidant solution (KIO3) until there is a blue tinge that persists for about a minute. Each mL of oxidant solution used is the equivalent of 5 ppm of suphur dioxide in the original wine.
Determination of Total Sulphur Dioxide.
Take 10 mL wine + 15 mL 1.0 M NaOH ( 40 g of solid sodium hydroxide in 1.0 L water, KEEP IN PLASTIC BOTTLE), let mixture stand for 10 minutes. Add 5 mL sulphuric acid solution, a pinch of sodium bicarbonate and potassium iodide, 1 mL starch solution. Titrate with oxidant solution until a persistent blue tinge. Again 1 mL oxidant = 5 ppm SO2.
NOTE: For red wines the colour change is difficult to see and it is recommended to illuminate the solution from the side or below with a strong yellow light. If still in doubt compare titration solution against a control sample of wine treated with the same reagents but with x mL added water instead of x mL KIO3 oxidant solution.
Contributed by Dr. Sandy Kirk