Acidity is an essential aspect of wine balance, and is present naturally in grapes as Tartaric acid and Malic acid. Acid is added to musts and wines to correct natural deficiencies, especially in grapes from warm climates.

Tartaric is the most common acid added to musts and wines, and the addition should be made after a bench trial is done to confirm the desired acid level. Tartaric acid contributes to cold instability, so care should be taken when adding Tartaric acid to white wines close to bottling.

Malic acid is also found in grapes, and is typically converted to lactic acid during Malolactic Fermentation. Malic acid additions can contribute to microbial instability if the wine is already ML dry, but it's a good choice for additions being made to already cold-stable whites that will be membrane filtered.

Citric acid is found in trace quantities in grapes and is not typically added to musts or wines, though some winemakers choose to use it. Citric acid is converted into diacetyl by lactic acid bacteria, so elevated levels can result in an increased buttery character if added before MLF. Adding citric acid after MLF can stimulate lactic acid bacteria (it's food to them, after all)... so maintain good SO₂ levels and sterile filter, if you can.

In any case, you should always set up and taste bench trials before making an addition to the production lots.

Adding acid is not always the answer, sometimes you might want to reduce the acidity. The three (main) agents of wine deacidification are Potassium Carbonate (K₂CO₃), Potassium Bicarbonate (KHCO₃) and Calcium Carbonate (CaCO₃).

Potassium Carbonate and Potassium Bicarbonate are best suited for high TA, low pH juices or wines. Adding K₂CO₃ or KHCO₃ encourages bitartrate crystallization (cold stabilization), so additions should be made prior to cold stabilizing.

Calcium Carbonate is best suited for high TA, high pH juices or wines. CaCO₃ additions should be made following the double salt procedure in which the entire amount of CaCO₃ is added in stages to a portion of the total juice or wine volume. This encourages precipitation of the Calcium Tartrate salts which would otherwise form later, perhaps in bottle.

Acid solutions are an easy way to prepare acid trials. Rather than weighing out small amounts of acid into glasses of wine, you can create a solution and dose the solution into the glasses of wine instead.

For example, 1 mL of a 10 g/100mL Tartaric acid solution will add 1.0 g/L TA to a 100mL sample.

You want to be sure that the add you're about to make is going to taste as good as you think it will. And there's no better way than to make up an acid trial and be sure that your addition is going to improve the wine.