Titratable Acidity: Trouble with the better way?

I’ve mentioned that I’m getting inconsistent results with my CO2 acid testing apparatus, and I’ve been trying to figure out why. I noticed that the measured volume of CO2 increased in the first few minutes. That’s an important problem because the test works by using that figure to determine how much acid was in the wine or must. Ideally, the final volume would increase almost immediately by the volume of the sample plus the volume of liberated CO2 then remain constant. Not only does the volume rise in the first five minutes, but then it falls over the next thirty minutes.

Gathering data

I had some ideas about this, but wanted to get a clear picture of exactly how the final volume changed over time. I’ve been recording six final volume measurements at the same time intervals over the past two months. For example, on March 2, I tested my Merlot. The initial volume was 0.2 ml, and right after adding the sample I measured the final volume as 2.4 ml. At +5 minutes, +10 minutes, +15 minutes, +20 minutes, and +30 minutes I measured:

2.9 ml, 2.9 ml, 2.8 ml, 2.8 ml, and 2.7 ml

Since the initial volume is arbitrary, it’s more helpful to view the data as a difference from the initial volume or as a percentage of it. Here are the same data expressed as a percentage of the initial volume:

120.83%, 120.83%, 116.67%, 116.67%, and 112.50%

I ran this same test nine more times on different wines and musts. Here are the results expressed as a percentage.

Normalized Volume Measurments
Immediate +5 Minutes +10 Minutes +15 Minutes +20 Minutes +30 Minutes
100.00% 120.83% 120.83% 116.67% 116.67% 112.50%
100.00% 98.18% 96.36% 94.55% 92.73% 89.09%
100.00% 116.67% 116.67% 112.50% 112.50% 108.33%
100.00% 103.23% 98.92% 96.77% 93.55% 92.47%
100.00% 104.55% 97.73% 95.45% 90.91% 90.91%
100.00% 95.24% 90.48% 88.10% 85.71% 83.33%
100.00% 110.00% 106.67% 103.33% 100.00% 96.67%
100.00% 107.69% 104.62% 104.62% 104.62% 103.85%
100.00% 121.88% 125.00% 128.13% 128.13% 128.13%
100.00% 96.81% 91.49% 89.36% 88.30% 87.23%

On six occasions, the final volume rose (indicating more acid) five minutes after the initial measurement then fell over the next thirty minutes. Three times (the red text in the table), the final volume steadily fell after the initial reading. And during one test (indicated in gree), the final volume continued rising for fifteen minutes, then held steady. What’s going on? It doesn’t surprise me that the apparatus can’t hold pressure perfectly, so the gradual fall that I see in almost all the cases makes sense.

Am I just being impatient?

The rise in the first five minutes that I saw in most of the tests made me think that the chemical reaction between the acid and the baking soda was slower than I thought. That would explain six of the tests and sort-of explain that (green) one that rose for fifteen minutes and plateaued. But the three (red) tests that showed a steady decline just don’t fit.

Is it something in the water?

I drain the device after each use, and fill it with water just before a test. Most of the time I get water from my kitchen faucet, but sometimes I use a utility sink in the basement. Water is water, of course, but the kitchen faucet has an aerator on it and the utility sink does not. Could the aerated water from the kitchen be releasing air during the test and affecting the final volume? Might there have been some air bubbles in the tubing that I overlooked? That could explain all ten test results. Being especially quick with the test and using kitchen water (or not doing enough to dislodge all the air bubbles in the tubing) might have caused the fifteen minute rise in the green test. The six times that I saw a five minute rise might have been me going at a more normal pace, allowing some air to bubble out while I got ready for the test. Water from the utility sink would not have cause a rise at all, and that would dovetail nicely with the red tests.

Putting it to the test

I didn’t think that my water source would matter, so I didn’t make a note of it when I recorded my data. So I’m going to run more tests and collect more data. This time, I will leave the water in the apparatus between tests. This ought to insure that no air bubbles out and affects my results. If I get the same sort of results, then I can rule out the air in the water. If, on the other hand, all my tests show a gradual decline after the initial reading, like the red tests, then I think I may have solved this mystery.

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2 thoughts on “Titratable Acidity: Trouble with the better way?

  1. Erroll Post author

    Hello Jerrold,

    Thank you for posting that detailed evaluation. I’ve since spoken with the gentleman who built the device for me, and I now think I’ve been having a problem with leaks just as you did. Checking for leaks is on my to-do list, and I plan to revisit the … um, you know I still don’t have a good name for the darn thing.

    I couldn’t see, in the photos of your CO2 acid test device, a way to correct for pressure inside the device. The CO2 acid test device I use has a movable reservoir, which you can see in the far left of the photo. Moving it up or down until the liquid in the reservoir is even with the liquid in the column indicates atmospheric pressure in the device. The idea is to take your initial and final readings at atmospheric pressure. If you don’t do this, then the final reading will be at a slightly higher pressure, and there may be some compression of the gas inside the device.

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