Gasometer for Micro-Dumas Determinations JOSEPH G. S.ANDZ.4 Ah’D JOSEPH F. .ALICISO, Fordham University, N e w York, N. Y.
T
HE use of a gasometer in the micro-Dumas determination for standardizing the blank and enabling the analyst to establish more accurately the end of the determination has been introduced by Xiederl and eo-workers (3,4). Mercury is utilized as the confining liquid. I n this laboratory (1) carbon dioxide is supplied by a system in which the gas is stored over water. It was thought that a measuring apparatus employing the same confining liquid would be more satisfactory. Such an apparatus is described beloTv, It has the added advantage of being always under carbon dioxide pressure of the main gas supply. Once set, the gasometer is aln-ays ready to deliver a measured volume of carbon dioxide, since one chamber is automatically filled a t all times.
Discussion Gas is delivered from the measuring apparatus a t approximately the same pressure as if it were delivered directly from the reservoir. Since the system is completely closed t o the atmosphere, n-ater can be used as the confining liquid. -4leveling bulb is therefore unnecessary, because the gas is never supporting more than about 60 cm. ( 2 feet) of water (in the reservoir). The change in pressure during emptying of the reservoir
Description of Apparatus The apparatus is constructed of tn.0 32 X 200 mm. test tubes, two three-way, 120”, elbow-bore stopcocks (C and D),a piece of glass tubing, and two three-hole rubber stoppers, as shon-n in Figure 1. The lover test tube is filled about three-fourths full with boiled Lvater, the apparatus is assembled, and the stoppers are lacquered.
FIGCRE1. DIAGR.UIO F ,IPP.~R.ATCS Sumbered a r r o w indicste p w l t i o n of elbow bore i n stoncock.
Method of Procedure Air is flushed from the system by allowing carbon dioxide to flow through the apparatus rvith the stopcocks first in positions 1 and 2, then in positions 3 and 4. Flushing can also be accomplished by alternately filling and emptying each chamber as in actual operation. Once flushed adequately, the apparatus need not be flushed again. As shown in the figure, the upper tube is ready to deliver the measured volume of gas. With the stopcocks in positions 1 and 3, gas is permitted to flow from the carbon dioxide supplv, forcing the water in the lower t,uhe from level A to level R. There is a corresponding movement of the m t e r in the upper tube from B’ t o A‘, displacing the measured volume of gas. The distance A B is marked on either the upper or lower tube (or both), and represents the desired amount of gas (ca. 50 ml.). The next measured volume of gas is taken from t,he lower tube (which has become charged during the emptying of the upper tube) by turning the stopcocks to positions 2 and 4. During this operation t,he upper tube has become charged. An intermediate position of either stopcock serves to stop the flow of gas cornplet,ely. In an idle period, stopcock C should be closed to prevent diffusion of air into the gasometer. For the complete standardization of the Dumas method it \vas decided that not only the flushing period during the actual determination should be fixed, but also the initial flushing prior to combustion. Since the gasometer is used principally t o avoid the necessity of judging the size of poorly defined “microbubbles” whose size varies according t o the bore of the inlet tube of :he azotometer (Q, n-e should depend on it also for the initial flushing. After the introduction of the sample, 1.5 measures of gas (78 mi.) are sent through the cold combustion tube t o discharge the air. The tube is heated for 2 minutes while part of the remaining half measure is slo~vlypassing into the azotometer. Excess carbon dioxide from this half measure is discharged into the atmosphere, except for the last few milliliters, m-hich are sent into the azotometer. Stopcock C is closed (by turning t o an intermediate position), the azotometer is refilled, and the combustion is stal’ted. In this manner the error is incorporated entirely in the blank.
TABLE I. Hippuric acid
Acetanilide
‘4BSOLVTE BL.kNK
Observed 0.583 0.522 0.488 0,418
Calculated 0.563
0.776 0.542 0.431 0.345
0.765 0.524
0.505
0.470 0.400
0.403 0.327
DETERMINATIONS
Difference 0.020 0.017 0.018
-0.8%
- 0.004 0.005 0.004 0 . 0 1 8 - 0.003
0.021 0.018 0.018
- 0.006
- 0.004
- 0.003 0,018 - 0.003
Absolute Blank 0.015 0.013 0.014 0.015
0.015 0.014
0.016 0.015 Av. 0.0143
then causes a change of about 3 ml. in the volume of gas delivered. If the apparatus is calibrated for the average head of water in the reservoir, the volume delivered will vary only *1.5 ml. This variation does not interfere in the Dumas determination, Loss of water from the gasometer is negligible, since the gas from the reservoir is saturated with wat’er a t room temperature. The measuring apparatus has been in use in this laboratory for the past year. Absolute blank determinations, made as indicated by Niederl (a),give an average of 0.014 =t0.001 ml. (five determinations), This blank is confirmed by actual test runs on very pure substances. After subtracting the corrections for vapor pressure and adhesion of potassium hydroxide (0.8 per cent, d ) , the difference between the observed and calculated volume of nitrogen represents the absolute blank. Results obtained from analysis of hippuric acid and acetanilide are given in Table I.
Acknowledgment The authors wish to thank Francis W. Power, S. J., for help and advice in this work.
