Paragraph 9 BORON (B) VALUE OF THE SODIUM HYDROXIDE SOLUTION (4.4)

Transfer by pipette 20 ml (2.0 mg B) of the calibration solution (4.5), into a 400 ml beaker and add several drops of methyl red indicator solution (4.1). Add 3 g of sodium chloride (4.7) and the hydrochloric acid solution (4.2) up to the point of change of the indicator solution (4.1).

Make up the volume to about 150 ml and bring gradually to the boil so as to eliminate 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.

Prepare a blank solution in the same way, substituting 20 ml of water for the calibration solution. Let V₀ be the volume required.

The boron value (F) in mg/ml of the standard NaOH solution (4.4) is as follows:

F (in mg/ml) = 2/(V₁ − V₀)

1 ml of exactly 0.025 M sodium hydroxide solution corresponds to 0.27025 mg B.