(12) Mitchell, John,,,Jr., Smith, D. M.,
"Aquametry, Interscience, New Yorlr, 194s. (13) Moberg, A I . L., Knight, W. P., ISindsvater, H. iM., ANAL. CHEX 28, 412 (1956).
(14) Orton, IC. J. P., Jones, M., J . Chem. Soc. 101, 1708 (1912).
(15) Stubblefield,F. hi., IND. ENG.CHEV., ANAL.ED. 16, 366 (1944). (lG) Warren, G. G., Cun. Chem. Process Inds. 29, 370 (1945). (17) Weidner, B. V., Hutchison, A. W., Chandlee, G. C., J . Am. Chem. Soc. 56, 1265 (1934). (18) Ibid., 60, 2877 (1938).
(19)
White, L., Jr., Barrett, W.J . , ANAL. CHEM.28, 1538 (1956).
RECEIVED for review August 18, 1956. Accepted November 30, 1956. Division of Analytical Chemistry, 130th meeting, ACS, Atlantic City, N.J., September 1956.
Spectrophotometric Deterrriination of Zinc and Other Metals with Alpha-, Beta-, Gamma-, Delta-Tetraphenylporphine CHARLES
V.
BANKS and RAMON
E.
BISQUE
lnsfifufe for Afomic Research and Deparfmenf of Chemisfry, Iowa Sfafe College, Ames, Iowa
b Trace amounts o f zinc can be determined by utilizing the spectrophotometric properties o f the zinc complex of a, 0, y, 6-tetraphenylporphine in glacial acetic acid. Procedures deserving of special mention are those for the determination of trace amounts o f zinc in cadmium, magnesium, rare earth, beryllium, iron, yttrium, and the alkali metals. A method for the indirect determination of several other metals in certain compounds i s also described.
T
HE SIMPLER PORPHINES and their metal derivatives have been the subject of a number of investigations in the past 15 years. Their relationship to the more complex porphyrins, hemoglobin and chlorophyll, and the presence of numerous porphyrin metal derivatives in natural petroleum lends broad appeal to these studies. The porphines and their metal derivatives have intense, well-defined absorption bands in the visible region of the spectrum which serve in their characterizntion. These bands have not previously been utilized in analytical methods for the determination of metals. Spectra of many of the simpler porphines and their metal derivatives have been reported (1-3, 6-7, IO, IS, 15, 17) and several recent publications are concerned specifically m-ith a,p,r,b-tetraphenylporphine (TPPH2) (2, 6, 7, IS, 14). The present investigation is concerned with the formation of the zinctetraphenylporphine complex, ZnTPP, in glacial acetic acid and the preliminary inyestigation of analytical applications to determine trace amounts of zinc spectrophotometrically, One indirect application by which metals other than zinc can be determined in certain com-
522
ANALYTICAL CHEMISTRY
absorptivity of a,PJr,6-tetraphenylporphine in glacial acetic acid a t 655 mp is in the order of 27,000, while that of the zinc complex a t 551 mp is approximately 14,000. The change in absorbance which takes place a t 551 mp when 5 y increments of zinc are added to 25 ml. of a 3.5 x lO-5Ad solution of .,p,r,6tetraphenylporphine in glacial acetic acid is s h o m in Figure 2, curve A. The upper extremity of this calibration curve deviates from a straight line plot as the excess reagent is depleted. The addition of an acetate salt increases the concentration of the solvent anion and functions as a base according to the theory of solvent systems (8), thereby shifting the equilibrium of
pounds is discussed. The nomenclature used in this work is that recommended by Aronoff (4). T t IlORY
When zinc acetate is added to a solution of a,P,yJ6-tetraphenylporphine in glacial acetic acid, the following equilibrium is established in 60 to 70 minutes a t room temperature: Zn++
+ TPPHZ = ZnTPP + 2H+
(1)
As equilibrium is approached the absorption band 0' the reagent a t 655 mp diminishes TI iile the absorption band of the complex a t 551 mu increases without any appreciable shift in wive length (Figiire 1). The molar
WAVE LENGTH I N
Figure 1. Absorption spectra o f complexes
01,
mp
8, y, 6-tetraphenylporphine and
7 X 10- GMTPPHz in glacial acetic acid 7 X 1O-'6MTPPH2, in excess zinc acetate, 2 X 10-M sodium acetafe C. 7 X 10"6M TPPHz, in excess copper acetate, 2 X lO-3M sodium acetofe
A.
