AOAC Official Method 942.07 Isopropanol in Lemon Extracts
Titrimetric MethodFirst Action 1942
Final Action
(Applicable to lemon extract in absence of acetone.)
A. Isolation of Analyte
Place extract containing £8 g total alcohols [approximation of alcoholic content may be made from specific gravity determination and reference to 913.02 (see Appendix C)], into separator in stem of which is cotton pledget wet with H2O. Add 25 mL 10% NaCl solution and 25 mL petroleum ether. Shake well, let layers separate, and drain lower layer into flask. Repeat extraction with 3 additional 25 mL portions NaCl solution or until alcohol is completely extracted.
Add H2O to combined aqueous extracts until volume is ca 150 mL. Connect flask to vertical condenser and distil into 100 mL volumetric flask, removing flask when distillate is 2–3 mL below mark. Dilute to volume and mix.
B. Qualitative Test for Acetone
To 2 mL distillate add 0.5 mL 5% alcoholic o-nitrobenzaldehyde solution and 1 mL 10% NaOH solution. Mix; then shake with small volume CHCl3. If CHCl3 turns blue, acetone is present.
C. Determination
Pipet 10 mL distillate into 500 mL Erlenmeyer containing 50 mL ca 0.33M K2Cr2O7 and add 100 mL H2SO4 (1 + 3). Stopper flask, swirl, and let stand 30 min. Add 100 mL 30% FeSO4×7H2O solution. Connect flask to vertical condenser through foam trap. Slowly distil ca 100 mL into 500 mL volumetric flask containing 200–300 mL cold H2O. Dilute to volume, mix, and pipet 25 mL into glass-stoppered flask containing 25 mL 1M NaOH; add 50 mL standardized 0.1M I2 with swirling. Let stand 15 min. Add 26 mL 1M HCl and at once titrate residual I2 with standardized 0.1M Na2S2O3, adding starch solution when I2 color is nearly discharged. Each mL 0.1M I2 consumed in reaction = 1.001 mg isopropanol.
