(2) Boltz, D. F., Mellon, M. G., IND. ENG.CHEM.,ANAL.ED. 19,.873 (1947). (3) Efroymson, E. F., “Stepmse Multiple Regression Program,’’ available from SHARE-IBM program, Esso Research and Engineering Co., July 1, 1958. (4) Fiske, C. H., Subbarow, Y., J. Bid. Chem. 66, 375 (1925). (5) Kahler, H. L., IND.ENG. CHEM., ANAL.ED. 13, 536 (1941).
(6) Kitson, R. E., Mellon, M. G., IND. ENG.CHEM.,ANAL.ED. 16, 466 (1944). ( 7 ) Kolloff, R. H., A . S.T. M.Bull. 237, 74 (TP-94-TP-100) April 1959. (8) Lundgren, D. P., Loeb, N. P., ANAL. CHEM.33, 366 (1961). (9) Schwartz, M. D., IND.ENG.CHEM., ANAL.ED. 14,893 (1942). (10) Strickland, J. D. H., J . Am. Chem. SOC.74, 862, 868, 872 (1952).
(11) Weiser, H. J., J . Am. SOC.34, 124 (1957).
Oil Chemists’
(12) Welche2 F. J., “Organic Analytical Reagents Vol. I, p. 229, Van Nostrand, New York, 1947. (13) Woods, J. T., Mellon, M. G., IND. ENG.CHEM.,ANAL.ED. 13, 760 (1941).
RECEIVEDfor review April 16, 1962. Accepted August 3, 1962.
Optical Emission Spectrographic Method for Analysis of Microgram Deposits on Electron Tube Parts ALFRED M. LIEBMAN Radio Corp. of America, Elecfron Tube Division, Harrison, N . 1.
b This paper describes a rapid, precise, and accurate spectrographic method for the determination of seven elements (Bo, Sr, Ca, Mg, Mn, Si, Ni) found in small sublimed deposits on electron-tube parts. These deposits constitute a variable matrix because the above elements vary over unusually wide concentration ranges (from tenths to hundreds of micrograms). Matrix effects are suppressed by a relatively high concentration of cobalt in the solution used to remove the deposit. The cobalt serves as a buffer and an internal standard in a modified graphite spark technique. The applicability of this method for the quantitative analysis of deposits on other substrates is also discussed.
s
form on the internal parts of an electron tube during its processing and operating life. When these deposits form on the plate, they can cause secondary emission. When they form on the insulators, they can cause interelement leakage. The effects of films which form on grids depend on the tube type and the composition of the film. Until a short time ago, these tube deposits could only be qualitatively or semiquantitatively analyzed by rapid and convenient techniques. Although the results of these analyses were helpful in many cases, they Kere not accurate enough for many critical tube studies. Thus, it was necessary to develop a new quantitative method having much improved precision without sacrificing the speed necessary for the routine analysis of many samples. The elements which had to be determined in microgram quantities by this method-Ba, Ca, Sr, Mg, Mn, Si, and Ni-were chosen for several reasons. Barium, calcium, and strontium were selected because they are the major constituents of the cathode MALL DEPOSITS
1370
ANALYTICAL CHEMISTRY
coating which partially sublimes during tube operation. The sublimation rate of these three elements is in turn influenced by the reduction of their oxides by magnesium, silicon, and manganese which diffuse from the cathode itself. Because these reducing agents may also sublime, they were included in the analysis. The analysis of nickel was also required because direct sublimation of the nickel cathode base material could also occur. A literature search showed that many attempts had been made to analyze sublimates in receiving tubes. A wide variety of conventional and imtrumental methods has been developed. Only the barium in tube deposits could be determined by a wet microchemical method ( 3 ) . Mass spectrometric methods either reported qualitative data (15) or were calibrated to give ratio measurements (1) of Ba to BaO and Sr to SrO. Radioactive tracer techniques (6) giving qualitative data have been reported. Although a tracer technique (fl)which provided quantitative results for barium, calcium, and strontium has been developed, the sublimate analysis cannot be performed until 1 week after the tube has been subjected to the required treatment. This week delay is necessary to allow the Bal@ and its radioactive daughter, Lala, time to attain radioactive equilibrium. Seutron activation techniques (19) have been employed for sublimate analysis, but this method requires preliminary chemical separations which are time-consuming. Two methods based on electrical measurements have been reported. The first method (12) detects increased thermionic emission from a tungsten filament target; however, this method is applicable only to the analysis of barium sublimates. In the other technique ( I 6 ) , only the electrical resistance of the sublimed film can be determined. An opticalemission spectrographic ( I 7) method
has been developed for the determination of barium and strontium. Jaycox (9) later extended this method to a general semiquantitative technique which included all other chemical elements of interest. This latter method, however, involves several operations after the removal of the deposit from the tube, such as evaporating the solution in a weighed amount of buffer and firing a t 700” C., and is more time-consuming than the method described here. illso, there are mixing errors involved in this procedure which are discussed later in this paper. An x-ray emission spectrometric method (2) developed a t RCA can only be used in the analysis of Ba, Sr, Ni, and Mn in tube deposits. This concludes a representative survey of the literature on the analysis of tube sublimates. Although more papers have been published on this subject, no one as yet has reported an analytical method which is capable of determining microgram quantities of the seven elements in electron tube sublimates with the convenience, rapidity, and precision necessary for tube problem work. Accordingly, a new analytical method was needed for this purpose. An optical emission spectrographic method was chosen because of its excellent sensitivity and good precision a t low concentration ranges. EXPERIMENTAL
