PROCEEDINGS OF THE AMERICAN ACADEMY OF ARTS AND SCIENCES. VOL. XXX. PAPERS READ BEFORE THE ACADEMY. AID IN THE WORK DESCRIBED IN THIS PAPER WAS GIVEN BY THE ACADEMY FROM THE C. M. WARREN FUND FOR CHEMICAL RESEARCH. I. CONTRIBUTIONS FROM THE CHEMICAL LABORATORY XIX. ON THE DETERMINATION OF SULPHUR IN VOLATILE ORGANIC COMPOUNDS.* BY CHARLES F. MABERY. Presented April 11, 1894 THE great quantity of products introduced into the petroleum industry from the fields in Ohio and Canada yielding the sulphur oils has involved many sulphur determinations, and the necessity of a rapid method capable of affording results of extreme accuracy, especially in oils containing a small fraction of one per cent of sulphur. Several of the older methods leave nothing to be desired in point of accuracy, but they are not sufficiently expeditious for service in manufacturing operations, or in investigations which depend upon immediate information concerning the percentage of sulphur. The first attempt to determine sulphur in organic compounds by combustion in oxygen was made by C. M. Warren,† the sulphuric acid formed being absorbed within the combustion tube in plumbic peroxide. * This paper is one of the series on the composition of the sulphur petroleums. †These Proceedings, VI. 472. The oxides and acids formed by combustion were first distilled and collected in bromiue water as an oxidizing agent by Sauer and this method was still further improved by Mixter, † who avoided the use of a rubber cork in the forward end of the combustion tube, carried forward the volatilized substance by a current of carbonic dioxide, and suggested more efficient means for oxidation by bromine and absorption. All these methods depend upon the formation of sulphuric acid and precipitation as baric sulphate, which involves considerable labor when a large number of determinations are necessary in a limited time. To overcome this difficulty Burton suggested a modification of the method of Sauer, which consists in absorbing the oxidized sulphur in a standard solution of potassic hydrate and titrating the excess of alkali with standard sulphuric acid. Besides these methods the only other suitable means for the determination of sulphur in oils with large percentages of sulphur is the well known method of Carius, in which the substance is oxidized in a closed tube by means of fuming nitric acid. In its applicability to all classes of compounds, and in the accuracy of results of which it is capable, this method leaves little to be desired except perhaps in the analysis of oils containing less than one hundredth of one per cent of sulphur. On account of the limited weight of substance that can be oxidized in a Carius tube another method must be selected for substances containing less sulphur. Our experience has shown that the Carius method may be relied upon in sulphur determinations to yield concordant results within a few hundredths of one per cent. Oxidation of the less volatile oils containing a small percentage of sulphur, without doubt, may be accurately accomplished in an open vessel, but with larger amounts of sulphur the action of nitric acid is so violent that it must entail loss by volatilization, unless indeed the sulphur oil is considerably diluted by a sulphur-free oil, in which case the solvent must be completely oxidized. The great number of sulphur determinations in crude oils and products obtained from them, connected with the extended examinations which have occupied my attention during several years past, has demanded a careful comparison of the various methods as to their efficiency and economy of time. Particular attention has been given to details of the Carius method, with the precautions necessary in its successful application to the analysis of sulphur oils. The first requi Fres. Zeit. Anal. Chem., XII. 32. † Amer. Chem. Journ., II. 396. Ibid., XI. 72. site is a furnace of suitable construction to maintain an equal temperature, easily controlled in all the tubes within the furnace, without a great loss of heat by radiation. For this purpose and for Carius analyses in general I have recently had a furnace constructed which differs in certain features from any other I have seen, and it shows such a high degree of efficiency that a brief description may not be entirely devoid of interest. The body is of the ordinary cylindrical form, 75 cm. long and 25 cm. in diameter, of heavy sheet iron, and it is surrounded by two outer jackets of sheet iron each enclosing a half-inch space, and extending beneath on either side to within 6 cm. of the heating tube; it is supported upon legs of strap iron three sixteenths of an inch thick and two inches wide, each entirely encircling the body at either end for rigidity. These two air spaces retain the heat so effectually that the hand may be borne on the outside of the furnace when the thermometer within indicates a temperature of 200°. The iron tubes are as usual of gas pipe, with threads at either end with caps easily movable by the fingers. With a small hole in each cap for the escape of gas, these tubes retain all glass in the most violent explosions. When several tubes are in the furnace at the same time a record of them may conveniently be kept by suspending metal tags numbered consecutively