Testing TA with a pH meter

Ok, I have a pH meter which makes testing red wines easy. However, to save NaOH solution (and wine), it makes sense to use like 10 ml of wine, and some 8.2 pH dilution fluid to cover the probe. Got it. However, I've had a tough time making 8.2 solution from distilled water and NaOH. First, the distilled waters I've tested don't show as 7.0 pH. Does the lack of any ions throw off the pH meters?

I also think it is the lack of buffer of any kind that makes adjusting DI water to 8.2 tough (or maybe I'm a spaz). Why not use tap water? If the pH of the dilution solution is 8.2, does it matter what it is?

Sorry for such technical questions in the beginners forum, but I couldn't find a "Vintner's Science" forum ;-)

I have a few wines going, and if I bring out the pH meter, I'd like to sample them all. Today, I finally got some water to read 8.2 and used it to read a sample. Ok, but now the water wasn't 8.2 anymore (must have got a drop of wine in there or something). Instead of making more solution, I simply poured my finished sample (at 8.2 pH) into the next sample to dilute it. Anything wrong with that? Results seemed to work. Even did a commercial wine last and the TA was reasonable.

TA and PH are not really correlated..... unless you are talking a "Strong" acid like Hydrochloric or Nitric acid.

PH measures how easily and completely the acid ionizes....

TA measures how much base it takes to neutralize an acid to a certain level....

What happens is that most acids in wines, ciders, and fruits in general aren't "Strong" acids - they are "Weak" acids... This means they don't ionize all the way initially - and they form buffer solutions at a certain equilibrium PH.... Once you have a buffer solution - they soak up a whole bunch of acid or base without changing PH.... until the "Buffer" is all used up.... meaning the acid is ionized all the way.

What you will see in your PH meter is that as you add base to your wine - the PH will initially rise with each drop, until it hits the buffer PH... Then, you will add a whole bunch of drops and the PH won't change.... Then, once you have used up all the buffer - the PH will start to rise again with each drop.

The end result is that you might see a giant TA with a moderate PH because there may be a large amount of this weak acid in the wine that can't all ionize at once - but that large amount of "weak" organic acid makes a powerful buffering action that soaks up a lot of base.

Hope this makes sense.

John

I think pkeeler is using 8.2 (maybe should be 8.3) as the end point for his titration and having trouble getting a stable reading. A magnetic stirrer is good to stir the NaOH through, but a common problem is deterioration of the electrodes in the pH meter - they lose their accuracy if not cared for properly. You can get cheap kits with indicators and litmus paper that do a reasonable job of measuring TA, maybe a better bet than trying to get a lab quality pH meter and keep it in good condition.

Thanks for the replies to what was probably a poorly worded post. I guess distilled water in plastic does absorb CO2 from the air and its pH lowers. But the pH is not buffered and as you stir it comes out, sending the pH skyrocketing once the CO2 is gone but the NaOH is not. Plus, I was using small amounts, so a drop of NaOH was just too much. What I actually did was just find a more slender test jar and use wine without dilution. Then, nothing to worry about.