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How to do a ppm calculation for sulphur dioxide in wine

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lab123:
I've recently read an article called 'sulphur dioxide and tanin' by G.W.A Fowles. it talks about a titration between iodine and wine to determine the sulphur dioxide content. It gives this equation to work out the sulphur dioxide content in parts per million(ppm):

sulphur dioxide in ppm = 12.8* volume of iodine added(in cm3).

Does anyone know where does the value 12.8 come from?  

Mitch:
This is the general formula for parts per million, but....

ppm = mass of the solute/ mass of the solution *10^6 parts per million

...If your density is approximately 1.0g/ml  you can just use this nifty trick
1ppm = 1mg/liter

Let me know if you need more info. :)

lab123:
Hi! Can you show me how 12.8 is derived?

Bystander:
"12.8" is a meaningless number without information on the volume being titrated, the reaction, and the strength of the titrant (I doubt very much that the analysis is being done with elemental iodine).

hmx9123:
I used to TA a lab that actually calculted several things based on titration, including alcohol concentration and SO2 concentration.  There were no 12.8 figures in there.  I'm sure that number came from the concentration & volume of the iodine solution used.  If you're interested, I can post the part of that lab that dealt with the sulfur dioxide concentration.

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