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Copper sulfate (CuSO4) works well but should be kept below 1 mg Cu per liter (about 2 mg CuSO4 per liter.) for safety reasons.

Why do AO at all? https://vinmetrica.com/product/vinmetricas-sc-100-sulfite-analyzer/

Your wine is probably highly buffered (11 g/L tartaric with a pH still at 3.8!), so that the rule of thumb (1 g/L tartaric decreases pH by 0.1) may not be not applicable here. What is your TA right now? If it's already above 10 g/L tartaric you're going to see resistance to pH lowering by any...

You can check your pH calibration with saturated cream of tartar https://vinmetrica.com/troubleshooting-guide/#CreamOfTartar

The first pKa of 6.37 is an apparent pKa for the overall reaction H2O + CO2 <==> H2CO3 <==> H+ + HCO3- . The actual pKa for dissociation of carbonic acid H2CO3 is much lower, on the order of 3.5 - whether this is significant in most wine is not clear.

A relevant article by Daniel Pambianchi: SO2_Management

Vinmetrica offers the SC-50 MLF kit. SC-50 MLF tester We also carry the Sentia system, a little pricey but worth a look for the serious home winemaker.

Corks can pop and spritz develop at WAY BELOW 1 g/L malic acid. Paper chromatography is unlikely to reliably detect below 0.2 g/L, in my experience.

Check the pH meter calibration with saturated cream of tartar https://vinmetrica.com/troubleshooting-guide/#CreamOfTartar

19 g/L is 1.9%TA

Should be pH 13, but most pH meters don't do well above 12 anyway so don't worry too much. I get 6.0 g/L TA from your numbers: (24 mL * 0.1 N / 30 mL sample) * 75 g/eq tartaric acid (not 150 g/mol because there's two protons per molecule; and N is in units of equivalents per L).

If it doesn't taste sweet, maybe it's not. Check SG or ABV by some other method, maybe?

You can do it either way, as long as it's consistent. In fact, we at Vinmetrica are now recommending that, especially with the new electrodes, you not move the probe during calibration or measurement. Simply place in the solution, stir briefly and wait for a stable calibration/reading.

Our winery dogs at Little Oaks Winery

Your VAs of 0.4 and 0.5 g/100 mL are way high (numbers above 0.15 are usually noticeable and not allowed per CFR). Gusmer reporting malic at <0.3 g/L is not running a sensitive-enough test. You really want to see malic below 0.05, and at least below 0.1, to be sure MLF doesn't spontaneously...

1/4 teaspoon is about 1.4 g KMBS x 0.57 SO2/KMBS = .798 g SO2 in 6 gal x 3.8L/gal = .798g/22.8L = 35 mg/L SO2. Most commercial yeasts can handle this much, and after 8 hours it would be less anyway. When we sulfite our musts we shoot for 50 mg/L SO2 just by the way.

I would use pH 4 buffer in a pinch, but prolonged storage in that buffer can deplete the reference electrode's buffer, leading to errors. So get replacement storage solution as soon as possible. Vinmetrica uses essentially 2M potassium chloride in pH 4 reference solution and it is available...

Yes that's correct. Check our support tab at vinmetrica.com; contact us 760-494-0597, or [email protected] if you need help. -Rich from Vinmetrica

I bought what I believe is the same unit (~$15 on Amazon) just for curiosity, to see how/if it works. The instructions are quite rudimentary, and I could not get calibration to work, 2-point or 3-point. When reading their buffers on our Vinmetrica SC-300, the pH 4.00 was fine, but in my hands...

Ascorbic acid is a bad choice because it is a very weak acid, so won't bring your pH down as well as acid blend does. Wouldn't recommend it in the future. I'm guessing the taste will be weird, but if it's OK then it won't hurt otherwise.