Determining volatile acidity in wine via distillation

Aug 2017
1
0
Lebanon
Hello,

I'm a winemaker, but not an enologist.. so my chemistry skills are a bit limited. I'm trying to assess the amount of volatile acidity (VA) in wine (volatile acids in wine include several but the major acid is acetic acid, therefore we assume volatile acidity ~= acetic acid concentration in the wine). One method is by distillation, we take a wine sample, place it in a still of sorts (I have a mothe type still), pass steam through the sample which boils the sample and captures the distillate in an erlenmeyer flask. We then titrate with a 0.1 N NaOH solution to determine the amount of volatile acidity in the wine. Every time I do it I get a different result. I even tried it with a sample of diluted acetic acid: i ran the still and captured 250ml of distillate, when i opened the boiling chamber to release what was left, there was a very strong smell of acetic acid (meaning most of the acetic acid actually remained in the boiling chamber instead of distilling into the capture flask).

As additional info: wine often contains sulphites, which can erroneously increase the final result, so we neutralize the sulphites by adding a few drops of 0.3% hydrogen peroxide to the sample. The method also says to add a few crystals of tartaric acid but I'm not sure why.. tartaric acid is not volatile and wouldn't affect the distillate.

Where am I going wrong? It seems to me this type of analysis is too fickle to be reliable.. There are too many parameters that could go wrong: steam escaping somewhere along the way (reducing volatile acidity that makes it into the capture flask), incorrect NaOH concentration for the titration, too long or too short distillation time, etc...

Can someone provide some tips on doing this right? Below is the exact procedure I follow:

1. Degas wine sample (to remove CO2 which can affect result)
2. Add 10 ml wine sample to steam distillation flask
3. Add a few crystals of tartaric acid & couple of drops of hydrogen peroxide
4. Distill and capture distillate in sealed flask
5. Once captured 150 ml of distillate, stop and titrate with 0.1 N NaOH

The formula I use to calculate VA is as follows:

VA = 60 * (Vt * N)/(Vw * F) in g/L

Where:
Vt = mililiters of NaOH titrated
N = Normality of NaOH (0.1 N)
F = Formality factor of NaOH
Vw = volume of sample (10 ml)

If sample size is 10ml and NaOH normality is 0.1, equation is simplified to VA (g/L) = 0.6 * Vt

The expected range for the result should be between 0.2 and 1.5 g/L

Thanks,
Eddy