It's not as simple as that. When you're talking about TA you're talking about the complete acid content of the substance.
Yes, it is as simple as that and no, you are not talking about the complete acid content of the sample (see below).
If you wanted an accurate measurement of malic acid specifically you would have to use spectrophotometry or paper chromatography.
But you don't. You want the total acidity. Total acidity is not the total amount of malic or lactic or any other kind of acid. It the amount of alkali required to bring the pH of a sample to a reference pH - here I believe it to be 8.3.
Suppose I dissolve 1 gram of malic acid in a liter of water and give it to you with the request that you determine the total alkalinity. The solution pH will be 2.81 and you will have to add 14.91 mEq of strong base to get the liter to pH 8.3. As the equivalent weight of malic acid is 134/2 you conclude that the total acidity of this sample is 14.91*134.09/2 = 999.64 mg/L which is, of course, very close to, but not exactly equal to the amount of malic acid that I put into the liter in the first place.
Now I return to the back room and prepare another sample with 1 gram of malic acid and 302 mg of NaOH dissolved in 1 liter of water to you and request that you determine the TA of this sample. This time you measure the starting pH as 4.3 (more typical of apple juice) but it only takes 14.91 - 7.54=7.37 mEq of base because I already slipped 7.54 meq (302 mg NaOH) into the mix in the back room. You conclude the TA is 7.37*134.09/2 = 494.12 mg/L. This is half the amount of acid I added but is, nevertheless the total acidity of the sample I gave you. Now if you fully understand how this works and the only acid in the sample is malic you will know how to compute the total amount of malic which went into the sample from the 'total' acidity and the pH using the two pK values for malic acid. IOW it appears your confusion stems from your misapprehension that 'total acidity' means the sum of the malic acid, bimalate and malate in the sample IOW the amount of malic acid that someone originally put into distilled water before adjusting pH to whatever pH the sample is presented at. That is not what it means. It means the amount of base that must be added to the sample to get it to pH 8.3.
What I'm talking about is if you already have a fairly high TA and want to lower the pH you're going to have problems, because malic and tartaric have different pKa's and disassociation constants, tartaric is better for making pH adjustments because it doesn't affect TA as much as something with a higher pKa.
I think we covered in the previous post that what OP wants to do is manipulate TA, not pH. Either acid will do to manipulate TA as adding 1 gram of malic increases TA by 1 gram as malic and adding 1 gram of tartaric increases TA by 1*134/150 grams as malic. We don't care about buffering because we are not concerned about pH.
Essentially, the higher the TA, the greater the buffer capacity of the substance, and therefore the more resistant it is to pH change via additions of acid.
We don't care about buffering because we are not concerned about pH in the sense that we want to regulate it to some value. At the same time we (or rather OP) do in the sense that the TA
is the buffering of the sample to pH 8.3.
However, if you want to believe that adding acid ALWAYS lowers the pH, feel free.
That's a pretty good thing to believe. Could you cite an example where adding Løwry-Bronsted acid doesn't lower pH?
Just inquiring and trying to help.
We know you are trying to help but apparently you don't understand what 'total acidity' is and so are muddying the waters. That is actually OK since it got a bit more of the science aired here.