Procedure for Semimicrodetermination of Sulfur in Organic Compounds ROBERT iM.LINCOLN, A. S. CARNEY1, AND E. C. WAGNER Department of Chemistry and Chemical Engineering, University of Pennsylvania, Philadelphia, Penna.
Gooch Crucibles for Sulfur Semimicrodeterminations
A
SATISFACTORY procedure for the semimicrodetermination of sulfur in organic compounds by use of the sodium peroxide bomb has hitherto been lacking, and no bomb equipment suitable for the purpose has been described. The feasibility of using the sodium peroxide macro- or microbomb for decomposition of semimicrosamples has doubtless been tested by many analysts. The few opinions available here with respect to the use of the microbomb in this way are contradictory. Some preliminary test analyses made with semimicrosamples and a macrobomb gave acceptable results when the charge consisted of about 0.03 gram of sample (taurine), 0.2 gram of potassium perchlorate, 0.2 gram of sucrose, and 8 grams of sodium peroxide. As this procedure appears to involve a deliberate imposition of unfavorable conditions for precipitation of barium sulfate, it has not been examined further. This paper presents descriptions of a bomb assembly and an analytical procedure which permit the determination of sulfur in solid or liquid organic substances by decomposition of semimicrosamples in admixture with sodium peroxide, potassium perchlorate, and sucrose in a metal bomb, the sulfur being weighed as barium sulfate. The magnitudes of sample, combustion charge, and apparatus are intermediate between those of the familiar macroprocedure (7) and the microprocedure of Elek and Hill (6).
Porcelain Gooch crucibles of 10-cc. and 15-cc. capacities were used; the smaller size is preferred. Since in this method an attempt must be made to weigh the Gooch crucible to about 0.02 mg. it is important that the asbestos mat be properly formed, to avoid loss due to passage of h e asbestos fibers through the perforations, and especially that the prepared filter be brought
Parr Sodium Peroxide Semimicrobomb Inquiry disclosed the fact that the Parr Instrument Company, Moline, Ill., had previously designed a bomb assembly with a cup of about &cc. capacity, and presumably suitable for semimicroanalysis. One of these outfits was secured and was found satisfactory. The form of the apparatus, and some of the principal dimensions, are shown in Figure 1. The dimensions indicated are taken from blueprints provided by the Parr Instrument Company, from which the apparatus may be obtained. The cup is made of either 98 per cent nickel or 30 per cent nickel steel, and the cover of either 98 per cent nickel or nickel-plated brass. Neither stainless steel nor any other alloy of chromium is suitable for parts which are t o be exposed to the ignited sodium peroxide mixture. The screw collars are made of chromium-plated bronze. Lead gaskets were used; rubber gaskets are available and would doubtless serve about as well. In either case, since the charge is ignited by applying a flame to the bomb, care must be taken not to overheat the gasket. FIGURE 1. Present address, Pennsylvania State College, State College, Penna.
358
PARRSODIUM PEROXIDE SEMIMICROBOMB
ANALYTICAL EDITION
May 15, 1941
to constant weight under the conditions of its use in analysis. Constancy of weight should never be assumed. A solution, in which the volume, temperature, and concentrations of sodium chloride and hydrochloric acid approximate those of the liquid to be filtered in analysis, is passed through the filtering crucible, which is then washed, dried, ignited, and weighed by the procedures to be used in the analysis. This treatment should be repeated until the wei ht is constant within 0.05 mg. The importance o f these precautions is shown by the following data, obtained with four 10-cc. Gooch crucibles. Each crucible was prepared as usual, and was then treated with 150 cc. of a liquid whose temperature, acidity, and salt concentration a p proximated those of the analysis liquid. It was washed, dried, and weighed as described below. Treatment with Conditioning Solution
Gooch I
MQ.
MQ.
Before first weighing +0.02 -0.28 -0.03
-0.44 -0.01
MQ. 1 2 3
Changes in Weight Gooch I1 Gooch I11
....
Gooch I V
MQ. -0.02 . . I .
