[sibie]

Dear Gurus & All,

I have a question in mind and appreciated if you all can help on it.

Current product contains HNO3, CH3COOH and H3PO4.
Current method to determine the CH3COOH are as follows,
1) Determine the HNO3 by using UV.
2) Vaporate the CH3COOH until its boiling point is reached.
3) Using titration to determine the H3PO4.
4) CH3COOH assay = Total Assay - (HNO3 assay + H3PO4)

Please advise if this method is reliable, consistant and what are the cons & pros of it.
Is there a standardized method for all chemistry lab to determine the assay from a mixture?

Thank you so much,
Sibie.

[Panoramix]

Why don't you titrate with some base and an electrode as an indicator?

The pka of the acids are quite different and you should get a nice curve indicating the concentration of all the acids, don't remember the pka of all the protons in H3PO4 but google should give the answer.

[lavoisier]

Dear Gurus & All,

I have a question in mind and appreciated if you all can help on it.

Current product contains HNO3, CH3COOH and H3PO4.
Current method to determine the CH3COOH are as follows,
1) Determine the HNO3 by using UV.
2) Vaporate the CH3COOH until its boiling point is reached.
3) Using titration to determine the H3PO4.
4) CH3COOH assay = Total Assay - (HNO3 assay + H3PO4)

Please advise if this method is reliable, consistant and what are the cons & pros of it.
Is there a standardized method for all chemistry lab to determine the assay from a mixture?

Thank you so much,
Sibie.

Concerning Panoramix's suggestion, I'm afraid HNO3 will overlap with the first dissociation of H3PO4, and CH3COOH with the second dissociation of H3PO4. I may be wrong, though. Just try and see.

I know that conductimetric titration is a bit more helpful with mixtures of weak and strong acids, because it relies on the differential conductivity of anions from the acids, but here it's a real mess...

The other possibility could be to determine the anions, e.g. precipitate the phosphate as some metal salt and then do a selective titration of the remaining acids. But I guess it's not very practical.

Looks more like a question for analytical chemists, anyway, doesn't it?

[Panoramix]

Acetic acid:
4.75
http://home.planet.nl/~skok/techniques/laboratory/pka_pkb.html

Phosphoric acid:
H1: 2.12
H2: 7.21
H3: 12.67
http://home.planet.nl/~skok/techniques/laboratory/pka_pkb.html
Wikipedia has almost the same values

Nitric acid:
-1.5
http://chemweb.unp.ac.za/chemistry/Physical_Data/pKa_values.htm

The pKa are at least 2.5 orders different, so you should get a good titration curve.

[Borek]

Why not use pKa values link on the left menu?

2.5 unit is not enough. IMHO you will see something like that, without clearly separated endpoints (curve is calculated assuming identical concentrations of all acids, if they are not identical, endpoints can combine).

[Panoramix]

Why not use pKa values link on the left menu?

Because I was too blind to see them, next time I will be happy to use them.

My personal experience is that with some experience acids with a pKa difference of 1 can be titrated in DMF with TBAH or ethanolic KOH with potentiometric indication with reliable results.

[Borek]

For some reason I have assumed it is water solution so it will require removing water before non-aqueous solvents/tritration can be used. That could be difficult as acetic acid is relatively volatile - but you are right that it may be worth of trying.

Conductometric titration seems interesting (and much easier) as well.

I doubt anything can be reliably told about the problem as long as we don't know anything about relative concentrations of acids and about other substances present in the solution (water being one of them ).

[sibie]

Dear All,

Thanks for the replies.

Yes, Lavoisier is right, it is much more for the Analytical Chemist to answer but I do hope to find out the answer from this forum too

Typically from the mixture, I am most interested in getting out the true value of Acetic Acid.

There are a few methods like TOC, Evaporation and CE. However I am not sure is there any verified, standarised, recognised methodology by worldwide lab.

Just like ISO9001 or ISO17025. It is widely recongised.

Thank You So Much,
Sibie