Iron Arc as a Standard Source for Spectrochemical Analysis RlORRIS SLAVIN, Bureau of Mines, U. S. Department of the Interior, University of Maryland, College Park, &Id.
IK
A R E C E N T communication ( 5 ) ,the author showed that the spectral energy emitted by a n element when it is vaporized in a carbon arc is directly proportional to the weight of the element consumed. It was proposed t o use this observation as a basis for quantitative analysis, making the energy determination, which need be only relative, in terms of a constant, reproducible light source used as an external standard. This external standard was a quartz mercury arc lamp, which, though satisfactory for solution of that problem, nevertheless left much to be desired when it was used for routine analytical work. It is appropriate to list the properties that a source intended as a n external standard for spectrochemical analysis should have.
tion effects. The iron wire used for standardizing volumetric solutions is an acceptable and convenient material. Before use, the bead must be “seasoned”--that is, it must be burned in the arc until all spitting of molten metal has stopped. This violent action is probably due to oxidation of carbon and other gas-forming impurities in the iron, because when a gently burning arc is finally obtained, a matter of 3 minutes or so, the bead no longer has the appearance of metallic iron. As the necessary facilities were a t hand, a n x-ray diffraction pattern was made of one of these beads; the results showed that it is a mixture of FelOa and Fe304,principally the latter. The electrical circuit is identical with that of the ordinary low-voltage analytical arc, consisting of a variable series resistance in a 220-volt direct current line. An ammeter is inserted in the circuit and is the sole control instrument. Electrode spacing is controlled by projecting a n image of the arc on a neighboring wall carrying marks to indicate the spacing. Once the resistance is adjusted, no other control is necessary but to feed the upper electrode down at the same rate a t which it is consumed, and this is most easily done by feeding a t a rate that keeps the ammeter needle a t the predetermined point on the scale. The various emission studies were carried out by means of photographic photometry. -4 step-sector spectrum of the iron arc %-as included on each plate, the step-densities of a chosen line (Fe 3017.63) were measured, and a characteristic curve was plotted. Relative intensities were then determined by interpolating the measured density of the unknown line in this curve. The process is described fully elsewhere ( 2 ) . I n the graphs presented below, the intensity values, where necessary, hare been recalculated in terms of a standard exposure = 100, in order that all the graphs may be readily comparable. For this standard exposure the time was 1 minute, the current 4 amperes, and thB arc gap 6.5 mm. The current range a t which the arc will operate smoothly is limited. The relation between intensity and current is shown in Figure 2. The lowest point, at about 2.3 amperes,
1
Primarily, of course, it should be capable of reproduction, and the degree of reproducibility should be greater than the over-all precision of the whole process. CATHODE On this basis, the allowable error for present-day practice should be of the order of 2 or 3 per cent. In addition, there should be no slow deterioration m t h age, a fault of incandescent lamps and of some enclosed metallic arcs. Radiation should consist of a line spectrum rather than a continuous one, because intensity of spectrum as photographed is a function of slit width, and this function is not the same for both kinds of radiation. The type of radiation ANODE should be the same for standard and analytical spectra; otherwise slit width would have to be standardized, a matter of some difficulty and an additional complicating factor. The source should operate on direct current, as emission from an alternating FIGURE 1 current source fluctuates periodically and thus cannot be used in conjunction with the usual photometric rotating disks (step adjustable, and wedge sectors) because of the uncontrollable stroboscopic effects. The spectrum should be rich in lines, distributed over as broad a wave-length range as possible. Finally, the device should be simple t o set up; it should be inexpensive, and should require a minimum of change in the equipment of the arc stand and optical bench, so that the change-over from standard to analytical source may be accomplished quickly and easily,
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The source that fitted these requirements most closely was the iron arc, the only question-a crucial one-being on the score of reproducibility. I n spite of the nide use of the iron arc in spectroscopy, there appears to have been no recorded work on this property. I n his original description of the well-known arc form that bears his name, Pfund (4)mentioned that when the anode was in the shape of a globular bead of metal, a very steady arc was obtained; but he made no quantitative study of emission characteristics. Accordingly, this was the starting point, and the final form of the arc as discussed here is essentially similar to Pfund’s. The arrangement which experiment showed to be most satisfactory is outlined in Figure 1. The upper electrode, which is the negative, is a 0.125-inch graphite rod. The lower electrode is a spherical metal bead formed by feeding iron wire or cuttings into the burning arc. This bead is supported in a shallow cavity (formed by an ordinary twist drill) in the end of a 0.3125-inch graphite rod. It is important that the iron forming the bead be pure to obviate fluctuations in emission due to differential volatiliza-
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FIGURE 2 131
