Tartaric acid powder.
Dosage: 10 g/10 l increases the acidity with 1 g/l.

The acidity of must/wine is always expressed in grams of tartaric acid/litre. In French books, you will find the acidity expressed in sulphuric acid ÷ 1.53.
Acidity as tartaric acid = acidity as sulphuric acid x 1.53.