V O L U M E 25, NO. 6, J U N E 1 9 5 3 Table 11. csaon
conditions, the average value of E” being 0.22 us. NHE volt comp u e d to the calculated value of 0.21 volt.
Oxidation Potential of Lead Dioxide as Function of C N ~ O and H CPb(I1) CPb(I1)
0.1 0.1 0.01 0.01
a87
10-5
10-4 10-6
lo-‘
Zero Current Potential (SCE) +O. 160 0.12 0.19; 0. 155
ACKNOWLEDGMENT Eo(us. NHE)O
Acknowledgment is made to the Graduate School of the University of Minnesota. for a grant in support of this work.
$0.221 0.216 0.220 0.220 0 . 2 1 9 =t0 . 0 0 2
Mean Using Eilriation 4 and artivity coefficients of hydroxyl and biplurnhite on.$ of 0.81 and 0.89 in 0.1 M a n d 0.01 I\. no4ium hy,iroxide, respectively. a
LITER.ATL‘RE CITED
Delahay, P., and Stiehl, G. L., J . Am. Chem. Sac., 73, 1755 (1951 ).
Hume, D. iX , and Harris, W.E., ISD.ENQ.CHEM.,AXAL.ED., 1 5 , 4 6 5 (1943).
current curve of the supporting electrolyte-even if the corresponding waves are partly rate-controlled. If this were true for thepresent system, the zero current potential (or oxidation potential), E,=o,of the lead dioxide electrode should vary according to the rehtion :
E,,o
=
E”
- 0.030 log U O H -
- 0.030 log
UHP~OP-
(4)
E” in Equation 4 should be identical with the standard potential of Reaction 1, which can be calculated accurately from free energy data. Vsing Latiiner’s (6) values with the polarographic sign convention, we calculttte E” = +0.207 volt os. normal hydrogen electrode. I n Table I1 are given the authors’ data of potentids of lead dioxide a t varying alkali and lead concentrations. It is seen that Equation 4 accounts satisfactorily for the potentid of the plumbite-lead dioxide system, under experimental
Kolthoff, I. M., and Jordan, J., J . Am. Chem. Sac., 74, 382 (1952).
Kolthoff, I. hI., and Lingane, J. J., “Polarography,” 2nd ed , Vol. I, pp. 5 2 , 2 1 7 , Kew York, Interscience Publishers, 1952. Ibid., Vol. 11,p. 528.
Latimer. W.bi.. “Oxidation States of the Elements and Their Potentials in Aqueous Solutions,” 2nd ed., pp. I50 ff., Kew York, Prentice-Hall, Inc., 1952. Levich, B., Acta Physicochim. U.R.S.S., 17, 257 (1942); 19, 117 (1943): Discussions Faraday Soc.. 1 , 3 7 (1947).
Lingane, J. J., Chem. Revs.. 29, 1 (1941). Lord, S. S., Jr., O’iXeill,R. C., and Rogers, L. B., ANAL.CHEY., 2 4 , 2 0 9 (1952).
Schwarzenbach, G., and Freitag, E., H F ~Chirn. . Acta, 34, 1492 (1951).
Tsukamoto, T., Kambara. T., and Tachi, I., Proceedings of 1st International Polarographic Congress, Prague, Vo1. I, p. 525,1951. RECEIVED for review November 24, 1952. Accepted January 1 6 , 1953.
Suppression of Cyanogen Bands in the Direct Current Graphite Arc by lithium Chloride ROBERT G. KEENAN’ AND CHARLES E. WHITE University of Maryland, College Park, M d . The production of extremely dense cyanogen bands in the 3500 to 4800 A. region of the emission spectrum, when a graphite arc serves as the excitation source, has rendered many valuable persistent line spectra valueless for qualitative and quantitative spectrographic analysis. lllethods to circumvent this difficulty include employment of less sensitive lines in other regions of the spectrum, use of the generally less sensitive spark source, use of metallic electrodes, at a great loss of sensitivity, or exclusion of nitrogen by maintenance of an atmosphere of steam, oxygen, or helium around the arc. None of these methods is completely satisfactory for routine
A
31 IJOR difficulty, encountered in the use of graphite electrodes in spectrochemical analysis, is the production of cyanogen band spectra, when graphite is burned in air, due to the formation and subsequent excitation of the cyanogen radical in the atmosphere of the arc. These bands, occurring in the 3500 to 4800 -4.region, are five in number and emanate from their respective “band heads” a t 3590, 3883, 4216, 4600, and 4740 A. The last two are produced only under severe conditions of expoNure. The first three are the most prominent, the most extensive, and the most readily produced. They are very dense and their fine line structure extends throughout the 3500 to 4216 region. Theqe bands mask many valuable persistent line spectra, thereby making difficult the qualitative identification of minor and trace quantities of metallic elements in this region. As most of this 1 Present address, Division of Occupational Health, U. S. Public Health Service. Cincinnati, Ohio,
determination of trace elements. The method described employs a lithium chloridegraphite matrix and permits qualitative and quantitative spectrography throughout the cyanogen band region. It is suitable for routine application. Cyanogen bands have been suppressed sufficiently to permit quantitative spectrography of vanadium, molybdenum, and titanium, employing lines which occur normally within and between band structures. Triplicate analyses show that good results are obtained over the electrode content range of 0.1 to 3.0 micrograms. The function of the lithium chloride is to lower the potential drop across the arc.
