Does anyone know a really straight forward approach to titrating a polyprotic acid?

Question

Does anyone know a really straightforward approch to titrating a polyprotic acid specifically phosphoric acid?

Answer 1

The easiest way is to use a pH meter, and graph the pH of the solution vs. volume of base added. Look for the 3 places on the graph (one for heach acidic proton in the acid) where it is the steepest, which show the volumes of base that titrated each proton (endpoints). The volume of base added at the first of these three steep points can be used to calculate concentration (in the same way you would for a monoprotic acid) The three points should be the same distance apart (for example, at 25, 50, and 75 mL of base added), which you could use to get basically 3 replicate titrations on the same sample.

If you need it, the pKa of each proton equals the pH at 1/2 of the equivalence point for that proton (12.5, 62.5, 87.5 mL in this example). These values can be used to identify the acid by comparing them to tables of known values.

Straight forward? Maybe, maybe not. But since you used the word polyprotic in a sentence, I'm thinking you are probably looking for both concentration and pKa's. I'm also hoping you'll be able to follow this.

I suppose another way to do this would be with an indicator, if you know what acid you've got and just need the concentration. You need to choose your idicator carefully, so the pKa of the indicator is as close as possible to the pKa of the proton you are interesed in titrating (the first proton would be simplest, since you can then treat the titration as if the acid were monoprotic).

Answer 2

If you're trying to get the three equivalence points and nothing else, three indicators that change in the vicinity of the equivalence points would be very necessary, but for a titration curve a laptop, logger pro and a pH meter would be the way to go. You'd have to figure out the Ka of phosphoric acid, dihydrogen phosphate and hydrogen phosphate to figure out the three equivalence points for the graph to figure out what indicators to use though.

Answer 3

Well, if you're learning it in class, then your teacher will probably want you to look for the 3 equilenance points on the titration curve. Notic those! They look like this:

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|7777777 *
|77777777***
|7777777777*****
|7777777777777********
|7777777777777777******
|777777777777777777777****
|777777777777777777777777**
|7777777777777777777777777*
|7777777777777777777777777*
|7777777777777777777777777*
|7777777777777777777777777*****
|7777777777777777777777777777********
|777777777777777777777777777777777****
|77777777777777777777777777777777777**
|777777777777777777777777777777777777*
|777777777777777777777777777777777777*
|777777777777777777777777777777777777*
|777777777777777777777777777777777777****
|777777777777777777777777777777777777777*****
|777777777777777777777777777777777777777777*********

Where the y-axis is the pOH (oops I got the curve up-side-down, if it were right-side-up, the y-axis would measure pH) and the x-axis is the amount of base added. Also, be sure to use a strong base, i.e. a alkaline or alkaline-earth hydroxide. Dispense in a buret, measure pH constantly.

Answer 4

It depends on if you want the first, second, or third equivalence point. Basically, do three titrations, one for each equivalence point, using indicators for each desired equivalence point. The equivalence point will be determined using the Kb's of H2PO4(-), HPO4(2-), and PO4(3-). You need to find the pH of the soln when it contains "only" H2PO4, HPO4, and PO4.

Answer 5

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