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Paragraph 7 PROCEDURE

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Determination

7.1

Place in a 400 ml beaker (5.3) an aliquot portion (a) of the extract (6.1) containing 2 to 4 mg B. Add 150 ml of water.

  • Add several drops of the methyl red indicator solution (4.1).

  • In the case of extraction with Method 26b, acidify by adding 0.5 M hydrochloric acid (4.2) up to the point of change of the indicator solution, then add a further 0.5 ml of 0.5 M hydrochloric acid (4.2).

  • After adding 3 g of sodium chloride (4.7), bring to boiling to drive off the carbon dioxide. Allow to cool. Place the beaker on the magnetic stirrer (5.2) and insert the precalibrated pH meter electrodes (5.1).

  • Adjust the pH to exactly 6.3, first with the 0.5 M sodium hydroxide solution (4.3), then with the 0.025 M solution (4.4).

  • Add 20 g of D-mannitol (4.6), dissolve completely and mix thoroughly. Titrate with the 0.025 M sodium hydroxide solution (4.4) to pH 6.3 (at least 1 minute stability). Let x1 be the volume required.