Tartaric acid occurs free and as potassium hydrogen tartarate in grape juice.
Properties of tartaric acid.
Preparation from grape juice.
Argol which is potassium hydrogen tartarate (Cream of tartar), is formed as a sludgy precipitate during the fermentation of grape juice in wine making . It is dissolved in boiling water and neutralised with milk-of-lime. Calcium tartarate is precipitated, while potassium tartarate remains in solution. The calcium tartarate is separated by filtration and the filtrate treated with calcium chloride when potassium tartarate is also precipitated as calcium tartarate.
The calcium tartarate obtained in the two steps is treated with a calculated quantity of dilute H2
to liberate tartaric acid. The calcium sulphate precipitate that is formed is removed by filtration, and the filtrate concentrated by evaporation to get the crystals of tartaric acid.
Preparation from maleic acid. Maleic acid
produced industrially by oxidation of cyclohexane is treated with alkaline KMnO4
to get meso-tartaric acid.
Similarly, fumaric acid yields racemic tartaric acid.
Tartaric Acid properties.
Physical properties of tartaric acid. Tartaric acid
is a white crystalline substance, mp 171 °C, having acidic taste. It is soluble in water and alcohol, but sparingly so in ether. Since it has two asymmetric carbon atoms, tartaric acid shows optical isomerism. The natural tartaric acid is the dextro variety, α =+ 12.7.
Chemical properties of tartaric acid. Tartaric acid molecule has two secondary alcohol groups (>CHOH) and two -COOH groups, and behaves accordingly.
Formation of salts.
It forms two series of salts when treated with alkalis.
Action of heat.
When heated, tartaric acid converted to pyruvic acid.
However, on strong heating, tartaric acid chars, giving smell of burnt sugar. (test)
Reaction with HBr.
It reacts with hydrobromic acid to form 2.3-dibromosuccinic acid.
When heated with Hl, tartaric acid is reduced to malic acid then succinic acid.
On oxidation with HNO3
it is converted to tartonic acid and then oxalic acid.
(ii) When warmed with ammoniacal silver nitrate, tartaric acid is oxidation to tartonic acid and silver mirror is formed . (test)
(iii) Fenton’s reagent
) oxidises tartaric acid to dihydroxymaleic acid.
(6) Formation of complexes.
In fehling solution ( CuSO4
+ sodium potassium + NaOH) the soluble copper hydroxide first formed dissolves due to the formation of a soluble complex,
Tartaric acid uses.
Tartaric acid is used.
- In carbonate beverages and effervescent tablets.
- In backing powder.
- As mordant.
- In silvering of mirrors.
- In tanning.
- For preparing tartar emetic, cream of tartar, Rochelle salt etc.