QUALITATIVE ANALYSIS WITH THE "SPECTRANALf ''
8
BRUCE McDUFFIE2 Emory University, Emory University, Georgia
T w m w - F o m cations encount,ered in the usual qualitative analysis scheme have been studied with the recently developed Todd "Spectranal" (lo), an instmment consisting of a Bunsen-type spectroscope with an attachment for sparking solutions a t a part,ially submerged electrode. The features of the instrument are descrihed briefly, data are presented on its performance with the above-mentioned cations, and some possible uses of the instrument as a teaching aid are suggested.
of the inst,rumeut under a removable cover, and a power supply unit nhirh operates from 110 volts, a. c. or d. c., is built into the base. Bot,h the eyepiece and the wale may he focused, the slit width is adjustable, and t,he eyepiece arm may be moved horizontally to bring any part of the visihle sperirum into view. The spectral range is from about 4000 A. to 7000 A,, and the dispersion of the prism is such that the sodium I)-lines can just he resolved.
DESCRIPTION OF THE INSTRUMENT
Figure 1 is an over-all view of the Todd "Spectra nal," Model A, Improved, showing (in clockwise fashion) the eyepiece of the instrument, the lamp for scale
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elements in SOIL''.-Figure 2.
i r e 1.
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illuminat~ion,and the esritation chamber positioned in front of the slit. A glass prism is located in the top
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View of Excit.*ian Ch.mh.r
A diagrammatic view of the t,est-tube-like excitation chamber is shown in Figure 2. An appropriat,e solution of the sample is poured into this chamher t,o the desired level, about 1 to 2 ml. of solut,ion being required. When the electrodes are connected to the power supply and t,he voltage turned on, the submerged tip of t,he excitation electrode heromes surrounded by sparks or a glow characteristic of either t,he electrode or of certain substances in the sample solution. The light emitt,ed passes through the excit,ation rhamber and cooling heaker, then into t,he spert,rosrope to he resolved into its ron~ponentmave lengths. Two general methods of analysis are available with the "Spectranal." If a sample is in metallic form, a wire or thin strip of the sample ran be analyzed directly by 11sing it as the excitat,ion electrode in an appropriate sparking medium. On the other hand, if a sample is in the form of a solution, it may be aualymed by sparking it a t a special excitation elertrode xhich gives no spect,ral lines that interfere with the analvsis of elements in the solution. A platinum xvire is generally used as this special excitation dertrode. PRINCIPLE OF EXCITATION
The use of solutions in spectroscopy is not new.
Bmed in part on a paper presented a t the Southwide Chemi~ ~(7) and ~pollok and ~ ~ , ~ (9) l used ~ solutions ~ ~~ eal Conference, Wilson Dam, Alabnma, Ootober 19, 1951. graphite or prior to sparkz P~~~~~~*ddress: washington and ~ ~~ ~ fllyash. l ~i to ~ moist,en ~~ , ~ ~ met,al ~ electrodes ~ ington, Penneylvania. ing, and ohsenred spectral lines characteristic of sub-
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SEPTEMBER, 1953
stances in the solutions. Other workers (1-6, 11) used solutions as one or both of the electrodes and ohtained characteristic spark spectra. Lundeghdh (8) studied the flame spectra of solutions, laying the foundation for present-day flame photometers. The "Spectranal" uses a new method of excitation. A 5.45-ohm resistor connected in series reduces the line voltage to an optimum value for producing the characteristic line spectra of the elements. These line spectra are intermediat,e in intensity between the relatively cool gas flames and the very hot arc or spark spectra of conventional spectrographic methods. By nsing this intermediate excitation method each element produces only a few characteristic or analytical lines with a minimum number of lines of secondary brightness. Any solution of sufficient conductivity can he used as the sparking medium. A 10-15 per cent solution of nitric acid is satisfactory, although solutions of other acids, sodium hydroxide, and even neutral salts can be used. The sparking causes a gradual disintegration of the platinum excitation electrode, and this is quite pronounced in hydrochloric acid, ammonium nitrate, and potassium nitrate solutions. A platinum-iridium electrode might be more resistant