portion of the spectrum iu well blackened by these bands, quantitative analysis by means of lines in this region is practically impossible when conventional procedures are employed. Elements whose most persistent lines occur within these band structures are determined quantitatively in most cases by the use of less sensitive lines of the element in other portions of its spectrum; the employment of the spark source rather than the arc, with the production of less dense cyanogen bands but at a sacrifice of sensitivity; or the use of metallic electrodes, such as copper or silver, which also reduce the intensity of cyanogen band spectra, but a t a great loss of senqitivity. I n order to utilize this spectral region for qualitative and quantitative analysis, two previous methods have been developed for the suppression or elimination of cyanogen band spectra. T h e first is that of Ashton ( I ) , who found that lead oxide, when mixed with plant ash samples, upon exposure in a 6-ampere. 4,s-
888
ANALYTICAL CHEMISTRY
volt direct current arc, yielded spectrograms whose cyanogen band spectra were suppressed sufficiently to permit the densitometric reading of the chromium 4254.34 A. and molybdenum 3902.96 A. lines. However, inspection of the spectrograms obtained in this investigation shows that the lead oxide suppressed mostly the background between the bands; it is in the regions immediately to the right of the 4216 and 3883 A. band heads that the lines of chromium and molybdenum occur. The second method involves the exclusion of nitrogen from the atmosphere around the arc. Several investigators ( 2 , 8, 9, 11) have reported effective cyanogen band suppression by the maintenance of atmospheres of steam, oxygen, or helium around the arc; a great reduction in band densities resulted and precise quantitative spectrography was made possible. Ailthough this method is highly effective, the operation of a chamber around the electrode assembly for the purpose of maintaining a nitrogen-free atmosphere is inconvenient for routine spectrochemical analysis. The use of spectroscopic buffer salts is a well established technique for the accomplishment of such objectives as the minimizing of the mutual interference effects of sample elements or the promotion of more uniform rates of volatilization. A judicious choice of buffer can increase the sensitivity of procedures for certain metals by increasing their volatility, if difficult to volatilize, or by promoting a more uniform rate of distillation of metals that tend to vaporize too rapidly into the arc stream. The experimental evidence submitted in this paper demonstrate8 that the lithium chloride-graphite matrix system is not only highly effective in suppressing cyanogen band spectra, but also acts as an enhancing agent in the spectrochemical determination of certain minor metallic constituents. APPARATUS
The apparatus employed in this investigation was that usually found in a spectrographic laboratory and included a Bausch and Lomb large Littrow spectrograph complete with quartz optics and a condensing lens system, built into the safety-type arc and spark stand, operated to focus the central portion of the arc on the spectrograph slit; a Jarrell-hsh Model 200 direct-reading comparator-densitometer; and a micropipet assembly to permit precise control over the addition of standard solutions to the rlectrodes. PRELIMINARY EXPERIMEYTATION
In preliminary qualitative experiments, the graphite electrode craters were waterproofed with a 20% solution of paraffin wax in benzene and packed with the individual reagent grade salts being tested. Two and one-half micrograms of lead and 4 micrograms of cadmium in a standard solution were added, and the loaded electrodes were dried at 105' C. for 1 hour. Sixty-second exposures were then conducted in a 4.5-ampere, 220-volt direct current arc, with the width of the slit of the spectrograph set a t 10 microns. Two exposures were made with each salt as matrix, in which 3-mm. and 6 m m . craters mere used. There appeared to be no significant advantage with either crater, in so far as band suppression was concerned; however, the lead 4057 A. and cadmium 3261 A. lines appeared slightly more dense when the 3-mm. crater was employed. The spectrograms indicated the following decreasing order of effectiveness in cyanogen band suppression: potassium carbonate, sodium carbonate, lithium chloride, and silver nitrate. The cyanogen bands in the spectrograms prepared from magnesium, manganese, nickel, and vanadium salt matrices were typical of those ordinarily produced in the direct current arc. The alkali metal spectrograms also showed the cadmium 3261 A. line to be present in the lithium chloride spectrogram but absent from the potassium and sodium carbonate spectrograms. While the lead 4057 A. line was present in the spectrograms of each of the three alkali metal salts, it was significantly stronger when lithium chloride was the matrix. In view of these observations it was decided to compare the relative effectiveness of the chlorides and the carbonates of each of the three alkali metals with respect to minimal spectral line
