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• Angie Cerinza Acosta

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27.1.18 AOAC Official Method 950.07 Acidity (Total) of Beer First Action 1950 Final Action

A. Indicator Titration Method

Bring 250 mL H2O to bp and continue boiling 2 min. From fast-flowing pipet add 25 mL beer previously decarbonated by shaking and filtering, 920.49 (see 27.1.01). After emptying pipet, continue heating 60 s, regulating heat so that solution resumes boiling during final 30 s. Remove from heat, stir 5 s, and cool rapidly to room temperature. Add 0.5 mL 0.5% phenolphthalein. Titrate with 0.1M NaOH against white background. Make frequent color comparisons with sample of equal volume and dilution to which has been added approximately anticipated amount of alkali but no indicator. Titrate to first appearance of faint pink. Read buret. Add 0.2 mL more alkali; color should then be permanent, definite pinkish red, indicative of overtitration. Take first buret reading as end point. Observe strictly all details of method. However, 100 mL H2O, 10 mL beer, and 0.2 mL indicator may be used in place of amounts specified above. (Use potentiometric titration method, B, for beers of dark color which, even when diluted, may not permit judging phenolphthalein end point with necessary precision.) Report results: (1) as lactic acid, to nearest 0.01% (1 mL 0.1M alkali = 0.0090 g lactic acid); or (2) as mL 1M alkali, to nearest 0.1 mL, necessary for neutralization of 100 g beer. B. Potentiometric Titration Method

Use pH/reference electrode system. Decarbonate beer completely by shaking, 920.49 (see 27.1.01). Using 50 mL undiluted sample (or such amount as best suits titration assembly), titrate potentiometrically with 0.1M NaOH to pH 8.2. Add alkali in 1.5 mL portions to ca pH 7.6, and in 0.15 mL portions from there to pH 8.2. Make sure that complete equilibrium and good convergence are attained before reading buret at exactly pH 8.2. Report results as in A. Precautions.—Observe all details of good potentiometric technique, including following: Standardize potentiometer against fresh 0.05M potassium acid phthalate, 964.24(c) (see A.1.04), before and after any set of titrations; read potentiometer to nearest 0.02 unit; use flexible shielding around electrode leads and motor cords; ground motor and motor cords, preferably to H2O pipes; avoid contact between electrodes and glass beaker; use proper stirring speed to ensure quick mixing but to avoid foaming which may temporarily trap some of alkali added; stop titration at ≤pH 8.6 to minimize alkali contamination of glass electrode; check batteries frequently. Follow manufacturer’s instructions for potentiometer used. Reference: ASBC: Beer 8.

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