Spectrographic Limit of Identification of Potassium DANIEL P. NORMAN AND W. W. A. JOHNSON New England Spectrochemical Laboratories, West Medway, Mass.
I
T IS well known (6) that spectrographic tests are of high
taining potassium as an impurity were weighed direct1 into the cathode, Potassium salts were dissolved in distilcd water (which had been found potassium-free) and 0.1 ml. of solution of the appropriate concentration was placed in the cathode, which was then dried at 110" C. The loaded electrode was replaced in the arc stand, the spectrograph shutter opened, the arc started, and the spectrograph plate-holder racked down every 40 seconds until the sample was completely consumed. The exposure time used was the optimum exposure dictated for the authors' spectrograph by the twin opposing factors of maximum exposure for faint lines and minimum exposure for the background to permit maximum visibility for faint lines. In general, the bulk of the potassium was volatilized during the first exposure.
sensitivity for elements in the first two columns of the periodic table and of low sensitivity for elements in the last few columns of the periodic table, and ( 2 , 4, 7 , 9) that the intensities of the emission lines of any element are a function of the spectrographic and photographic techniques uaed, the matrix in which the element appears, the anions with which it is associated (S),and the intrinsic brightness of its lines. There is general agreement among spectroscopists in this country that the direct current arc is the most satisfactory source to use in qualitative spectrographic analysis, and that when dealing with very small amounts of materials the lower electrode, which holds the sample, should be the cathode ( 7 , Q ) . I n the case of potassium, Steadman, Hodge and Horn, (8) have reported that they could detect from 1 to 5 micrograms in the directo current arc, using the potassium lines 4044.1 and 4047.2 A. Using these lines, however, the authors found that the spectrographic sensitivity of potassium varied by factors of several hundredfold and that, in general, the sensitivity WAS not so great as i t was reported to be. They therefore investigated the spectrographic limit of detection of potassium in the direct current arc for both the sensitive (6) doublets: x Int. x Int. 4044.14
800
4047 20
400
and
7664 91
YO00
7698 98
5000
the strongest lines of the element, arising from the lowest energy values. These lines Fill hereafter be rgferred to simply as the doublets at 4000 A. and at 7600 A. These latter lines are seldom used because specially sensitized plates are required to photograph them, and the dispersion of prism spectrographs is low in that wave-length region.
Procedure The spectrograms were taken in the$& order of a 3-meter grating spectrograph (f), dispersion 5.6 A. per mm. The 4000 A. doublet was photographed on Eastman 33 plates, the 7600 A, doublet on Eastman Spectroscopic I-N plates. Both types of plates were processed in accordance with the manufacturer's directions. When photographing the 4000 A. doublet the overlapping second order spectrum was absorbed by a Cornjng No. 738 filter placed before the slit; in the case of the 7600 A. lines by a Wratten Cine-Red filter. An enlarged image of the arc was formed on the slit of the spectrograph with a quartz lens and was so adjusted that the image ?f the cathode fell just off the edge of the Hartmann slide delimiting the slit length. The direct current arc was operated at 15 amperes (250-volt input). The voltage across the electrodes varied from 40 to 50 volts, depending on the sample being arced. The interelectrode gap was maintained manually at 3 mm. as the electrodes burned away, with the aid of an enlarged image formed on a target by an auxiliary lens. The electrodes were 0.6-cm. (0.25-inch) diameter spectrographic graphite electrodes manufactured by the National Carbon Co. In the lower (cathode) electrode a crater 5 mm. wide and 4 mm. deep was drilled by a special fixture. The upper electrode was pointed in a pencil sharpener reserved exclusively for this purpose. For each sample a fresh pair of electrodes was introduced in the arc stand, and preburned for 40 seconds to volatilize any traces of metal that might have been introduced in preparing the electrodes. Then a 40-second spectrogram was taken to record any impurities intrinsically present in the electrodes, and the cathode was removed and cooled rapidly in a metal block. Samples con-