suppression and maximum hand suppression. Thirty spectrograms were prepared, using these matrices under varied exposure conditions, but with a constant slit width of 10 microns. Four micrograms of cadmium and 2.5 micrograms of lead were added to each matrix as in the previous experiments. A portion of a typical plate in which the spectrograms of the chlorides and carbonates of lithium, sodium, and potassium are contrasted is shown in Figure 1. Inspection of the spectrograms shown partially in Figuie 1 and the others prepared in this phase of the investigation led to the following conclusions: The chloride matrices generally enhance the spectral lines of lead and cadmium, except in the case of lithium; lithium chloride provides less background and suppresses the cyanogen bands more effectively than doeq lithium carbonate; potassium salts almost eliminate the cyanogen bands, but, unfortunately, also eliminate most line spectra; sodium chloride reduces general background below 3900 il. more eff ectively than does lithium chloride; and the greater the amperage, the greater the density of line and cyanogen band spectra. I n view of these conclusions, it was decided to discontinue lengthy experimentation with potassium salts and to determine by the variation of exposure conditions tvhether lithium chloride, sodium chloride, or an equal aeight mixture of the two chlorides ~vouldyield optimum band suppression and a t the same time permit sufficient sensitivity in detection of trace elements. EVALUATION OF OF'TIMUBI EXPOSURE CONDITIOKS
This phase of t,he investigation involved repeated experimentation with lithium chloride, sodium chloride, and equal weight mixtures of these two salts while the conditions of spectrographic exposure were varied. The exposure conditions evaluated included the use of diaphragms in front of the quart.z refrarting prism to reduce extraneous radiation; the spectrographic slit Ividth; the depth of t.he graphite electrode crater, the sample electrode serving as anode in all exposures in the complete investigation; the amount of salt matrix loaded int'o the cr:iter; the electrode arc gap; and amperage. The development of t,he optimum exposure conditions for the cyanogen band region led ultimately to the selection of lithium chloride as the most suitable matrix. During this evaluation study, lithium oxalate, sodium carbonate, potassium chlo~ide, lead chloride, and synthetic urine ash were again tried in the e w n t that the improved conditions of exposure might result in very little choice of one matrix over another. In the individual exposures with each of the indicated matrices, a solution containing the chlorides of numerous metals was used as a source of line spectra. This mixture of metals was obtained by dissolving the ash of a crude paper filter in 10 ml. of concentrated hydrochloric acid and diluting to 50 ml. with distilled n-ater. This solution contained calcium and iron as major constituents and such metals as copper, manganese, zinc, aluminum, sodium, molybdenum, and titanium in minor or trace concentrations. All salts under test as possible matrices were ground in an agate mortar and stored over concentrated sulfuric acid in a desiccator. All electrode craters were bored uniformly, using a Bausch and Lomb electrode shaper. The electrodes used were obtained from National Carbon Co. and were of the highest degree of purity available. The upper electrodes were pointed in a pencil sharpener reserved for spectroscopically pure electrodes. The sample electrodes served as anodes, which were waterproofed, after tioring, by immersion of the upper portion in a 2070 solution of paraffin wax in benzene. Aft,er air-drying for a few minutes, the powdered salts were introduced into the crater while the bottom of the electrode was tapped on the bench to obtain uniform packing. All additions of standard solutions throughout this investigation were made to the salt-charged crater by means of the micropipet assembly, so that precise measurements of 0.05 ml. of solution were possible. The pipets used were Pickard-Pierce blood pipets with graduation marks a t 0.05 and 0.10 ml. -411 additions were made between these t,Tvo levels to avoid any error due t,o leaving liquid a t the tip. The sample electrodes ivere dried at. 105' C. for 1 hour after the addition of standard solution.
V O L U M E 25, NO. 6, J U N E 1 9 5 3
889
l...l..