B.
CONCENTRATION OF ZINC,
Figure 2.
Y / 2 5 ml.
Zinc added to 3.5 X 1 O-5M TPPH, in glacial acetic acid A.
E.
Zinc acetafe Zinc perchlorate
pletely. This concentration was chosen such that the resulting ionic strength was high enough to render further addition of electrolytes ineffective in altering the activities of the system (9). Subsequent addition of sodium acetate further increased the ionic strength of the system but displayed its basic effect by shifting the equilibrium t o the right again. The latter effect, however, reached a maximum after which reversal again took place, indicating that some factor other than ionic strength alone is responsible for the over-all reversal effect (Figure 4). This other factor does not appear to be the formation of acetate complexes of zinc-tetraphenylporphine similar to the pyridine complexes observed by Miller and Dorough (IZ),because the absorption band of the complex a t 551 mp does not shift to a different wave length a t high acetate concentrations. Quantitative comparison of the two effects indlmtes that the influence of ionic strength is the major factor. MATERIALS AND APPARATUS
Glacial Acetic Acid. Fifty milliliters of acetic anhydride n7as added t o each 5-pound bottle of reagent grade glacial acetic acid a t least 12 hours before use. Rubber gloves were worn a t all time,s to prevent uncomfortable acetic acid burns.
1.01
0.8
.38
-0 .
-3 LO
.36
lMOAc)
Figure 3. Plot of log (metal acetate) vs. absorbance at 551 m p showing effect of metal acetates on zinc-TPPHz equilibrium in glacial acetic acid A.
Cadmium
E. Sodium C.
Magnesium
.24 LOO ( W . a ~ C )
Equation 1 to the right. This effect can be observed esperimentally by comparing the absorbances a t 551 mp of two identical solutions of a,p,y,& tetraphenylporphine to which zinc has been added, in one case as zinc acetate arid in the other as zinc perchlorate. Figure 2 shows an increase in absorbance which is much greater in the former case. The addition of sodium acetate to a system in which zinc ion and a,p,y,& tetraphenylporphine are in equilibrium serves to shift the equilibrium in Equation 1 to the right until a maximum amount of the zinc present is
complexed. Further addition of sodium acetate finally reverses the equilibrium as shown by Figure 3. This reversal is due to a t least two factors: that of increased ionic strength which ninrkedly decreases the activities in a solvent with as low a dielectric coilstant as acetic acid (dielectric constant = 6.1), and the formation of ion pairs due to excess acetate concentration. The significance of the latter effect was shown experimentally by maintaining an effectively constant ionic strength in the system by the addition of sodium perchlorate a t a concentration great enough to reverse the equilibrium com-
Figure 4.
