This monograph is based on the corresponding, internationally-harmonized text developed by the Pharmacopoeial Discussion Group (PDG). Editorial modifications have been made in order to be in line with the style used in The International Pharmacopoeia .

Chemical name. Sodium dodecyl sulfate. C₁₂H₂₅NaO₄S. CAS Reg. No. 151-21-3.

Description. White or pale yellow, powder or crystals.

Solubility. Freely soluble in water giving an opalescent solution, partly soluble in ethanol (~750 g/L) TS.

Category. Anionic surfactant.

Storage. Sodium laurilsulfate should be kept in a well-closed container.

Requirements

Definition. Sodium laurilsulfate is a mixture of sodium alkyl sulfates consisting mainly of sodium dodecyl sulfate (C₁₂H₂₅NaO₄S). It contains not less than 85.0% of sodium alkyl sulfates, calculated as C₁₂H₂₅NaO₄S.

Identity tests

1. Carry out the test as described under 1.7 Spectrophotometry in the infrared region. The infrared absorption spectrum is concordant with the spectrum obtained from sodium laurilsulfate RS or with the reference spectrum of sodium laurilsulfate.

2. Place 2.5 g of the test substance in a silica or platinum crucible and add 2 mL of sulfuric acid (~500 g/L) TS. Heat on a water bath, then cautiously raise the temperature progressively over an open flame. Ignite, preferably in a muffle furnace, at 575 °C to 625 °C. Continue heating until all black particles have disappeared. Cool, add a few drops of sulfuric acid (~100 g/L) TS, and heat and ignite as above. Add a few drops of ammonium carbonate (158 g/L) TS, evaporate to dryness, and ignite as above. Cool, dissolve the residue in 50 mL of water R and mix. The solution yields reaction B, described under 2.1 General identification tests as a characteristic of sodium.

3. Prepare a 100 g/L solution of the test substance. After acidification with hydrochloric acid (~420 g/L) TS and boiling for 20 minutes, no precipitate is formed. Add barium chloride (120 g/L) TS; a white precipitate is produced.

Alkalinity. Dissolve 1.0 g of the test substance in 100 mL of carbon dioxide-free water R and add 0.1 mL of phenol red/ethanol TS1; not more than 0.5 mL of hydrochloric acid (0.1 mol/L) VS is required to change the colour of the indicator.

Non-esterified alcohols. Dissolve 10.0 g of the test substance in 100 mL of water R, add 100 mL of ethanol (~750 g/L) TS and shake the solution with 3 quantities, each of 50 mL, of light petroleum R2, adding sodium chloride R, if necessary, to promote separation of the 2 layers. Wash the combined organic layers with 3 quantities, each of 50 mL, of water R, dry over anhydrous sodium sulfate R, filter and evaporate on a water-bath until the solvent has evaporated. Heat the residue at 105 °C for 30 minutes and cool; the residue weighs a maximum of 0.4 g (4.0%).

Sodium chloride and sodium sulphate

Sodium chloride. Dissolve 5.00 g of the test substance in 50 mL of water R, add nitric acid (~130 g/L) TS dropwise, if necessary, until the solution is neutral, and add 5.0 mL of sodium chloride (5.84 g/L) TS. Titrate with silver nitrate (0.1 mol/L) VS, while stirring vigorously to disperse the silver chloride, using 0.1 mL of fluorescein sodium (2 g/L) TS as indicator, to the first appearance of turbidity with solution colour change from yellowish-green to orange through yellow. Perform a blank determination. Each mL of silver nitrate (0.1 mol/L) VS is equivalent to 5.844 mg of NaCl.

Sodium sulphate. Dissolve 1.00 g of the test substance in 10 mL of water R, add 100 mL of ethanol (~750 g/L) TS and heat at a temperature just below the boiling point for 2 hours. Filter while hot through a glass filter with a porosity of 4 µm to 10 µm and wash with 100 mL of boiling ethanol (~750 g/L) TS. Dissolve the precipitate by washing with 150 mL of water R, collecting the washings in a beaker. Add 10 mL of hydrochloric acid (~70 g/L) TS, heat to boiling, add 25 mL of barium chloride (120 g/L) TS and allow to stand overnight. Collect the precipitate by filtration (maximum pore size 16 µm) and wash with water R until the last washing shows no opalescence with silver nitrate (0.1 mol/L) VS. Dry the precipitate, ignite to constant mass at 500 °C to 600 °C by raising the temperature gradually, and weigh as barium sulfate. Each mg of barium sulfate is equivalent to 0.609 mg of Na₂SO₄.

The sum of the percentage contents of NaCl and Na₂SO₄ is not greater than 8.0%

Assay. Dissolve 1.15 g of the test substance in water R, warming if necessary, and dilute to 1000.0 mL with the same solvent. To 20.0 mL of the solution, add 15 mL of dichloromethane R and 10 mL of dimidium bromide/sulfan blue TS and mix. Titrate with benzethonium chloride (0.004 mol/L) VS, shaking vigorously and allowing the layers to separate before each addition, until the pink colour of the dichloromethane layer is completely discharged and a greyish-blue colour is obtained. Each mL of benzethonium chloride (0.004 mol/L) VS is equivalent to 1.154 mg of sodium alkyl sulfates, expressed as C₁₂H₂₅NaO₄S.