Use of Prism Diaphragms. Repeated exposures were oonducted, in which the larger or the smaller diaphragm supported on tho prism mount WB.E employed. It war first considered a possibility that these diaphragms might arsist in the suppression of band spectra by reducing extraneous radiation. Experimentation with a variety of other exposure conditions indicated that the diaphragms had little, if any, effect. and their use was discontinued. Spectrographic Slit Width. Successive exposures were made, using the range of 30-to 5-micron d i t widths. It was determined that with alkali metal salt matrices and using a 5-micron slit, the cyanogen band spectra were suppressed greatly and the background was negligible; moreover, with the right combination of electrical arcing conditions and with a lithium chloride matrix, elements present in small or trace quantities produced easily readable lines. Slit widths greater than 10 microns produced cyanogen bands which were too great in intensity for any useful purpose. Electrode Crater Depth. Exposuresrepeated many times, while determining the optimum arc gap and amperage, indicated that a crater 3 mm. deep, 6 mm. in diameter, and with B 1-mm. center post, helped to produce a steadier arc and 8 more uniform rate of distillation of the alkali metal salt than did either the 6or the IO-mm. craters. Quantity of Matrix. Experimentation with varying quantities of salt used as matrix showed that the quantity used was strictly a matter to be governed by the length of the.exposure period. I n the case of 30second exposures, a half-filled orster WRS =ti, factory, as the lithium chloride wy&~ not completely burned out by the end of this period. Electrode Arc Gap and Amperage. These two variables were evaluated together, because they are closely related. A 10-mm. arc gap a t 9 amperes was found to produce 8. steadier arc, more dense line spectra, and less dense cyanogen band spectra than a 4-mm. gap operating a t 4.5 amperes, These findings were especially true in the case of exposures wit.h lithium chloride matrices and a 5-micron spectrograph slit. Inspection of spectrograms involved in this phase of the in-
ation demonstrated conely that lithium chloride almost as effective 8;s n chloride in the suppreesF cyanogen hand spectra. i also apparent that specne densities were signifigreater when lithium ie constituted the matrix. iditional experimentation herefore confined to the lithium chloride as the metal salt component of
se&h Tas thus begun for a suitable diluent which would permit the lithium chloride to continue to function as a cyanogen hand suppressor, hut would minimize the suppressing effect on the line spectra. An ideal diluent for this purpose would he a nonmetallic substance of good burning qualities, which would contribute few, if any, line spectra in the region. Two materials which might possess these qualities were avaihble in the laboratory: two Iota of a specially pure grade of silica and spectroscopically pure graphite obtainable from the drilling of the electrode craters. A series of experiments was then conducted to evaluate the suDeriority of either graphite or the silica. as diluent and t o establish the most suitable weight ratio of the lithium chloride to the diluent selected. The optimum exposure conditions described above were maintained throughout these experiments
A series of standard solutions was prep~red,which contained
tions &re
memrkd contained only trace quantities of sodium, so
studies, provided a means of e s t a h h i n g the weight ra&o of lith: ium chloride to diluent capable of yielding maximum sensitivity in trace element detection. Lithium chloride and graphite or silica were weighed separately on a Roller-Smith direct reading hslsnce. The desired comhinrttion8 were mixed quickly on glazed paper, so as to minimize the absorption of moisture by the lithium salt, More than 75 individual mixtures were prepared in this manner. The electrode craters were one half to two thirds filled with a specific mixture. The weight ratios of lithium chloride to graphite and of lithium chloride to silica were varied from 1:l t o 1:4. The standard solutions were added and the charges were dried as described. Reproducible spectra. line densities were obtained only with the lithium chloride-graphite matrix, although the cyanogen hands were well suppressed with both types of matrices. Lithium carbonate was tried also in this series, hut was still inferior to the lithium chloride as a hand suppressor; with graphite &S diluent, the chloride permitted the registration of markedly stronger lines in all four metallic constituents of the standard solutions.
ANALYTICAL CHEMISTRY
,890
In the spectrograms of the lithium carbonate experiments the lead lines were missing and the molybdenum, manganese, and aluminum lines were about 25, 50, and 90% the density, r e spectively, of those obtained when lithium chloride was used in the charge. It was further observed that the most dense lines of these four metals occurred in the 1 to 3 lithium cbloridegraphite exposure, while band suppression was greatest with the 1 to 1 lithium chloride-graphite matrix. It was decided to .adopt a 1 to 2.5 lithium chloride-graphite weight ratio for the quantitative experiments described below.
