Bench methods
Sulfite solution, by weight
The solution you draw free SO₂ additions from: potassium metabisulfite weighed, dissolved and made up to a set volume. A percent here means grams of the SALT per 100 mL. A 10% solution is 10 g made up to 100 mL, and each milliliter carries 57.6 mg of SO₂. The calculator, the protocol and the addition log all work at this strength, and it's the number to write on the bottle.
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
- A stock solution of known strength, measured out in milliliters for a dose in ppm.
- Takes
- Ten minutes, and a little longer for the salt to dissolve in cold water.
What you need
- Potassium metabisulfite, fresh. A tub that's been open for a season has lost strength.
- A scale that reads to a tenth of a gram
- A volumetric flask or graduated cylinder for the volume you're making: 100 mL, 500 mL or a liter
- Distilled or deionized water, cool
- A bottle with a tight cap, labeled, kept cold and dark
- Gloves and eye protection. The tub's safety data sheet lists anything else you need.
What the percent means
Weight per volume: grams of metabisulfite in every 100 mL of FINISHED solution. The same words are sometimes read as percent of SO₂ by weight. That's a different solution, 17.4 g of salt in 100 mL to make ten percent SO₂, and almost no one mixes it. Fermata always reads a percent as grams of salt, the way you weighed it, and asks you to enter the strength.
| Strength (% w/v) | Metabisulfite per 100 mL | Per liter | SO₂ in each mL |
|---|---|---|---|
| 5% | 5 g | 50 g | 28.8 mg |
| 10% | 10 g | 100 g | 57.6 mg |
| 15% | 15 g | 150 g | 86.4 mg |
Make it up
Weigh the salt: 10 g for every 100 mL you're making, or 100 g for a liter of the usual 10% solution.
Weigh to a tenth of a gram. The scale sets the strength, and every step after this just keeps it.
Dissolve it in about two-thirds of the water, swirling until the liquid is clear.
Use cold water, and be patient. Warm water drives off SO₂ as the salt dissolves (that's the smell), and whatever escapes weakens the solution you just weighed out.
Add water up to the mark, cap it, and turn it over a few times.
Fill UP TO the volume. Don't add that volume of water. The salt takes up room, so a liter of water with the salt stirred in is more than a liter. That's why the strength is stated per volume of finished solution (w/v).
Label it with the strength, the salt and the date. Keep it cold, capped and in the dark.
A bottle labeled only "sulfite" is whatever strength the last person remembers. The protocol and the calculator both ask for the strength on this label, and show it beside every milliliter they work out.
Use it
A dose in ppm is milligrams of SO₂ per liter of wine. Divide the milligrams you need by the milligrams in each milliliter to get the volume to draw. The calculator and the protocol both show this working beside the answer. A 10% w/v solution is 10 g of potassium metabisulfite made up to 100 mL. Metabisulfite is 57.6% SO₂ by weight, so each milliliter carries 57.6 mg SO₂.
Measure the milliliters with a cylinder or syringe, add them while mixing, and log the dose on the lot in milliliters.
Log it against the sulfite solution on your Supplies shelf. The lot page works out the ppm from the milliliters, the volume at dosing and the strength recorded for that solution. It never uses a rate typed in at the time.
Where it goes wrong
Warm water, or a long stir with the cap off, makes the solution weaker than you weighed.
Metabisulfite gives off SO₂ as it dissolves, and faster when it's warm. The sharp smell over the flask is your dose escaping. You can't tell how much was lost without titrating the solution itself, so a solution made up warm has an unknown strength.
Adding the volume of water to the salt, instead of making up to the mark.
Dissolved salt takes up room. A liter of water with the salt stirred in is more than a liter, so the strength comes out lower than you weighed. On the usual 10% solution it's a few percent short, and you can't see it, because the label still shows what the scale said.
An opened tub of metabisulfite loses strength.
It loses SO₂ to the air from the day you break the seal, and a tub left open for a season can be well below its label strength. No arithmetic can correct for that, because there's no way to know how much it has lost. Buy small tubs, keep them sealed, and replace one that smells strongly.
A bottle with no strength written on it.
Every milliliter drawn from it is worked out at a strength someone assumed, and the two readings of "ten percent" differ by more than a third of a dose. The protocol and the calculator show the strength beside every figure so you can check it against the bottle's label.
Metabisulfite is 57.6% SO₂ by weight. That comes straight from the chemistry (K₂S₂O₅ at 222.32 g/mol carries two SO₂ at 64.07), so no outside source is needed. The figure is for potassium metabisulfite. Sodium metabisulfite is a different salt with a different fraction, and Fermata doesn't work out solutions for it. Reading a percent as grams of salt per 100 mL is a convention, the one most cellars mix to. This method states it rather than assuming it, because the other reading is a real convention too, and the two differ by more than a third of a dose. Everything here assumes fresh salt. A tub open since last season is weaker than its label by an amount no one can calculate, so the only fix is a fresh tub.