Handling and Weighing Absorption Tubes in Microdeterminations of Carbon and Hydrogen DOUGLASS F. HAYMAX, Merck & Co., Inc., Rahway, N. J.
T
HE standard method of handling and weighing absorption tubes in microanalysis is that of Pregl (14). This
to be determined by changes in the temperature, by the percentage of moisture in the air, and by the length and diameter of the constrictions in the ends of the tubes. Lieb and Soltys (11) have demonstrated the importance of constrictions of the proper size. I n this laboratory absorption tubes with constrictions 0.15-0.20 mm. in diameter and 4 mm. long have always been used. Both tubes were wiped and placed on the rack beside the balance, the water tube being first, and were then weighed in the same order. The time interval on each tube between completion of its wiping and its final weighing was about 4 minutes. The water-absorption tube had a tendency to pick up weight over an indefinite period of time, especially if any static charge was present. With constant temperature and relative humidity about 45 per cent the increase over a 2minute period a t the time of weighing was between the limits 0.000 and 0.005 mg. The ascarite tube was exceptionally constant if there was no static charge present; the increase in weight over a 2-minute period a t the time of weighing was between the limits 0.000 to 0.003 mg. With constant temperature and humidity above 45 per cent and no static charge on the tube the weight was constant for as long as 5 minutes. It has been found that consecutive weighings taken a t the exact time interval during each routine analysis have given analytical results which check well within the limits of accuracy as shown by Table I. About 10 minutes’ time for each analysis has been saved, which has been found to be an important item in routine analysis.
method was an excellent beginning. However, in an effort to remove possible errors, various workers have developed improvements of definite value, mainly along two lines: t o find better absorption agents, and to produce a tube which could be closed to the atmosphere. The original absorption agents, calcium chloride and soda lime, have been almost entirely replaced now by magnesium perchlorate (8, IO, 12) and ascarite. The anhydrous granular magnesium perchlorate has almost double the moistureabsorbing capacity of the trihydrate salt and is the preferable form to use. In an attempt to make it possible to close the tube to the atmosphere, tubes have been made, the ends of which could be closed by mercury seals (6, IO), by hollow ground-glass stoppers (2, 6, 7 ) , and by wires (4, 13). A recent development has a special absorption tube, designed and described by Friedrich (8), in which a single stopcock closes both the entrance and the exit to the tube. The method of handling absorption tubes used in this laboratory has been developed during the course of making several thousand carbon and hydrogen determinations, and although there may be few separate operations which are new, the method, as a whole, has been found to be a distinct improvement over the regular method given by Pregl (14). The principal difficulties encountered in this laboratory in handling and weighing absorption tubes have been (1) lack of constancy in temperature, (2) long conditioning time of Pregl tubes beside the balance, (3) the certain removal of lint and dirt from glass surfaces, and (4) elimination of static charge from glass surfaces.
Cleaning and Wiping The method of cleaning and wiping the tubes has been modified to make the above weighing system possible. The absorption unit was removed from the combustion train and 50 cc. of air were pulled through it to remove the oxygen, as described by Niederl and Roth (12). After washing the hands in cold water, the separate absorption tubes were wiped from end to end with a freshly washed flannel from which a few drops of water could still be squeezed. The tube was immediately wiped by the first set of soft chamois skins with a very few soft strokes, until the chamois glided smoothly over the surface. All strokes had to be partly rotatory and from the center toward the ends only, never in the reverse direction. The open ends of the tube were then cleaned with a tuft of cotton on a wire, and this treatment was followed by a second wiping with a second set of chamois, when even more gentle treatment had to be given in order t o prevent the formation of static charges. Following this wiping it was found that any foreign materials remaining on the glass surface could be easily seen by viewing the tube against a narrow beam of light from a dark background. For this purpose a simple apparatus was constructed as follows: A 60-watt incandescent bulb as a source of light was placed in a tin box 12.5 X 15 X 12.5 cm. (5 by 6 by 5 inches), The front of the box contained an opening on which a ground-glass panel was mounted. The glass panel was entirely covered by a dull black opaque paper, except for a slit 2 mm. wide by 12 cm. long and a second opening about 2.5 cm. (1 inch) lower, which was masked from view
Constancy of Temperature Constant temperature proved to be an important factor in both handling and weighing of absorption tubes. A constant-temperature room was to be desired but, if sudden fluctuations were avoided, a gradual change in temperature did not seem to affect the weighings noticeably. The balances in this laboratory were placed in a separate room which opened into the main laboratory. This room had inside walls only, with no ventilation except through the doorway which was kept open to the main laboratory. The temperature in this room fluctuated very little but still tended to be near that of the outside room. There were no drafts. A constant zero point on the balances was obtained, except on abnormal days when the outside temperature variation was very large.