ANALYTICAL EDITION
DECEMBER 15, 1940
Literature Cited (1) Hamill, W.H.. and Alicino, J. F., ISD. EXG.CHEY.,Anal. Ed., 9, 290 (1937). (2) Kiederl, J , B,, and xiederl,Victor, 461\Zicroh.Iethods of Quantitative Organic Analysis", pp. 74-5, New Tork, John Wiley & Sons, 193s.
777
(3) Kiederl, J. B., Trautz, O., and Saschek, W., Mikrochemie, Emich Festschrift, 219 (1930). (4) Trautz, O., Mikrochemie, 9, 300 (1931). ( 5 ) TTeyland, C., "Quantitative analytische hlikromethoden der
organischen Chemie in vergleichender Darstellung", 94-8, Leipzig, Akademische Verlagsgesellschaft, 1931.
pp.
Adapting Polarizing Microscope for Use as a Polarimeter .4LEX.lKDER 3IARION, Queens College, Flushing, N. Y .
T
HE polarizing microscope can be adapted for use as a polarimeter by the addition of a simple analyzer constructed from a few square centimeters of Polaroid. The addition of such an analyzer greatly increases the versatility of the polarizing microscope, which is a more common laboratory instrument than the polarimeter. A primary advantage of the microscopic method is the small quantity of sample which will suffice to fill the specimen tube, only 150 cu. mm. being necessary. The analyzing unit consists of a metal frame which can be attached firmly to the graduated stage hy means of a knurled machine screw, ordinarily used in fastening a mechanical or Federoff stage to the instrument. The height of the frame must be selected so as barely to clear the top of the cell. The two small pieces of Polaroid are located beneath a hole dr!lled in the metal frame concentric with the optical axis of the microscope. The sections of Polaroid are cut so that their planes of polarization include an angle of approximately 5' when the segments are mounted in place with a slight overlap. The cell is essentially a length of 2-mm. glass tubing cemented in a hard-rubber rod and then fastened to a microscope slide for easy manipulation. The height is selected so that the rack and pinion gears of the microscope adjustment are engaged and allow focusing, taking into consideration the thickness of the cover glass which is on top. The frame is bent from a strip of aluminum ( 2 . 5 em. viide), so that the distance between the microscope stage and the top of the frame is 5.25 cm. This provides sufficient clearance for the cell which is 5.15 cm. overall and has an effective cell length of 5.0 cm. B 5-mm. hole is bored in the frame and the Polaroid fastened beneath.
In use, the microscope is focused upon the slightly overlapping intersection of the two pieces of Polaroid. This junction should approximately bisect the field and when the stage is rotated the mid-point of the intersection should remain in the center of the field. Coupled with the manner of mounting of the Polaroid, this procedure gives a field roughly halved, in which the intensity of the light will be uniform only at the zero point.
A magnification of X 100 is sufficient to make the end of the tubing cover the entire field. Higher magnification serves no useful purpose, since it complicates the adjusting of the microscope and does not produce any refinement in procedure. Best results were obtained when a compromise plane of focusing was selected midway between the Polaroid and the top level of the liquid in the cell. This caused the junction of the polarizing films to become indistinct but brought the end of the cell more closely into focus. It is for this reason that it is recommended that the metal frame be constructed to be as close as possible to the top of the cell, allowing foi the cover glass, and that the Polaroid be mounted on the underside of the frame.
TIBLE I. RESCLTS OBTAISEDKITEX .\PPAILIT~-S Dextrose, 2ZCc Dextrose, 16.75i Dextrose. 12.5% Dextrose, 8 . 3 % Levulose, Z5CG lIaltose, 2 5 5 llaltose, 25Yc Xaltose, 12 5% Sucrose, 2.55
Calculated 6' 30' 40 25'
;:;;:
Observed R O 32' 4; 22' o:
1:
Error, 0 /C 1.8 1.1
8.5 2.3
110 30'
110 3s'
2.6
1 7 0 15' l i o 15' S o 38'
16' 19' 16' 15' 51 46' 100 13'
5 4 5 4 1 9
100 23'
2.9
Armsement of Polproid Sections
YOW
f ram
Hicroscope r%e
The solut'ion can be made u p in a 1-ml. volumetric flask by weighing out 100 to 250 mg. of the solid on the analytical balance and dissolving in sufficient water to make the proper volume. To transfer the solution, a portion is sucked up in a capillary tube, the tip of xhich is then placed in contact with the bottom of the cell, and the liquid is carefully expelled. As the cell is filled, the capillary is slowly withdrawn, always keeping the tip below the surface of the liquid. Advantage is taken of surface tension to create a hill of liquid above the top of the cell. The cover glass is slipped on top and the excess liquid absorbed by a piece of filter paper. This draws the cover glass tightly against the top of the cell and also ensures the complete filling of the cell. I t is important to obtain the volume of liquid entirely free of air bubbles; a suitable check is to hold the cell directly at a light, when a translucent disk of light free of any dark spots will indicate complete filling. To obtain the zero point, the cell, filled with distilled water, was manipulated until it was in the field of view and the stage rotated until both halves of the field were equally extinct. Monochromatic light was obtained from a sodium vapor lamp; the intensity of this light was regulated by the substage iris until the zero point illumination was sufficient t o provide a sharp zero point. A simplification in procedure resulted when the zero point found using the water cell was identical to that determined without any cell. This suggested the feasibility of dispensing xith the cell in locating the zero point. By this method the cell need be filled only once with the unknown solution, and dilution errors 11-ill be minimized.
