So basically with this setup you fill the bottom bubbler with 20 mL of wine, and 10 mL of phosphoric acid (25% solution). The upper bubbler gets 10 mL of 0.3% hydrogen peroxide (H2O2) solution and a couple drops of a green indicator solution.
We use an aquarium air pump that's been dialed in to 1 liter of air per minute to bubble air from the bottom flask to the upper for 15 minutes.
You have a burette filled with 0.01N NaOH and we record the starting volume. Let's say its 3.0 mL.
After the 15 minutes of bubbling, you pull out all your stoppers and turn off the pump. Grab the now purple H2O2 solution in the upper flask.
Titrate the purple H2O2 solution with the 0.01N NaOH from the burette while swirling until you get it to turn back to a bright green color, and make sure it actually stays that color. If it goes gray, give it another drop.
Then measure your volume of NaOH in your burette, lets say it 5.5 mL.
Take your initial reading and subtract it from your final reading. 3.0 - 5.5 = 2.5.
Multiple this number by 16. So 2.5 x 16 = 40
You have 40 ppm free SO2 in your wine. Obviously the more precise you are with your measurements and how well you keep your reagents, the more accurate your results will be.
I buy local from a store called All World Scientific which sells this setup. We replaced a few things like the pump and graduated cylinders but everything you would need is in their package deal. You will need to dilute the 30% hydrogen peroxide by 100 which can be tough without the right measuring equipment. We use single channel pipettors for all that stuff but you don't need to. I don't know what other kits are out there but once you do this a few times it's a piece of cake, and you have full control. I have been using this same set of glassware for over a decade and just have to reload on reagents. Here is a link to All World's package deal.
http://www.wine-testing-supplies.com/Item/assembly4
