You’re right to suspect that both the potassium metabisulfite powder and solution can lose their power over time. The solution, which you make by mixing the powder with water, is particularly unstable. All that antioxidant power we love means that when it comes in contact with air, which it usually does in a storage container (despite our efforts to prevent this), it’s getting oxidized. This means that it is losing some of its power. As sulfur dioxide is soluble only at relatively low temperatures, the liquid solution is also particularly sensitive to high storage temperatures. Store it for a week at 65 ºF (18 ºC) or above and it’s guaranteed that you will lose a significant amount of microbe-killing and antioxidant power. How strong your solution smells can be an indicator of strength, but it is an inconsistent one as we all have different tolerances to the odor. I suggest trying to get a handle on the strength of your solution by measuring the free sulfur dioxide.
Most winemaking books suggest using a sanitizing solution made out of water, citric acid and potassium metabisulfite, where the free SO2 is around 75-100 mg/L and the pH of your solution is under 2.9. The dry potassium metabisulfite powder itself can also lose its strength over time with exposure to air, and hence, its ability to retard microbial growth and oxidation. Because of this, it’s wise to buy the powder in small pouches or bags that make sense for your scale of winery and then to only make up as much liquid solution as you will need for any given job. This way you have a good feel for the amount of sulfur dioxide you really have in that solution. An aqueous solution of potassium metabisulfite powder can lose as much as half of its strength in two weeks when stored at 65–70 ºF (18–21 ºC).
For your current dilemma, I suggest getting a new batch of powder if it can’t be used to make a solution that is strong enough for your winemaking needs. Small pouches and cool, if not cold, storage conditions for liquid and powdered potassium metabisulfite can make a big difference in the life of your sulfur dioxide.