,Plot of log (sodium acetate) 551 mp in glacial
vs. absorbcince at
acetic acid Acid is O.O08/rl in sodium perchlorate, contains 2 0 y of zinc in equilibrium with 2.8 X 10% TPPHz
Metal Acetates. Reagent grade metal acetates mere dissolved in glacial acetic acid. I n some instances i t was necessary t o add small amounts of mater t o effect solution. An iron acetate solution was prepared by dissolving finely divided electrolytic iron ii:. dilute acetic acid by proVOL. 2 9 , NO. 4, APRIL 1957
* 523
longed refluxing. Water was removed by addition of acetic anhydride. a,P,y,&Tetraphenylporphine (molecular weight, 614.17). This reagent was prepared and purified by the method of Priesthoff and Banks (IS). A 1.75 X 10-411f stock solution was prepared by refluxing 1 liter of glacial acetic acid containing 0.1075 gram of free-base a,pLy,6-tetraphenylporphinefor 8 hours. A a-ml. portion of this solution diluted to volume in a 25-ml. volumetric flask gives the desired concentration for the procedures outlined below. Standard Zinc Solution. A standard zinc solution mas prepared by dissolving 0.336 gram of zinc acetate dihydrate in 1 liter of glacial acetic acid. The acid was kept hot and water added slowly in 0.5-ml. aliquots until the solid dissolved. Microburets used were 5-ml. IGmble Exax graduated t o 0.01 ml., and volumetric flasks and pipets mere of the Pyres class A type. A Cary Model 14 recording spectrophotometer was used. DETERMINATION
OF ZINC IN METALS
Acetic Acid-Soluble. Metals found t o be soluble in glacial acetic acid include the alkali metals, magnesium, and most of the rare earth metals. Magnesium will be treated as a n example. The effect of increasing magnesium concentration on the zinc ion-oc,P,r,6-tetraphenylporphineequilibrium is shown in Figure 3. At magnesium concentrations below 2 X 10-4Jf the change is negligible and corrcctions arc unnecessary. At higher concentrations, however, the magnesium concentration must be held fairly constant in all samples if the results are to be related to a calibration curve. An alternative procedure mould involve making corrections from a curve such as is shown in Figure 3. CALIBRATION CURVE. Prepare by dissolving pure magnesium metal in acetic acid to yield a standard solution of tlie desired concentration. Transfer into a series of 25-mI. volumetric flasks aliquots of the standard zinc acetate solution containing 5-7 increments of zinc. Add appropriate aliquots of the magnesium and 01, P,y,&tetraphenylporphine solutions to each, mix, dilute to volume, allow to stand for 1 hour, and read the absorbance a t 551 mp against a reference solution of glacial acetic acid, using ti-cm. cells. Plot these absorbances us. mierogran~sof zinc per 25 nil. The resulting calibration curve will resemble curve A in Figure 2, although absorbance readings may be l o m r for a given quantity of zinc and the sensitivity decreased slightly. Solutions sliouId be kept less concentrated than 1 x 10-2M if possible. PROCEDURE. Dissolve samples, dilute to volume in a 100-ml. Volumetric flask, and remove aliquots to yield the same concentration of magnesium when 524
ANALYTICAL CHEMISTRY
T a b l e I. Direct Dlstermination of Zinc Zinc, y hletala Added Found Error hfg
Be
Cd
40 50 60
70 40
60 80 100
50 60 70 80
43
+3
72
+2 +2
35
-5
79
-2 -1 -3
50 62
58 97 49 61 69 82
0
-1 +1 -1
+2 Following concentrations were aaed: magnesium, 5 X lO-3M; beryllium, 5 X lO-3M; cadmium, 2 x 10-311. a
diluted to 25 ml. as was used in the preparation of the calibration curve. Add the same amcunt of a,P,y,&tetraphenylporphine, dilute to volume, allow to stand for 1 hour, and read the absorbances as before. Table I shows tlie recovery of zinc in four samples in Nhich the zinc was added to the solution of acetic acid while the magnesium metal was dissolving. A correc:t,ion mas necessary for a trace amount of zinc present in the magnesium meld. Formic Acid--Soluble. hletals found t o be soluble in hot, dilute formic acid are b?ryllium, iron, and yttrium. Berylliim will be treated as an example. T h ? metal dissolves in formic acid and the formate formed on evaporation 1s soluble in dilute acetic acid. Formic acid in concentrations greater thar 5 x 10-2iM begins to affect the zinc ion-alp,7,6-tetraphenylporphine equilibrium. Consequently, the bei,yllium concentration must be kept lower than 5 X 10-ziV, preferably about 2 X 10-3M, and as constant as possiksle in order to obtain the best analytical results. CALIBRATIONCURVE. Prepare by dissolving pure btmryllium metal in formic acid to yield :t standard solution of the desired concentration. Into a series of beakers, add aliquots of the standard zinc acetate solution containing 5-7 increments of zinc and add appropriate and identical aliquots of the beryllium solution to each kcaker. Evaporate to dryness, being careful not to heat the dry residue for any appreciable length of time. Dissolve the residue in dilute acetic acid and take to dryness a second time. Dissolve the residue in glacial acetic acid or in dilute acetic acid followed by addition of acetic anhydride. Transfer to a series of 25-ml. volumetric flasks, add 5 rnl. of a,&y,b-tetraphenylporphine solution, dilute to volume, and continue as for the determination of zinc in magnesium. PROCEDURE. 1)issolve samples, dilute to volume in a 100-ml. volumetric flask, and remove aliquots to yield the
