Analysis of phosphate in a phosphoric/citric acid mixture

A. If you have dissolved metal in it also, I doubt if there is any titration method that would work. You could try titrating the similar strength raw acids bu them selves and then titrating a virgin mixture. Since the phosphoric has 3 eq points, I rather doubt it. You should use recording equipment or take huge amounts of readings.

Thanks James, I was afraid of such inconvenience. Thanks again for the answer. Mil gracias

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A. There is a pretty good gravimetric method for phosphate that I used some years ago to determine phosphoric acid in a phosphoric/propionic acid mixture. I regret to say that I don't have the details of it in front of me. You can likely find such with a literature search. It involves precipitating a complex ammonium phosphomolybdate. I do recall that control of acidity, and careful washing of the precipitate are critical.

There are also good colorimetric methods for phosphate. However, they are only applicable to samples with PO4 3- (as P) up to about 0.5 mg/l, which means that you would need to dilute the sample by many orders of magnitude to get to the answer. These methods are presented in detail in
<== 'Standard Methods ...'.

? Dave, I have not been in the Standard Methods in many years, but I seriously doubt if it has a method for the exact amounts of those two acids in a solution with metals and accelerators, promoters and inhibitors. I would be happy to be wrong.

A. If one would to do a very careful volumetric dilution, I think one would get a pretty good phosphate answer. I believe that those other solution elements you mention would be diluted out of relevance.

To be safe, the method of additions could be used.

A. Dave,

The method you mention is one of several classical methods for determination of P in ferrous alloys, and is well-proven and accurate.

It should be used by somebody with some experience of manipulations in wet-chemical laboratories, else they would not be able to handle the procedure.

It would be useful to have a bit of an idea of the P level in the OP's solution. A useful amount of P to have in the analysis sample (which in the case of a liquid would determine the most appropriate volume to take after a volumetric dilution) is 0.5 to 2 milligrams (and hence 1.5 to 6 milligrams of H3PO4).

Guillermo, do you have laboratory facility and personnel to take this further?

Bill, Dave,

Thanks a lot for your answers. I have a wet lab, but not as complete as I would like. One important device I am missing is the colorimeter. Bench top pH, ORP, conductivity are in place. Titration and gravimetric as well.

Thanks again. Mil gracias!