The tube component on which the sublimate is deposited is placed in a commercially available cylindrical plastic vial 1 X inch (available commercially from Spex Industries, Inc.). Structural members of most tubes (grids, plates, shields, and micas) can be placed inside this container by cutting or bending the parts. Then 0.50 ml. of a 2% by volume nitric acid solution containing 0.600 mg. per ml. of cobalt is pipetted into the vial. The vial is closed and shaken for 1 minute. This procedure dissolves the sublimate on the surface of the tube element without
age, 220 v.a.c., 10, 60--; analytical gap, 2 mm.; auxiliary gap, 3 mm.; voltage in primary, 175 volts, 5 discharges per half cycle. A detailed description of the a.c. spark source is given by Fowler and Wolfe (7). The commercially available spark source used in this paper (available from the Jarrell-Ash Co. under the trade name Varisource) is similar to that of Fowler and Wolfe except that the auxiliary gap is irradiated by a mercury-vapor lamp to improve the reproducibility of the spark discharge. A Jarrell-Ash 3.4-meter Ebert mount spectrograph is used for this work. A 15,000-line-per-inch flat grating blazed for high intensity in the first order a t 3100 A. is the dispersing element. The reciprocal linear dispersion in the first order is 5.0 A. per mm. The spectrographic conditions are: exposure time, 30 seconds; split filter transmitting 1 0 0 ~ oand 15.9y0 of the incident light placed over the entrance slit; Eastman Kodak 33 plates; source to slit distance, 20.5 inches; focus on collimator; vent in arc stand open full to permit maximum exhaust of vapors from analytical gap. Photometry. The analytical and internal standard lines were read on a Jarrell-Ash projection-type densitometer in units of per cent transmittance. These readings were converted to intensity ratios by an emulsion calibration curve with the linear range extended by a Seidel transformation (IO). The emulsion calibration was performed using the two-step method described by Churchill (4) and modified by Schmidt (18). An iron arc a t 3.0 amperes d.c. served as the light source. Selected iron lines in the 2900 A. to 3100 A. spectral region were used for the preliminary curve. Because the response of the Eastman Kodak 33 plate is unusually uniform over a wide spectral range, a separate emulsion calibration for the visible region was not necessary. This uniformity is illustrated in Figure 1 where the two analytical curves for
severe attack of the base material of that tube element. However, because of the effects of even minute contamination from the base material, nickel and manganese cannot be determined in nickel-clad steel plate deposits, and magnesium and silicon cannot be analyzed in mica sublimates. In some tubes run for long periods of life testing, the surface film can possibly diffuse into the grid and form a n alloy. This alloy would still be dissolved by the dilute nitric acid solution and therefore would not affect the accuracy of the analysis. Analysis of a sublimate deposited on the glass bulb of a tube requires a special sample preparation procedure. First, the bulb is cut cleanly near the base of the tube by a hot-wire glass cutter and removed for analysis. Then 0.50 ml. of the 0.600 mg. per ml. Co solution is pipetted directly into the bulb itself. This solution is swirled t o dissolve any sublimed deposit. The 0.50 ml. of solution, whether in the glass bulb or in the plastic vial, contains the sublimate and constitutes the analytical sample. A 0.1-ml. aliquot of the above solution is taken for a single determination in a manner similar t o that described for the graphite-spark technique ( I S ) . In this technique, 50 pl. of solution are pipetted onto each of two flat-topped graphite electrodes which have been precoated with a drop of 5% Apiezon N solution in benzene. This precoating prccedure prevents seepage of the sample solution into the graphite electrodes. Because tn-o such electrodes oppose each other in the spark discharge, 0.1 ml. of sample is used for each determination. All samples are run in duplicate so that sufficient sample solution is left if a repeat analysis is necessary. Unlike the previously mentioned graphite spark technique, 0.180-inch instead of 0.25-inch diameter graphite electrodes are used. The smaller diameter reduces wandering of the discharge and thereby improves the precision of the method. All electrodes are placed in a Transite electrode block on a hot plate where the drops of sample solution are dried. The electrodes are then excited under the following conditions: source, a.c. spark; r. f . current, 6.5 amp.; capacitance, 0.010 yf.; inductance, 155 ph.; resistance, residual; open circuit volt-
Table I.
