Why doesn't tartaric acid addition lower my pH?

Hi Clark,

I have been working a blog post to the PA wine industry regarding how winemakers can deal with high potassium wines.  For the past few years, we've been dealing with wines that have an extremely high pH (4.0+) and usually the TA is low, too (<6.0 g/L).

Now, I've tried to make tartaric acid additions to these wines, but often, the TA will go up and the pH will hardly move.  Of course, then I am dealing with a high pH, high TA situation.  Ironically (or perhaps, no so), we have been getting more phone calls regarding high potassium levels in the vineyard or in harvested fruit, so I'm currently guessing many are dealing with high potassium situations.  I recalled that the use of ion exchange or ED may be applicable for high potassium wines to reduce the potassium concentrations and potentially alter the pH to a more reasonable level in the wine.  I'm contacting you to 1) see if this is the case, 2) if there are any great resources that you can recommend for winemakers to alter this problem in the cellar, and 3) if there are any companies on the eastern part of the U.S. that can offer these services (ion exchange, ED) to winemakers in the region?

   Any help that you can provide will be GREATLY appreciated.  I've already referenced your book and few online articles that I have found regarding the situation.  Otherwise, I hope this harvest is treating you well.  Best, Denise Gardner, Penn State Enologist


Hi Denise,

How very fine to hear from you.  I am quite impressed with the program you’ve put together and read your weekly blog updates with relish.

Your question involves a basic tricky problem which confronts winemakers all the time.  They often report adding tartaric acid to no effect.  This is an illusion.  No need for IX or ED technology - the solution is easy once you understand what’s going on.  This is the first thig I deal with in Fundamentals of Modern Wine Chemistry and it takes a while to explain.  Let me try.

Tartaric acid has the following dissociation equilibria:

H2Ta  <=>  H+  +  HTa-  <=>  2H+  +  Ta2- , with pKas of 3.0 and 4.2.  The bitartrate curve (HTa-) peaks at pH 3.6 and is rather low at pH 4.0. 

Let’s break these down into the two separate equilibria:

Eq1:    H2Ta  <=>  H+  +  HTa-               pKa = 3.0

Eq2:    H+  +  HTa-  <=>  2H+  +  Ta2-   pKa = 4.2

In the normal case, K+ has been exchanged for H+ so you have a lot of bitartrate buffer but K+ is the counter ion.  This combination should precipitate as KHTa, but it doesn’t because the pH is so high that much of it is in the TA2- form.

Now think about what happens when you add a little more tartaric acid (H2Ta).  Say you add 1.0 gm/L.  Initially, the pH will shift down, but you have a lot of buffer present, so maybe you get to 3.85.  Now you have a even more tartrate present and a higher percentage in the bitartrate form, plus lots of K+, so you get a precipitation. 

Since you’re lowering the TA by removing a titratable proton (in KHTa), the TA comes back down.  At the same time, you are removing material from the left side of Eq2.   Mass action will make this equilibrium shift to the left to compensate like any buffer.

Notice that this decreases the free H+ from 2 to 1.  A decrease in H+ is the same as an increase in pH. Net effect is that your TA goes down and your pH goes up.  It’s entirely possible to end up back where you started. 

Now Eq1 mass action would make your pH go DOWN, but it only operates strongly at low pH.  In sum, when you cold stabilize (precipitate KHTa) above 3.6, your pH goes up.  If you start below 3.6, your pH goes down.  Magic.

So basically, your problem is that you aren’t adding enough tartaric.  Because you chickened out and added less tartaric than you need, you think nothing happened. But that’s not true, because you have removed a lot of potassium.  You just need to keep going.

Generally speaking, the right addition is to 3.6.  At this pH, you get the maximum KHTa precipitation and no pH shift.

Note:  If the TA is higher, sometimes the cause is high malate rather than high K+.  In that case, double salt (aided by crossflow clarification) or a malic-loving yeast is the solution pre-fermentation, or ED afterwards.  But always test first by correcting a sample to pH 3.6 with tartaric and doing an overnight freeze/thaw.

Let me know if this needs further clarification.