Absorption Tube Tares in Carbon and Hydrogen Microdetermination W. 3f. M.&CNEVINAND J. E. VARNER, Ohio State University, Columbus, Ohio
THE
microdetermination of carbon and hydrogen as described by Pregl makes use of two absorption tubes, one for water and the other for carbon dioxide. Glass vessels containing lead shot are used as tares. The use of a third absorption tube as a control was described by Friedrich ( 2 ) , who used specially designed tubes that could be closed for weighing. Niederl and Niederl (presumably using Pregl tubes) have suggested the use of a third tube not only as a control but as a tare in order to overcome the effects of high humidity (5, p. 114). However, the performance of their absorption system using Pregl type tubes has not been described in the literature. The authors have studied this system extensively and find that its use makes the handling and weighing of the absorption tubes much more flexible. As a result, this part of the carbon and hydrogen procedure is no longer regarded as most subject t o error ( 3 , p. 123). Pregl-type tubes are used, since they are the simplest in design and operation. Drierite (20-mesh)is used as water absorbent and Ascarite (20-mesh) as carbon dioxide absorbent. Cotton plugs are used at either end and the stoppered ends are sealed in with Kronig’s cement. The tare tube is filled one half with Ascarite and one half with Drierite, with the Ascarite toward the exit end. The weight of the tare tube is adjusted so as to be slightly lighter than the two absorption tubes. When the tubes are put on the combust,ionline, the tare is also put on the line following the Ascarite tube, and is treated in every way like the absor tion tubes. At the balance it is placed on the right-hand pm, wiile the absorption tubes go as usual on the left. When standing overnight, the tubes are left open to the air on a rack near the combustion apparatus. Dust is kept off by covering the tubes and rack with a folded piece of paper.
Table I shows data representing the changes in weight of the absorption tubes over a period of time counting from the moment the tubes were placed on the balance. The first time noted under each experiment represents the time elapsed between placing the tube on the balance and observing its weight.
Discussion The data of Table I indicate that constant weight is reached about as rapidly as the weights can be added and the weighing operation completed. There is no pronounced tendency for the tubes to change weight, as has been found for other absorption systems (1). Thus it is not necessary to have a weighing time schedule. This makes the procedure flexible and therefore advantageous for beginners in the technique, whose working speed may be unusually slow. The greatest change noted in the water tube after 1-hour standing is +0.019 mg. For the four observations, the average change is $0.008 mg. For the carbon dioxide tube the corresponding values are noticeably less. This means that the operation of running carbon and hydrogen
TABLE I. CONSTANCY OF ABSORPTION TUBEWEIGHTSUSIXGA THIRDTUBEAS CONTROL A N D TARE COS Tube
HrO Tube Elapsed Expt.
time
Elapsed
Change
time
1
When these observations were first begun, the need for the elaborate process of wiping the tubes prescribed by Pregl (5) was questioned. It quickly appeared, as Royer (6) has also observed, that wiping did not improve the results and often resulted in a drift in the apparent weight, thus requiring a longer waiting period. When the wiping !vas omitted the tubes became constant within 2 to 3 minutes of putting them on the balance-that is, as soon as the a-eighing could be completed-and remained constant over a relatively long period of time. The only wiping is given the outside of the tips of the capillaries between the thumb and first finger of the gloved hand, and the inside with a “pipestem” cleaner. A pair of clean white cotton gloves is worn when handling the tubes. The omission of wiping niakes possible the use of absorption tubes made of Pyrex despite the statement of Niederl and Niederl (3, p. 136).
2 3
4 5 G
7
8 9 10 11
224
Change
-0.001 -0.005 -0.008
3 5 8 8 30 8
60 8 60 8
60 8 60 8 90 8 90 8 20 hours 8 20 hours
-0.001 -0.003 0.006
0.019
0.007 0,002
-0.003 0.013 0.024 -0.020
-0.038
3 S 12 24 12 64 12 64 12 64 12 64 12 94 12 94 12 20 hours I2 20 hours
-0.001 -0,002
-0.002 0,000 -0,004 0,012
0.007 0,004 0,014 0,010
0.015
March 15, 1943
ANALYTICAL EDITION
analyses may, so far as the absorption tubes are concerned, be interrupted without affecting the results. Thus it is not always necessary to have a continuous period of several hours in order to perform carbon and hydrogen analyses. This is of some importance to students whose time is likely to be interrupted. (While such interruptions do not affect the tube weights, some combustion tube fillings, especially those containing lead dioxide, must be conditioned immediately before running an analysis.) The relative constancy of the tubes over a period up to 20 hours indicates the definite absence of any trend in the weight of the tubes. It also indicates that the Pregl tubes may safely be left open to the air for several hours without protecting them with rubber caps. A great many analyses of research compounds prepared in this laboratory have been run with the absorption system described. The carbon values have agreed regularly within 0.3 per cent relative error and the hydrogen within 1 per cent relative error. However, the high hydrogen values usually associated with high temperatures and humidities (4) are still obtained with this system of tares, just as with the lead shot tares, and the proper correction must be determined using a known compound. The high hydrogen values are therefore probably not a direct result of high humidity. This system of tare requires the use of extra weights from the set up to 300 to 400 mg. to tare the continuous gain in weight of the absorption tubes. The use of so many weights as a tare has not produced a noticeable error in the results. The elimination of wiping apparently makes the use of Pyrex absorption tubes satisfactory. The omission of the
225
waiting period shortens the total time for tne analysis by a t least 10 minutes.