to attack in hydrochloric acid solutions. The spectral lines appear on a dark background underneath an illuminated, linear scale. This scale is positioned with reference to a hydrogen index line, 6563 A,, which is always present when aqueous solutions are sparked. The scale illumination can be adjusted with a variable potentiometer, but this is not necessary except in the detection of very faint lines, when the scale lamp can be turned down or off. A reference table supplied by the manufacturer gives, with few exceptions, the scale positions and nTave lengths of the three or fonr most persistent visible lines of each of the detectable elements. BEHAVIOR OF COMMON CATIONS
Although 53 elements are reported as being detectable with the "Spect,ranal" (lo),the present study mas restricted to the behavior of 24 common cations from the qualitative analysis scheme. Practical lower limit,s of detection are given in Tahle 1for the cations studied. These limits were determined by sparking lcnoli-n concentrations of each ion in a 2 A f nitric acid medium. Sitrate solutions of the ions were used in preparing the test solutions except in the cases of As+++, Sh+++, and Sn+4for which chloride solutions were used. A platinum excitation electrode of 0.025-in. diameter mas employed in these experiments. It was sparked in pure 2 M nitric acid before each test with each ion to insure the absence of electrode contamination. The room was not darkened for these studies. Since the exact limit of detection would vary somewhat wit,h the amount of external illumination, the diameter of the excitation electrode, the duration of the sparking, and the condition of the observer's eye, B range of values has been given; evem so,
there is some uncertainty. Mercury (I) 'nd mercury (11) solutions give identical spectra with the "Spectranal," and this would be the case for all elements having multiple oxidation st,ates. TABLE 1 PracticalLower Limits of Detection of 24 Common Cations with the Todd "Spectranal, Model A, Improved" -
The interference of one element with the detection of another element does not appear to be very frequent, but it was noted that a high concentration of magnesium causes sodium or bismuth lines to be less intense. Zinc and cadmium lines, on the other hand, are not affected appreciably by the presence of magnesium, and a high concentration of sodium has little or no effect on the intensity of bismuth lines. A complete study of interference effects mas not attempted. Each element, of course, has its characteristic "Spectranal" spectmm, and many of the spectra, are beautiful with their variety of colored lines of differing intensities. With some elements only a few lines appear, with others many appear. The spectra of calcium and strontium solutions contain several glow regions as well as definite lines. Certain of the transition elements-chromium, cobalt, iron, and nickel-have many "fine-lines" in their spectra. Platinum itself gives more than 40 visible lines, 12 of them quite intense. However, platinum seldom interferes with the detection of other elements. Either the platinum lines fade out, or they'do not coincide with the lines of the other elements in question. It hrss been noted that, using a 0.026-in. diameter platinum excitation electrode, the platinum lines fade out when rather concentrated solutions of barium, calcium, chromium, cobalt, copper, magnesium, silver, or strontium are sparked. Tantalum has been used as the excitation electrode, for it is one of the few metals that give practically no lines with the "Spectranal," but it is not as durable as platinum tovard the sparking and is not as sensitive as platinum for the detection of many of the cations listed in Tahle 1. The spark c,haracteristics of the various elements giving lines are not all alike. Using a 0.025-in. diameter platinum electrode the best lines are observed with some elements when the e1ect)rode is sputtering, with others just as it. begins to g I o ~ and , with still others after the glow sets in. The best conditions for detecting the various elements studied are summarized in Tahle 2. In a,ddit,iont,o t,hese effects,it has been noted
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JOURNAL OF CHEMICAL EDUCATION
that solutions df aluminum, cadmium, and potassium inhibit the glowing of the electrode.
tice. The instrument might well be used by students in the preliminary examination of alloys or minerals prior to a quantitative analysis.