The four crucibles did not behave uniformly; two attained constancy of weight after one treatment with the conditioning solution, while the other two required an additional treatment. The asbestos mat thus conditioned may be used for a considerable number of analyses. After five or more precipitates have accumulated, the greater part of the barium sulfate may be removed, dislodging not more than a superficial layer of asbestos. Enough fresh asbestos is introduced to restore the mat to its original thickness, and the crucible is brought to constant weight, an operation which should involve little or no trouble.
Conditions for Analysis Preliminary experiments showed a mixture of 4 grams of sodium peroxide, 0.2 gram of potassium perchlorate, and sucrose and sample totaling 0.2 gram (0.02 to 0.05-gram sample) t o effect satisfactory decompositions in the semimicrobomb. The conditions for precipitation and handling of the barium sulfate obtained from such charges are chosen with a view to controlling some of the errors which are unavoidable when the small precipitate of barium sulfate is formed in the presence of considerable sodium chloride, some barium chloride, hydrochloric acid, and sodium sulfate. To avoid excessive contamination of barium sulfate by salts (9, 10, 18, 21), precipitation is effected in a volume of about 150 cc. (a dilution greater than in the macromethod, Q), and with a dilute precipitant added rapidly, as suggested by Hintz and Weber (8). This procedure has been found advantageous for macroprecipitations under like conditions (8, cf. 19), though perhaps of reduced advantage in the present method because of the delay in the appearance and subsidence of the small precipitate of barium sulfate ( 1 4 ) . The “reversed precipitation” appears t o have some merits (16,19), but i t has been reported to increase contamination by anions (16, 19) and so was not tried. The acidity (1 cc. of concentrated hydrochloric acid in a volume of 150 cc.) is such that increase in the solubility of barium sulfate and occlusion of “free” sulfuric acid (sodium hydrogen sulfate, 2 ) and of chlorides (10 ) are not favored. T o minimize coprecipitation by adsorption on slowly growing crystals (12, 20, 24, 26), and t o render the precipitate more promptly filterable, granulation of the precipitate is hastened by addition of picric acid (19). Results which indicate the value of this procedure are given in Table IV. No provision is made for avoiding interference of nitrate (6),which need be present only to the extent that it may be formed during decomposition of nitrogenous -e. g.: nitrocompounds. No provision is made for avoiding interference by iron or chromium. The familiar devices for avoidance of contamination of barium sulfate precipitates by iron (1, 17, 23) are inconvenient or uncertain. Errors due to chromic ion are avoidable in part by addition of alkali acetate (I?‘), but this increases the concentration of salts present. Chromate
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ion may cause serious contamination (4) because of coprecipitation of mixed crystals of isomorphous barium sulfate and barium chromate (3, 11, IS). It is therefore advisable to use a bomb which yields little or no iron or chromium, and to replace a cup or cover found to yield considerable of either. The nickel bomb and nickel-plated cover specified above are satisfactory; stainless steel and illium parts are not satisfactory.
