AOAC Methods for Review in Codex STAN 234_11-2018

AOAC Official Methods Listed in CXS 234 for Milk and Milk Products

8

A.1.11

Molarity (mol/L) = g KCl × 1000/mL AgNO 3

× 74.555

AOAC Official Method 941.18 Standard Solution

Volhard Method

of Silver Nitrate First Action 1941 Final Action

D. Reagents ( a ) Ferric alum indicator solution .—Saturated solution of FeNH 4 (SO 4 ) 2 @ 12H 2 O in H 2 O. ( b ) Potassium or ammonium thiocyanat e standard solution .—Prepare ca 0.lM solution, 942.26B ( see A.1.02). Determine working titer by accurately measuring 40–50 mL standard AgNO 3 solution, adding 2 mL ferric alum solution and 5 mL HNO 3 (1 + 1), and titrating with the thiocyanate solution until solution appears pale rose after vigorous shaking. E. Standardization Accurately weigh enough KC1, B ( a ), to yield titration of ca 40 mL (ca 0.3 g for 0.1M solution) and transfer to 250 mL glass-stoppered Erlenmeyer with 40 mLH 2 O. Add 5 mLHNO 3 (1 + 1) and excess AgNO 3 solution. Mix, and let stand few minutes protected from light. Filter through Gooch prepared with medium pad of asbestos previously rinsed with 2% HNO 3 . Wash flask and precipitate with several small portions of 2% HNO 3 , passing washings through crucible until filtrate and washings measure ca 150 mL. Add 2 mL ferric alum solution and titrate residual AgNO 3 with thiocyanate solution. From titration, together with ratio of the 2 solutions, calculate net volume AgNO 3 solution. (Errors of blank are compensating and may be disregarded.) From net volume AgNO 3 , calculate molarity as in C . Reference: JAOAC 24 , 100, 631(1941).

A. Preparation of Standard Solution Dissolve slightly more than theoretical weight of AgNO 3 (equivalent weight, 169.87) in halogen-free water and dilute to volume. Thoroughly clean glassware, avoid contact with dust, and keep prepared solution in amber glass-stoppered bottles away from light. B. Reagents ( a ) Potassium chloride .—Recrystallize KCl 3 times from H 2 O, dry at 110 ° C, and then heat at ca 500 ° C to constant weight. Equivalent weight KCl = 74.555. Or, preferably, use NIST SRM 999. ( b ) Potassium chromate solution .—5% solution of K 2 CrO 4 in H 2 O. C. Standardization Accurately weigh enough KCl to yield titration of ca 40 mL (ca 0.3 g for 0.1M solution), and transfer to 250 mL glass-stoppered Erlenmeyer with 40 mLH 2 O. Add 1 mLK 2 CrO 4 solution and titrate with AgNO 3 solution until first perceptible pale red-brown appears. From titration volume, subtract mL of the AgNO 3 solution required to produce end point color in 75 mL H 2 O containing 1 mL K 2 CrO 4 solution. From net volume AgNO 3 , calculate molarity: Mohr Method

 2005 AOAC INTERNATIONAL

10/9/2018