Tartaric acid
L(+)-tartaric acid
Raises acidity and lowers pH using the acid grapes already contain.
Typical dose
About 1 g/L raises titratable acidity by roughly 1 g/L and drops pH by around 0.1. Bench-trial before treating the batch.
When to add it
Into the must before fermentation, or the finished wine before cold stabilisation.
In practice
The acid to reach for, because it is the one grapes make and the one least metabolised by anything in the cellar. It also seeds tartrate instability, so add it before cold stabilising rather than after, or the crystals it forms drop in the bottle. The pH moves less than the acidity because the must is buffered, which is why every addition is trialled and not simply calculated.
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
Malic acid
Raises acidity with a sharper, greener sourness than tartaric.
Citric acid
Small acid corrections and iron-haze prevention, but microbially unstable.
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.