Bench methods
Free SO2, by titration
The sulfur in the wine that's still free to protect it. Two methods are in common use, and they don't give the same result. How far you can trust the number depends on which one you ran, so both are described here.
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
- Free SO₂ in mg/L. It goes in the free SO₂ box on the lot's lab panel, and it's the number the molecular SO₂ calculator starts from.
- Takes
- Ripper: five minutes. Aeration–oxidation: twenty-five minutes, most of it waiting.
Which one to run
| Ripper | Aeration–oxidation | |
|---|---|---|
| Speed | Five minutes | Twenty-five minutes |
| Kit | A burette and three reagents | A glass manifold and a pump |
| On a white | Close enough to act on | Accurate |
| On a red | Reads HIGH, by an amount that varies with the wine | Accurate |
| If ascorbic acid was added | Can't be used | Accurate |
If you make reds and only have a Ripper kit, you can still use it. Just remember that on a red its number reads high.
What you need
Ripper: iodine to a starch endpoint
- Iodine at 0.02 N, standardized, in an amber bottle
- 1% starch indicator, made up within the last two weeks
- 25% sulfuric acid, or 1:3 phosphoric
- 50 mL pipette and a white-bottomed flask
Aeration–oxidation: the reference method
- The AO manifold, and an aquarium pump or an aspirator
- Sodium hydroxide at 0.01 N, standardized
- Hydrogen peroxide, 3% diluted to 0.3%
- Mixed indicator: methyl red and methylene blue
- 25% phosphoric acid
Ripper
Pipette 50 mL of wine into the flask.
Add 5 mL of acid and 5 mL of starch, and titrate straight away.
The acid starts releasing bound SO₂. A sample left standing after you add the acid reads high, and the reading keeps climbing, so don't wait.
Add iodine until the first blue-black color that holds for twenty seconds.
On a red, watch for the color break, a sudden change, rather than for blue. Stir instead of swirling, because a whirlpool hides the moment it turns.
Free SO₂ in mg/L = milliliters × normality × 32.03 × 1000 ÷ sample in mL. At 0.02 N on 50 mL, that's 0.02 N × 32.03 mg ÷ 50 mL = 12.8 mg/L for every mL, so 2.4 mL means 31 mg/L.
| Iodine | Sample | Each mL is worth |
|---|---|---|
| 0.02 N | 50 mL | 12.8 mg/L |
| 0.01 N | 50 mL | 6.4 mg/L |
| 0.02 N | 25 mL | 25.6 mg/L |
Aeration–oxidation
Pipette 20 mL of wine into the reaction flask and add 10 mL of phosphoric acid.
Fill the trap with 10 mL of 0.3% peroxide and three drops of indicator. Bring it to its green endpoint with hydroxide before you turn on the air.
Dilute peroxide is slightly acidic. Neutralizing the trap first works like neutralizing the water in the acid titration: everything the burette measures afterward is the sulfur.
Draw air through for ten to fifteen minutes, with the flask COLD.
Heat drives off bound SO₂ as well as free, which is how the same setup measures total SO₂. Heating the flask is the only difference between the two runs, so a warm bench or a hot plate left on gives a number somewhere between free and total.
Titrate the trap back to the same green with hydroxide.
Free SO₂ in mg/L = milliliters × normality × 32.03 × 1000 ÷ sample in mL. It's the same arithmetic as Ripper, because one millimole of SO₂ counts as two milliequivalents in both methods. At 0.01 N on 20 mL, that's 0.01 N × 32.03 mg ÷ 20 mL = 16.0 mg/L for every mL.
| Hydroxide | Sample | Each mL is worth |
|---|---|---|
| 0.01 N | 20 mL | 16.0 mg/L |
| 0.02 N | 20 mL | 32.0 mg/L |
| 0.01 N | 25 mL | 12.8 mg/L |
Run the same sample again with the flask at boiling to measure TOTAL SO₂. It goes in the total SO₂ box on the lot's lab panel.
Then read it against the pH
A free SO₂ figure alone doesn't tell you whether the wine is protected. What protects it is the MOLECULAR SO₂, and the molecular share falls steeply as pH rises. The same free figure is plenty for a white at pH 3.2 and too little for a red at pH 3.8. The molecular calculator does this arithmetic, and the lab bench judges each lot against its own pH rather than one fixed level.
Where it goes wrong
Ripper reads HIGH on a red, and no factor can correct it.
Phenolics and other reducing compounds in the wine use up iodine along with the sulfur, so the titration counts them as free SO₂. The error always runs high, never low, but its size depends on the wine, so you can't just subtract it. This matters because a wine that seems to have plenty of SO₂ but is actually short looks protected on every screen. That's the very problem the bench's pH-based check is there to catch, and an inflated reading hides it.
Ascorbic acid makes a Ripper number meaningless.
It's a reducing agent and titrates almost as if it were free SO₂, so a wine dosed with it can read well while holding almost no sulfur. Aeration–oxidation isn't affected, because the SO₂ is swept out of the wine before anything is titrated.
Fermata stores one free SO₂ figure and doesn't record which method gave it.
Free SO₂ is one box on the lot's lab panel. If you measure a lot by Ripper in October and by aeration–oxidation in December, the trend shows a jump that isn't a change in the wine. Use one method per lot where you can. Where you can't, write the method in the reading's note.
Old starch and stale iodine both blur the endpoint.
Starch loses its sharpness within a couple of weeks, and iodine breaks down in light. Both fail the same way: a vague endpoint that gets called late, which reads as more sulfur than the wine has.
Adding sulfite doesn't tell you the new free SO₂ level.
Some of what you add binds right away, and how much depends on the wine. An addition records that someone acted. Only a titration tells you where the wine ended up, so the bench asks for a fresh reading instead of working it out for you.
The 32.03 comes straight from the chemistry: SO₂ is 64.06 g/mol, and one millimole is two milliequivalents whichever oxidant does the work, so no outside source is needed. The reagent strengths, sample volumes and aeration time are the usual bench setup, standard practice rather than figures from a published sheet. If your kit comes with its own procedure, follow the kit.