Accuracy in titratable acidity (TA) determination

How does one obtain accurate results when analyzing grape juice or must for titratable acidity (TA) when the concentration of a sodium hydroxide titration solution is suspect due to extended storage? This article and the related laboratory  article “Preparation preparation and calibration of  sodium hydroxide  solutions”  describe how to prepare and/or  simply and quickly  calibrate your existing NaOH titration solutions to provide accurate and reliable results; a useful thing to do perhaps at the start of wine making season

It starts with a  standard concentration solution of an acid  such as tartaric acid and the procedure that follows is  titration of a sample of the standard acid solution to an end point of pH 8.2 with a pH pen or to a  colour change end point with  phenolpthalein indicator.

Prepare a standard solution of, say, 8.0 g/L of tartaric acid.  Now measure the TA of this standard solution by titrating it with your sodium hydroxide solution, using the procedure that you are accustomed follow to get a result we will call TA(observed). If your result is 8.0 g/L, congratulations; your NaOH solution is fine.

If not, however, a useful correction factor is created and it is the ratio TA(standard)/TA(observed). This number will be less than 1 if your NaOH is too dilute and more than 1 if too concentrated. Record this number and use it during wine making season to correct results of TA analysis on juice or must. You may keep using the sodium hydroxide solution that you have on hand unless it is wildly unacceptable.

Example:    TA(observed) after titrating the 8.0 g/L standard is 8.4 g/L.

The ratio is TA(standard)/TA(observed) = 8.0/8.4 = 0.94 so the NaOH is too dilute leading to TA(observed) values that are too high. These results must be reduced by multiplying by the 0.94 correction. (You might want to record the correction factor on the label of the NaOH solution).

Applied to a juice where the measured TA is, say, 10.5 g/L then the corrected TA is 0.94 x 10.5 g/L =10 g/L.

Similarly when the ratio found is more than one, your TA(observed) values are too low and must be increased by multiplying by the correction factor.

Contributed by Ken Stepushyn and Sandy Kirk

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