OCTOBER 15, 1937
ANALYTICAL EDITION
After cooling, the lower centimeter of the tube was cut off and the edges were fire-polished. Small “tear drops” of Pyrex glass were made approximately to fit the interior of the orifice. The b a l preparation of the crucible consisted in connecting it to a suction tube with a short section of heavy rubber tubing, and forming an asbestos mat above the tear drop. The tip of the tear drop should not extend below the bottom of the crucible; otherwise the filter mat would be disturbed when the crucible was set down. Checked in analytical work by comparison with commercial platinum crucibles with a platinum-iridium sponge filter plate, it showed little difference in performance. It successfully withstood ignition, if heated gradually a t first. Loss in weight was practically nil. The exterior could be easily cleaned by wiping, pFeventing possible retention of solid material apt to “creep” into the tube of a tube-type filter. Hygroscopic materials were easily transferred from the desic-
491
cator to a small weighing bottle in equilibrium with the atmosphere of the balance case. The total load was still less than half the capacity of a 20-gram microbalance. The weighing bottle had the further advantage of obviating the difficulty of obtaining constant weight (anhydrous surface, electrical charges?). Its general performance attested its value for analyses requiring a number of crucibles a t one time or for use in class instruction.
Literature Cited (1) Emich, F.,“Microchemical Laboratory Manual,’’ p. 25, New York, John Wiley & Sona, 1932. (2) Ibid., p. 28. (3) Ibid., p. 30. (4) Ibid., p. 68. ( 5 ) Ibid., p. 76. RECE~IVED June 9, 1937. Released for publication by courtesy of the Director, Agricultural Experiment Station, University of Minnesota.
Microdetermination of Organic Sulfur WILLIAM SASCHEK, College of Physicians and Surgeons, Columbia University, New York, N. Y.
T
HE Pregl gravimetric method (8) of determining sulfur
in organic compounds by catalytic oxidation in the spiral combustion tube occurs in two main operations: combustion of the substance and determination of the sulfuric acid formed. The general advantage of this method is somewhat impaired by experimental difficulties encountered in the second part of the original procedure, which involves washing out the combustion tube, evaporating the collected wash liquid t o a very small volume or to dryness before precipitation, and transferring the precipitate to a Neubauer filter crucible. If precipitation is done before evaporation, an evaporation ring of precipitated barium sulfate tends to form and adhere tenaciously to the wall of the vessel, causing difficulty in quantitative transfer. These S c u l t i e s have led to the application of various devices such as alternate washing with water and alcohol, use of a “feather,” use of automatic transfer devices (7, IOt I I ) , or precipitation with a “celluloid sol’’ (9). All these devices for facilitating quantitative transfer of barium sulfate precipitate, which is inherently difficult, are inconvenient and include possible sources of error. The various open fusion and bomb fusion methods (3, 12) tend to increase the possibilities of error, as they require precipitation of the small amounts of barium sulfate in the presence of proportionately large amounts of other salts. These conditions of high nitrate-ion concentration (anion most highly coprecipitated with barium sulfate), insufficient digestion, and the impossibility of reprecipitation are most favorable for extensive eo-precipitation. T o circumvent difficulties connected with the quantitative transfer of barium sulfate, titrimetric methods of determining the sulfuric acid after the combustion have been reoommended (4, 6, 8). These have only limited applicability to halogen-free and nitrogen-free substances, or are cumbersome and time-consuming. Heller and Meyer (6) suggest transferring the barium sulfate after evaporation into a large crucible of 25-cc. capacity and drawing off the wash liquid by means of a porcelain flter stick (9),but fail to give sufficient detail. During the past 7 years the following technic, which avoids many of the above difficulties, has been practiced and taught to students in this laboratory, and has, therefore, already
FIQURE 1 found its way into many other laboratories. As the necessity of transferring the barium sulfate precipitate from one vessel to another is eliminated, it is generally preferred to t h e Neubauer crucible technic.
Procedure
For precipitation a thin-walled porcelain crucible (preferably with black interior glazing) of about 15-cc. capacity (1) and a small porcelain filter stick (9) are used. The crucible should be cleaned of precipitate from previous analyses and rinsed with distilled water, the exterior wiped with a clean cloth. The filter stick may be cleaned b y brushing off any adhering precipitate and then washed by sucking water
INDUSTRIAL AND ENGINEERING CHEMISTRY
492
through it in both directions to rid the porous filter surface of precipitate as much as possible. Reverse washing is facilitated by the use of a suitable “adapter.” The crucible, containing the filter stick, should be dried at 150” to 200’ C. for about 10 minutes. For this purpose an oven, which can be made easily and practically without expense from a tin can, is convenient and adequate; Figure 1 explains some of its details. An electric hot plate will also serve nicely as a heating base for the oven. The crucible is then placed on a piece of clean metal, covered, and allowed to cool. A metal cooling block (Figure a), with glass cover for protection against dust, allows more secure handling of the crucible.
