Re: How would I measure the concentration of acetic acid in vinegar?

Hello, Ron.

You are more astute than our standard titration methods!  When titrating, 
the (basic) titrant only "recognizes" total acidity in the product being 
tested. 

In titrations where there is more than one acid present in the sample, 
calculations are based on the predominant acid.  In vinegars - whether they 
are produced from cider, grapes, malt, sugars, etc. - the main acid is 
acetic acid.  As you already know from your classes, each ml of 0.1N NaOH 
titrant is the equivalent of 0.006005 g of acetic acid.  

Standard vinegar is usually about 5% acetic acid.  In food production 
plants, they use more concentrated vinegars, right up to glacial acetic 
acid, diluting it as needed.

To determine the quantities of individual acids present in a sample, 
you would use chromatography.  High Performance Liquid Chromatography 
(HPLC) is commonly used now for analyzing components of many types of 
samples, including those with organic acids (such as acetic acid).  After a 
sample is prepared, it is run on the HPLC with RI/UV detection (Refractive 
Index/Ultraviolet).  A graph printout then shows peaks of the various 
components; the components are identified - as acetic acid, tartaric acid, 
etc. - by the time at which the peaks "come off" and they are quantified  
by comparing the size of the peaks against a standard.

So, Ron, to measure the concentration of acetic acid in vinegar more 
precisely than titration allows, you will have to look at using a 
chromatographic technique.

Carol Crouse 
The Food Chain Ltd.