[daz]

Hi there,
My chemistry is a bit rusty but at my work I wish to determine  the amount of acid (in gms) (mainly citric) in a beverage.
I'd be titrating the beverage with 0.1M NaOH solution using phenophthalein as the indicator. Once I reach  the endpoint, how do I calculate how much acid is in the beverage?
If there is more than one acid in a beverage ( eg. citric & malic acid), will titrating determine the overall level of acidity?
Also what solutions would you titrate with to isolate the amount of various acids in a beverage? e.g. malic, acetic, citric acid?
Any help is appreciated. thanks

[lavoisier]

Phenolphtalein has a pKa of 9.7, therefore in principle you will titrate all acids which form a (sodium) salt below this pH.

If you add NaOH 0.1 M to a 0.01 M aqueous solution of citric acid (pKA's 3.13, 4.76 and 6.40), the acid will be completely titrated (trisodium citrate) at pH 9.1. The next drop of base will make the pH rise above the pKa of the indicator, and you will see the endpoint.
The mmoles of citric acid can be then calculated by dividing by 3 the mmoles of NaOH used.

VNaOH (mL) * 0.1 mmol/mL = mmoles NaOH

mmoles citric acid = mmoles NaOH / 3

m citric acid (mg) = mmoles citric acid * 192.124 mg/mmol
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m citric acid in your sample (mg) = VNaOH (mL) * 0.1 * 192.124 / 3

Unfortunately, a lot of other acids commonly found in beverages (carbonic, phosphoric, malic, ascorbic, tartaric...) have pKa's low enough to be titrated (partially or totally) together with citric acid.

So I think you'd better know the qualitative composition of the sample if you want your measure to be meaningful.

Hope it helps.