Analytical and 'Internal Standard Lines
(All wavelengths are given in A.) Co 3044.0 F Co 3995.3 F internal standard internal standard Ba 3891.8 F (25 t o 250 Mn 2949.2 fig.!
Ba 4054.0 F ( 0 . 7 5 to Mg 2852.1 F 25 pg. 1 Ca 3968.5 F Si 2881.6 Sr 4077.7 F ( 0 25 to Sr 3464.6 F ( 5 . 0 5 . 0 pg.) to 50 pg.) Ni 3050.8
strontium are shown: one based on spectral lines in the ultraviolet, the other on lines in the visible region. Even though the same emulsion calibration curve was used to obtain intensity ratios for both curves, the lines are almost parallel. The same uniformity for this emulsion is also evident in the visible region to a wavelength of 4554 A. Th'is can be seen in Figure 2 where analytical curves at 3891 A. and 4554 A. for barium exhibit the same parallelism shown in Figure 1. Since this work was done, the Eastman Kodak 33 plate is no longer commercially available. However, its replacement, the Spectrum Analysis NO. 3 plate (6),is only slightly faster and has almost identical contrast and uniformity of spectral response. Table I shows a list of the analytical lines and their respective internal standard lines. I n this table, the designation F means that the line is read in the filtered half of the exposure (15.9% of the incident light transmitted). All other lines are read in the 100% step. Because of the wide spectral range of the analytical lines, two internal standard lines are used: One cobalt line is used for the ultraviolet, the other for the visible region of the spectrum.
BARIUM 4554,0A,-MiCROGRAMS
603
-
400
0 c Cc 4
g
2
I00
380
360 040
0 020
0 4 0 060
Figure 1 .
10
20
40
60
10
20
40
60
BARIUM 3891 EA-MICROGRAMS
100
STRONTIUM-MICROGRAMS
Analytical curve for strontium
Ranges: Sr 4077.7 A., 0.25 to 5.0 pg.; 5r 3464.5 A., 5.0 to 50 pg.
Figure 2.
Analytical curve for barium
Ranges: Ba 4554.0 A., 0.75 to 25 pg.; Ba 3891.8 A., 25 to 250 pg.
VOL. 34, NO. 1 1 , OCTOBER 1962
1371
6 00 I
u a
4 00
O ?
-a 2.00 $ mt?
?om 0
0
2 00
1.00 0 80 0.60
0 40
,
,
0.40 060 IO 20 4.0 6.0 MAGNESIUM-MICROGRAMS Figure 4. Analytical curve for magnesium
020
10
Range: 0.20 to 10 pg.
Standards and Working Curves. Standards for the concentration ranges of interest were prepared by volumetric addition of stock solutions of each chemical element. Standard chemical techniques were used in the preparation of the stock solutions. Meticulous care was used t o prevent contamination. The total acid concentration was kept constant at 2y0 by volume HI\’O1; the cobalt concentration was also identical (0.600 mg. per ml.) in all standards. Analytical reagent grade or JohnsonMatthey “spec-pure” chemicals were used for the preparation of the stock solutions. All chemicals were analyzed by optical emission spectrography to check their purity and by x-ray diffraction t o confirm their composition. Most of the chemicals used satisfactorily passed these tests. However, x-ray
Table II.
Element Ba Sr
Ca
MP. Mn Si
Xi
Concentration Ranges and Precision Data
Concentration ranges, pg. 0.75 to 250 0.25 to 50 0.10 to 5 . 0 0.20 to 10 0.10 to 10 0.25 to 3 . 5 0.50 t o 50
Relative standard deviation, Y
~k4.270 15.0%
28.9%
26.3% * i .1%
16.8% *5.4y0
Table 111.