sample solution into the crater, using separate pipets for each sample. Each solution was shaken a few times, before the pipet was tilled, as the espryllic alcohol used to reduce the surface tension of the solution droplet is insoluhle in water. The electrodes were dried in the oven for 2 hours a t 105' C. It was found advantageous to raise the temperature gradually from 85' to 105' C. during the first 15 or 20 minutes of the drying period to avoid spattering. When the dlying was completed, the spectrographic exposures on Eastman 111-F spectroscopic plates were conducted. Although a %mioron slit may be used for the determinations when 1 or more micrograms of the metal is on the electrode, a 10-micron slit was used in the exDosures of the syn-
QUANTITATIVE ANALYSIS IN CYANOGEN BAND REGION
The quantitative procedure has been applied to samples from a variety of sources. For the purpose of the present paper, the data presented are those obtained upon analysis of synthetic samples containing metals whose most persistent lines appear either within or between the cyanogen hand structures. Preparation of Synthetic Samples. Recrystallized lead chloride, reagent grade ammonium molybdate tetrahydrate, van& dium pentoxide, snd basic titanium sulfate were used in making -the standard samples. A series of eight synthetic samples WBS so prepared that the weight of each of the four metallic constituents per unit volume was the same in a given sample. The concentration range extended from 0.1 to 5.0 micrograms per a 0 5 ml. The solvent was approximately 1 N hydrochloric acid. In addition, every sample co?tained 0.85 mg. of t i l p e r 0.05 ml. a n d 2.0 ml. of Zoctanol per liter of solution. The tin was added to *ve bulk to the residues remaining in tbe upper region of the matrix after evaporation. ZOctanol was mcorporated into the .samples to reduce the surface tension of the solution droplet during the loading of the electrodes. This series of samples served both as standards for the preparation of the analytical curves and as "unknowns" for determination of the four metals in an independent series of spectrograms. Quantitative Procedure. Spectroscopicdly pure graphite 12 inches. were cut into convenient lengths. Uniform craters 3 mm. deep; niith a center post 1 mm. in diameter, were drill& into each, using an electrode shaper. Any loose graphite was removed from the craters, which wen: then waterproofed by m e r s ing the upper portions of the .electrodes in a zO% solution .of paraffin wax (Tavern Wax Brand) in benzene. After airI drying until the benzene had evaporated, the craters were 2 .approximately two thirds filled with the l i t h i u m c h l o r i d e .graphite mixture, whose weight 5 ratio was 1 part of pulverized 6 lithium chloride to 2.5 parts of 7 g f a p h i t e . These two ingre0 &nts had been weighed previ.ously on an analytical balance, 9 mixed and ground together in a IO mullite mortar, and stared in If .a desiccator containing coneenI2 trated sulfuric acid desiccant. 13 This mixture was added to the ,electrodecratersfrom the tipof 14 .a Scoopula and the electrcdes were tapped on the bench surI face to permit packing of the charge. The absolute amount 18 .of this mixture in a crater iS IS not critical as long as burnout 20 .doesnot occur during the subse21 quent exposure. When added as described here, the weight 22 of the charge was 20 to 22 mg. 23 Previously cleaned and ovendried P i c k a r d - P i e r c e blood 25 pipets were then used, ta ,a,d mlwtrotles. _.__...-. ~,,.. bv
gap of 10 mm. Clean, -freshlypointed, 'spec&oscopieally pure graphite electrodes served as the upper cathodes. The central portion of the arc was focused on the spectrograph slit during the exposures to obtain maximum light intensity. Preparation of Analytical Curves. Six of the synthetic samples, whose concentrations were 3, 2, 1, 0.5, 0.25, and 0.1 microgram per 0.05 ml., with respect to lead, molybdenum, vanadium, and titanium were subjected in triplicate to the quantitative procedure. The per cent transmittance values of the fallorring lines were read with the densitometer: vanadium 4111.i85, 3703.584, and 3183.982 A,; molybdenum 3902.963 and 3798.252 A,; titanium 3653.496 A,; lead 4057.820 A,; and copper 3273.962 A. Copper occurred as an impurity in the tin and in the lithium chloride; its content in the electrode charge was sufficiently constant for it to Serve as the internal standard. The spectrograms obtained from the triplicate exposures of the standard series and numbered 1 to 18, inclusive, are shown in Figure 2. All exposures for the production of the spectrograms reproduced in Figure 2 were z full 30 seconds each; a rotating shutter was not used in this investigation. The per cent transmittance values of the analysis lines "we converted to the logarithms of the intensity ratios of each soecificanalysis line to the comer 3274 A. line, wing an emulsio n calibration-H and D curve, prepared by
$
IT
24
. . ..
