Ethanol

Safety

Results

Methods

Under acidic conditions ethanol is oxidised to acetaldehyde by potassium dichromate.

3CH₃CH₂OH + K₂Cr₂O₇ + 4H₂SO₄ → 3CH₃CHO + Cr₂(SO₄)₃ + K₂SO₄ + 7H₂O

The acetaldehyde is itself oxidised to acetic acid.

3CH₃CHO + K₂Cr₂O₇ + 4H₂SO₄ → 3CH₃COOH + Cr₂(SO₄)₃ + K₂SO₄ + 4H₂O

As the dichromate oxidises the reactants it is reduced to trivalent chromium Cr³⁺ which is green.

The intensity of the green colour can be measured with the colorimeter and compared to known concentrations to measure the concentration of ethanol in the original sample.

It should be noted that other organic molecules can also be oxidised by dichromate with similar results, though the only example we have found where this is significant is the oxidation of glucose. The amount of ethanol in a growing culture of yeast can be measured but a small proportion of the green colour obtained will come from the presence of glucose. You can compensate for this by measuring the amount of glucose using DNSA and adjusting accordingly.

	1M H2SO4 should be labelled 'Irritant'
	2M NaOH should not be used with pupils before Yr9 (S3, age 14) and should be labelled 'Corrosive'. The method works without the addition of NaOH though the green colour is less apparent.

The method employed here used 0.5cm³ of sample to which was added 1cm³ of 1M H₂SO₄ containing 2% potassium dichromate. Incubation was at 50°C for 30 minutes. The colour change can be measured at this point or after the addition of 2cm³ of 2M NaOH (corrosive) to remove the remaining dichromate and reveal the green chromium Cr³⁺.

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