Microdetermination of Carbon and Hydrogen ADALBERT ELEK Rockefeller Institute for Medical Research, New York, N. Y.
N THE p a s t f e w y e a r s
1
used because Its greater f l e x i b i h permits ready adjustment for the conditions ~mposedby its nature. The regular heating mortar, containing decahydronaphthalene, is used to keep the temperature of the lead Ileroxide at about l90” c This arrangement has also been recommended by Roth ( 7 ) . The furnace is connected through a clock-controlled >witch, which automatically starts it at any set time and maintains operation for any desirable period of time. If the set time is properly chosen, the combustion train will be ready for use \Then the analyst commences to work; the heating of the furnace can be discontinued on nonworking days.
Experiences in the microdetermination of carbon and hydrogen with suggestions and modifications as to method and technic are described. The technic using two boats and two capillaries has contributed to the accuracy of carbon and hydrogen determinations for easily subliming or volatile substances,
s e v e r a l p a p e r s have aPwith peared ($, 5, 6Q8, personal experiences in the field of microanalytical c h e m i s t r y . Such exchanges of information are valuable and illuminating and often promote the development of new ideas. I n the course of many thousands of organic and inorganic microanalyses during the past 12 years, several unpublished modifications of the original Pregl technic have been devised in this laboratory. Much of the experience derived from 8 years of organic and inorganic macroanalytical work was found to be more or less directly applicable to the newer micromethods, and bears out the contention that a thorough competence in macroanalysis is advantageous for success in microanalysis. The laboratories of the Rockefeller Institute for hIedical Research were among the first in this country to realize the importance of Pregl’s methods and to perform microanalyses (6).
Balance For all the microanalyses made by the author a single Iiuhlniann microchemical balance has been employed n-it11 complete satisfaction, and beyond periodic cleaning by the sole user has never required any repair. I t is kept in a glass house (locked after n-orking hours) and adequate precautions are taken to reduce Tibration to a minimum. When in use, the zero point is adjusted before commencing n-eighing instead of calculating the results n-ith a shifted zero point. The same zero point must be maintained throughout any single det’ermination. Tiedcke ( 8 ) ,among others, has stressed this point.
The Combustion Train The usual Pregl pressure regulator is used. A large U-tube is inserted at its distal end; to absorb traces of carbon dioxide and moisture and to protect the bubble counter, it is filled with Ascarite and “indicating” Drierite. Drierite is more convenient than calcium chloride, phosphorus pentoxide, or Anhydrone because the distinct color change readily shows the condition of the absorbent, while it does not deliquesce like hydrated calcium chloride, which often blocks the passage of gas. It. can be u e d t o the last granule and does not have to be preheated. The comhustion tube employed is equipped with a side arm and is made of Supremax glass as described by Pregl ( 7 ) . The tube filling follows the recommendations of Pregl, except that it has been found advantageous to use a fine-meshed, pure silver gauze instead of silver thread. This is cut to appropriate length and rolled tightly until it is sufficiently thick to fit snugly inside the combustion tube. It is reduced in hydrogen in the usual manner before use. The eilver gauze has a larger surface area and is also easier to handle. Since 1926 a simple electric heater made in this laboratory has been uyed instead of the long gas burner. It consists of an alundum tube (7 inches long and 0.625 inch inside diameter) on which is wound Nichrome wire. (That part of the combustion tube which is within the heating unit is wrapped with asbestos paper.) llround it is alundum cement, then asbestos paper, both cement and asbestos being thick enough to prevent the radiation of heat. This heater is connected through a rheostat to regulate the temperature (between 650” and 700” C.). Alongside the combustion tube within the heater lies a thermocouple which is connected to a pyrometer. Recently, however, the heater has been equipped with a pyrometer and automatic control unit. The electric heater ensures a uniform temperat,ure all around the tube without distorting it throughout its life, which was not true of the gas burner. For the combustion of the sample a gas burner is
Absorption Tubes Much has been done in attempts to develop a satisfactory technic for the manipulation of absorption tubes. The experiences in this laboratory support the findings of Hayman (1) and Hernler ( 2 ) concerning the importance of keeping the humidity about 50 per cent to dispel static charges. Other factors enter to a greater or lesser extent, hovever. S o t the least of these is the differing susceptibility of various types of glass. Many different glasses TTere tried; the most suitable was Jena thermometer glass. By using tubes made of this glass, and as a further safeguard by grounding the balance, difficulties arising from static charges were satisfactorily eliminated. Katurally, the absorption tubes must be cleaned and handled meticulously. The capillary ends of the ahsorption tubes are made from special uniform-bore (0.2-mm.) capillary tubing and then sealed to the body of the tubeq. This ensures greater regularity than i j practicably attainable when the ends are dram n out from the body of the tube in the usual manner.
Oxygen It has been a general complaint that the figures obtained for hydrogen are often too high-sometimes as much as 0.5per cent. Early experience with macrocombustions showed that this condition could be traced to varying amounts of hydrogen in the oxygen drawn from the cylinders. The oxygen had been prepared by electrolysis, and the hydrogen present as impurity was presumably derived from incomplete separation of the gases a t the electrodes. By using oxygen manufactured from liquid air this difficulty was readily overcome in macrocombustions and, carrying over this experience into the microtechnic, high hydrogen results have not been encountered in this laboratory. The small amount of nitrogen present as impurity in liquid-air oxygen is unimportant. It is, however, important to possess personal cylinders which are never t o be used for the storage of electrolytic oxygen. The less experienced must naturally learn to insert the sample into the combustion tube with the minimum of time in order to avoid exposing the tube unduly to the room air.