I
~1 I 'I
II ll ll
I
l
l
V O L U M E 2 5 , N O . 6, J U N E 1 9 5 3
891
those obtained a t considerably higher concentrations with a pure graphite matrix. This bfo Detn. % of 110 Detn. % of 1' Detn. % of V Detn '1. of c o m p a r i s o n is brought out Electrode (3902.96 A . ) , True (3798.25 A,), True (-1111.79 .4,), True (3703 58 .I 1 , True Y Value Y Value Value Value rather markedly upon observaContent. Y 3.28 109 3.11 103 2 91 97 3.02 101 tion of spectrograms num.. 2.01 101 1.97 911 ... ... 0.87 87 0 'J1 91 0:93 93 bered 19 to 25 in Figure 2. 1 00 0.90 110 0 36 112 0 58 116 0.55 0 50 0.51 102 I n the preparation of these 116 0.26 104 0.25 100 0.25 0.26 104 0.29 0.10 0.12 120 .. .. 0 098 98 ... ... spectrograms, increased quan103 Mean 104 103 100 tities of the same pynthetic T i Detn. Pb Detn. V Detn. samples were added to a pure (3183.98.4.1, (3653.5A.j, (4037 82.i.j. 1 Y Y graphite matrix. The elec3 00 3.28 109 2.98 99 3 02 101 trode content of each of the four 2 00 ... ... I 00 0.88 ' 88 1'65 io3 @:io F(g metals was 25, 20, 15, 9, 5, 4, 0 50 0.53 106 0.62 1?4 0 6.3 130 and 3 micrograms in spectro0 23 0.25 100 0.26 10.4 (Curve too steep? 0.10 0.093 93 .. ... grams numbered 19, 20, 21, 22, Mean 99 ' 108 23, 24, and 25, respectively. i Values reported are averages of determinations conducted in triplicate Inspection of these spectrograms showed that the line spectra from the pure graphite matrix became extremely weak below 15 micrograms; that The cviivrntional step sect'or method and the calculating I w n i t l the t'itanium 3653 .\., vanadium 3703 A., and molybdenum T O ei?ec.t the conversion. The average of each triplicate set of 3903 A. lines, at 15 to 23 micrograms, were not so strong as, or a t rheze logarit,hmic values x a s calculated, except in a few instances most no stronger than, the 3-microgram lines produced from the where m apparently gross error justified the rejection of an lithium chloride-graphite matrix; that it is impossible to detect intlividual value. These average logarithmic values were then the vanadium 4111.8 A. and the molybdenum 3798 A. lines a t plotted against the logarithms of the electrode content of the any of these concentrations in spectrograms containing fully metal.: to yield a series of seven analytical curves, n-hith are developed cyanogen hnnd.: and that the presence of the inshown i n Figure 3. Inspection of Figure 3 shows the analytical curves reprocluceil therein to approsimate a general slope of 45", with the esception cn of the iead 4057 and molybdenum 3798 curves. This disparity 0 MQ 3 7 9 8 . 2 5 2 CU 3 2 7 4 could poisibly be minimized by a modification of the conditions 0 Mo 3 9 0 2 . 9 6 3 / Cu 3 2 7 4 of the procedure or by the employment of a more suitable internal 4 V 4111.785/Cu3274 etantlarrl. The chief objective in the preparation of these curves A V 3183.982/ Cu3274 was to establish whether or not quantitative analyses were posi0 V 3 7 0 3 . 5 8 4 / Cu 3 2 7 4 hle, u:ing ,spectral lines rvhich occur normally either s.ithin or 0 TI 3653.496/ Cu3274 hetween the cyanogen band structures as produced by conPb 4 0 5 7 . 8 2 0 / Cu 3 2 7 4 vent ional matrices. Results, The series of synthetic samples was again subjected to the qmntitative procedure in triplicate, but on a separate spectroscopic plate, the group of samples now being considered as "uiilinoxnF." The spectrograms thus obtained were treated by the densitometric procedure described previously. The iesulting densitometric data were used to estimate the quantitie.5 W I I I I I I of the re~prctivemetals using the established analytical curves. 35 5 I 5 d n indication of the accuracy of the quantitative procedure is INTENSITY R A T I O given i n the table of analytical results, summarized in Table I , Figure 3. Analytical Curves Established by Rleans of :'or each of the determinations. Lithium Chloride-Graphite hlatrix Thr ;ind~-ticalresults reported in Table I were obtained a; the average values of the determinations conducted in triplicate. creahed quantities of tin, incident to the addition of volumes of The -ptctial lines used for these determinations are given in the the standard solutions in excess of the 0.05 ml. used in the preparaappropriate columns. The sensitivity of the method is 0.10 tion of the standard spectrograms, had no effect in suppressing microgram of molybdenum or vanadium and 0.25 niicrogrmi in cyanogen bands. I t is apparent that the lithium chloridethe m - e of titanium, Densitometric readings in the region of graphite matris makes possible quantitative spectrography c90yotransmittance were obtained for the lead line a t an electrode throughout