Citric acid
Small acid corrections and iron-haze prevention, but microbially unstable.
Typical dose
Kept small, up to roughly 1 g/L; the US limits citric additions. Bench-trial.
When to add it
Finished, sulphited wine only, never one that will see malolactic.
In practice
Bacteria convert citric acid into diacetyl and acetic acid, so it belongs only in a finished white that is sulphited and will not undergo malolactic. Its honest use is chelating iron to hold off a metal haze, not acidification, where tartaric is safer.
Linked faults are the ones this addition either treats or, used wrongly, causes. The fault page says which, and whether it can be undone.
Others in acids and deacidifiers
Tartaric acid
Raises acidity and lowers pH using the acid grapes already contain.
Malic acid
Raises acidity with a sharper, greener sourness than tartaric.
Lactic acid
Gentle acidity for fine-tuning, especially after malolactic.
Potassium bicarbonate
Deacidifier: lowers titratable acidity and raises pH.
Calcium carbonate
Deacidifier for large acid reductions, via the double-salt method.
Potassium carbonate
Deacidifier, stronger per gram than the bicarbonate.