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Vol. 14, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
fected b y temperature changes of *3O C. for pressures of nitrogen dioxide below 10 mm. It will be observed that the logarithm of the transmission is not a linear function of the partial pressure of nitrogen dioxide. The linear relation is to be expected only for strictly monochromatic radiation or for the case where the absorbing species has a n absorption coefficient that is constant over the whole spectral band used for analysis. Dixon (3) has shown t h a t the absorption coefficient of nitrogen dioxide changes rapidly with wave length, so that the nonlinearity of this function is t o be anticipated. The limiting slopes (at high transmission values) of the curves of Figure 3 lead t o the following limiting absorption coefficients, k: Filter I
I1
I11
capable of reaction. It is applicable to other systems for determining the partial pressure of nitrogen dioxide. An accuracy of *0.05 mm. in the measurement of the partial pressure of nitrogen dioxide can be obtained where the filters are used under the following conditions: Filter
P N O ~(Mm.
Product of H g ) X I (Cell Length, Cm.)
I 1 t o 90 I1 46 t o 130 I11 110 to 2205 a Accuracy with filter I11 is k O . 1 mm. in the measurement of the partial pressure of N0z for p X I products between 220 and 460.
T o attain this accuracy, however, it is advisable to calibrate the cell and filters because of the possible variation in the transmission of the commercial filters.
k 0.0089 0.0057 0.0014
log,, I l l 0 X 7 1 , where k is calculated from the relation k = - ?) p is the partial pressure of nitrogen dioiide (reduced to 0' C.) in millimeters of mercury, and 1 = 22.3 cm., the cell length. The authors have used the procedure described t o follow the nitrogen dioxide partial pressure in various gas mixtures
Literature Cited (1) Bodenstein, M., Z.physik. Chem., A100, 68 (1922). (2) Clifford, P.A.,and Brice, B. A., IND.ENG.CHEM.,ANAL.ED., 12, 218 (1940). (3) Dixon, J. K., J. Chem. Phya., 8, 157 (1940). (4) Harris, L.,and Siegel, B. M., J . Am. Chem. SOC.,63,2520 (1941). (5) Miiller, R.H., IND.ENG.CHEM., ANAL.ED., 11, 1 (1939). (6) Willey, E. J. B., and Foord, 9. G., Proc. R o y . SOC.,A135, 166 (1932). CONTRIBUTION No. 475 from the Research Laboratory of Physical Chemistry, Massachusetts Institute of Technology.
A Cutter for Spectroscopic Electrodes E. S. HODGE, Kentucky Agricultural Experiment Station, Lexington, Ky. ANY workers using carbon electrodes for spectrographic analysis employ a small crater t o hold either a dry solid sample or a small liquid volume to be evaporated. Two devices for cutting such craters, described previously (1, a), require either a lathe of special design or a drill press capable of turning the tool and holding the carbon in line in a chuck. The tool described here is simpler than either of these, both in manufacture and in use. Its use requires neither a lathe nor a drill press n-ith a stationary chuck vhose axes must be kept aligned.
smooth and perpendicular to the axis of the rod. Any roughness should be taken off with emery paper. The uniformity of depth in the crater will be determined by the flatness and the right-angle surface of the electrode. A carbon rod is drilled by slowly pushing it into the rotating tool until the maximum depth is cut. In preparing craters on the spectroscopic carbons and special graphite electrodes a sharp drill holds its edge very well. However, because of the hardness of the special carbon spectroscopic electrodes the drills become dull and it iq desirable to have several well-sharpened drills of the desired size a t hand. Drills should be replaced after: 20 to 30 cuttings, or when the crater walls begin to break. I t is important to use a sharp drill and not to attempt t o turn down the crater wall.
The accompanying figure shows an inner sleeve which is fastened by a setscrew to an ordinary twist drill of the size of the crater to be made. The depth of the crater is regulated by the position of the sleeve along the drill. An outer sleeve for guiding and centering the electrode is fastened to the inner one. The inside diameter of this outside sleeve is the same as the diameter of the rods t o be drilled. In use the inner sleeve is fastened to the drill t o give the desired crater depth, then the guide sleeve is attached by means of a setscrew. The drill is placed in the headstock of a lathe or the chuck of a drill press or even a chuck of a polishing wheel shaft. A desirable speed is about 1200 r. p. m. The electrodes t o be cratered should be cut with a fine-toothed saw (a coping saw is excellent) in a right-angle miter guide, so that the ends are
This device has been found especially satisfactory since i t does not require that two axes be kept aligned. By using drills only for this purpose a regular shop drill press can be used without fear of contamination of the electrodes, even those of the highest purity. Khile similar devices are known to have been used for some time b y Cholak, Mankovich, and others, this is presented as a very satisfactory means of cutting the harder special spectroscopic carbon electrodes. The writer wishes to thank Fred hlangelson of the College of Engineering, University of Kentucky, for technical assistance.
Literature Cited (1) M a j o r s , X . R., a n d H o p p e r , T. H . , IND. ENQ.CHEM.,ANAL.ED., 13,647-8 (1941). (2) Myers, A. T., and Brunstett.er, B. C., Ibid., 11, 218-19 (1939). THE investigation reported here is in connection with a project of the Kentucky Agricultural Experiment Station and is published by permission of the director.