Methods and Technic TECHNIC OF THE T w o BOATS.Substances which sublime readily must be burned very slowly with a small flame located 3 to 4 cm. away from the boat. On one occasion, a pyrazine derivative sublimed so readily and traveled through the tube so rapidly that, even with the greatest precautions, the com-
INDUSTRIAL AND ENGINEERING CHEMISTRY
52
bustion was incomplete and the value obtained for carbon was low. There had been reported in the literature ( 4 ) just such difficulties with this type of compound. The following procedure has been utilized to solve them : The substance is weighed in an ordinary platinum boat and sprinkled with fine, previously heated copper oxide. Another, somewhat smaller, platinum boat is inserted into the former, so that the handles are at opposite poles. By conducting the combustion slowly, accurate results have been obtained with such compounds. This arrangement has been termed the “technic of the two boats.”
TECHNIC OF THE T w o CAPILLARIES. During the last few years several thousand samples of organic liquids, having a large variety of structure, have been analyzed. Some could be burned in the usual way, while others were very volatile. The following procedure has proved useful: The sample is weighed in a regular capillary. The tip is broken off and the capillary is placed in another somewhat larger capillary about 1 cm. long which is closed at one end. The larger capillary is filled with copper oxide or, if the substance is a halide, with
precipitated and previously dried silver. These two capillaries are placed in a suitable long platinum vessel. This arrangement has been termed the “technic of the two capillaries.”
VOL. 10, NO. 1
Certain organic halides (especially bromides and iodides) containing a high percentage of halogen yielded rather high values for carbon despite the rolls of silver gauze in the combustion tube. The additional silver placed in the larger of two capillaries by means of the technic described above absorbs most of the halogen and lengthens the life of the silver gauze. The two capillary arrangements also diminish the speed of vaporization of the volatile liquid sample; hence a more complete combustion can be expected.
Literature Cited Hayman, D. F., IND. ENG.CHEM., Anal. Ed., 8, 342 (1936). Hernler, F., Mikrochemie, Pregl Festschrifi (1929). Kirner, W. R., IND.ENQ.CHEM.,Anal. Ed., 5, 363 (1933). Kolb. A.. Ann. Chem.. 291. 276 (1896). ~, Niederl, J. B., IND.ENG.CHEY.,Anal. Ed., 7, 214 (1935). (6) Niederl, J. B., and Roth, R. J., Ibid., 6, 272 (1934). (7) Pregl, F., and Roth. H., “Quantitative Organic Microanalysis,” 3rd English ed., Philadelphia, P. Blakiston’s Son & Co.. 1936. (8) Tiedcke, Carl, iWikrochemie, 16, 171 (1934). (1) (2) (3) (4) (5)
RECEIVED October 15, 1937. Presented in part before the New York-New Jersey Section of the Microohemical Society, April 11, 1937.
Turbidimetric Titration of Small Amounts of Nicotine By the Use of a Photoelectric Cell LYLE D. GOODHUE Bureau of Entomology a n d P l a n t Quarantine, U. S. Department of Agriculture, Washington, D. C.
A n inexpensive photoelectric apparatus, including a special titration cell, is used for the turbidimetric titration of small amounts of nicotine. The unknown nicotine sample is added to an excess of silicotungstic acid and the excess of the latter is titrated with standard nicotine formate. Results that check to about 5 micrograms can be obtained. Flocculation of the precipitate is prevented by the addition of Irish moss extract, and the tendency to crystallize is retarded by using formic acid instead of hydrochloric acid as in the analysis by the gravimetric method.
C
ERTAIX fumigation and dusting experiments in progress a t the laboratory of the Division of Insecticide Investigations required a rapid method for the determination of small quantities of nicotine. A survey of the numerous available methods, which include gravimetric, nephelometric, and colorimetric procedures, did not reveal a method of analysis without objection in one respect or another. Although not previously reported for nicotine, titration to maximum turbidity appeared to be a rapid and accurate method, if certain conditions, such as very low solubility and stabilization of the precipitate, could be realized. The use of the photoelectric cell to indicate the point of
maximum turbidity during the titration of SO4-- with B a + + was recently suggested by del Campo, Burriel, and Escolar (1). Large volumes (200 cc.) were used, however, and, since stabilization of the precipitate was not complete, a continuous stirring device was necessary. N o details regarding the apparatus or the type of photoelectric cell were given.
Experimental I n the turbidimetric titration of nicotine with silicotungstic acid, i t was found that more accurate results were obtained when the nicotine solution was added to the silicotungstic acid than when the reverse procedure was used. A deiinite quantity of silicotungstic acid in excess of the unknown nicotine sample was placed in the titration cell and the excess titrated with standard nicotine solution. The quantity of nicotine solution equivalent to the silicotungstic acid was determined by a blank titration. The end point of the titration was obtained by plotting the scale readings of the photoelectric apparatus against cubic centimeters of standard nicotine. DESCRIPTION OF APPARATGS. The circuit shown in Figure 1 employs a highly sensitive gas-filled photoelectric cell, 918, with one power amplifier, 1F4. Because of its instability, a gas-filled cell is not usually recommended for an instrument of this kind, but if a separate battery is used to supply the potential across the photoelectric cell, and the grid of the amplifier is connected with the anode, a nearly constant plate current is obt’ained. In this way the effects caused by the small changes in resistance of the vacuum tube and the photoelectric cell pract’icallyneutralize each other. The over-all sensitivity of the set can be varied within wide limits by varying the load resistance, RI. About 40 megohms have been found t o give good sensitivity without sacrificing stability. The sensitivity can be increased by using an indicator,