TABLE I. LIMITOF DETECTION OF POTASSIUM K Introduced as
Limit of Detection 4044-4047
7664-7698 Micrograms 0.6 0.08 0.08 0.2 0.8 0.2 0.04 0.04
KC1 120 KOH 180 95 KaPO4 320 KMnO4 240 KzCrzOi 200 KCzHsSO; 10 NazCO1 I6 NazCzO4 40 0.04 VzOa 20 0.06 ~iaes5 a Bureau of Standards borosilicate glass, standard 93
Column 2 Column 3 200 2250 1200 1600 300 1000 250 400 1000 330
Results The limit of detection of potassium was studied for six of its salts and for four compounds containing i t as a minor impurity. The observed limits of detection are listed in Table I. The first column tabulates the form in which the potassium was introduced in the arc, the second column lists the limit of detection for the 4000 A. doubjet, the third column lists the limit of detection for the 7600 A. doublet, and the last column gives the ratio of the two limits of detection given in the preceding columns. The figures represent the smallest amount of potasaium (introduced in the form listed in column 1) which can always be detected with certainty. One half of these amounts may or may not be detected. One quarter of these amounts will usually, but not invariably, not be detected. Whenever one line of a doublet was present, the second line invariably was present. The striking variation in the spectrographic sensitivity of potassium demonstrated in Table I can be attributed primarily to uncontrollable changes in the conditions of excitation in the arc produced by variations in the constituents being volatilized, This phenomenon is most prominently shown b y the two samples whose major constituent is sodium. While the volatile sodium is coming off in the arc, very little carbon is volatilized from the electrodes and the cyanogen bands are correspondingly faint, thus allowing faint potassium lines to be seen. I n general i t m4y be said that the limit of detection of potassium at the 4000 A. doublet is set primarily by the intensity of the heavy cyanogen bands (8) rather than b y the faintness of the potassium lines. This point is further dernonstrated by the fact that where the theoretical ratio of intensities of the 7600 A. doublet to the 4000 1 .doublet is roughly 10 t o 1, the observed ratio of the limit8 of detection at 4000 A. 152
ANALYTICAL EDITION
February 15, 1943
t o t h e limits of detection a t 7600 A. (column 4, Table I) varies from 200 t o 2200.
Summary The spectrographic limits of detection of potassium in the direct current arc for the doublets 4044-4047 and 7664-7698 have been studied for six potassium salts and four compounds containing potassium as minor impurities. The limits of detection vary widely, depending on t h e form in which the sample is introduced. T h e limit of detection for the 7600 A. doublet is several hundred times lower than for the 4000 A. band, largely because the latter are masked by he%vy cyanogen bands. The limit of deteckion of the 4000 A. doublet varies from 10 to 300 micrograms for the materials investigated, and for the 7600 A. doublet it ranges from 0.04 t o 0.8 microgram.
153
Literature Cited (1) Baird, W. (2)
(3)
(4) (5)
S., “Proceedings of Sixth Summer Conference on Spectroscopy”, p. SO, New York, John Wiley & Sons, 1939. Brode, “Chemical Spectroscopy”, New York, John Wiley & Sons, 1939. Brode, W. R.,and Hodge, E. S., J. Optical SOC.Am., 31,58 (1941). Lewis, “Spectroscopy in Science and Industry”, p. 39, London, Blackie and Son, 1936. Lundell, G. E. F., and Hoffman, E. J., “Outlines of Methods of Chemical Analysis”, p. 177, New York, John Wiley & Sons,
1938. (6) M. I. T. Wavelength Tables, p. xix, New York, John Wiley & Sons, 1939.
(7) Pierce, W. C., Torres, 0. R., and Marshall, W. W., IND.ENG. CHEM., ANALED., 1 2 , 4 1 (1940). (8) Steadman, L. T., Hodge, H. C., and Horn, H. W., J. Biol. Chem., 140, 71 (1941). (9) Strock, “Spectrum Analysis with the Carbon Cathode Layer”, London, Adam Hilger, 1936.