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FIGURE 3
represents the least value of current at which the arc will niaintain itself. I n the range represented by the linear portion of the curve, the arc burns very steadily with a hardly audible hissing sound, a slight, rapid wavering of the ammeter needle of about 0.03 ampere about the median value, and very little wandering of the anode spot on the surface of the molten bead. The cathode spot s h o w no tendency to wander. This is the current range a t which constant emission is obtainable. Beyond about 6 amperes, where the break in the linearity of the curve occurs, the arc becomes irregular, the hissing sound becomes louder, and the anode spot begins to wander, sometimes striking to the graphite support. The ammeter needle reflects this irregularity by rapid, wide fluctuations, thus making it very difficult to maintain the current a t any fixed value. As read from the slope of the curve, a change of 1 ampere causes a change of emission of about 50 per cent. The unavoidable swing of about 0.03 ampere therefore causes a change of 1.5 per cent about the standard value. However, this variation is averaged over the period of exposure, so that the error due to current fluctuations should be less than 1 per cent. The extrapolated curve does not pass through the origin but still shows a value of 2 amperes a t zero emission, which is interpreted to mean that this portion of the total current goes t o make up thermal losses (by conduction and by incandescent radiation) a t the electrodes. If this portion is subtracted from the total current-that is, if the curve of Figure 2 is shifted laterally to pass through the origin-the proportionality between current and emission becomes direct, up to the break in the curve. The voltage drop across the electrodes could be used just as well as a parameter of emission, as, according to the arc equations of Ayrton (1) and of Nottingham ( 3 ) ,current and voltage are interdependent. However, contact resistances in the arc stand, particularly between the iron oxide bead and the graphite support, would then become unpredictable factors, so that it is safer to use current as the control. When the emission was examined with respect to time, the curve of Figure 3 was obtained. This indicated that a short warming-up period was required to establish thermal equilibrium a t the electrodes, about 1.5 minutes being enough. Beyond that, the emission became constant, the experimental points shown in the graph deviating less than 1 per cent from the mean. Although the duration of the test was 10 minutes, which is ample for most purposes, this period can be extended greatly. However, hand-regulation for more than a few minutes becomes very tiresome, so that for longer periods a better plan is to provide mechanical feed. I n this connection, evaporation of the bead will provide no difficulty, as the rate of evaporation is less than 2 mg. per minute a t 4 amperes. The influence of electrode spacing on emission was investigated next, and the results are shown in Figure 4. As read from the graph, the rate of change, is about 5 per cent per
VOL. 12, NO. 3
millimeter change in arc gap. This figure has only qualitative significance, as it depends to a great extent on the method of illuminating the spectrograph. However, it is evident that the electrode gap should be carefully standardized. This can be done to within a few tenths of a millimeter with no great difficulty by projecting a greatly enlarged image of the arc on a marked screen. A large part of this variation is caused by the fact that emission along the length of the arc column is not uniform. This point was investigated by focusing an image of the arc on the slit (all the other spectra were taken with the arc focused on the dispersing medium, in these experiments a grating), then dividing the resulting spectrum line into a number of equal segments and measuring the intensity of each segment. Figure 5 shows this variation in emission along the arc column. It will be noted that the curve is not symmetrical about the midpoint of the arc gap; the intensity is peaked near the anode. This is in general agreement with the experience of Strock (6) and others. Therefore, if the arc is to be used as a day-to-day standard, capable of giving reproducible results even though the setup be dismantled and reassembled between exposures, the portion of the light actually photographed must always be taken from the same part of the arc column. No quantitative significance is intended by the presentation of this curve; it is given here only to emphasize the fact that distribution of emission from an arc is not a t all uniform along its length. Other lines would undoubtedly show other types of distribution. To minimize the error due t o this effect, as large a portion of the arc emission as possible should be used in making the exposure. Perhaps the best arrangement, not always feasible with existing condensing systems, is t o use the whole of the radiation, excluding only the incandescent light from the poles.
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From the tests as presented here, it may be concluded that the reproducibility for a series of exposures taken without disarranging the optical setup should be within 1 per cent; exposures taken with an impermanent setup should be reproducible to 3 per cent. The size of bead may affect emission to a slight extent because thermal losses are a function of the exposed surface. However, no such effect was noted when the emissions from beads of various sizes were compared. The beads ranged in weight from 250 t o 500 mg. With a smaller bead, the arc tended to strike to the graphite support; v i t h larger beads, the entire mass did not fuse, so that the upper surface was more nearly plane than spherical.
