AOAC Official Method 933.07 Malic Acid (Inactive) in Fruits and Fruit Products
Polarization Method
First Action 1933
(As method is empirical, all directions must be rigidly followed, particularly with respect to dilutions. Substitution of volumetric flasks of capacities different from those specified is not permissible.)
A. Reagents
(a) Lead acetate solution.—Dissolve 40 g Pb(CH3COO)2×3H2O in H2O, add 0.5 mL CH3COOH, and dilute to 100 mL.
(b) Tribasic lead acetate standard solution.—Prepare solution from tribasic Pb(CH3COO)2, (c). To 5 g of the salt in 500 mL Erlenmeyer add 200 mL H2O and shake vigorously. Neutralize 3 mL 0.5M H2SO4, diluted with 200 mL H2O, with the solution, using methyl red as indicator. Note volume lead solution required. In determination use 2 mL in excess of this volume. (Solution should be freshly prepared.)
(c) Tribasic lead acetate.—Dissolve 82 g Pb(CH3COO)2×3H2O in 170 mL H2O. Prepare 100 mL dilute NH4OH solution containing 5.8 g NH3 as determined by titration (methyl red). Heat solutions to 60°C, mix thoroughly, and let stand overnight. Shake vigorously to break up precipitate, and filter on Büchner. Wash once with H2O and suck dry, then twice with alcohol, and finally with ether. Let dry in air.
