GAUNT: ESTIMATION OF SULPHUR IN RUBBER 9 ESTIMATION OF SULPHUR IN RUBBER. BY R. GAUNT, M.Sc., PH.D. (Read at the Neeting, December 2, 1914.) THE estimation of sulphur in vulcanised rubber is a matter of considerable im- portance in the rubber industry, and several attempts have been made to substitute the Cerius by a more expeditious method. The best known method is that of Henriques, described by Frank and Marckwald (Zeitsch.f. angew. Chem., 1889, 11, 802), in which the rubber is oxidised by means of nitric acid, the oxidation being completed by subsequent fusion of the product with alkaline nitrate. This method, however, was criticised by Hinrichsen and Memmler (‘I Der Kautschuk und seine Priifung,” Leipeig, 1910, p. 144). Several modifications have been proposed to avoid fusion.Rothe (Zoc. cit., p. 140) suggested oxidising the rubber by heating with a mixture of magnesium nitrate and nitric acid. Hinrichsen (Chem. Zeit., 1909, 33, 735) oxidised the rubber by means of nitric acid through which an electric current was passed. According to Stevens (ANALYST, 1914, 39,74), however, a varying loss of sulphur occurs during oxidation of rubber products with nitric acid, owing to the formation of volatile substances containing sulphur. Other methods of effecting the oxidation of rubber consist in heating the rubber with alkaline oxidising agents (Konek, Chem.Zeit., 1903, 27, 648 ; Alexander, Gzcmmi Zeit., 1904, 18, 738 ; Pontio, Kaye and Sharp, India Rabber J., 1912, 44,10 GAUNT: ESTIMATION OF SULPHUR IN RUEBER 1189).I t is difficult, however, to carry out these processes without loss from local explosions and spurting. The estimation of sulphur in organic substances by burning in a current of oxygen and absorbing the sulphur dioxide formed does not appear to have found much application in the case of rubber. This method was suggested by Dennstedt, who burnt the substance in a special apparatus by means of a double supply of oxygen, and absorbed the sulphur dioxide by means of lead peroxide contained in a boat placed in the combustion-tube.The difficulty in the method with regard to rubber lies in the fact that when rubber is heated a, volatile decomposition product is evolved which may escape combustion altogether, or form an explosive mixture with the oxygen.This difficulty, however, is eesilgovercome by carrying out the combustion in the following way : The rubber (0.2 to 0.3 grm.) is contained in a small tube of hard glass closed a t one end. This is placed in the centre of the combustion-tube, which is 30 to 35 cms. long, and drawn out at the rear end end fitted into a small flask, which by another tube is connected with another small flask.Twenty-five C.C. of 20 vol. hydrogen per- oxide are placed in each of the flasks. In the rear end of the combustion-tube in front of the constriction is placed a loose plug of platinised asbestos. Dry oxygen is led through the tube, and the part immediately under the mouth of the inner tube containing the rubber is heated hy means of a bunsen burner. The rubber itself is then gently heated by another burner until melting and decomposition begin.The gas evolved ignites at the mouth of the containing-tube and burns with :a luminous, but, if care be taken, a smokeless flame. This flame is maintained by gently heating the rubber until all the gaseous products of decomposition are evolved. The residual mass is then more strongly heated until it is completely burnt.Any carbon which may have deposited on the inner surface of the combustion- tube or on the asbestos is also burnt by gradually moving the burner along the tube. The sulphur dioxide is absorbed by the hydrogen peroxide forming sulphuric acid, which may be estimated by titration with standard alkali, or, after boiling with hydrochloric acid to decompose the excess of peroxide, may be estimated gravi- metrically.I t is necessary, however, to correct for any sulphuric acid or sulphates which may be present in the hydrogen peroxide. I n the case of a rubber containing much mineral matter, it is obvious that the sulphates in the residual ash must also be estimated. This usually takes about half an hour. The following results were obtained : I.PURE SULPHUR. Amount Taken. Amoiint found. (a) 0*100 grm. ... ... ... ... 0.101 grm. (b) 0.110 ,, ... ... ... 0.111 ... 11. SOFT RUBBER (WITHOUT FILLINGS). Percentage of Sdphur . By Carius’s Method. By Combustion. 9.8 ... ... ... ... (1) 9.7 (2) 9.9FOOD AND DRUGS ANALYSIS 1 1. 111. SOFT RUBBER (CONTAININU 34 PER CENT. MINERAL MATTER). Percentage of Sulphur. By Clarins’s Method. By Combustion. 9.4 ... ... ... ... ... 9.4 The mineral residue in the tube was dissolved in hydrochloric acid and added to the main solution. IV. VULCANITE. Percentage of Sulphur. By Carius’s Method. By Conibustioa. 46.2 ... ... ... ... (1) 46.4 (2) 45.6 In the case of an ashless rubber the whole process, using the volumebric method of estimating the sulphuric acid, can be carried out in an hour and a, half. SCIESTIFIC AND TECHNICAL DEPARTMEXT, IYPBRIAL INSTITUTE, S.W.