Bench methods
Volatile acidity, by distillation
The acid a wine makes on its way to vinegar. It's the one bench titration Fermata describes that you can't run straight from the glass. Acetic acid titrates exactly like tartaric, so the volatile acids have to be distilled away from the fixed ones first. That separation is the measurement.
These are the standard bench procedures in plain words, not a copy of any one published protocol. If your reagent kit comes with its own procedure, follow the kit. It was written for the normality in that bottle. The end of each method explains which of its figures come from the chemistry and which are standard practice.
Wear eye protection and gloves for every one of these methods. Sodium hydroxide is caustic, iodine stains and irritates, and the acids and peroxide burn. The chromatography solvent is flammable, so use it in a hood, not on an open bench. Each reagent's safety data sheet is the authority on handling and storage. These steps don't replace it.
- Produces
- Volatile acidity in g/L as acetic. It goes in the VA box on the lot's lab panel.
- Takes
- About twenty minutes a sample once the still is hot.
What you need
- A steam still for volatile acidity, and its heat source
- Sodium hydroxide at 0.02 N, standardized
- Phenolphthalein
- 10 mL volumetric pipette
- A flask that holds 150 mL of distillate
- For the sulfur correction: iodine at 0.02 N, 1% starch, and 25% sulfuric acid
Prepare the sample
Degas it the same way, and for as long, as for total acidity.
Carbonic acid is volatile, so it distills over with the acetic and titrates along with it. Everything the total acidity method says about removing carbon dioxide applies here, and it matters more. There's no blank to absorb it, and the numbers here are ten times smaller.
Pipette 10.00 mL into the still's inner chamber.
Distill
Bring the generator to a boil and collect distillate up to your still's mark, around 100 mL on most stills.
Collect to a VOLUME, not for a set time. The volatile acids come over early and the rest is water, so stopping short loses acetic and reads low. The amount of distillate doesn't enter the arithmetic, only the amount of wine, because all the volatile acid should have come over.
Titrate the distillate hot, with hydroxide, to a phenolphthalein pink that holds.
Titrate it hot because less carbon dioxide from the air dissolves in a hot liquid, and any that does would titrate as acid. This is the one bench titration Fermata describes where the indicator works better than the meter, because the distillate is clear whatever color the wine was.
Work it out
VA in g/L as acetic = milliliters × normality × 60.05 ÷ sample in mL. Acetic acid has ONE acid proton where tartaric has two, which is easy to get wrong. At 0.02 N on a 10 mL sample, that's 0.02 N × 60.05 mg ÷ 10 mL = 0.12 g/L for every mL, so 3.8 mL means 0.46 g/L.
| Hydroxide | Sample | Each mL is worth |
|---|---|---|
| 0.02 N | 10 mL | 0.12 g/L |
| 0.02 N | 20 mL | 0.06 g/L |
| 0.1 N | 10 mL | 0.60 g/L |
| 0.1 N | 20 mL | 0.30 g/L |
Fermata stores VA in g/L, like total acidity and malic. US compliance figures are usually given per 100 mL, which is this number divided by ten. So for the same wine, 0.46 g/L is 0.046 per 100 mL. The panel's VA box shows its unit, because a per-100-mL figure typed into it reads ten times too low and still looks believable.
Correct for sulfur dioxide
Free and bound sulfur dioxide also distill over and titrate as acid, so the number above is high by however much sulfur the wine had. Titrate the same distillate with iodine to a starch endpoint, just as for free SO₂. If both solutions are the same normality, subtract the milliliters of iodine from the milliliters of hydroxide, and work the answer out from what's left.
That corrects for the FREE sulfur dioxide. A full reference correction also accounts for the bound sulfur and for sorbic acid, and this method doesn't cover that arithmetic. If the figure has to hold up for someone else, check the correction with your own reference or the lab that will report it.
Fermata has ONE box for VA and can't tell a corrected figure from an uncorrected one. Record the corrected figure if you ran the correction. If you didn't, write that in the reading's note, just as you'd note which free SO₂ method you used.
Where it goes wrong
What you smell and what you measure are different things.
When people say a wine smells volatile (nail polish, solvent), they're usually smelling ethyl acetate. It's an ESTER, not an acid, so it doesn't titrate and isn't in this figure at all. A wine can smell strongly of it at a normal VA, and a wine with high VA can smell of very little. Measure one and smell the other. Neither replaces the other.
Sorbate reads as volatile acidity.
Sorbic acid distills over and titrates like acetic acid, so part of the VA on a wine stabilized with potassium sorbate is the sorbate itself. It's the same kind of error as the sulfur dioxide, but the iodine step doesn't correct it, because iodine doesn't pick up sorbate. Compare a VA on a sorbated wine with the one taken before the addition.
Carbon dioxide comes over with it.
Degassing comes first here too, as in the total acidity method, and skipping it does more damage here. Carbonic acid distills, titrates and lands in a figure whose whole normal range is a few tenths of a gram per liter. A wine straight off ferment or under a gas blanket can carry enough to move the answer by more than any difference you'd act on.
A distillation that stopped early reads low.
The volatile acid comes over in the first part of the distillate, and the rest is water. That's why you collect to a volume rather than for a set number of minutes. A still run short gives a clean, quick, believable number that's simply too low. Unlike most mistakes at the bench, nothing about the result looks unusual.
The 60.05 comes straight from the chemistry: acetic acid is 60.05 g/mol with a single acid proton, so no outside source is needed. The sample volume, the reagent strength and collecting to the still's mark are standard bench practice. Fermata RECORDS a volatile acidity and states no limit for it. There is a federal limit. Check your own TTB reference for what it is and which wines it covers, because quoting it here would mean the software deciding a compliance question for you. If your still or your reagent kit comes with its own procedure, follow it.