from the holes in the caps by means of bent wire. Figure 1 shows the arrangement of the outer air spaces with the position of the heating tube. The furnace is heated by means of a gas stove heater 45 cm. in length, with thirty-two gas jets that will burn continuously with a flame 2 mm. high, giving a temperature within the furnace of less than 60°; by interposing an asbestos or an iron plate a considerably lower temperature may be maintained. The heating tube is supported on two iron straps bolted to the legs, one at either end of the furnace; by means of it the heat is very equally distributed with little waste, and the glass tubes being thus evenly heated there is less danger of loss by explosion. A temperature of 200° may be obtained within twenty-five minutes after lighting the jets, and it may be maintained with jets fifteen millimeters in height, requiring a small consumption of gas; the hand may be held without discomfort for some time directly beneath the heater. The variation in temperature at different heights within the furnace is small; with the thermometer at 275° at the level of the upper tubes, the tempera FIG. 1. ture at the level of the lower tubes is about 9° higher. For temperatures higher than 275° a second heating tube is necessary. It is frequently convenient to be able to regulate within close limits the flow of gas for the required temperature without further attention after lighting the jets. The device shown in Figure 2, which suggested itself for this purpose, consists in attaching to the end of the gas valve by means of a screw thread a brass cap with an index of stout copper wire moving in front of a graduated circle with a radius of about six inches. 0 With glass tubes of large size, those we use are 1.5 mm. inside diameter, well sealed and with strict adherence to certain conditions which have elsewhere been described by A. W. Smith and me,* there is little danger of an explosion. The quantity of nitric acid should not be in excess of twenty times the weight of the substance taken, and after heating to 175° for fifteen hours the tubes are opened, best without removing from the furnace, - resealed and heated again to 250° during five to The serious objection to the Carius method for sulphur is the slow process of oxidation, and it seems hardly possible to hasten the operation by raising the temperature, since glass tubes will not stand the great pressure. FIG. 2. ten hours. In studying various methods depending upon the oxidation of sulphur by combustion I have found that nothing less than complete oxidation gives reliable results. Many experiments on fractional combustion have shown clearly that compounds with high percentages of sulphur do not yield concordant results, even when the sulphur compound is diluted with a sulphur-free oil. I have found Burton's adaptation of the Sauer method reliable and expeditious, and with certain modifications presently to be described it is perfectly satisfactory for the analysis of oils of high as well as low percentages of sulphur. In Figure 3 the inlet tube for oxygen or air is shown as entering through the rear stopper, as proposed by Mixter, and extending just to the centre of the constriction. In the combustion of some of the oils which we have under examination, the temperature must be maintained as high as the most infusible Bohemian glass will stand, and at such temperatures the smaller tube within is distorted if it is * Amer. Chem. Journ., XVI. 83 placed in the forward portion of the combustion tube in the zone of greatest heat, if it terminates at the narrowest point of the constriction, continuous combustion is insured by thorough admixture of the volatilized substance with oxygen. Complete oxidation is still more certain in rapid combustion if that portion of the tube in front of the narrower part is left somewhat longer than is preferred by Sauer, Mixter, or Burton. The tubing we have in use is somewhat thicker in the wall than that in ordinary use, and larger, with an inside diameter of 18 mm. It is important that the oxidation proceed as rapidly as is consistent with complete absorption, and we find that this is best accomplished in a large U tube partly filled with broken glass. Our U tube is 34 cm. in height, 25 mm. inside diameter, and with 50 c. c. of the absorbent solution a rapid gaseous stream may be passed through without danger of loss. For low sulphur oils we use a solution of sodic hydrate of such a strength that 1 c. c. equals 0.0010 gram of FIG. 3. sulphur, and for higher percentages a solution in which 1 c. c. equals 0.0050 gram. Methyl orange has been used as an indicator in all our determinations; the change in color in titrating an alkaline solution with this indicator is well defined and exceedingly delicate. The titrations may be made in the U tube without transferring the solution after washing in the acid from the combustion tube. To carry forward the volatilized substance it is advantageous to introduce a slow current of carbonic dioxide, as proposed by Mixter, and we have sometimes used a combustion tube closed with a rubber cork in front and sometimes a bent tube. With substances containing a high percentage of sulphur it is doubtless safer, as Mixter suggests, to avoid the use of a cork in front. |