First, let’s look at your question about the pH of water. We all learned in high school chemistry that water has a pH of 7.0, which is totally neutral, neither acidic nor basic, on the pH scale. When measuring the pH of water in the real world, say in streams, lakes or even distilled water, the pH can be anywhere in the neighborhood of 5.0–8.0, a far cry from “totally neutral.”
The problem is, pure liquid water is extremely rare in the natural world. The pH of water in our daily lives is affected by many things from dissolved minerals and organic matter (like water-borne microorganisms) to dissolved gasses. This is, in fact, the main source of acidity in distilled water, which, as you correctly mention, can often have a pH of 5.0–6.0. Distilled water likes to react with carbon dioxide in the ambient environment at typical atmospheric pressure levels and will trap carbon dioxide molecules, dissolving them and essentially turning “pure” distilled water into a weak solution of carbonic acid.
“But Wine Wizard!” you implore. “Doesn’t that mean that when I run my TA analysis at home I will get an erroneous result because I’m actually adding acid in the form of my water?”
Before I cause you unnecessary panic by answering “Yes” (which is the technically correct response, by the way), I’ll calm you down by telling you that boiling your distilled water helps drive out most of the dissolved carbon dioxide.
To be even more sure that you’re starting your titration free of interference from acidified water, it’s important to add a few drops of dilute NaOH (0.01 M) to the 100 mL or so of water that you use to run each TA analysis. If you’re using a phenolphthalein indicator, add your indicator to the water and add the dilute NaOH until you barely see the water turn pink. This is now your endpoint — when you add the wine sample be sure to titrate back to that same pink color. If you use a pH meter to indicate when your titration has reached the endpoint of 8.2 pH, (what I recommend, especially since red wine is almost impossible to test using the phenolphthalein method) again, add dilute NaOH to get your water to read 8.2 pH. It’s all right to over-run this endpoint in the water, as the solution at this juncture is unbuffered and a reading of 8.2–8.7 won’t significantly affect the final answer. However, when you add the wine sample to the 100 mL of your adjusted water and then titrate it, it is critical to try to get back to 8.2 (or your colored endpoint) as accurately as possible. This is important because the solution is now buffered by acid and small errors in the amount of titrate added will greatly affect the final reading.
The above obviously bridges us into your larger concern — that of a wine’s acidity seeming to increase over time. In my experience, it’s not uncommon for a red must’s TA to climb during fermentation. My viticulturist friends tell me that in some varieties, and especially in specific years, the grape tissues will selectively sequester and then release some compounds before others during fermentation. In fact, this year I had some Cabernet Sauvignons from California’s Paso Robles area that came in with a TA of 0.40 g/L, but as the fermentation progressed the TA climbed up to 0.65 g/L, a more typical number. I have never seen, however, a wine’s TA in-crease as drastically as you mention during the aging process — which leads me to think that the problem is analysis errors or that there is acid getting into your wine in a way that you are not aware of.
For starters, let’s revisit the above information about properly performing a TA and point out things that can skew the result. Make sure that not only your distilled water but also your wine sample are thoroughly degassed as dissolved carbon dioxide in the wine sample will give an erroneously high TA. Wine samples are usually degassed using a vacuum aspirator – if you don’t have access to a lab with this set up, the best way to degas at home is to let the sample come to room temperature (carbon dioxide is more soluble at cooler temperatures) and shake vigorously, “burping” your sample bottle repeatedly. If your wine is fermenting you must freeze the sample to knock down any yeast activity before you degas.
Another thing that often contributes to a bad TA result is an old chemical reagent. It’s normal for the NaOH solution with which we titrate to weaken over time so for this reason, it’s important to standardize the reagents yourself or buy fresh, pre-standardized ones from reputable laboratory supply companies. If you’re using an NaOH solution that is weaker than you think it is, this could explain the apparently high TA — it will take more of a weak NaOH solution to titrate the same wine to achieve the target endpoint.
With respect to your pH, most errors come from the following few sources. A common culprit is failure to calibrate the pH meter often enough. You really ought to calibrate your equipment daily or at least before each time you use it. Another source of error is calibrating the pH meter with old, off-concentration buffers. The pH 4.0 and 7.0 buffer solutions for calibration will chemically change and become unusable in two to three months, so always make sure you have fresh buffer. Storing the buffers covered away from light and heat, as well as never pouring any buffer you’ve used to calibrate the pH meter back into the mother storage bottle, will go a long way towards keeping the buffers sound and contaminant-free. Another big source of pH meter fallacy is not following the specified use and storage instructions for your particular instrument. pH probes are some of the most delicate pieces of equipment in the lab and need exacting care and attention in order to function at their best.
If you feel you’ve ironed out your lab bugs and still don’t know where your higher acids are coming from, take a hard look at your winemaking practices post-fermentation and see if some rogue acid could be making its way into your wine through unforeseen avenues. Do you rinse your equipment with a strong citric acid and sulfur solution and then neglect to rinse out the containers adequately before filling them? Do you use an acidic storage solution for your barrels that could seep into the wine once they are filled for aging? Is there a possibility that the SO2 solution you’re adding to the wine when you rack has been mixed up with another solution you use for sanitizing? Many prepared SO2 sanitizing mixes have ascorbic, citric or tartaric acid added to them to help the SO2 be more effective (more antimicrobial molecular SO2 is available at lower pH levels). Also, if you add SO2 to your lots in tablet form, check the composition of your particular brand. Some tablets are a blend of sulfur dioxide and ascorbic acid (added to make the SO2 more effective).
Lastly, take a close look at your wine’s VA (volatile acidity levels). Post-fermentation, certain microbes can metabolize various compounds in wine (ethanol, for one) and turn it into acetic acid, or vinegar. Typical increases in VA during primary and secondary fermentation are anywhere from 0.02 g/L to 0.075 g/L and will be reflected in your total acidity (TA) analysis. Though it’s unlikely to happen (as one would tend to throw wine out that becomes this bad), it’s possible for a wine that is really high in VA to contribute as much as 2.0 g/L to a wine’s total acidity. You’ve got quite a rare and interesting situation here and not one that I’ve ever encountered to such a degree in my winemaking career. I hope the comments above help you diagnose your problem. I wish you luck as you begin to rule out the possibilities.