TABLE 2 Best E l e c t d e Conditions for Detecting Various Elements (using a 0.025-in.d i m . Pt excitation electrode)
ACKNOWLEDGMENT
A. Whjle the electrode sputters, before the glow sets in: Cu, Hg R. Just as the electrode begins to glow: Bi, Cd, K, Pb, Zn C. After the glow sets in: Ag, Al, Ba, Ca, Co, Cr, Fe, Mg, Mn, Na, Ni, Sr. Sn
The work of this paper was begun at Emory University and completed at Washington and Jefferson College. The author gratefully acknowledges the courtesy of the Todd Scientific Company in supplying the Model A, Improved "Spectranal" on which experiments at the latter institution were performed. LITERATURE CITED
the With a 0.035-in' diameter platinum excitation is more uniform. The electrode does not become heated to redness and effects such as those summarized in Table 2 are not as pronounced. l.he ,,latinurn electrode becomes it is sparked in some solutions, particularly solutions containing ions of aluminum, cobalt, copper, iron, magnesium, manganese, or nickel. However, even in most serious cases, this contamination disappears if the electrode is scraped lightly or is sparked a few minutes in fresh nitric acid. Certain elements-bismuth, cadmium, mercury, potassium, sodium, and tindo not contaminate the electrode even when rather concentrated solutions of the elements are sparked. USE AS A TEACHING AID
The simplicity of design of the "Spectranal" makes it an ideal instrument for demonstrating the elementary principles of spectroscopy to small groups of beginning chemistly students. Do students realize that the spectral lines which appear in most general chemistry textbooks are merely images of a slit, and that the lines can be broadened or narrowed by varying the slit width? These and other principles become clear after a few manipulations. A suggested experiment is outlined as follows: Place light in front of slit and observe the visible spectrum through the eyepiece. 2. Remove cover of spectroscope, exposing prism, and trace the optical path. 3. Spark a 2 M HNOs solution and observe the H-index line and the P t lines. Kote the effoct of varying the slit width. 4. ~ d 2dmg. each of strontium, and mwesium ions to the HNOa solution, ob~ervingthe characteristic line8 after each addition. Focus the eyepiece to resolve the sodium D-lines. 1.
Teachers of qualitative analysis should find the instrumeut useful in demonstrating the theory of "flame tests" and in checking the composition of student unknows. Furthermore, use of the instrument by individual students is a definite possibility. The technique of operating the instrument can be acquired rapidly, solutions can he prepared easily, and results can be obtained in a matter of minutes after a little prac-
( 1 ) BRODE,W . R., AND J. G. STEED,Ind. Eng. Chem., Anal. Ed., 6 , 157-9 (1934). ( 2 ) DE GRAMONT, A., Compt. rend., 144, 1101-4 (1907); 145, 117lL3 ((1907). ( 3 ) DUFFENDACK, 0 . S., A N D K. B. THOMSOX, Proc. Am. Sac. Testing Mats., 36 (II), 301-9 (1936). ( 4 ) DUFFENDACE, 0.s., F. H. W~LEY,AND J. S. OWENS,Ind. Eng. Chem., Anal. Ed., 7 , 410-13 (1935). ( 5 ) GERLACH, W., A N D W. GERLACH, "Clinical and Pathological Applications of Spectrum Analysis," Adam Hilger, Ltd., London, 1934. ( 6 ) GERLACH, W., AND E. SCHWEITZER, "Foundations and
Methods of Chemical Analyais by Emission Speotrum," Adam Hilger, Ltd., London, 1931. ( 7 ) HAUTLEY, W. N., Phil. Trans., 175,50 (1884). (8) LUNDEGI~RDH, H., "Die quantitative Speetrslsnalyse der Elemente. Part 11," G. Fisoher, Jenct, 1934. ( 9 ) POLLOK, J. H., AND A. G. G . LEONARD, Sei. Proc. Rog. Soe. Dublin, 11,217-36 (1907). ( 1 0 ) Todd Scientific Co., "Operating Instructions for Improved Model Todd 'Spectranal,"' rev. ed., Springlield, Pa., 1952.
( I I ) TWYMAN, F., AND C.
s. HITCHEX,PTOC.Roy. SOC.( ~ o n d m ) ,
A133, 72-92 (1931).
ADDENDA
After this article was in press, the Todd Scientific Company announced the manufacture of an Improved '
