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%TA not adding up?

Ok, so I'm rusty at my chemistry. I'll be honest about it. But, I'm having trouble getting the %TA and pH of tartaric acid to be consistent with typical wine musts. Hopefully someone can point out where I've gone wrong below.

For example, take 1gm of tartaric acid and dissolve it in 1L of water ( = 0.1% TA by definition).

Since the molecular weight of tartaric acid is 150gm/mol, then the molarity of this solution is 1/150 = 0.00667 N.

The pKa of tartaric acid is 2.98 for single ionization. (We can safely ignore the even weaker doubly ionized tartrate as the pKa is 4.36.) The dissociation constant then is Ka = 10^-2.98=0.001047.

So, let's find the [H+].

[AH] [H+] + [A-] is related by Ka

So, setting up the equation: x = [H+] Io = 0.00667 Ka = 0.001047

Ka = [H+]*[A-]/[AH] Ka = x*x/(Io-x)

x = sqrt((Ka/2)^2 + Ka*Io) - (Ka/2)

Plugging in Io and Ka, x = 0.00217, which means the pH is 2.66.

That is the inconsistent part, as wine typically is 0.6% TA and has a pH in the 3.4 range. How can I get a pH of 2.66 with a 0.1% TA? Where have I gone wrong? Thanks in advance!

-Greg

Reply to
Greg
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Greg, My last chem class was in 1951, so I bet my chemistry is more rusty than yours Much of the tartaric acid in grapes is in the form of potassium bitartrate. The more active H+ of tartaric acid has been replaced by a K+. Margalit goes through a sample wine pH calculation in his book "Concepts in Wine Chemistry." See Pages 13 - 15.

Reply to
Lum Eisenman

Pretty much unless you have a ton of analytical equipment you will never be able to determine pH from TA.

First, like Lum says, there is a lot of the acid in wine is in the conjugate base form, with either potassium (K) or calcium (Ca) as the counter ion.

Second, the assumption you make is that the tartaric acid is the only acid in solution. Which is definately not the case. You have malic and citric acid in significant quantites, along with other minor acids formed during fermentation (such as lactic and acetic).

Reply to
Droopy

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