[felitix]

Hello,

  I'd like to ask if the method called " total acid as phosphoric" which should test %phosphoric with some %oxalic in the liquid product is correct:
1. I have a liquid product of many ingredients containing according to specification 8.5% phosphoric acid and 3% oxalic acid.
2. When product of the above specification is manufactured - a sample comes for test %Phosphoric Acid
3. The test result is always high (12.5% or 13.6%). The method used to test %Phosphoric is titration with 0.5N NaOH. The formula is %H3PO4=(9.*(ml NaOH)(0.5)/ gram sample. Titration goes until pH 4.8 of the solution is achieved
4. Is the assumption correct that the high result is due to the fact that Hydrogen ions of oxalic acid are neutralized too?
5. If the assumption is correct can the test method be changed from %Phosphoric to %Total acid as phosphoric? What are we really getting by applying this method of testing to the solution including both phosphoric and oxalic acids?
6. I assume that the method only accounts for phosphoric acid, and by setting the pH to 4.8 we were supposed to get the phosphoric acid concentration after the first ionization. If so the result is not the same as %phosphoric in the solution. Am I right?
7. As we have solution of both acids does it make any sense to apply this method? I believe the result is telling nothing, because:
- by setting pH to 4.8 and titrating solutions of both acids we even don't know what part of the phosphoric acid has been neutralized and what part of oxalic has been neutralized. In my opinion we can't say the obtained result in this way is %total acid as phosphoric. Am I right? Please send me any comments/corrections to the way I think.
Thank you,
felitix

[Arkcon]

To start with, we'd need to know the name of the method, or some of the reagents used.  Just a few facts, we don't need the whole procedure.  It seems that you're using a simple titration with NaOH, so that will definitely be affected by the presence of oxalic acid.

[felitix]

Thank you for taking this topic. The method is simple titration with 0.5N NaOH.
It says:
1. Add appropriate amount of sample (which is 2 gr) to 100 ml of de-ionized water in a beaker.
2. Add the acid to the water.  NEVER ADD WATER TO ACID.
3. Set on stir plate and mix continuously.
4. Place pH meter in solution and titrate with 0.5N NaOH until pH of 4.8 is achieved.

% H3PO4 = (9.(ml NaOH)(N NaOH)
                        (gr sample)

Sample is the product containing among other ingredients phosphoric and oxalic acid. If the above test is applied to the sample, is the result telling us anything? If it is telling can anyone specify what are we getting by doing this test?

[Borek]

Titrating to 4.8 in the solution WITHOUT oxalic acid would give a correct result, as 4.8 is more or less the equivalence point for the first proton of phosphoric acid, it has reasonably well visible inflection point on the titration curve. In the presence of oxalic acid your final result is a sum of phosphoric acid and around 180% of the oxalic acid (strange as it looks, that's correct - at pH 4.8 you have neutralized first proton of oxalic acid completely and the second proton in around 80%).

[felitix]

Thank you for the detailed explanation 
I have one more question to have a complete understanding of what is going on here:
If my product has 8.5% Phosphoric acid and 3% Oxalic Acid, my total % of acids is 11.5%.
Using the method described in my previous post I neutralize the first proton of phosphoric acid, neutralize completely the first proton of oxalic acid and about 80% of the second proton. So if I want to know what is the % acids in my sample after the test how to calculate that?
I can see that about 20% of oxalic will not be neutralized, so I should have 2.4% of oxalic acid - correct?
I don't know about the phosphoric? If I neutralized only the first proton can I assume I have 1/3 of 8.5% meaning 2.8%. It would come up that my % acid after the test is 2.4%+2.8%=4.9%. It does not correspond to what I'm getting (12.5 or higher)

[Borek]

If my product has 8.5% Phosphoric acid and 3% Oxalic Acid, my total % of acids is 11.5%.

When titrated to pH=4.8 you will find 8.5+1.8*3=13.9 - so well above 11.5%

Note that this system is quite sensitive to the changes in ionic strength, so it is very difficult to calculate anything with a reasonable precision, it won't be exactly 13.9%. Still, general idea will hold, you should expect result that is higher than the real total acid concentration. And that's what you are getting.

[felitix]

Thank you for the excellent explanation :-)
So for phosphoric 8.5 is valid after the first proton dissociation?

[Borek]

So for phosphoric 8.5 is valid after the first proton dissociation?

Only if there is no oxalic acid present. Take a look at the titration curve I posted earlier.

[AWK]

You can find amount of oxalic acid in this mixture by a permanganate titration.

[felitix]

Thank you for truly excellent explanation! :-)