Procedure Transfer to the bomb cup 0.2 gram of powdered potassium perchlorate. Introduce the sample (finely powdered if a solid. for manipulation and analysis of liquids see below), which should be sufficient to yield from 20 to 100 mg. of barium sulfate, and should be weighed to 0.02 mg. Add sucrose in such amount that the sum of sample plus sucrose is 0.2 gram. Weigh out rapidly 4 grams (*0.1 gram) of granular sodium peroxide (the grade low in sulfur) and transfer to the cup. A satisfactory procedure, especially in humid weather, is to support the bomb cup on the left-hand pan of a trip scale, place a small beaker on the righthand pan, and introduce water to counterpoise the bomb cup, into which the sodium peroxide is weighed directly and rapidly. A measuring cup of proper capacity would be a convenience. Make certain that the gasket is in good condition, and then adjust the cover in place and secure it firmly by tightening the screw collars. To mix the charge shake the apparatus vigorously, and finally tap the cup against the table top to settle the charge. Support the apparatus in a vertical position by means of a clamp and ring stand, a t such height that the cup can be heated by the flame of a blast lam seated on the table top. It is advisable to wear goggles, a n t t o place an iron plate or other protective device in front of or around the bomb while the charge is ignited. The explosion hazard which attends the use of this apparatus cannot yet be estimated. Sodium peroxide macrobombs sometimes explode with dangerous violence, but the factor of safety should be considerably greater with the smaller bomb because of its relatively heavier construction and the reduced charge which it contains. The first semimicrocup used developed a perforation in the bottom after about a hundred decompositions, and this presumably followed a gradual weakening of the wall which might have led to explosive rupture. Adjust the blast lamp flame so that it is 7.5 cm. in length and 3 to 4 mm. in diameter. First bring the tip of the flame to a point about 1 cm. below the bomb for about 15 seconds, and then raise the flame so that it touches the bottom of the bomb. This gradual heating appears to prevent the projection of a portion of the charge against the inner surface of the cover, to which some of it may adhere and thus escape combustion. Heat the bomb in this way for 30 seconds; a heating period much longer than this may melt the gasket. Cool the apparatus, finally, in tap water, and remove the cover. Wash the lower surface of the cover with hot distilled water, collecting the washings in a 250-cc. beaker. Wash the bomb externally (discard these washings) and then, with the aid of a thick glass rod, place the cup on its side in the beaker. Add water to a volume of about 50 cc., cover the beaker, and allow the contents of the cup to dissolve, if necessary warming gently a t first to start the action. Using the glass rod lift out the bomb cup and wash it externally. Then grasp it between the fingers and wash the interior, collecting the washings with the main extract. Cover the beaker and allow the liquid to cool somewhat. Insert the glass rod obliquely, so that it projects through the lip of the beaker, push the watch glass slightly away from the rod, and then cautiously pour 8 cc. of concentrated hydrochloric acid along the sloping rod, so that the acid enters the liquid a t a point well under the cover. This procedure minimizes loss due to spraying. Test a drop of the liquid on Congo red paper, and if necessary add more acid until the liquid is just acid in reaction. To the solution add 1 cc. of concentrated hydrochloric acid in excess, and warm on the hot plate to expel most of the carbon dioxide. Filter the liquid through paper, receiving the filtrate in a 250-cc. beaker, and wash the first beaker, the watch glass, and finally the filter, with hot water. To the filtered solution add 15 cc. of a saturated aqueous solution of picric acid (about 1 per cent), and dilute the liquid to a volume of about 140 cc. Cover the beaker and heat the solution to boiling. Meanwhile measure out 7 to 10 cc. of 0.1 N barium chloride solution, heat to boiling, and then add it rapidly, with stirrinq to the hot sulfate solution. Digest the liquid at or near 100 C. for an hour or more, when the precipitate may be removed by filtration. Alternatively allow the analysis to stand until the following day before filtering; in this case the picric acid is dispensable. Decant the liquid through a weighed Gooch crucible (10-cc.),
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Vol. 13, No. 5
INDUSTRIAL A N D ENGINEERING CHEMISTRY
TABLE I. ANALYSISOF POTASSIUM SULFATE &so4
Sulfur
Bas04
MU.
Mu.
Mu.
Mu.