same concentration of beryllium when diluted to 25 ml. as was used in the preparation of the calibration curve. Transfer these aliquots to beakers, evaporate to dryness as before, redissolve in acetic acid, and transfer to 25ml. volumetric flasks. Add 5 ml. of a,p,y,&-tetraphenylporphine solution, mix, dilute to volume, and measure the absorbance a t 551 m p after 1 hour. Results for four consecutive samples in which the final beryllium concentration was 5 x I O - ~ Mare shown in Table I. Metallic samples of beryllium and iron dissolve rather slowly in formic acid and should be taken in a finely divided form if possible. Iron also dissolves directly in acetic acid although much more slowly. The color which develops when iron formate is dissolved in acetic acid is characteristic of iron acetate solutions in glacial acetic acid. A Correction must be made for absorbance a t 551 mp due to this color. This correction is discussed below. Other Metals. Cadmium mas the only example encountered in the category of metals which yield nitrates easily converted to oxides soluble in acetic acid. However, there may be others. The close association of cadmium and zinc in nature due to their chemical similarity and the scarcity of spcctrophotomctric methods for determining trace amounts of zinc in the presence of cadmium renders this application deserving of particular mention. By reference to curve A in Figure 3 the advantage of maintaining cadmium concentrations in the range of 2 X lO-4M is readily seen, because in this range variations in cadmium concentration have little effect on the zinc ion-a:,p,~,6-tetraphenylporphineequilibrium. However, much higher concentrations of cadmium have been used without encountering experimental difficulties. CALIBRATIONCURVE. Prepare b y dissolving pure cadmium metal in nitric acid (or cadmium nitrate in water) to yield a 1 x 10-2iLI standard solution of cadmium. Transfer into a series of glazed crucibles aliquots of the standard zinc acetate solution containing 5-7 increments of zinc. Add 5 ml. of the cadmium solution to each crucible and evaporate to dryness. Ignite the residues a t 500" to 600°C. for 15 minutes, allow to cool, add 5 to 10 ml. of glacial acetic acid, and warm to dissolve. Transfer into 25-m1. volumetric Aasks, add 5 ml. of a, P,r,&tetraphenylporphine solution, mix, dilute t o volume, and continue as for the other calibration curves. PROCEDURE. Weigh 50-mg. samples into crucibles and dissolve in dilute nitric acid. Evaporate to dryness and ignite as before. Dissolve samples from crucibles with hot glacial acetic acid, transfer to a 100-ml. volumetric flask, dilute t o volume, mix, and transfer a 10-ml. aliquot into a 25-ml. volumetric flask. Add a,ply,6-tetraphenylporphiiie
mix, dilute to volume, allow to stand for 1 hour, and read the absorbance a t 551 mp as before. Results for four consecutive samples in which the final cadmium concentration was 2 x 10-311f are shown in Table I. INDIRECT DETERMINATION
OF OTHER METALS
Many organic salts, chelates, complexes, and other compounds which dissociate as bases in glacial acetic acid can be analyzed for their metallic component by observing their basic effect on the zinc-alp, r,&tetraphenylporphine equilibrium through changes in the absorbance at 551 mp. Increased concentration of a favorable anion serves to decrease the effective hydrogen ion concentration and thereby increase the amount of zinc complexed. This effect is shown by the straight line portions of curves A and B in Figure 3. Hence, an indirect spectrophotometric determination can be devised to complement or substitute for gravimetric procedures to determine nietals other than zinc in certain compounds. Results for eight consecutive determinations of sodium are shown in Table 11. Similar procedures have been applied to the determination of potassium and ammonium as acetates, cadmium and lead as carbonates, and sodium as the chelate of dipivaloylniethane. Preparation of calibration curves for such indirect methods requires that the compound used as a standard and the compound to be determined be completely dissociated in glacial acetic acid to yield anions of comparable basicity (11). As a difference of 0.1 y of zinc in 25 ml. would introduce a significant error in the indirect determination of another metal, extreme care is required in measuring the amount of zinc added.
