Chemistry Titration help

Question

I am doing an experiment at school and I just needed some help with my titration calculation (we are doing a titration with wine with NaOH solution, we are using 10 ml of wine that is to be titrated. With this we are testing different ethanol concentration including 11% 12% 13% 14% 15% 16%, for the titration).
My teacher has said 'we can measure the tartaric acid concentration at the start of the experiment as no oxidation has occurred and we can calculate it's pH using Ka1 (which I have). But then he says this calculated pH would differ from the pH meter results *[(1) could you please explain this to me why].*

He also stated the pH cannot be calculated after the oxidation is taken place as acetic acid would be present. He said that calculating the pH mixture of acids is impossible as two concentrations, two equations (with differing ratios) and two Ka values would have have to be combined somehow ... *(2) I really need help in how to do this calculation when the oxidation is taken place! Could you please help me with this calculation?

The Ka values:

Ethanoic (acetic) acid: Ka 1.75 x 10-5

Tartaric acid: Ka1 9.20 x 10-4 & Ka2 4.31 x 10-5


Thank You very much! 

Answer 1

1.  From the titration you should be able to calculate the concentration of tartaric acid (TA) and then using Ka1 you can calculate pH.  Since TA is a weak acid, adding NaOH makes the Na salt and thus a buffer is formed (not sure if you have covered buffers yet).  At any rate, ignoring that fact, you would calculate [H+] from Ka = [H+][Tartrate^-]/[TA] and from the [H+] you would calculate pH.  This would be different from the pH meter for a couple of reasons.  It could depend on the standardization of the NaOH you use, and it could be because you didn't use Ka2 (which differs from Ka1 by only a factor of less than 10). This second dissociation will add to the acidity so pH meter will probably show a slightly lower pH than the calculated pH.

 

2.  You cannot calculate pH if an unknown amount of oxidation has taken place. As explained by your teacher, you will have two acids with differing ratios (unknown ratios) and two Ka values.  Now, if you knew the ratios (i.e. how much TA had been oxidized), then you could calculate the {H+] contribution from each acid (TA and acetic), and hence the pH.  As a note in passing, I'm not sure that acetic acid is a product of TA oxidation, but assuming it is, this is what you'd have to do.

 

If this doesn't make sense, then contact me and provide additional information, and maybe I can help further.