Relation of Shape to the Passage of Grains through Sieves GORDON RITTEN’LIOUSE Sedimentation Division, Soil Conservation Service, Greenville, S. C.
N
OTSS’ITHSTANDING the widespread use of sieves for sizing sands and other particulate materials, many problems relating to the meaning and interpretation of sieving data remain unsolved or partially solved. One such problem concerns t h e actual diameters of grains passed or retained b y sieves. Although many factors are known to influence t h e size of grain passed or retained, very few data have been published, either on t h e net effect of these factors or on their relative importance. Recently microprojection measurements on a natural stream sand showed t h a t from 30 t o 50 per cent of the grains retained on sieves of a 1/z series had intermediate diameters larger than the rated openings of t h e next largest sieve (4). Thus the effective size of the sieve openings, as measured b y the intermediate grain diameters, was considerably larger than the rated size of the openings. T h e passage of these oversize grains through the sieves was thought to be due mainly to two factors-(1) the relative importance of the few oversize holes in t h e sieves, and (2) the passage of flattened grains diagonally through the sieve openings. This article presents some data on the quantitative importance of these two factors and outlines several types of investigations in which shape-calibrated sieves can be used advantageously.
Laboratory Technique A 60-gram sample of channel sand from the Rio Grande near Los Lunas, N. M., mas placed in a nest of standard wire-mesh sieves, in which the size of openings increased from fine Lo coarse approximately as the 4 2 . After 14 minutes of shaking in a Rotap mechanical shaker, the sand retained on each sieve was removed and placed in a separatory funnel partially filled with acetylene tetrabromide (specific
gravity 2.92). The “heavy” minerals that sank and the “light” minerals that floated were removed to separate filter papers, washed with alcohol, and dried. After removal of magnetite, the heavy mineral separates consisted dominantly of ilmenite, pyroxene, and hornblende, and smaller amounts of garnet, apatite, mica, and other minerals. The light mineral separates were composed of quartz, feldspar, calcite, and altered grains. Parts of the lighh and heavy mineral separates were split out with an Otto microsplit (a), mounted in a three-dimensional measuring wedge (3), and placed on the stage of a petrographic microscope equipped with a mechanical stage and micrometer ocular. Each grain was identified and the long diameter, a, the short diameter, c , and the intermediate diameter, b,, that controls passage of grains through sieve openings were measured. The ratio of breadth to length, bJa, and the flatness ratio, c/b., were computed for each grain. These ratios are dimensionless numbers that have a possible range from one to zero. All grains measured in each size grade were classified according to flatness ratio (0.999-0.900, 0.899-0.800, etc.) and the average intermediate diameter was computed for each flatness class. The averages for the smaller flatness ratios were based mainly on measurements of heavy mineral grains. These data are presented in Table I and Figure 1. For comparison, the arithmetic mean diameter of each size grade,
TABLE I. AVERAGEINTERMEDIATE DIAMETERS
FOR
DIFFERENT FLATXESS RATIOS
Size Grade 0.246-0.175 mm. 0.175-0.124 mm. 0.124-0.088 mm. 0.351-0.246 mm. Grains Average Grains Avera e Grains Average Average Grains intermediate meas- intermediate meas- intermediate meas- intermeAate ured diameter ured diameter ured diameter measured diameter IVO. Mm. NO. Mm. No. Mm. No. Mm.
7
Flatness Ratio 0.999-0.900 0.899 0.800 0,799-0:700 0.699-O.600 0.599-0.500 0.499-0.400 0.399-0.300 0.299-0.200 o.ig9-0. in0
48 67 99 83 57 26 8 15 9
0.314 0.326 0.341 0,358 0,367 0.385 0.408 0.419 0.425
45 61 82 83 48 30 6 2
..
0.228 0,243 0.252 0.264 0.277 0.267 0.296 0.290
...
57 71 102 108 73 28 11 4
6
0.166 0.175 0.184 0.193 0.200 0.210 0.227 0.211 0.258
70 83 100 93 42 42 11
... ...
0.115 0.128 0,136 0.138 0.143 0.151 0.153
... ...
