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THE DETERMINATION OF SULFUR
CHAPTER
VI
T H E DETERMINATION OF SULFUR All methods for the analysis of organic sulfur involve the complete oxida tion of the substance and the determination of the resulting sulfuric acid. The usual procedure for estimating the acid is to convert it to barium sulfate and weigh it as such. There are several ways of doing this but the most popu lar appears to be the Carius method in which the oxidation and formation of barium sulfate is accomplished in one operation. This is brought about by heating the substance in a sealed tube with fuming nitric acid and barium chloride. The resulting barium sulfate is re moved from the bomb and after certain puri fications it is weighed. Both micro and macro methods involving this outline are largely used and, in studying possibilities for a semimicro procedure, it appeared to be the best all-around choice. Such a procedure has been developed1 and has been used sufficiently to conclude that for general purposes it is entirely satis factory. Apparatus. The apparatus required for the method is the same as that described in connection with the determination of halo gens by the Carius method except that stand ard micro platinum Gooch crucibles with asbestos pads are used in place of the sintered A B glass filtering tubes. F I G . 20 Procedure. A sample of the material, made Hooks for Handling Micro into a pellet of approximately 25 mg., is Platinum Crucibles placed in the sample tube, which is intro duced into the glass bomb containing approximately 100 mg. of barium chloride and 0.3 ml. of fuming nitric acid. The bomb tube is sealed, heated to 300°C. for 6 hours or more, cooled, and opened, and the barium sulfate is transferred to a 30 ml. beaker in the manner described in the directions for halogens. The contents of the beaker are evaporated to dryness, on a sand bath, and the residue is treated with 10 ml. of 5 per cent hydrochloric acid. The liquid is again evaporated to dryness, after which the residue is digested with hot water, and the barium sulfate is transferred to a weighed
Λ
1
E. P. Clark, J. Assoc. Official Agr. Chem. 18, 476 (1935). 63
64
SEMIMICRO QUANTITATIVE ORGANIC ANALYSIS
platinum Gooch crucible with the aid of a small stirring rod, a fine stream of water from a wash bottle, and a little alcohol. The crucible is prepared by placing in it a mat of asbestos, washing it with water and alcohol, heating to redness for 5 minutes, cooling upon a brass block, and weighing it in the same manner as directed for glass filter tubes in the preceding chapter. Another crucible prepared and treated like the one in which the barium sulfate is collected must be used as a tare. The heating is done most conveniently in a porcelain crucible placed in an electric heater, such as a Cenco hot-cone heater. The introduction and removal of the crucible are best done with a small hook, as shown in A, Fig. 20, made of No. 15 B & S gauge wire. A similar straight hook, shown in B, is convenient for placing crucibles on the balance and removing them. After the barium sulfate has been transferred, the crucible is heated to redness, cooled, and washed with 5 per cent hydrochloric acid. The crucible is again heated to redness, allowed to cool, and weighed. The percentage of sulfur in the sample is calculated from the quantity of barium sulfate found. Note. A micro method employing a sodium peroxide fusion for converting sulfur to sulfate has been described by Elek and Hill.2 According to these authors, this procedure gives satisfactory results and should therefore be readily adapted to semimicro quantities. The author has had no experience with the method, but as soon as opportunity permits, it is his intention to investigate it. 2
A. Elek and D. W. Hill. / . Am. Chem. Soc. 55, 3479 (1933).
VI
T H E DETERMINATION OF SULFUR All methods for the analysis of organic sulfur involve the complete oxida tion of the substance and the determination of the resulting sulfuric acid. The usual procedure for estimating the acid is to convert it to barium sulfate and weigh it as such. There are several ways of doing this but the most popu lar appears to be the Carius method in which the oxidation and formation of barium sulfate is accomplished in one operation. This is brought about by heating the substance in a sealed tube with fuming nitric acid and barium chloride. The resulting barium sulfate is re moved from the bomb and after certain puri fications it is weighed. Both micro and macro methods involving this outline are largely used and, in studying possibilities for a semimicro procedure, it appeared to be the best all-around choice. Such a procedure has been developed1 and has been used sufficiently to conclude that for general purposes it is entirely satis factory. Apparatus. The apparatus required for the method is the same as that described in connection with the determination of halo gens by the Carius method except that stand ard micro platinum Gooch crucibles with asbestos pads are used in place of the sintered A B glass filtering tubes. F I G . 20 Procedure. A sample of the material, made Hooks for Handling Micro into a pellet of approximately 25 mg., is Platinum Crucibles placed in the sample tube, which is intro duced into the glass bomb containing approximately 100 mg. of barium chloride and 0.3 ml. of fuming nitric acid. The bomb tube is sealed, heated to 300°C. for 6 hours or more, cooled, and opened, and the barium sulfate is transferred to a 30 ml. beaker in the manner described in the directions for halogens. The contents of the beaker are evaporated to dryness, on a sand bath, and the residue is treated with 10 ml. of 5 per cent hydrochloric acid. The liquid is again evaporated to dryness, after which the residue is digested with hot water, and the barium sulfate is transferred to a weighed
Λ
1
E. P. Clark, J. Assoc. Official Agr. Chem. 18, 476 (1935). 63
64
SEMIMICRO QUANTITATIVE ORGANIC ANALYSIS
platinum Gooch crucible with the aid of a small stirring rod, a fine stream of water from a wash bottle, and a little alcohol. The crucible is prepared by placing in it a mat of asbestos, washing it with water and alcohol, heating to redness for 5 minutes, cooling upon a brass block, and weighing it in the same manner as directed for glass filter tubes in the preceding chapter. Another crucible prepared and treated like the one in which the barium sulfate is collected must be used as a tare. The heating is done most conveniently in a porcelain crucible placed in an electric heater, such as a Cenco hot-cone heater. The introduction and removal of the crucible are best done with a small hook, as shown in A, Fig. 20, made of No. 15 B & S gauge wire. A similar straight hook, shown in B, is convenient for placing crucibles on the balance and removing them. After the barium sulfate has been transferred, the crucible is heated to redness, cooled, and washed with 5 per cent hydrochloric acid. The crucible is again heated to redness, allowed to cool, and weighed. The percentage of sulfur in the sample is calculated from the quantity of barium sulfate found. Note. A micro method employing a sodium peroxide fusion for converting sulfur to sulfate has been described by Elek and Hill.2 According to these authors, this procedure gives satisfactory results and should therefore be readily adapted to semimicro quantities. The author has had no experience with the method, but as soon as opportunity permits, it is his intention to investigate it. 2
A. Elek and D. W. Hill. / . Am. Chem. Soc. 55, 3479 (1933).