Citric Acid can be used in sanitizing solutions to lower the pH, which will make the SO2 level more effective. It is also used to increase acidity at a ratio of 0.1% for each teaspoon per gallon in wine. Be careful with how much you add to your wine because you will get a very tart flavor if you use to much.
9028 - 2 oz
This works well to help lower the pH of wine. Measure well because unlike other acids this can change the wines flavor.
When I add acid, I add mostly Citric, with a touch of Malic and Tartaric to round it out. I wouldn't dream of doing this without a pH meter.
I use several acids to adjust my Wines pH rather than rely on a blended powder, so I have more control. Citric acid is noticeable above a not so low threshold so add a little at a time in a sample then scale up.