Sodium Lauryl Sulphate

  Ph Eur                                                                                          

DEFINITION

Sodium Laurilsulphate is a mixture of sodium alkyl sulphates consisting chiefly of sodium dodecyl sulphated , C12H25NaO4S (Mr 288.4 ).It contains not less than 85.0 per cent of sodium alkyl sulphates , calculated as C12H25NaO4S.

CHARACTERS

A white or pale yellow powder or crystals , freely soluble in water giving an opalescent solution , partly soluble in alcohol .

IDENTIFICATION

A. Dissolve 0.1g in 10 ml of water R and shake . A copious foam is formed .

B. To 0.1 ml of the solution prepared for ldentification test A, add 0.1 ml of 1g/l soulution of methylene blue R and 2 ml of dilute sulphuric acid R Add 2 ml of methylene chloride R and shake . An intense blue colour develops in the methlene chloride layer .

C. Mix about 10 mg with 10 ml of ethanol R. Heat to boiling on a water-bath , shaking frequently. Filter immediately and evaporate the ethanol .Dissolve the residue in 8 ml of water R, add 3 ml of dilute hydrochloric acid R, evaporate the solution to half its volume and allow to cool. Separate the congealed fatty alcohols by filtration . To the filtrate add 1 ml of barium chloride solution R1. A white, crystalline precipitate is formed.

D. Ignite 0.5 g. The residue gives reaction (a) of sodium (2.3.1).

TESTS

Alkalinity Dissolve 1.0 g in 100 ml of carbon dioxide-free water R add 0.1ml of phenol red solution R . Not more than 0.5 ml of 0.1 M hydrochloric acid is required to change the colour of the indicator.

Non-esterified alcohols Dissolve 10 g in 100 ml of water R, add 100 ml of alcohol R and shake the solution with three quantities, each of 50 ml, of pentane R, adding sodium chloride R, if necessary , to promote separation of the two layers. Wash the combined organic layers with three quantities,  each of 50 ml, of water R, dry over anhydrous sodium sulphate R, filter and evaporate on a water-bath until the odour of the solvent is no longer perceptible . Heat the residue at 105 for 15 min and cool. The residue weighs not more than 0.4 g (4 per cent).

Sodium chloride and sodium sulphate Not more than a total of 8.0 per cent of NaCl and Na2SO4.

Sodium chloride. Dissole 5.00g in 50 ml of water R, add dilute nitric acid R dropwise until the solution is neutral to blue litmus paper R, add 2 ml of potassium chromate solution R and titrate with 0.1 M silver nitrate.

1 ml of 0.1 M silver nitrate is equivalent to 5.844 mg of NaCl.

Sodium sulphate. Dissole 0.500g in 20 ml of water R, warming gently if necessary , and add 1 ml of a 0.5 g/l solution of dithizone R1 in acetone R. If the solution is red , add 1M nitric acid, dropwise, until the solution becomes bluish-green. Add 2.0 ml of dichloroacetic acid solution R and 80 ml of acetone R. Titrate with 0.01M lead nitrate until a persistent violet-red or orange-red colour is obtained . Carry out a blank titration.

1 ml of 0.01 M lead nitrate is equivalent to 1.420 mg of Na2SO4

ASSAY  

Dissolve 1.15 g 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 chloroform R and 10 ml of dimidium bromide-sulphan blue mixed solution R.. Titrate with 0.004 M benzethonium chloride, shaking vigorously and allowing the layers to separate before each addition, until the pink colour of the chloroform layer is completely discharged and a grayish-blue colour is odtained.

1 ml of 0.004M benzethonium chloride is equivalent to 1.154 mg of sodium alkyl sulphates, calculated as C12H25NaO4S.

STORAGE