Summary The behavior of a carbon and hydrogen absorption system, in which a third Pregl-type tube is used both as a control and tare, is described. If the tubes are not wiped, they are constant in weight as soon as they can be weighed. Thus they may be weighed without the usual waiting period. The tubes remain fairly constant in weight up to periods of 1 hour and show only slight variations over much longer periods. Hence, a rigid weighing program is not required. The omission of wiping makes possible the use of Pregl tubes made of Pyrex. The use of a third tube as tare and control does not eliminate the high hydrogen values usually obtained at high atmospheric humidities with other absorption systems. The use of extra weights to tare the gain in weight of the absorption tubes does not introduce a noticeable error. Literature Cited (1) Clark, R. O.,and Stillson, G. H., IND.ENG.CHEW,ANAL.ED., 12. 494 (19401. \----. (2) Fri’edrich, A,, Mikrochemie, 19, 23 (1935). (3! Niederl and Niederl, “Micromethods of Quantitative Organic Analysis”, 2nd ed., New York, John Wiley & Sons, 1942. (4) Power, F.W., IND.ENG.CHEM.,ANAL.ED., 11, 660 (1939). (5) Pregl, F., “Die quantitativen organische Mikroanalyse”, 3rd ed., p. 48, Berlin, Julius Springer, 1930. (6) Royer, G. L., Norton, A. R., and Sundberg, 0. E., IND. ENG. CHEM..ANAL.ED., 12, 689 (1940).
Microdetermination of Hydroxyl Content of Organic Compounds Acetic Anhydride-Pyridine Mixture as Reagent JACK W. PETERSEN, KENNETH W. HEDBERG, AND BERT E. CHRISTENSEN Oregon State College, Corvallis, Ore.
T
HE simplest procedures for the determination of the hydroxyl content of organic compounds are those based on esterification. As yet little attention has been given to their application on a micro scale, a field in which they would be especially useful. Several macro- and semimicromethods (1, 2, 4, 6 , 7 ) employing both acetic anhydride and acetyl chloride have been described in the literature. Extensive esterification experiments with acetyl chloride hare been reported recently from this laboratory (1). Attempts to use this reagent on a micro scale, however, have led to several other limitations besides those already mentioned (1). Attention was therefore directed to a study of the esterifications with acetic anhydride-pyridine mixture. Both Peterson and West (4) and Verley and Bolsing ( 7 ) have published methods based on the use of this reagent. Stodola (6) has reduced the procedure to a micro scale. Since extensive testing of this mixture has not been previously reported, the behavior of a large number of typical alcohols and phenols treated with acetic anhydride-pyridine solutions was studied. As a result of this work a simple microchemical technique based on the use of a hermetically sealed tube has been developed in this laboratory which gives fairly satis-
I
factory results for the microdetermination of the hydroxyl content of organic compounds.
Reagents c . P. acetic anhydride, redistilled and acetate-free, kept in wellstoppered (screw cap) bottle; c. P. pyridine, redistilled and water-free; and 0.04 N sodium hydroxide, carbonate-free.
Apparatus The reaction vessel consists of a melting point tube, 3 mm. in diameter and 6 cm. in length, made from a soft-glass test tube. Three medicine droppers, for the delivery of alcohol, acetic anhydride, and pyridine, respectively, are made by drawing one end of a 6-mm. soft-glass tubing to a fine capillary and equipping the other end with a rubber policeman. Glass plungers, 1.0 mm. X 0.5 cm., are made from soft-glass rod. A microcentrifuge. Analytical Procedure Introduce 2 t,o 10 mg. of the compound into a weighed reaction tube by means of the dropper, or, in the case of solids, employ the technique described by Niederl for filling Rast tubes ( 3 ) . Centrifuge and again reweigh the tube. Using the same technique, add a proximately 20 to 25 mg. (4to 5 drops) of pure acetic anhydride {om the second dropper, recentrifuge, and weigh
