March 1, 1920] 



THE INDIA RUBBER WORLD 



357 



THE METHOD IN DETAIL. 



The sample, weighing about 0.5 gm., is cut finely with scissors, 

 or is crumbled on the mill, and transferred to a 500 cc. Erlenmyer 

 destruction flask (Pyrex glass)." Ten cc. of the zinc oxide-nitric 

 acid" solution is added and the flask whirled rapidly to thoroughly 

 moisten the sample. If convenient, the mixture may be allowed 

 to stand over night at this point. By so doing the sample becomes 

 partially decomposed, which permits the addition of fuming nitric 

 acid with no danger of ignition of the sample. Fifteen cc. of 

 fuming nitric acid is then added (all at once) and the flask 

 whirled rapidly to keep the sample immersed in the solution in 

 order to avoid ignition by too rapid oxidation. 



With certain samples, it may be necessary to cool the flask 

 under a stream of tap water. When the solution of the rubber is 

 complete, tive cc. of a saturated water solution of bromine is 

 added and the mixture is evaporated slowly to a foamy syrup". 

 If particles of organic matter remain at the end of the evapora- 

 tion, a few cc. of fuming nitric acid arc added and the solution is 

 reevaporated to the same consistency as before. The flask is 

 then cooled and a few crystals of potassium chlorate are added to 

 assist in the oxidation of the sulphur and the decomposition of 

 any nitrates. 



The mixture is then evaporated to dryness over a Tirrill 

 burner, using an asbestos gauze. While in this position, the con- 

 tents of the flask is baked at the highest temperature of the 

 burner, until all nitrates are decomposed and no more nitrogen 

 peroxide fumes can be detected". When the "baking" is complete, 

 the flask is cooled and the residue taken up with fifty cc. of (1 :6) 

 hydrochloric acid and heated until solution is complete". The 

 solution is then filtered, made up to 300 cc, and precipitated with 

 barium chloride in the usual manner, observing the customary 

 precautions. The barium sulphate precipitate is washed with boil- 

 ing water until no cloudiness results on testing the filtrate with 

 silver nitrate solution. 



DISCUSSION OF THE METHOD. 



It was desired to employ the minimum quantity of zinc oxide 

 necessary to effect the complete oxidation of the rubber and sul- 

 phur. Several preliminary tests were made with different amounts 

 of this substance. Best results were indicated with two gms. 

 of zinc oxide to ten cc. of nitric acid. The use of these quantities 

 was confirmed by the results obtained for the combined sul- 

 phur in a rubber mixture which contained 1.903 per cent sul- 

 phur, when estimated by the method of Waters and Tuttle. 





Grams Combined 



Zinc Oxide Sulphur, 



Used. Per Cent. 



lated with hydrochloric acid and the sulphates reprecipitatcd. This 

 determination was carried out in duplicite, and the results are 

 tabulated below. From these results, it is apparent that the 

 difference between the two, when expressed as "per cent sulphur," 

 is negligible, showing that the barium sulphate, as originally pre- 

 cipitated, is practically free from impurities. 



Original Fused and 



Precipitate Sulphur, Barium Sulphate Sulphur, 



Barium Sulphate Combined Reprecipitatcd Combined 



Sample in Grams. Per Cent. in Grams. Per Cent. 



A 0.0805 2.040 0.0781 1.979 



B 0.0801 2.030 0.0781 1.979 



The possibility of error from the foregoing sources having 

 been determined and found to be neglijjible, sulphur estimations 

 were made on several different mixtures. In all instances, unless 

 otherwise stated, the mixture subject to analysis was composed of 

 9IY2 parts rubber and 7^^ parts sulphur. The condition of the 

 mixture, and the nature of the sulphur (total or combined) was 

 varied acording to the experiment. 



Our inability to readily obtain satisfactory results for the total 

 sulphur in mixtures composed of rubber and sulphur only, by 

 methods not employing subsequent fusion, led us. to first examine 

 mixtures of this type. The total sulphur, as i^stimated by our 

 method, in both unvulcanized and vulcanized mixtures, was found 

 to be in good agreement with the quantity originally added. In 

 the case of unvulcanized samples, however, we found it best to 

 employ the modification recommended in foot- note 14. Typical 

 results for total sulphurs are tabulated in Table I. 



To test the accuracy of our method for the determination of 

 combined sulphur, results obtained by it were compared with 

 similar determinations made by the method of Waters and Tuttle. 

 The results of this comparison are shown in Table II, and are 

 such as to require no comment in regard to them. 



From this, it is evident, that with less than two gms. of zinc 

 oxide the results obtained are apt to be low. 



The barium sulphate precipitates obtained by our method were 

 then examined qualitatively for the presence of zinc. The "Rin- 

 mann Green" test for zinc gave negative results, indicating the 

 absence of this substance as an impurity. The true barium sul- 

 phate contents of the barium sulphate precpitates obtained by our 

 method were also determined quantitatively in the following 

 manner. The combined sulphur on a sample of rubber was esti- 

 mated in the usual way. The barium sulphate precipitate so 

 obtained was then fused with one to one sodium carbonate-potas- 

 sium nitrate mixture, the melt dissolved in water, filtered, acidu- 



Per Cent Free Per Cent Per Cent Per Cent 



Mineral Per Cent Sulphur Combined Total Total 



Pigment Sulphur by Bromine- Sulphur Sulphur Sulphur 



Sample Added to Adcied to Oxidation by New by by New 



No. Mixture. Mixture. Method. IVlcthod. Addition. Method. 



361 Zinc Oxide 2.56 0.893 1.664 2.557 



466 Zincf)xide 2.56 0.740 1.801 2.541 



483 Zinc Oxide 3.75 2.486 1.221 3.707 3.830 



524 Litharge 7.14 7. 188 



The figures shown in the Tables I and II were further substanti- 

 ated by those recorded in Table III. From this table, it is also 

 evident that satisfactory results may be expected when our method 

 is employed for the determination of combined and total sulphur 

 in conjunction with the bromine oxidation method of the United 

 States Bureau of Standards" for the determination of free sul- 

 phur. 



' Circular of United Stat. 



of Standards, No. 38, 1915, page ( 



