Determination of total acidity & volatile acids in vinegars
Objective :
1) to determine the total acidity in samples of vinegar by direct titration with alkali
2) to determine the volatile acids,mainly acetic acid, in samples of vinegar.
Introduction & principle:
The total acidity of vinegars is derived both from the original fermentation process & from acidic salts
present in the original liquor used for fermentation. It may be determined titrimetrically using
phenolphthalein as an indicator.
The determination of volatile acids is useful in the detection of adulteration with mineral acids or
certain organic acids. The natural acidity of vinegar is mainly due to the presence of acetic
acid,CH3COOH, which is volatile. Contaminating mineral acids & certain organic acids are not
volatile & are not removed by evaporation. Hence they contribute to the acidity present after
evaporationof the sample. The difference in the acidity present before & after evaporation may be
taken as the volatile acetic acid.
Material:
1. 0.1 M of NaOH
2. Phenolphthalein indicator
3. Water bath
4. Porcelain evaporating basins.
Method:
Determination of total acidity
1. weigh 25 gm vinger.
2. add 50 ml of distilled water
3. titrate with 0.1M NaOH , using phenolphthalein as an indicator
4. calculate percent acidity as acetic acid(MW=60.05)
Wt. of acetic acid=(0.1M NaOH X ml titre X MW ) / 1000
%of total acidity =(wt. of acid / wt. of sample) X 100
Determination of volatile acids
1) determine total acidity in a sample of vinegar as above.
2) take a similar quantity of the vinegar & place it in a white porcelain evaporating basin.
3) evaporate the vinegar to small volume on a water bath
4) add about 25 ml of distilled water & repeate the evaporation.
5)repeate the addition of water & evaporation(i.e. the 3rd evaporation)
6add about 25 ml of distilled water & titrate with 0.1M NaOH, using phenolphthalein as an indicator
7) the difference between the titration in step 6 & that in step 1 arise from the volatile acids & should
be calculated as acetic acid .calculate the percent acidity as acetic acid, taking into consideration that
1ml of 0.1M NaOH is equivalent to 0.006005 grams of acetic acid.
% of acidity of non volatile acid ={(titre X 0.006005)/wt.g of sample} X100
% of volatile acid = % of total acidity - % of acidity of non volatile acid
REFERENCE:
Lees,R, "Food Analysis", (1975), Leonard Hill