Determination of Sulfur by Wet Combustion with Perchloric Acid PER OLOF BETHGE Swedish Forest Products Research Laboratory, Analytical Laboratory, Stockholm, Sweden
Difficulties in the determination of sulfur in wood and pulp samples by the conventional method of Grote and Krelreler have been overcome by applying a wet combustion technique, using perchloric and nitric acids. The sulfur is reduced to hydrogen sulfide, by boiling with a reduction mixture, and determined iodometrically. The method gives quantitative yields for pure organic compounds and is suitable for a great variety of samples with sulfur contents from 0.01 to 100%. Sample weights can be varied from 9 mg. to 5 grams.
I
S T H I S institute the sulfur content of wood and pulp saniples
has hitherto been determined by the method of Grote and Iirekeler ( 4 ) . The sulfur content of these samples is sometimes fairly low, of the order of 1% or less, so that 1 gram or more is required for each analysis. This leads to lengthy combustions. In some samples the considerable ash content made the combustion tubes become opaque so that they had to be frequently replaced, leading to increased costs. Because of these disadvantages it was decided to try wet combustion technique6 for wood and pulp samples. First a method described by Klingstedt ( I O ) was applied to pure organic substances, such as sulfosalicylic acid and thiourea: the results were often low and the length of time required for digestion, 2 to 4 hours, was considered disadvantageous. When the resulting solution Tvas evaporated to dryness, a more or less black residue, obviously containing charred decomposition products, was sometimes obtained, an indication that the oxidation w:is not complete. \Vith this experience i n mind, attention was turned to method3 using perchloric acid for the wet combustioil. Such methods for the determination of sulfur have been described 1)-: Iiahiitic+ and com-orkers, first in a method applicable to vulcanized rubber ( 7 ) and l a t a for samples of biological origin ( I I i These methods were developed further by Wolesensky ( 1 9 ) and Smith ( 1 7 ) for Figure 1. Wet the determination of sulfur in coal arid decomposition i n 1934 Kahane and Kahane (.9) deapparatus scribed a general method for deterniining- sulfur in organic substances. Preliminary experiments in this institute confirmed the observation of Kahane t h a t sulfur-containing gases escape if the wet decomposition with perchloric acid is done in open vessels. The escaping gas is presumably sulfur dioxide, as no hydrogen sulfide could be detected. According to Staudinger and Niessen (18), carbonyl sulfide may be formed in some cases. This substance is a relatively stable gas and it is possible that it escapes when the oxidation is carried out in an open vessel. The experiments uere continued with the apparatus previously described (3) (Figure 1). When this apparatus wap used recoveries of sulfur were quantitative (see Tables I11 and IV), even when iodic acid was omitted, so it was considered superior to the one described by Kahane.
After the wet combustion the reaction flask contains a mixture of perchloric acid and the sulfuric acid to be determined, together with a small amount of nitric acid and the decomposition products of perchloric acid. After dilution with water it is possible to determine the sulfate content. directly by precipitation with barium chloride solution. This procedure is, however, timeconsuming and laborious and it would be more desirable to transform the sulfur into hydrogen sulfide. This may be accurately determined by volumetric or colorimetric methods which are much more sensitive than gravimetric methods for the determination of sulfate. Boiling with a reducing mixture containing hj,driodic acid as a method of reducing sulfates to sulfides was proposed by Lorant in 1929 (I$), and this technique has been further developed for analytical purpose by Luke ( I S ) , Roth (15), and Johnson and Kishita (6). The method of Luke ( I S ) for the determination of sulfur in rubber was of special interest because here reduction takes place in the presence of perchloric acid. This would appear to involve great risk of explosion. Experiments showed, however, that when the reducing mixture of Luke, which contains hydriodic, hypophosphorous, and hydrochloric acids, was boiled with perchloric acid in the proportion 3 to 1 there was no tendency to violent reaction; in fact, no reaction a t all took place. When the experiment was repeated iyith mixtures containing more perchloric acid, a slow formation of iodine was obtained. I t is obvious that if the reducing mixture is boiled with perchloric acid in the proportion 3 to 1, the temperature, about 120" C., is so low that the oxidizing properties of perchloric acid do not appear. I n wet decomposition experiments the oxidizing properties of perchloric acid begin to be noticeable a t about 150" to 180" C. Having carried out thousands of sulfur determinations, the author is convinced that there is no risk of violent reaction in boiling perchloric acid with the reducing mixture in the stated proportions. Khen the reducing mixture was added directly to the mixture i n the reaction flask after the wet decomposition, in order to boil off the hydrogen sulfide, some difficulties arose. Residual nitric. acid and other oxidizing substances such as chlorine caused a formation of free iodine bvhich reacted with the hydrogen sulfidc already formed. Hydrobromic acid, which reduces nitric acid and other oxidizing substances formed n-ithout affecting the sulfuric or the perchloric acid, was found to be a most suitable reagent for destroying these substances. iifter this treatment the hydrogen sulfide could be boiled off without any formation of iodine. The quantitative experiments on the reduction of sulfuric acid t o hydrogen sulfide were carried out in the apparatus shoxn in Figure 2. In order to avoid any oxidation of the hydrogen sulfide formed, nitrogen, previously freed from oxygen by the method of Meyer and Ronge ( 1 4 ) , n-as used as carrier gas. Although no abnormal results were obtained when nitrogen was used without prior purification, it seemed wise to standardize the purifiratiori procedure, since the quality of the nitrogen may vary in different gas cylinders. I n order to reduce the amount of hydriodic and hydrochloric acids carried over into the receiver, water cooling uas introduced, but it proved unnecessary to scrub the gas with water or dilute hydrochloric acid to remove acid fumes. The amount of these carried over during a 15-minute distillation i3 more than neutralized if the receiver contains 20 ml. of 3N sodium hydroxide solution. As the gases escaping from the a p p a r a t u ~ are harmless, it can he operated on an open laboratory bench 119
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ANALYTICAL CHEMISTRY Table I.
Reduction of Potassium Sulfate without Additions iTheoretica1, 18.38% S)
Amount Taken, hlg. 9.96 10.10 11.19 11.96 11.71 11.40
Table 111. Influence of Some .idditions before Wet Decomposition T'ariations.
S
Found, %
Yield,
18.37 18.39 18.41 18.39 18.39 18.43
99.95 100.05 100.16 100.05 100.05 100.27
%
Variation
?
Table 11. Reduction of Potassium Sulfate (Hydrobromic acid and perchloric acid added. Theoretical, 18.38% 5) Amount Taken, Mg. Found, % Yield, 7% 12.33 9.26 11.71 10.91 11.01 9.78
18.29 18.51 18.44 18.20 18.41 18.38
4
99.51 100.16 100.33 99.02 100,16 100.00
The design of the receiver and the titration of the hydrogen sulfide have been described ( 2 ) . T o ensure quantitative recovery of sulfur with the reduction mixture of Luke, experiments were carried out in which dried potassium sulfate was boiled with 7 ml. of the reduction mixture i r i the apparatus mentioned The results are given in Table I.
319
G
4 5 4 + 5
According t o procedure given for semimicro samples Preoxidation with NaOH H20z Preoxidation with Brz H202 500 me. of glucose added Arnmo