the region normally masked by the cyanogen bands, content of 0.1 microgram, but the analytical curve n-ac too steep and that the lithium chloride functions as an enhancing agent for ?'or quantitative use. An internal standard, other than copper, those lines which can be detected normally between the band might render the procedure more suitable for t,he determination structures. of lead. The 2-niiwograni samples are not reported in most inst:inces EXPL4NhTIOS OF CYAVOGEK BAND SUPPRESSION BY ALKALI Iiecau.e the analytical curves were plotted without the use of the METAL SALTS 2-microgram standards, except in the case of molyhtkriuni. Too much curvature was produced at' the upper end of the During the preliminary experimentation with the alkali metal working range when the other 2-microgram values were used. salts, consideration was given to the possibility that these salts exert a blanketing action. Such action could conceivably be .4ctualI!-, both the 2- and the 3-microgram samples of most of the produced by these relatively volatile salts; the effect might be rnetak yielded lines which were too dense for precise denxitomthat observed-a reduction of cyanogen bands due to a decrease etry. They are presented mostly to demonstrate the upper end of the working range with the employment of the lithium in the nitrogen concentration in the arc zone. T o test this hypothchloride-graphite matrix and to compare these linea visuall>-rvith esis, two extremely volatile salts, ammonium chloride and Table I.
Analysis of Synthetic Samples"
;g
*
ANALYTICAL CHEMISTRY
892 aluminum chloride, were exposed separately under the standardized conditions. Great quantities of fume were produced and condensed on the surface of the upper electrode and its support clamp. However, the spectrograms resulting from these two exposures contained very extensive cyanogen bands. Therefore, the suppression obtained with alkali metal salts must be explainable by some other mechanism. In a discussion of the choice of a carrier, Scribner and hlullin ( 7 ) have pointed out that “alkali salts tended to suppress boron lines, possibly because of the low ionization potential of the alkali metal in the arc.” Scribner (6) notes that “differences in the ionization potentials are usually small for elements determined in the spectrochemical analysis of metals and their compounds.” Whereas the majority of chemical elements have ionization potentials lying between 5 and 10 volts, the alkali metals have potentials whose values are as follows: lithium 5.39, sodium 5.14, potassium 4.34, rubidium 4.176, and cesium 3.88 (4). Scribner (6) also mentions that the presence of varying amounts of alkali metals in samples may result in marked reduction of higher energy excitation. As an example he cites the depressing effect on various lines in the analysis of portland cement when the concentration of potassium was varied from 0.2 to 1.0%. These facts are in agreement with the experimental observations of the present investigation, where the suppression of line spectra as well as cyanogen band spectra have been noted.
Table 11.
Voltage Drop across 10-%Plm.Arc with Separate Salt Matrices
Voltage Drop, Matrix D.C. Volts Sone SaC1-LiC1 (equal weight mixture) 33 32 NaCl 56 Powdered graphite 30 LiCl 56 TiOSO4 LiC1-graphite (1 :2.5 54 VOCl 30 parts b y wt.) 5 2 29 ._ LizCzO4 Cu(CzH8Oz)i 26 48 NazCOa Pb(CzHs0z)n 25 46 Zn(CnHa0z)z KC1 CSCl 40 AgXO1 25 6-2a RbCla 35 LizCOs 24 34 KzCOa PbC1z 0 Voltage read 0.4 volt for 10 seconds a n d then rose, b u t varied from 5 to 25 volts. This occurred during two attempt8 t o determine voltage drop with rubidium chloride matrix. Matrix
’
Voltage Drop, D.C. Volts 56
The direct current arc is recognized as being predominantly thermal in its mode of excitation. The maximum temperature in arc discharge column has been found to be around 7000” K. (6). In a study of the cyanogen and aluminum monoxide bands in the carbon arc, Tawde and Trivedi found that the central gaseous zone, where strong emission of cyanogen radiation occure, has a temperature of 6200’ K. (IO). In view of all these observations i t seems reasonable to propose that the suppression of the cyanogen band spectra by alkali nietal salts is caused by the lowering of the potential of the arc. This lowering is accompanied by a drop in the arc stream temperature, which is then insufficient to excite the cyanogen radical to the level attained in a normal arc. 4 n experimental test of this explanation was conducted. The voltage drop across the arc was measured during the burning of some of the different salt matrices which had been tried as possible suppressors. The electrical conditions of the arc were identical to those employed in the final method, as described in the quantitative procedure. The craters were packed with the separate salts and freshly pointed upper electrodes were used in each ca8e. The leads from a direct current voltmeter were inserted into the electrode supports prior to striking the individual arcs. The voltage drop measurements obtained repeatedly in this manner are reported in Table 11. The values given in this table show a pronounced lowering of the arc potential when one of,the alkali nletal salts constitutes the