Conditioning Time The regular tubes as designed by Pregl (14) have been chosen as the most practical absorption apparatus, for the tubes are of such simple construction that they are easily prepared for weighing. The main criticism advanced against the Pregl tubes has been the fact that they are open to the atmosphere from 10 to 20 minutes during conditioning beside the balance. In this laboratory the tubes picked up weight during this period of time. The amount of increase was found 342
SEPTEMBER 15. 1936
ANALYTICAL EDITIOX
by an upturned hood of black paper so that light was thrown up on the tube while the source was hidden from the eye. This box was fastened on the wall, with the light opening in a horizontal direction. I n the final wiping, the tube was held up within a few centimeters of the slit, so that the operator could not see the light directly. Any lint, fiber, or dirt which might still be clinging to the tube appeared in sharp relief against the black background, as the light from the opening passed over the top surface of the tube. Every piece of foreign material of any weighable size was easily seen and removed, so that this source of error was definitely removed. Rewiping and reweighing of tubes were found to be superfluous after this treatment. As an added precaution, the tube was stroked with a camel’s-hair brush before being placed in the balance, special care being given to the tip end which was last touched with chamois.
Elimination of Static Charge
343
A daily record of the relative humidity and degree of static trouble showed vividly that the lack of moisture in the air was primarily responsible for this static trouble. With cold clear winter weather the relative humidity was often below 40 per cent which was the lowest relative humidity a t which static charges were absent. A successful method of static elimination was found to consist simply in keeping the atmosphere in the room a t or above 45 per cent relative humidity, by allowing a flow of steam from a steam bath to raise the moisture content of the room to a relative humidity of 45 per cent or above. A steam valve permitted ample adjustment to keep the humidity quite constant. TABLEI. RESGLTS OF 54 ANALYSES TAKEBFROM THREE PAPERS (1, S, 15) Average Deviation from Theory C H 0 125 0,123
Average Deviation in Checks C H 0 099 0.144’
Tendency
C +0.06
H -0
01
Would have been 0.126 if one widely divergent check had been omitted where t h e results were on both sides of theory. a
In this laboratory the author has had a great deal of trouble with static charge on the surface of the tubes, which has been found to grow progressively worse as the relative humidity decreased. Hernler (9) reported better results a t high relative humidity than low, but did not mention static charge. In this laboratory, above a relative humidity of 45 per cent little static was noticed, but below 40 per cent the static charge became severe. The charge was frequently so great that even with care in wiping and discharging, the pan of a Kuhlmann balance was rotated on its axis by the approach of the absorption tube to be weighed. The weighing in this case, needless to say, was so disturbed that the first reading was often in error by as much as 1 mg., the observed weight being less than the true weight in general. At the end of a 15-minute wait this static charge was usually dissipated and a fairly accurate weighing obtained, but since the weighing was desired a t the completion of wiping, it was necessary to prevent static charges. A charged tube was detected in two ways. If a tube surface was charged the lint or fibers would stand perpendicular to the surface of the tube. This phenomenon was especially noticeable with the above-mentioned apparatus for viewing the absorption tubes. The second test for static charge consisted in bringing the tube