Calibration Curve. Dilute 50 nil. of the standard zinc acetate solution t o volume in a 500-ml. volumetric flask and use this solution to deliver aliquots containing 20 y of zinc into a series of 25-ml. volumetric flasks. Increase the concentration of the standard compound by adding respectively larger aliquots of a standard solution into each volumetric flask. Add 5 ml. of a,O,r,& tetraphenylporphine solution, mix, dilute to volume, allow to standfor 1 hour, and read the absorbances a t 551 mp in 5-cm. cells as described for the determination of zinc in magnesium. Prepare a calibration curve by plotting absorbances a t 551 mp us. micrograms of the metal being determined per 25 ml. Procedure. Dissolve an appropria t e amount of the sample in glacial acetic acid, or in dilute acetic acid followed by the addition of acetic anhydride, and dilute t o volume in a 100ml. volumetric flask. Into 25-ml. volumetric flasks add aliquots of this solution to yield a t least three different concentrations of the sample. Add a,& y,6-tetraphenylporphine, mix, allow
to stand for 1 hour, and read absorbances a t 551 mp. LIMITATIONS AND INTERFERENCES
Preliminary tests are necessary to determine whether the above procedures are applicable to alloys of the metals mentioned as being soluble in acetic acid or formic acid. The presence of other metals in proportions great enough to render samples insoluble in these solvents may necessitate the use of nitric acid. The latter alternative is limited by the fact that appreciable amounts of metals whose oxides are not soluble in acetic acid may render the procedures inapplicable. Incomplete conversion of nitrates of some metals to the oxides also causes interference due to the subsequent solvolysis of the metal nitrate in acetic acid. The cations which were checked for interference were those most likely to form complexes with a,p,y,&tetraphenylporphine in acetic acid (7). Quantitative study of the zinc-a,P,y,Gtetraphenylporphine equilibrium in solutions of nickel(II), cobalt(II), iron(I1) and -(III), and silver(1) a t concentrations up to 4 x 10-311.1showed no interference due to complex formation. Copper(I1) is the only ion encountered other than zinc which forms a complex a t room temperature. The absorption spectra of these two metal complexes are similar. The prominent band of the copper complex occurs a t 538 mp, and the molar absorptivity is approximately the same as for the zinc complex (Figure 1). Sickel(II), cobalt(II), and iron(I1) develop significant colors in glacial acetic acid and corrections for their absorption a t 551 mp may be necessary. Their colors are easily detected visually a t the following concentrations in a 50nil. \.ohmetric flask:
Ion Co(I1)
Fe(I1)
Ni(I1)
Color Pink Yellow Green
Molar Concn., AbsorpM x tivity a t lo4 551 Ill, 6 4 8
15 30 20
A close approximation of the copper (11) concentration (if appreciable) can be made by observing the absorbance of copper(I1) acetate a t 680 mp in concentrated solutions of the sample. The molar absorptivity of copper(I1) acetate a t 680 mp in glacial acetic acid is approximately 200. To determine the copper concentration more accurately then, the procedure of Smith and TVilkins (16), utilizing 2,9-dimethyl-4,7diphenyl-1,lO-phenanthroline,is recommended. Blanks are prepared by delivering aliquots of a standard copper (11)acetate solution from a microburet
Table II.
