This method uses a redox titration to find the concentration of ethanol in an aqueous solution. The ethanol is oxidised to ethanoic acid by reacting it with an excess of potassium dichromate in acid.
The amount of unreacted dichromate is then determined by adding potassium iodide solution which is oxidised by the potassium dichromate forming iodine.
The iodine is then titrated with a standard solution of sodium thiosulfate and the titration results are used to calculate the ethanol content of the original solution.
Because alcoholic beverages such as wine or beer contain other oxidisable substances that could interfere with the titration, the dichromate solution is placed in a flask and the alcoholic beverage sample is suspended in a small container above it (see diagram).The water and ethanol slowly evaporate and as the ethanol comes in contact with the dichromate it first dissolves, and is then oxidised. More ethanol evaporates until eventually all the ethanol from the beverage has left the sample and reacted with the dichromate. Since this transfer takes time, it is necessary to leave the flask with the suspended sample in a warm place overnight.
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Ethanol method (PDF 93kb)
Ethanol sample suspended over acidified potassium dichromate solution.
The left flask shows the brown-coloured solution resulting from the formation of iodine. The right flask shows how the brown colour fades to pale yellow as the iodine is titrated with thiosulfate (this is the stage at which starch solution should be added).
Upon addition of starch the solution takes on a blue-black colour due to the formation of a starch-iodine complex.
As more thiosulfate is added and we near the titration endpoint, the blue-black colour from the starch-iodine complex fades.
The endpoint of the titration is reached when just enough thiosulfate is added to react with all the iodine present and the solution becomes colourless.