up to a pith ball suspended on a silk string about 15 cm. (6 inches) long. If there was a heavy charge on the tube, the pith ball would swing through an arc of 5 or 6 cm. (2 or 3 inches) to contact the absorption tube, although occasionally the charge would be repulsive. This charge was especially noticeable on the surface of glass which covered the dry magnesium perchlorate, but was also heavy a t the tips of the tube. The first method of eliminating static charges studied, involved their removal from the surface of the tube. The balance, as well as the rack on which the tubes rest before weighing, was grounded. Cranium acetate and various radioactive ores were placed in the balance case, but none g y e any material dissipation of the charge on the tubes. The static could be discharged by touching the tube in several positions with the fingers, if the charge was only slight, but a heavy charge could never be removed in this manner. Another procedure to ensure a tube free from charge was to prevent its being built up on the glass surface. This was accomplished by wiping as previously outlined, with the added precaution of using chamois that had been kept in a covered dish on a rack above a shallow layer of water. I n extremely bad cases of static charge the chamois were moistened in a blast of steam before using, so that they felt slightly moist. A careful wiping with these chamois gave a clean tube that was free of static.
The analyses of Table I were made over a period comprising every season of the year. From these and other data compiled in this laboratory it has been found that there is no appreciable seasonal variation except when the static is not properly taken care of in the winter, when results tend to be high. That the high humidity found in the summer months has not affected the weighings in any manner, speaks well for the elimination of the rest of the absorption tubes beside the balance for the standard 15 minutes.
Summary B method of handling and weighing absorption tubes has been developed which includes the following features: (1) location of the balance in a room kept at a fairly constant temperature and where sudden changes in temperature were never observed; (2) use of Pregl absorption tubes, since they are the simplest tubes to clean; (3) weighing tubes immediately after wiping to eliminate the necessity of closing the ends of the tubes; (4) saving about 10 minutes on each analysis; ( 5 ) use of a special background for viewing absorption tubes during cleaning; and (6) elimination of static charge on the glass surface, by proper wiping and, still more important, by maintaining the relative humidity of the laboratory at or above 45 per cent.
Literature Cited (1) Addinall, C. R., and Major, R. T., J . Am. Chem. Soc., 55, 2153
(1933). (2) Blumer, F., Ber., 50, 1710 (1917). (3) Boese, 4.B., Jr., and Major, R. T., J . Am. Chem. Src., 56, 949
(1934). (4) Boetius, M., “cber die Fehlerquellen bei der mikroanalytischen Bestimmung des Kohlenstoffes und Wasserstoffes nach der Methodevon Fritz Pregl,” p. 27, Berlin, Verlag Chemie, 1931. (5) Cornwall, R. T., IND.ENG.CHEM.,Anal. Ed., 3, 4 (1931). 16) Flaschentmner. B.. Z. anaew. Chem.. 39, 720 (1926). i7) Friedrich, A:, Makrochemie, 10, 329 (1931). (8) Ibzd., 19, 23 (1935). (9) Hernler, F., Ibad., Emachfestschrzft, 148-51 (1930). (10) Kemmerer, G., and Hallett, F., IND. ENG CHEM.,19, 173 (1927). (11) Lieb, H . , and Soltys, A4., Mikrochemae, 20, 59 (1936). (12) Niederl, J. B , and Roth, R. T., IND.ENG. CHEM.,Anal. Ed., 6 , 272 (1934). (13) Niederl, J. B., and Whitman, B., Makrochemie, 11, 287 (1932). (14) Pregl, F., and Fyleman, E . , “Quantitative Organic Microanalysis,” 2nd ed., pp. 40-9, 76, Philadelphia, P. Blakiston’s Son & Co., 1930. (15) Wallis, E. S., and Fernhole, E., J . Am. Chem. SOC.,57, 1504 (1935).
RECEIVED April 28, 1936