Indirect Determination of Other Metals
Sodium Salta Formate
Taken 20 40 60 80 20
Benzoate
40 _. 60
so 5
Sodium, y Found Error 18 41 58 78 20
t-1 -2 -2 0
39 _.
-1
60 77
0 -3
-2
Both salts used a t concentration of
x
io-3~1.
Table 111.
Determination of Zinc in the Presence of Copper
Copper Present, y
Added
Zinc, y Found
Error
to give a final copper(I1) colicelitration corresponding exactly to that of the sample. This aliquot is added to a 25ml. volumetric flask containing the same amount of a,p,y,&tetraphenylporphine t o be used in the final determination of zinc. The absorbance of this blank a t 561 mp subtracted from that of the final sample solution a t 551 nip gives an absorbance value which may be referred to a calibration curve for the dete~minationof zinc. Copper concentrations low enough to escape visual detection by the color developed in glacial acetic acid are easily detected by a shift in wave length of the zinctetraphenylporphine absorption band from 551 m p toward 538 nip. This shift is indicative of the zinc-copper ratio in the sample. Table I11 shows the results or determinations a t several different zinc-copper ratios. LITERATURE CITED
(1) Albers, V. DI., Ihorr, H. IT., J . Chenz. Phvs. 4 , 422 (1930). . . (2) Zbid., 9, 497-(1941). (3) .4lbers, V. DI., Knorr, H. V., Fry, D. L.. Ibid.. 10. 700 (1942). (4)Aronoff,’ S., ’ C h e k . R’eus. 47, 175 f1950). \ - - -
rlronoff,‘S., Calvin, NI., J . Org. Chem. 8 . -205 - - (1943). - 1
\ - - - - I
Barnes, .T. W.,Dorough, G. D., J .
Ant. Cherra. SOC.72, 4045 (1950). Dorough, G. D., LIiller, J. R., Huennekens, F.AI,, Zbid., 73,4315
----
(14.511 \ r.
Franklin, E. C Zbid., 46,2137 (1924). Glasstono, S., ’“Thermodynamics for Chemiiits,” p. 414, Van Nostrand, New Pork, 1947. Knorr, 1%. V., Albers, V. LI., J. Chent. .Phys. 9, 197 (1941).
Kolthoff, I. RI., Bruckenstein, S., J . Am. Chem. SOC.78, 1 (1956). VOL. 29, NO. 4, APRIL 1957
525
(12) Miller, J. R., Dorough, G. D , Zbid . 74, 3977 (1952). (13) Priesthoff, J. H., Banks, C. V., Zbid., 76, 937 (1954). (14)Rotllemund, p., Menotti, A. R., Ibid., 63, 267 (1941).
(15) Ibid., 70, 1808 ( ?048). (16) Smith, G. F., Wilkins, D. H., ANAL. CHEII.25, 51C (1953). (17) JFralter, R.I., J . Ant. Chem. soc. 75, 3860 (1953).
RECEIVEDfor review June 23, 1956. Accepted December 6, 1956. Contribution No. 496. Work performed in hmes Laboratory of u. 8. Atomic Energy Commission.
Spectrographic Determination of Lead in Leaded Steel JAMES E.
PATERSON
Graham Research laboratory, Jones & laughlin Sfeel Corp., Piiisburgh 30, Pa.
b increased production of leaded steel has required development of a rapid method for the determination of lead in the range from 0.15 to 0.3570. Lead can b e determined spectrographically b y the procedure described in the range from 0.10 to 0.50%. A nitric acid solution of the sample i s used with a rotating disk electrode and Multisource excitation. The method is sufficiently rapid and economical for routine control use. With the usual laboratory conditions, a limit of error of ~ 5 . 9 %at the 95% confidence interval can be obtained a t a concentration of o,20470 lead.
