How to Determine if Supplied 70% Ethanol was the correct Proportion of Alcohol

[chunkytuna21]

You need to use 70% ethanol for an experiment. You only have a small quantity of 100% ethanol which is not enough to dilute and make the volume of 70% that you need. A chemistry friend of yours says he will make and distill some 70% ethanol for you. Unfortunately, your experiment doesn't work and you suspect the 70% ethanol is to blame. Suggest a way to determine whether the supplied 70% ethanol was the correct proportion of alcohol.


I'm thinking you could use specific gravity with pure water as a reference. Is that right? Any suggestions?

 

[Ygggdrasil]

Specific gravity is a good suggestion.

 

[PhaseShifter]

You would need to prepare a few references over a range of compositions, though.

I seem to recall that there is a small but significant change in volume when mixing ethanol with water. (10 mL of water and 10 mL of ethanol give something like 19.5 mL of the mixture, but it's been years since I observed this)

 

[Borek]

There are even hydrometeres scaled not in g/mL but directly in ethanol %.

No need for reference solutions - it is enough to check density table in any handbook.

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