Bench methods
Total acidity, by titration
All the acids in the wine, added up and reported as if they were all tartaric acid. It's the TA on the lab bench, the figure the acid calculator adjusts, and the main number for judging a wine's balance.
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
- Total acidity in g/L as tartaric. It goes in the TA box on the lot's lab panel.
- Takes
- About ten minutes, once the meter is calibrated.
What you need
- pH meter, calibrated today at pH 7 and pH 10
- Sodium hydroxide at 0.1 N, standardized
- 5 mL volumetric pipette
- Burette or digital titrator
- Distilled water, boiled and cooled: 50 mL for each run
- Magnetic stirrer
- Phenolphthalein, for white wines only, and only if you have no pH meter
Get the carbon dioxide out
Draw the sample and let it come to room temperature.
A cold sample holds more dissolved gas, and all of it titrates as acid.
Degas about 30 mL of it. Use one of these: a minute under vacuum with the flask swirling, two or three minutes in an ultrasonic bath, or several minutes of shaking hard in a part-filled flask.
Carbon dioxide is an acid, and the titration can't tell it apart from tartaric. A ferment that's still going, a wine just off the lees, and anything sparged or blanketed will all read high. These are often the very wines you're deciding an addition for. Any of the three methods works, because none of them changes the sample's volume.
Stop when swirling brings up no new bubbles.
Use this as a guide, not proof. Carbon dioxide can stay dissolved without bubbling if there's nothing for bubbles to form on, so a sample that looks still may not be fully degassed.
On a wine you haven't measured before, degas the rest of the sample again and run it a second time.
This is the check the bubbles can't give you. If the second degassing doesn't change the number, the first one was complete. Once you know how long your own equipment takes on your own wines, timing it is enough and you can skip this.
A still wine off the lees holds roughly 500 mg/L of dissolved carbon dioxide. At this endpoint that titrates as 0.85 g/L of tartaric. That's many times larger than any difference you'd adjust for, and none of it is the wine's own acid. That's why degassing comes first.
Set the bench up
Calibrate the meter at pH 7 and pH 10.
The endpoint is pH 8.2, which sits between those two buffers. The usual pH 4 and 7 calibration for reading wine pH doesn't reach 8.2, so the meter would be estimating there instead of measuring.
Pipette 50 mL of the boiled distilled water into the beaker. Bring it to pH 8.2 with the hydroxide BEFORE the wine goes in.
Distilled water left in an open bottle picks up carbon dioxide and turns slightly acidic. Neutralizing it first means everything the burette measures afterward is the wine's acid.
Pipette 5.00 mL of the degassed wine into the neutralized water, and note the burette's starting reading.
The pH drops sharply as the wine goes in, and the titration brings it back up to 8.2. Because the water was neutralized first, all the hydroxide you add from here goes to the wine's acid. Pipette carefully: the sample volume divides straight into the answer, and you can't correct it later.
Titrate
Add hydroxide with the stirrer running. Slow right down for the last half milliliter.
The pH moves fast near the end. Overshooting by 0.2 mL on a 5 mL sample adds 0.30 g/L, which is enough to change an acid addition.
Stop at pH 8.2 and let it hold there for twenty seconds.
If the reading drifts back down, the reaction hasn't finished. Stopping the moment it first touches 8.2 gives a low result.
Write down the milliliters used.
Work it out
TA in g/L as tartaric = milliliters × normality × 75.04 ÷ sample in mL. At 0.1 N on a 5 mL sample, that's 0.1 N × 75.04 mg ÷ 5 mL = 1.50 g/L for every mL, so 4.2 mL means 6.30 g/L.
| Hydroxide | Sample | Each mL is worth |
|---|---|---|
| 0.1 N | 5 mL | 1.50 g/L |
| 0.1 N | 7.5 mL | 1.00 g/L |
| 0.1 N | 10 mL | 0.75 g/L |
| 0.133 N | 5 mL | 2.00 g/L |
Record it on the lot's lab panel. Every box on the panel is optional, so you can record the acid on its own, without a gravity.
Where it goes wrong
Titrating to pH 8.2 and to pH 7.0 give different results.
US practice titrates to 8.2 and the OIV titrates to 7.0, and 8.2 always gives the higher figure. The gap changes with how the wine is buffered, so no factor converts one to the other. Fermata's screens assume 8.2. If a contract lab reports to 7.0, write that in the reading's note, because the reading itself can't show which endpoint was used.
Report it as tartaric, not as sulfuric.
The same titration reported as sulfuric acid gives a smaller number: the tartaric figure divided by 1.53. A sulfuric figure entered as tartaric reads about a third low, which makes the wine look flabby when it isn't.
You can't read phenolphthalein in a red wine.
The pink can't be seen through the wine's color, so the endpoint gets called early or late depending on who's looking. Use the pH meter on anything but a white.
Don't boil the wine and then pipette from it.
Boiling removes carbon dioxide more completely than anything else, but here it causes an error you won't notice. Water leaves as steam and concentrates what's left, so 5 mL of boiled wine holds more acid than 5 mL of the original wine. Boiling works when the wine and the water go in together and the whole mix is titrated, because the water leaves and the acid stays. But that's a different procedure, and it can't neutralize the water first.
Standardized hydroxide weakens over time.
It absorbs carbon dioxide from the air and weakens on the shelf, which makes the wine read higher in acid than it is. Restandardize it monthly, and sooner if the bottle has been open all vintage.
The 75.04 comes straight from the chemistry: tartaric acid is 150.09 g/mol with two acid protons, so no outside source is needed. The 8.2 endpoint, the sample volume and reporting as tartaric are standard US winemaking practice, not figures from a published sheet. If your reagent kit comes with its own procedure, follow the kit. It was written for the normality in that bottle.