How To Test For Sulfur Dioxide in Wine
Sulfur Dioxide is formed from one sulfur and two oxygen atoms. This is expressed as SO2. It is a colorless yet foul smelling gas that when reacted with water, forms sulfurous acid. It can then be oxidized into the highly corrosive sulfuric acid.
When SO2 is added to wine it can have several benefits providing it is in small amounts. It can control the growth of bacteria and yeasts, prevent the browning in fruit caused by certain natural enzymes. It can prevent oxidation in your juice and wine when given the proper amounts. When old wine is afflicted with acetaldehyde, SO2 can even reduce it’s smell.
To test for the free SO2 in wine you either need a SO2 meter or a Titret kit (approx. $17.00 for 10 test ampoules).
Using a kit take one ampoule and one valve assembly from the kit, and slip the valve assemble onto the neck of the ampoule. Gently push the valve assembly up to the indicator mark on the ampoule and stop.
Carefully bend the tip of the ampoule at the score mark (not indicator mark) to break it. (No need to have tube in the liquid at this time). Now take a sample of your wine and immerse the end of the valve assembly into your sample. Squeeze the valve at the bead assembly. Do not squeeze the bead valve on the valve assembly until it is immersed in your wine sample.
A small amount of wine will rush into the ampoule via negative pressure. Shake to mix the solution in the ampoule with your wine sample. The fluid will turn purple. Wait 30 seconds. If the fluid remains purple continue adding wine to the ampoule as directed above until the fluid turns either clear or the color of your wine. Remember to wait 30 seconds after each addition of your wine.
Once the fluid inside the ampoule changes to clear or the color of your wine, hold it straight up and see the reading at the point of the top of the liquid. This is the amount of free SO2 based upon ppm (mg/litre) currently in your wine. Now you may make adjustments to your wine if necessary.
UNDER NO CIRCUMSTANCES DRINK OR RETURN THE WINE SAMPLE USED IN THE TEST TO YOUR WINE. SAFELY DISPOSE THE USED AMPOULE. DO NOT ATTEMPT TO USE IT AGAIN.
ADDITIONAL INFORMATION ABOUT SO2
When you add sulfur dioxide to wine approximately half of it combines (bonds) with other compounds and becomes fixed. The remaining half of the SO2 remains as a free form. The free form SO2 will exist in 3 different forms, as a molecular SO2, in a bisulfite form and in a doubly ionized sulfite form. The molecular SO2 is the biological active form, and is what we are interested in, is how much of the free SO2 exists as SO2.
Measuring the amount of free SO2 is easy, however the amount that exists as molecular form is difficult to measure. Fortunately by knowing the wines ph and how much free SO2 exists, determining the molecular amount is easy.
The amount of free sulfur dioxide needed in wine to protect it depends upon a wines ph. As a wines ph value increases, so does the amount of SO2 needed to protect it.
Ph of your________Free SO2 Needed To Protect Wine__% That Is Molecular
Wine____________(As expressed as mg per litre)
The molecular SO2 is a gas and escapes each time the wine is exposed to the atmosphere. Hence the reason to check and add SO2 each time to maintain safe levels. Roughly 0.5 to 0.8 milligrams of molecular SO2 per litre will provide the adequete protection for a dry table wine (closer to .5 best), and at least .08 for sweet and blush wines providing normal conditions.
When sulfur dioxide is added to wine half of it bonds with elements in the wine such as acetaldehyde, sugar, oxygen and pigments. Generally one half quickly bounds with these elements and is no longer available as free SO2. If oxygen is added to the wine, this bounded SO2 can not bind with the oxygen to protect the wine so additional sulfur dioxide needs to be added.
¼ teaspoon of potassium metabisulfite in 5 gallons of wine equals 25ppm of free SO2, total SO2 50ppm. 5 Camden tablets crushed equals ¼ teaspoon.
At the time of pressing add 25ppm of free SO2 (50ppm total SO2)
At racking or after mlf add up to 25ppm free SO2 (50ppm total SO2).
During aging keep .5 to .8 molecular SO2
At the time of bottling bring up to .5 to .8 molecular, never more than .8 molecular.
Try to keep your total SO2 under 100ppm.
After you have bottled your wine, the free SO2 will drop 10 to 15ppm during the first 2 months.
If there is insufficient SO2 in a wine to bind the oxygen molecules, the oxygen will begin to oxidize the wine. As a wine ages less SO2 is needed to protect the wine as some SO2 exists as bound SO2.
Commercial wineries are governed by federal regulation as to how much sulfur dioxide is present in their wines as opposed to the home winemaker who isn’t generally concerned with total SO2 in their wines, merely the free SO2.
Here is a link to a sulfite calculator.