100.31 100.31 100.31
18.45(5) 18.45(5) 18.45(5)
133.90 134.06 134.10
18.39(1) 18.41(3) 18.41(9)
Sulfur Found
Difference
Mu. -0.06 -0.04 -0.04
wash the precipitate several times with hot water by decantation, and then tranlifer it to the filtering crucible with the aid of a fine stream of hot water. Dislodge any barium sulfate adhering to the beaker and stirring rod by thorough manipulation with a small policeman and transfer to the filter. Wash the precipitate in the Gooch crucible five times with hot water. Dry the crucible in an oven at 120' for 30 minutes, and then transfer to a large porcelain crucible supported on a triangle above a Bunsen burner. Cover the larger crucible with its lid, and heat for 30 minutes with the full flame of the burner. Allow the Gooch crucible to cool somewhat, transfer it t o a desiccator, and when it has cooled to room temperature set it in the balance case. After 15 minutes weigh the crucible to 0.02 mg. The procedure for precipitation and eventual weighing of barium sulfate, as described in the last two paragraphs, was tested in preliminary trials. Aliquot portions of a standard solution of potassium sulfate (recrystallized from hot water, dried a t 120°,ground in a mortar, redried a t 120°, and kept in a desiccator over calcium chloride) were analyzed, with addition of 5 grams of sodium chloride to simulate the conditions encountered in the analysis of organic compounds. The results of the final trials appear in Table I. BLANKANALYSIS. Blank analyses were conducted by the procedure described, using 0.2 gram of sucrose and omitting the sample. The blanks obtained in the experimental trials yielded precipitates not visible, but filtration showed presence of insoluble material. It seems advisable to conduct blanks in triplicate, and to use the averaged result, as individual results may show variations. Some results obtained by the entire procedure described above, applied to fifteen organic compounds, are collected in Table 11. PROCEDURE FOR ANALYSIS OF LIQUIDS. Liquid substances which are not volatile or sensitive t o atmospheric moisture, and which are not attacked by sodium peroxide upon contact, may be weighed from a Lunge-Rey pipet or other similsr device, directly into the bomb cup. Liquids which cannot safely be handled in this way must be weighed and introduced
into the bomb in sealed thin-walled glass ampoules, which can be broken during the mixing of the charge after the bomb is closed. The decomposition in the bomb then effects a silica fusion of the thinner parts of the glass bulb, only the thicker glass of the stem ordinarily surviving the action. [Rapid silica fusions of minerals might be possible if samples were ground impalpably fine. A specimen of orthoclase, submitted to decomposition in a macrobomb, was rendered water-soluble except for a small residue of the less finely ground materia1 (29).I Acidification of the aqueous extract leads to separation of gelatinous silicic acid, which must be removed before the barium sulfate is precipitated. The interference of silicic acid following decomposition of samples contained in glass ampoules is not mentioned in familiar descriptions of the sodium peroxide bomb procedure. This consequence of the use of a glass container for an organic sample may affect analyses for any element determinable by use of the sodium peroxide bomb, especially if a gravimetric method is to be employed. Some comments on the obstruction offered by silicic acid in the determination of halogen in liquid substances will appear in a later paper. The removal of silicic acid is effect,ed by the dehydration procedure familiar in mineral analysis. After extraction of the ignited charge acidifp the liquid, eva orate to dryness on a Bteam bath, and moisten the dry resigue with 5 cc. of concentrated hydrochloric acid. Again evaporate to dryness, and heat the residue for I to 2 hours on the steam bath. Extract soluble material in about 75 cc. of hot water, pass the extract through a paper filter, transfer the silica residue to the filter, and wnsli five times with hot water. Add 1 cc. of concentrated hydrochloric acid and 15 cc. of saturated picric acid solution. Dilute the liquid to 140 cc., and then proceed as described above. Some results which indicate the usefulness of this procedure are given in Table 111.
Effect of Picric Acid on Semimicroprecipitation of Barium Sulfate Some trials of the procedure described were made in the presence and absence of picric acid to test its effectiveness
OF SULFUR IN ORGANICCOMPOTJNDS TABLE11. SEMIMICRODETERMINATION
Compound
Sample
Mu. 41.61 28.55 p-Toluenesulfonamide 32.19 38.30 N p tolylbenzene- 4 9 . 0 5 4 8.41 sulfonamide (m. p. 27.12 122' corrected) 35.79 34.57 30,46 46.17 3 N methyl N p - 3 . 7 3 tolylhenzenesulfon- 4 1 . 7 1 amide 30.38 N methyl p aminophenol sul- 3 0 . 2 4 fate 33.92 Sulfanilamide Analvsis bv macro- 3 0 , 3 3 methbd
