NaOH

Relative molecular mass. 40.00

Chemical name. Sodium hydroxide; sodium hydroxide (Na(OH)); CAS Reg. No. 1310-73-2.

Description. White or almost white, fused masses, sticks, pellets, or flakes; they are hard and brittle, showing a crystalline fracture.

Solubility. Very soluble in water and ethanol (~750 g/l) TS.

Category. Alkalinizing agent; used in the preparation of sodium lactate solution.

Storage. Sodium hydroxide should be kept in a tightly closed container.

Additional information. CAUTION: Sodium hydroxide must be handled with care, avoiding contact with the skin. It is very deliquescent, strongly alkaline, and corrosive. It rapidly absorbs carbon dioxide.

Requirements

Sodium hydroxide contains not less than 97.5% of total alkali, calculated as NaOH, and not more than the equivalent of 2.5% of Na₂CO₃.

Identity tests

A. When tested for sodium as described under 2.1 General identification tests, it yields the characteristic reactions. If reaction B is to be used, prepare a 20 mg/mL solution.

B. A solution is strongly alkaline.

Heavy metals. Use 1.0 g for the preparation of the test solution as described under 2.2.3 Limit test for heavy metals, Procedure 1; determine the heavy metals content according to Method A; not more than 10 μg/g.

Arsenic. Use a solution of 2.5 g in 35 mL of water and proceed as described under 2.2.5 Limit test for arsenic; the arsenic content is not more than 4 μg/g.

Aluminium, iron, and matter insoluble in hydrochloric acid. Boil 5 g with 70 mL of hydrochloric acid (~70 g/l) TS. Cool, make alkaline with ammonia (~100 g/l) TS, boil, filter, and wash with a mixture of equal volumes of water and ammonium nitrate (50 g/l) TS. Ignite the residue to constant mass; not more than 5 mg.

Potassium. Dissolve 0.25 g in 5 mL of water, acidify with acetic acid (~60 g/l) TS, and add 5 drops of sodium cobaltinitrite (100 g/l) TS; no precipitate is formed.

Chlorides. Dissolve 0.35 g in a mixture of 2 mL of nitric acid (~130 g/l) TS and 20 mL of water, and proceed as described under 2.2.1 Limit test for chlorides; the chloride content is not more than 0.7 mg/g.

Sulfates. Dissolve 0.4 g in 20 mL of water, add 6 mL of hydrochloric acid (~70 g/l) TS, and proceed as described under 2.2.2 Limit test for sulfates; the sulfate content is not more than 1.2 mg/g.

Assay. Dissolve about 2 g, accurately weighed, in 80 mL of carbon-dioxide-free water R. Add 0.3 mL of phenolphthalein/ethanol TS and titrate with hydrochloric acid (1 mol/l) VS until the red colour of the indicator disappears. Note the consumption of acid. Add 0.3 mL of methyl orange/ethanol TS to the solution and continue the titration with hydrochloric acid (1 mol/l) VS until a permanent red colour is obtained.

Each mL of hydrochloric acid (1 mol/l) VS used in the second titration is equivalent to 106.0 mg of Na₂CO₃. Each mL of hydrochloric acid (1 mol/l) VS used in the combined titrations is equivalent to 40.00 mg of total alkali, calculated as NaOH.