Practical investigations
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I have conducted a titration with iodine to find the total and free sulphur dioxide content of white wine. To find the total content sodium hydroxide solution was added to the wine. At the beginning of the experiment a solution containing starch and EDTA was added to the wine. I know that the complex formed between starch and iodine indicates the end-point of the reaction. However, I am unsure what the purpose of the EDTA was. If you would like to view the method I used it can be found at www.tapdk.gov.tr/alkol/duyuru/analiz/20061013_tum_kurumlara_yeterlilik_testi_ek3-1-1.doc Also, i was wondering why the wine sample was diluted using 200 ml of ice-cold water before the last titration with iodine.
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Igloo writes ..........
The titration you have carried out is often known as the Ripper method and works perfectly well without the EDTA. I can only imagine that in your method the addition of EDTA complexes any metal ions in the wine which are susceptible to oxidation by iodine. The EDTA complexes formed stabilise the ions and prevent them from being oxidised, which would give rise to an additional set of unwanted competing reactions. Consequently the iodine used in your titration reacts only with the sulphur dioxide present, which is as intended.
The second half of the titration involves alternate uses of alkali (to remove the bound SO2 from molecules such as the carbonyls within the wine) and acid (iodine titrations need to be carried out at pH<7), which means that the solution is going to warm up because of the exothermic neutralisation process involved. Iodine titrations must be carried out at low temperatures, otherwise iodine will be inclined to vaporise from the reaction mixture before it has an opportunity to oxidise the sulphur dioxide within it. This would lead to higher (incorrect) titres.
Risk assessment
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updated: 13 April 2008
