Bepaling van zwavel

Overgenomen uit: Garratt DC. The quantitative analysis of drugs: 3rd edition. London, Chapman & Hill. 1964.

The original determination of elemental sulphur in pharmaceutical preparations, by oxidation to sulphate and determination of the latter by precipitation as barium sulphate, has mainly been superseded by more rapid processes depending on the conversion of the sulphur to thiocyanate or thiosulphate for their volumetric determination.
Conflicting opinions have been expressed on the accuracy of the volumetric methods, since low results have been found with them by some analysts, as compared with an oxidation process (which undoubtedly might give high values); but experience has shown concordant results with volumetric methods. They are advantageous for their simplicity and the results are unaffected by sulphates which may be present. The estimation of sulphur by methods depending on its degree of solubility in various solvents is quite inaccurate. Henvillel has drawn attention to the fact that precipitated sulphur is easily soluble in carbon disulphide and slowly soluble in ether and light petroleum, whilst sublimed sulphur is soluble to a great extent in all three solvents. Hence, for example, it is quite useless to try to extract the fat from a sulphur ointment in order to estimate the sulphur in the residue.
The gravimetric determination of elemental sulphur adopted by the U.S.P., depending on oxidation of sulphur to sulphate with hydrogen peroxide, is essentially the following:

Dry about I g over silica gel for four hours and transfer to a flask with 50 ml of 10 per cent ethanolic potassium hydroxide. Boil until the solution is transparent and the sulphur has dissolved, cool and dilute with water to 250 ml in a graduated flask. Oxidise 25 ml by adding hydrogen peroxide solution in excess (50 ml) and heat on a water-bath for one hour. Acidify with hydrochloric acid, dilute to 250 ml and precipitate as barium sulphate in the usual manner (see Sulphuric Acid). A blank is necessary as hydrogen peroxide often contains sulphate.
BaSO₄ x 0.1374 = S.

A method of oxidation using nitric acid and bromine (Evers and Elsdon2), since adopted by the U.S.P. for sulphur ointment, has been shown to give high results (Fleck and Ward3) unless nitric acid is removed before precipitation of the sulphate with barium chloride; barium nitrate is probably adsorbed and co-precipitated with barium sulphate. Oxidation to sulphate with bromine in sodium hydroxide solution (Allport*) avoids the nitric acid contaminant and yields results comparable with volumetric methods:

Treat a weighed quantity of material, containing about 0-1 g of sulphur, with 10 g of sodium hydroxide dissolved in a small quantity of water. Heat and add slowly, while rotating the flask, an excess of a solution containing 30 per cent each of bromine and potassium bromide until a clear solution is obtained. Dilute with water, acidify with hydrochloric acid, boil to expel bromine and precipitate the sulphate as usual.
BaSO₄ x 0.1374 = S.

A simple and rapid volumetric assay of elemental sulphur was devised by Fleck and Ward, which gives good results with both sublimed sulphur and precipitated sulphur. The process depends on the quantitative formation of thiosulphate when free sulphur is refluxed with a solution of sodium sulphite, paraffin being added to accelerate the combination. Excess sulphite is eliminated by formaldehyde and acetic acid and the thiosulphate is titrated with iodine:

Boil the weighed material, containing preferably about 0.1 g of sulphur, in a conical flask attached to a reflux air condenser, with 2 g of crystalline sodium sulphite and 30 to 40 ml of water, until the sulphur has dissolved completely, adding I g of soft paraffin to accelerate the reaction. Cool the solution and, if necessary, pour off from the paraffin.
Extract the paraffin twice with small quantities of hot water and cool; pour off the washings and add to the main bulk of liquid. Add 10 ml of 40 per cent formaldehyde solution and 10 ml of 20 per cent acetic acid. Dilute to 150 ml ensuring that the solution is quite cold before titrating without delay with 0.1N iodine.
I ml 0.1N = 0.003206 g S .

Starch is recommended as indicator, but it is preferable to titrate without it except in the case of Compound Powder of Liquorice, where it is necessary. The rapidity of solution of the sulphur depends upon its state of division and the nature of the material to be analysed, the time required being longest with flowers of sulphur. The simple expedient of using a broad 250-ml conical flask is an advantage to aid rapid solution of the sulphur.
The oxygen-flask combustion method (see Appendix IV) has been applied to a number of sulphur-containing materials of pharmaceutical and horticultural interest. The method is suitable for the determination of sulphur in organic compounds except those containing a high proportion of nitrogen.

Ointment of Sulphur, B.P.
Contains 10 per cent of sublimed sulphur in simple ointment.
The observation by Fleck and Wards that paraffin greatly accelerated the combination of sulphur in their method, makes the application of the method to the determination of sulphur in the ointment particularly useful. The further addition of paraffin is obviously unnecessary.

The oxygen-flask combustion method can be employed for this preparation using 50 mg. The absorbing liquid is 25 ml of water and 2 ml of 20 volume hydrogen peroxide, neutralised to methyl red with 0-02N sodium hydroxide. After rinsing, a further 100 ml of water is added, shaken for one minute and then boiled until the volume is reduced to about 50 ml. After cooling rapidly it is titrated with 0.02N sodium hydroxide using methyl red as indicator.
1 ml 0.02N = 0.0003206 g S.

Compound Powder of Liquorice, B.P.C.
Contains 8 per cent of sublimed sulphur.
The determination of sulphur is preferably carried out by the volumetric method of Fleck and Ward given above, since the other ingredients may contain small quantities of sulphate, 0-2 to 0-4 per cent having been found in commercial specimens of this galenical. As stated previously, the end-point is easily visible with starch as indicator in the titration with iodine. A more satisfactory end-point is obtained without the use of paraffin; one and a half hours' boiling with the sodium sulphite is necessary.