matrix. .Ilthough there are some discrepancies, this lowering of the voltage is greater as the atomic weight of the alkali metal increases. This trend is more apparent if the carbonate series is considered alone. A consideration of the values obtained 15 ith the chlorides leads to an understanding of certain experimental observations 1. The appreciably lower value for the potassium chloride matrix explains why this salt suppressed both cyanogen band and atomic spectra more completely than did the chlorides of sodium and lithium. 2. The chlorides of sodium and potassium produced less lowering of the arc potential than did the corresponding carbonates. This partially explains the experimental observation that chlorides generally enhanced the spectra of trace metals to a greater extent than did the carbonates. It is common practice to convert many metals to their chlorides to obtain increased volatility during spectrographic exposure. 3. The superiority of lithium chloride over lithium carbomte in the suppression of cyanogen bands and general background is explained by the lower voltage drop measured with the chloride. This reversal in the case of the lithium salts may be due to the decomposition of lithium carbonate to the oxide. 4. The partial suppression of cyanogen bands by silver nitrate and lead chloride is explained by their intermediate positions in the table. 5. The adherence of the values obtained with sodium and lithium chlorides in the neighborhood of 30 volts accounts for the amount of experimentation required prior to the selection of lithium chloride as suppressor.
Data are presented in Table I1 for only a few of the salts nhich were found to be of no value as band suppressors. The voltage measurement- in these cases are close to those of the powdered graphite matrix and of the empty crater. The validity of the latter measurements are considered to be satisfactory, as a standard text on spectroscopy gives a value of “about 55 volts” as the operating voltage of the plain carbon arc (5). I n order to determine whether the alkali metal suppression of the cyanogen bands was completely explained by the deprewion of the arc potential, i t was decided to consider the possibility of absorption of cyanogen radiation. To test this theory, t n o arcs were operated in series. The normal carbon arc, focused on the slit of the spectrograph, w,s operated in the usual position. A lithium chloride arc, with a 10-mm. gap, was operated directly in front of the slit, so that the radiation from the normal carbon arc had to pass through the dense lithium chloride vapor. Both arcs n-ere operated a t 4 amperes off of the one motor generator set. The spectrograms showed that the lithium chloride vapor did not absorb the cyanogen radiation. The data presented tend to confirm the theory that alkali metals suppress cyanogen band spectra by lowering the potential of the direct current arc. LITERATURE CITED
(1) Ashton, F. L., J . SOC.Chem. Ind., 58, 185T (1939).
(2) Conway, J. G , Moore, M. F., and Crane, W. mi. T., J . A m . Chem. Soc., 73, 1308 (1951). (3) Harrison, G. R., Lord, R. C.. and Loofbourow, J. R., “Practical Spectroscopy,” New York, Prentice-Hall, Inc., 1948. (4) Kaye, G. W. C., and Laby, T . H., “Tables of Physical and Chemical Constants,” 10th ed., p. 135, New York, Longmans, Green and Co., 1948. ( 5 ) Langstroth, G. 0.. and McRae, D. R., Can. J. Research, 16A, 17 (1938). (6) Scribner, B. F., Am. SOC.Testing Materials, Spec. Tech. Pub. 76 (1948). (7) Scribner, B. F., and Mulhn, H. R., J . Research Natl. B u t . Standards, 37, 379 (1946). (8) Smith, D. M., and Wiggins, G. M., Analyst, 74, 101 (1949). (9) Steadman, L. T., Phys. Rev., 63, 322 (1943). (10) Tawde, Pi. R., and Trivedi, S. A., Proc. Phys. SOC.(London), 51, 733 (1939). (11) Vallee, B. L., and Peattie, R. W. ANAL CHEV.,24, 434 (1952). RECEIVED for review September 15, 1952. Accepted February 26, 1953. From a t h e m submitted in partial fulfillment of t h e requirements for t h e degree of master of science. University of Maryland, J u n e 1952.
