A few notes on measuring acidity and pH of wines.
At Vinmetrica, we get a lot of inquiries about the acidity of wines and how to measure them. We have a blog on pH & TA here (and followed up here) on our website wherein you can find detailed discussions about various aspects of this subject. Here I’d like to touch on a few of these.
pH vs TA
the term pH represents the concentration of hydrogen ion (” H+ “, also called hydronium ion H3O+), though to be exact it’s H+ activity that your pH electrode senses; see Q&A below). Titratable acidity TA, on the other hand, measures the total amount of hydrogen ion available in free and bound forms. pH and TA both correlate to the tartness of the wine, but the kind of acidity the wine exhibits is generally more a function of its TA value. The pH value is important for taste as well, but more important is its effect on stability of wine, from the beginning of fermentation through to the final product. Very roughly speaking, we want our whites and rose’s to be in the pH range 3.1 to 3.4, while our reds are usually best in the range 3.3 to 3.7. And in general we’re hoping that our TA values are going to be 6 to 7 for reds and 6 to 9 for whites. Note that pH and TA of wines do not track each other in any consistent way. For example it’s possible for a low pH wine to have a low or a high TA value.
Tips on Measuring pH and TA
You no doubt are familiar with how to do pH and TA procedures from your manuals (here) so we won’t repeat the detail. But there’s a few points to keep in mind.
- It’s important to measure your pH and TA before you begin fermentation (as well as later), so that you can make needed adjustments from the outset. Waiting until the end of fermentation to adjust acidity runs the risk of wayward or incomplete fermentation, and a less desirable flavor profile.
- Always calibrate your pH system at lease once each day of use. Be sure to calibrate at two pH values, usually 4 and 7 as with the Vinmetrica product. And now, we recommend NOT stirring or swirling the sample or reference solution during measurement (we used to recommend that, but with the newer style pH electrodes it’s not needed, and old ones work OK also). Just put the pH electrode in, give a slight swirl to mix, then read or calibrate in the still solution.
- A good check on your pH calibration is to do the cream of tartar test.
- When measuring TA, don’t forget to degas the sample (by shaking/venting or as otherwise described in the manual) because you want to remove any excess dissolved CO2 that will show up erroneously as TA.
Adjusting pH and TA in wine
I suggest you do a web search on “adjusting pH and TA in wine” and you will see lots of good advice, including from us on our Support page. Also, I recommend Daniel Pambianchi’s Acidification-Deacidification Calculator under Tools On Techniques In Home Winemaking.
OK. Here’s the Q&A:
Q. Can’t I just dilute my wine with pure water if I want to raise the pH?
A. Alas, no, but you would think so, wouldn’t you? After all, pH is more or less the negative logarithm of the hydrogen ion concentration, so as that concentration goes down with dilution, the pH should rise, right? And pure water has a pH of 7, which should also raise the pH, right?
Ah, but wine is fairly well buffered by tartrate and other fruit acids at the pH range 3-4. This buffering effect is due to all that bound hydrogen ion, whose release resists changes in pH due to dilution. If you dilute 0.01M HCl (a strong acid) tenfold in water to 0.001 M, the hydrogen ion concentration will indeed go down and the pH will rise from 2 to 3 in this case. But if you dilute tartaric acid (a weak acid) in equilibrium with some potassium acid tartrate (as in wine to give, say pH 3.4) the pH will not rise.
And pure water does indeed have a theoretical pH of 7.0, but it has no buffering oomph to it, so it can’t influence the pH of wine, which does have the buffering oomph. This lack of buffering ability is also why you can’t use pure water to check your pH electrode’s calibration. Pure water absorbs CO2 from the atmosphere and rapidly drops in pH to the 5 – 6 range.
And finally, if you do dilute your wine with water, its pH may actually go down slightly! Most wines will show a drop in pH of 0.1 to 0.2 units with as much as a ten-fold dilution by pure water. This is because the pH (and the pH electrode’s response to it) is actually based on the activity aH+ of the hydrogen ion, rather than its concentration.
pH = -log(aH+)
The activity is related to the hydrogen ion concentration [H+] through its activity coefficient, written here as the greek letter gamma ‘γ’:
aH+ = γ * [H+]
The activity coefficient γ increases closer to 1.0 as any solution is diluted in pure water, and since the hydrogen ion concentration is not changing (due to the buffering ability of the wine acids and their salts as mentioned above), the measured activity becomes closer to the concentration; hence the pH decreases.
Ongoing developments for measuring pH
We are in the middle of adding new firmware features to the pH calibration choices of the SC-200 and SC-300. We expect to release these as firmware upgrades available in the second quarter. You’ll have more choices of calibration points to use, a selectable “autocalibrate” feature, and other things that will make your pH measurements even more precise and reproducible. So stay tuned!