E
anDm t o steel in amounts of 0.15 to 0.35% to improve its machinability. As increased tonnages have been produced, there has been more need for a rapid analytical procedure for research and control determination of the lead content. While procedures are available for the determination of lead in steel in trace quantities (3, 4), in the amounts present in leaded steels this element appears as a heterogeneous mixture rather than a solution, and the concentration varies sufficiently from spot to spot in a sample to make normal direct spectrographic analysis impractical. There is also a problem in preparation of the surface for analysis by either emission or x-ray spectroscopy. Care must be exercised to avoid smearing the lead over the surface during cutting or polishing the samples. It was decided to use solution samples t o avoid these problems. EAD IS
APPARATUS AND REAGENTS
Nitric acid, 1 to 1. Erlenmeyer flasks, 250 ml. Vacuum filtration assembly. The spectrographic equipment used t o develop this procedure consisted of a 2meter grating spectrograph; Multisource unit (Applied Research Laboratories, Glendale, Calif.); film densitometer; film developing, mashing, and drying apparatus; and a calculating board. Kodak D-19 developer. 526
ANALYTICAL CHEMISTRY
Acetic acid shortst3p, 5%-
Kodak rapid liquid fixer with hardener.
Rotating disk electrodes, United Carbon Products, Style 106, or National Carbon Co., Style L4075. PROCEDURE
A 2-gram sampli: of chips from the steel to be analyzed is dissolved in 20 ml. of 1 to 1 nitric acid in a 250-ml. Erlenmeyer flask. After solution the sample is filtered, then taken to the spectrograph where it is excited with a hfultisource dischuge (Model 4700) using a rotating disk electrode. The following excitation and exposure conditions mere used. Output voltage 900 volts 15 pf. Capacitance Resistance 25 ohms Inductance 440 ph. added Discharge point control 30 Slit width 40 microns Prespark 12 seconds Exposure 30 seconds Intensity control 50% transmittance Sample gap 3 mm. Speed of disk o r.p.m. Sample polarity Negative The first order spectrum is recorded on Kodak Spectnim Analysis #l film. The film is developed for 3 minutes at 68" I?. in D-19 developer, rinsed 20 seconds in a 5% acetic acid stop bath, and fixed for 30 seconds in Rodak rapid liquid fixer with hardener. After a 10minute wash in running water, the film is dried and interpreted. The lead line a t 2833.07 A. is u s d for the analysis line and the iron line a t 2827.9 A. is used for the internal stanc1:ird line. EXPE UMENTAL
In preliminary ,:speriments a portion of a sample in solution mas dried on unsealed, or wax-waled, flat electrodes. This method was not so precise as was required, nor mas it as rapid as it was felt the procedure could be. The rotating disk electrode was chosen over the porous cup electrode because it appeard that the disk would offer a greater flexibility in choice of excitation for the sample. A few tests made with each 'iype of electrode con-
firmed the superior flexibility of the rotating disk. DISCUSSION
Comparison of a spark discharge and the Multisource discharge mas made using the spark excitation outlined in Table I. A precision of 1 2 2 % a t the 95% confidence interval mas obt,ained. These conditions were adapted from those suggested by Paglinssotti (9).
Table 1. Source and Exposure Settings for High Voltage Spark Excitation (ARL Model 4700 source)
Capacitance Resistance Inductance Input voltage Sample gap Slit width Prespark Exposure Intensity control Speed of disk
0.007 pf. Residual 360 pli. added 150 volts 3 mm. 40 microns 12 seconds 30 seconds 100% transmittance 5 r.p.m.
Tests were made to determine whether the speed of rotation of the disk would affect the determination significantly. Speeds of 2, 5, and 10 revolutions per minute were used. Except for disk speed, the settings mere the same as used for the procedure. No significant effect was caused by the disk speed and a speed of 5 r.p.m. was used. The sample was made negative as suggested by Gillette, Boyd, and Shurkus (1). Tests with the sample positive indicate that more cyanogen background is obtained, as well as a markedly increased tendency for the sample to boil. The spectra obtained with the sample positive show only a slight increase in sensitivity and no increase in precision or accuracy. Standards were prepared by adding lead in nitric acid solution to meighecl samples of pure iron. These standards were then dissolved in 1 to 1 nitric acid in proper volume t o maintain a concen-
