Lead acetate Malic acid

While the wine contains several g/L of L-malic acid before MLR, it usually only contains between 200 mg/L and 300 mg/L of citric acid. Although the citric acid is only present in low concentrations, it is of considerable importance. On the one hand, its metabolic pathway leads to production of acetic acid, in other words, it increases the volatile acidity of the wine. However, the most important enological significance associated with fermentation of citrate is the production of diacetyl and other acetonic compounds, which affect the wine aroma. 

Hartmann and Hillig (1930) found the potassium acid tartrate procedure unsatisfactory because when alcohol is added pectins and other colloidal materials are precipitated and occlude other acids. To avoid this they recommend adding alcohol first to remove the pectins and to precipitate the acids in the filtrate with lead acetate. The precipitate is then dissolved and treated with hydrogen sulfide to remove the lead. Recovery of 97% to 100.2% of the tartaric acid from mixtures with malic and citric acid was reported. The procedure of Beis (1934) for the determination of tartrates and tartaric acid is based on the relative insolubility of the tartrates in alcohol. Titration before and after removal of the tartrates was used to obtain the tartrates by difference. Other procedures appear to be more suitable. 

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