VOL. 9, NO. 10
liquid employed should be so adjusted that the crucible will not become much more than about two-thirds filled. For checking the completeness of the rinsing, another larger washing of the combustion tube may be collected in the protecting test tube and tested for sulfuric acid. If a precipitate of barium sulfate forms, it may be added to the crucible during subsequent filtration. It has been found, however, in several hundred sulfur determinations, that complete removal of the sulfuric acid produced in the combustion can easily be effected with three to four washings of 1to 2 cc. as described. For precipitation of the barium sulfate the crucible is placed on a suitable steam bath, and about 0.5 cc. of 10 per cent barium chloride solution is added dropwise. Any combustion residue in the boat is extracted with small portions of wash liquid and filtered into (the reduced contents of) the crucible. After about 15 minutes the solution is allowed to cool and filtration is carried out as described below. In case of very low percentages of sulfur, the volume should be reduced before filtration. For filtration the filter stick is attached to a suitable suction device, and sufficient suction is applied so that the liquid filters at a rate of 1 to 2 drops per second. The precipitate is then washed in such a manner that the whole inside surface of the crucible is thoroughly moistened and rinsed three to four times with spa11 portions of the acidulated water, making a total wash volume of about 3 cc. The filtrate should be collected and examined in a separate vessel of the suction device, and must be perfectly clear. Traces of barium sulfate may pass through a filter used for the first time; in this case the filtrate is poured back into the crucible and refiltered before washing. The filter is then detached and left in the crucible, which is wiped outside with a clean moist cloth and placed in the oven. With moist preci itate in the crucible, drying should be started below 100’ C. to algw the moisture to be driven off slowly without spattering.
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cm.1
2
3
4
5
FIQURE2
The sise and form indicated have finally been evolved as a more convenient carrier for the boats and other small containers of analytical samples than the more clumsy glass hand desiccator of Pregl. The standard small metal block fits over the depression for the crucible (Figure 3). After drying, the crucible should be handled with suitable (curved) metal forceps until after weighing. After about 15 to 20 minutes, when the crucible has cooled to room temperature, it is placed on the balance for about 5 t o 10 minutes and then weighed to one-hundredth milligram. The filter stick is then removed from the crucible and kept in the balance until needed for filtration. Combustion is carried out according to Pregl. The absorption liquid of the combustion tube is transferred quantiSatively t o the weighed crucible in the following manner: The combustion tube is either clamped in a vertical position over the crucible, or held horizontally, and 1 to 2 CC. of water acidulated with hydrochloric acid (1 to 300) are blown into the wide end from a wash bottle fitted with a fine nozzle. For convenience a length of thin rubber tubing may be attached to the mouthpiece of the wash bottle. The combustion tube is rotated in such manner as to wash the entire inside surface, and the wash liquid is then transferred to the crucible by holding the tube vertical with the capillary end in the crucible. Care must be taken not to lose wash liquid outside the crucible. Since the kiquid flows out through the capjMFry rather slowly, expulsion may be hastened by blowing into the combustion tube through an air filter to which a length of flexible rubber tubing is attached for more convenient operation. This process is repeated four to five times to ensure complete rinsing. The amount of wash
FIQURE 3
When thoroughly dried, the crucible is allowed to cool and is weighed as before. Many trials have shown that another washing with 2 to 3 cc. as before should not cause a decrease of weight of more than 0.01 to 0.02 mg.
Literature Cited (1) Benedetti-Pichler, A., 2. ana2. Chem., 64, 412 (1924). (2) Eigenberger, E., 2. angew. Chem., 68, 22 (1926). (3) Elek, A , , and Hill, D. W., J . Am. Chem. Soc., 55, 3479 (1933). “Die Praxis der quantitativen organischen Mikro(4) Friedrich, analyse, 1st ed,, Leipdg, F. Deutioke, 1933. (5) Friedrich, A., and Watslaweck, O., 2. anal. Chem., 89, 401 (1932). (6) Heller, K., and Meyer, X., Ibid., 71, 117 (1927). (7) Meixner, A,, and Krocker, F., Mikrochemie, 5, 128 (1927). (8) Pregl-Roth, “Die quantitative organisohe Mikroanalyse,” 4th ed., Berlin, Julius Springer, 1935. (9) Schwarz-Bergkampf, E., 2. anal. Chem., 69, 327 (1926). (IO) Wagner, O., 2. angew. Chem., 36,494 (1923). (11) Wintersteiner, O., Mikrochemie, 2, 14 (1924). (12) Zahnd, H., and Clarke, H. T., S. Am. Chem. Soc., 52; 3275 (1930).
t.,
R ~ P C ~ P August I V ~ D 10, 1937.
