I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
680 Table
VII. Comparison of Moving Plate M e t h o d and H i g h Streaming Velocity A r c
Bample
Found in M.P.M. but Not in R.S.V.A.M.
Found in H.S.V.A.M. but Not in B1.P.M.
The spectrum of an unknown is first examined to determine what elements are present by reference to the master standard plates. The plate is then compared visually with the standard grading plates for the elements foiind, and an estimate is made of the standard to which each element intensity in the unknown most nearly approximates. Each standard has been assigned an arbitrary grading desigoation for report purposes, aa shown in Table V. An element graded as being nearer the 0.01% standard than to any other standard could be reported as approximately 0.01% or as f. An element judged to be about midway between 0.01 and 0.1% could be reported aa approximately 0.05 or aa f-w. I t is the general practice in this laboratory to report the symbol rather than the approximate percentage. To report an actual figure in per cent, even though modified by the word “approximately”, may be misinterpreted by some user of the results ae an exact quantitative result. There are several known sources of error. An effort is made to maintain uniform excitation conditions, plate sensitivity, and plate processing conditions, but these conditions are variable. The photographic errors are particularly serious. Differences im the major constituents of the sample introduce variations in the intensity of the spectra, which cannot be completely controlled or compensated for by any practical methods. The analytical reaulta on five samples selected at random from samples upon which both qualitative and quantitative analyses have been made are listed in Table VI. Two zinc-base, two copper-base, and one iron-base samples are included. These results indicate the degree of reliability to be expected from the semiquantitative indications given by this system of qualitative analysis. The average time required for the identification and grading of a qualitative plate is 20 minutes. MOVING PLATE METHOD
Occasional use is made of another technique designated aa the moving plate method. No claim is made for originality in using this technique but the. results obtained thereby in qualitative analysis are worthy of being recorded. Using simple counter-type graphite electrodes, the sample is volatilized with the direct current arc and the spectrum recorded in successive increments of exposure Over the entire 5- to 1 5 minute period which may be required to volatilize the entire ample. The author’s practice is to photograph a spectrum 2 a m . wide for each 30 seconds of exposure, moving the plate 2 mm. vertically every 30 seconds during the total arcing period. This technique results in a series of spectra showing the elements being volatilized during each increment of the arcing period. This sometimes gives greiiter sensitivity in detecting some elements, particularly those elements which volatilize over a short period at some portion of the arcing period, probably bccause the increment of spectrum exposure recorded at the time the particular element is being volatilized is not overexposed with gencral background. The results shown in Table VI1 indicate that the moving plate method is more likely to detect all the elements presriit than the high streaming velocity arc method. The additional elenieuts found, however, are tmw c 4 ~ ~ 1 1 1*c u t
Vol. 16, No. 11
giving very faint spectra. The moving plate method is much more timeconsuming and is pot used as a matter of routine unlese circumstances indicate the need for the very beat sensitivity and the most complete analysis. In grading a plate, the practice is to search each spectrum for each of the 53 elements. The intensity grading, for each element found, is made on the increment of exposure showing this element most strongly, using the same standard grading plates as those used for the high streaming velocity arc. No justification for the validity of relative conceiitration gradinge made in this manner can be made except to say that the gradi n g are generally in fair agreement with those made by the high streaming velocity arc method. ACKNOWLEDGMENT
Appreciation is expressed to M. L. Fuller for his painstaking care in the review of this work and for the preparation of the manuscript.
LITERATURE CITED (1) Haslsr, M. F.,J . Optical SOC.Am., 31,140 (1941). (2) Haler, M. F., and Harvey, C. E.. IND.ENO.C H ~ MANAL. ., ED., 13, 640 (1941).
Alundum DWIGHT WILLIAMS
Gas AND
Diffusers
GEORGE S. HAINES
Research Department, Westvaco Chlorine Products Corporation, South Charleston, W. V I .
AVERY
convenient and inexpensive gaa diffuser for labor& tory use can be made by sealing ordinary Pyrex glass t u b ing to Alundum extraction thimbles. Despite the fact that Alundum has about twice the expansion coefficient of Pyrex, satisfactory seah can be made between the two materials aa shown in Figure 1. The large sizes are made by blowing an enlargement in the tubing of desired size and butt-sealing this to the thimble. The smallest size is most conveniently made by lapsealing the glass tubing to the thimble. The resistance of this type of diffuser is indicated by the data in Table I, showing typical pressure drops across the various size thimbles when passing 1000 ml. of air per minute through water. The Alundum thimbles are available from the Norton Company, Worcester, Mass., for about 50 cents each. The Norton Company recommends Alundum diffusers for use in acid but not in alkaline solutions. ACKNOWLEDGMENT
-
0
sco~e,cm.
Figure 1
Table I.
The authors acknowledge the assistance L.A. Bednell, who did the necessary glass blowing, and the permission of Westvaco Chlorine Products Corporation to publisli this note.
of
Pressure Drop across Alundum Diffusert (Air rate 1000 ml. per minute) Uiffuer .Pressure P
,-Dimensions, .Mm. 19 x 90 16 X 70 15 X 90 6 X 32 0 X 32
Deacriptiou No 6530 RA98 No‘ 1170b RA98 No‘733R ha98 No’8133’ R.408,Randed No: 8133: RA98
i f m . of 89
02 107 130 162
dp
