Measuring the alcohol concentration of pre-made wine?

I am currently doing an assignment at school that involves using different means of measuring the alcohol conc. of several different types of wine. We are thinking of using the methods of, titration, ebulliometry and using a hydrometer, but is it even possible to use a hydrometer to find alcohol conc.? Because everywhere I have read on the Internet says that it needs to be measured before you add yeast and then after.

Is there any other methods that you can suggest that are quick, easy and effective?

There is some density method that we were thinking but opted out of that one because we have limited time.

Anyone know if we can use a hydrometer?

Thanks in advance. :)

Answer:
You can use a hydrometer. I think this is a standard technique in the wine and beer industry. You need to make some sort of standard curve, by testing the density of different solutions of ethanol (ie 1%, 5%, 10%, etc), and then comparing the density (more accurately, the specific gravity) of your solution (the wine) against that. You can probably find more accurate tables specifically for wine online, since wine also contains sugars, flavors, etc that may affect the specific gravity.
Yes you can use a hydrometer. You'll find one at any shop that sells home-brewing supplies.

If you have a balance and a volumetric flask or pipette you can do it just fine that way too.

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