3IARCH 15, 1940
ANAL\-TICAL EDITION
133
to make multiple exposures, as with astigmatic spectrographs. A further possible use is in absorption work, where two separate exposures, one through the solvent and one through the solution, can be taken in place of the single exposure with a split beam. Hence, it would obviate use of a cumbersome piece of apparatus. Still another use is in miscellaneous test work in the laboratory, such as the determination of relative spectral sensitivity of plates, relative efficiency of‘ condensing systems, and selective absorption of screens and neutral filters. DISTANCE ALONG ARC GAP-
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Acknowledgment
FIGURE 5
As Pfund pointed out, other metals can be used as the anode. The present author has tried several having spectra similar to iron. Cobalt and nickel, particularly the latter, behaved very well, chromium refused to fuse to a bead, and manganese proved entirely unsteady. No tests were made for reproducibility.
Applications of Iron Arc Besides the use to which the standard iron arc is put in this laboratory, it should prove highly suitable for plate calibration in conjunction with the usual internal standard procedure of spectrochemical analysis, particularly when one is forced
The author’s thanks are due to Howard F. Carl, of this station, for suggesting the iron arc as a standard and for the x-ray work.
Literature Cited (1) Ayrton, Hertha, “The Electric Arc”, London, Electrician Printing and Publishing Co., 1902. (2) Forsythe, W. E., “Measurement of Radiant Energy”, p. 246, New York, McGraw-Hill Book Co., 1937. (3) Nottingham, W. B., J . Am. Inst. Elec. Engrs., 42, 12 (1923). (4) Pfund, A. H . , Astrophys. J., 27, 296 (1908). (5) Slavin, Morris, IXD. ENG.CHEM.,Anal. Ed., 10, 407 (1938). (6) Strock, L. W., “Spectrum Analysis with the Carbon Arc Cathode Layer”, London, Adam Hilger, 1936.
PUBLISHED by permission ment of t h e Interior.
of the Director, Bureau of Mines, U. S. Depsrt-
( S o t subject t o copyright.)
An Empirical Mercurimetric Method for Zinc ALBERT C. TITUS AND JACK S. OLSES University of Utah, Salt Lake City, Utah
I
?i T H E application of the usual volumetric ferrocyanide
method to the determination of zinc dependable results can be obtained only with practice and by standardizing the procedure very carefully. However, a previously existing gravimetric method R as perfected by Vosburgh, Cooper, Clayton, and Pfann (6) in which the zinc was precipitated as its mercuric thiocyanate. By converting this to a volumetric basis time could be saved in an otherwise excellent method. Jamieson (2) dissolved the precipitate in an excess of potassium iodate in acid solution, extracted the liberated iodine by an ether layer, and titrated in this two-phase system with sodium thiosulfate until the disappearance of the purple color in the ether layer. The liberation of free hydrocyanic acid tends to make this method undesirable. Another conversion to the volumetric basis was that of Kolthoff ( 5 ) ,who determined the excess of precipitant in aliquot portions of the supernatant liquid above the white crystalline zinc mercuric thiocyanate, using a standard mercuric-ion solution with ferric-ion indicator. It would seem preferable t o determine some constituent of the pure precipitate rather than the excess of precipitating agent in the filtrate, since the latter would contain various possibly interfering ions derived from the sample being analyzed. The authors base their method upon the mercurimetric determination of the excess iodide remaining in a solution made by dissolving the filtered out zinc mercuric thiocyanate in a known amount of potassium iodide solution This empirical method necessitates the use of a simple straight-line equation to convert the milliequivalents of iodide apparently used up to milliequivalents of zinc. Its appli-
cation should be particularly useful in the routine determination of large numbers of zinc samples Were the method stoichiometric, the following equations would represent the course taken by the reactions: 4 I(excess)
+ ZnHg(SCN)d = Zn’+ + HgId-- + 4 SCX‘(precipitate) Hg++ + 4 I- = HgI,-(standard solution)
The indicator for the last reaction is self-contained in the system: Hg++
+ HgId--
= 2 HgI? (red
precipitate)
In practice two sources of error of opposite sign appear to be the chief reasons for the empirical relationship. The first is a positive error caused by the appearance of the end point before an equivalent amount of mercuric ion has been added, causing the calculated amount of used up iodide to be too the error is high As has been pointed out by llolthoff (8, 4, quantitatively related to the square root of the Hg14-- molarity at the equivalence point. By control of the volume a t the end point and of the amount of iodide added, the concentration of the HgIa-- is kept essentially constant in all runs, whether no zinc is present or a large amount is being determined. I n this way the positive error is kept constant. The other and negative error is proportional to the amount of zinc being determined, since it is due to the thiocyanate ion equivalent to the former. Kear the equivalence point the iodide-ion molarity decreases rapidly and so becomes too