~ 1 __Standard ~ 5 ment Added Found 1 0 0 90 Ba 050 046 Sr Ca 025 023 030 026 Mn Mg 080 027 0 3 0 08.5 SI 0 82 XI 10 1372
analysis (2) determined that the Johnson-Matthey spec-pure material contained 10% by weight Ba (NO&; this amount was corrected in the calculation of the standards. Also, the A.R. grade Na2Si03.9H20had to be standardized by a gravimetric analysis of its silicon content. Because a 0.10-ml. aliquot is taken for each determination from the total sample solution of 0.50 ml., the analytical results should be multiplied by a factor of five. In this procedure, the aliquot was automatically corrected by multiplying the values of the standards by five when they were plotted on the analytical curve. As a result, no additional operation was required for each calculation. Table I1 shows the analytical ranges for the elements analyzed by this method. Each point on the analytical curves shown in Figures 1 through 5 represents an average of six replicate intensity ratios t o reduce random errors. The curves for silicon and nickel (not shown) are linear over their complete ranges of concentration. A reproducible curve for silicon was ensured by shaking each standard vigorously before loading. This shaking gave a more homogeneous dispersion of the colloidal silica in the standards. The slight curvature at the extreme lower end of the analytical curves for Mg, Mn, and Ca is probably caused by latent background which could not be adequately corrected because it was so
Accuracy of the Method
(All figures are in pg.) Standard 6 Standard 7 Standard 8 Standard 9 Added Found Added Found Added Found Added Found 50 5 2 20 20 80 80 150 135 2 5 30 32 22 5 0 17 5 0 15 1 0 2 0 22 5 0 050 054 1 0 3 9 10 12 1 0 1 0 2 0 5 0 5 6 2 4 1 0 1 0 2 0 1 7 5 0 47 10 11 1 0 1 0 20 17 3 0 27 060 062 28 40 40 A 0 5 4 11 20 10
ANALYTICAL CHEMISTRY
light (approximately 97y0 transmittance). Because this slight curvature was reproducible, it did not affect the accuracy of the method. PRECISION AND ACCURACY
The precision of the method was determined by using data obtained from routine samples which were run in duplicate on different days and on different photographic plates. This was done t o ensure that the precision data represented the capabilities of the method in routine practice rather than under idealized conditions. Forty samples n-hich had been run in duplicate were used to assess the precision for the determination of each chemical element in the sublimates. The precision is expressed as the relative standard deviation ( v ) for the duplicate determinations in Table 11. Because there was no other independent analytical technique available for comparison, synthetic samples mere prepared t o assess the accuracy of the method. -4 two-step procedure Tvas employed t o test the accuracy of the entire analysis (including the removal of the deposit from the tube part). In the first step, 100 p1. of standard solutions were pipetted onto pieces of pure platinum foil. These foils were placed on a clean piece of flat glass which was then placed on a hot plate. -4lowheat setting was sufficient to obtain a residue on the foils without causing spattering. The foils were then placed in plastic vials with 0.50 ml. of cobalt solution and treated identically as any tube sample. In this way, the accuracy of the entire spectrographic procedure was evaluated. Table I11 shows the results for five such standards covering the entire analytical range. As shown, the average deviation for any element is within *lo% of the amount present.
This deviation includes the additional errors involved in pipetting the solutions onto the platinum foils; these errors would not occur in the preparation of the actual samples where the deposits are formed by sublimation within the tube. MATRIX EFFECTS
Because of the extremely wide concentration range for Ba (0.75 to 250 pg.) and Sr (0.50 to 50 pg.), an investigation of matrix effects was necessary. In addition, these two elements could have considerable effect on the results of the entire analysis because their ionization potentials are lower than the other elements being analyzed (Ba = 5.2 volts, Sr = 5.7 volts, whereas Si =8.1 volts, Xi = 7.6 volts, Ca = 6.1 volts, Mg = 7.6 volts, Mn = 7.4 volts.). Four special standards were prepared to determine the effect of the Ba and Sr content on the analysis of the other elements. The Ba and Sr contents of these standards %ere varied a t either extreme of their respective analytical ranges, while the content of the other elements was held constant. The standards were then analyzed as regular samples. The results are shown in Table IV and indicate that there is no significant effect (statistical bias) on the analytical results for Ca, Mg, Mn, Si, or S i introduced by even the most extreme variations in Ba and Sr content. The deviations observed are within the inherent random variations of the method itself. DISCUSSION
Heretofore, spectrographic methods, such as Jaycox’s work (9), for the analysis of many elements over wide concentration ranges in a variable matrix required dilution of the sample with a relatively high proportion of buffer in the solid state. I n the method described here, however, a solution technique is used both to dissolve a sample of variable matrix and to achieve the necessary dilution with the spectroscopic buffer. This solution technique is advantageous because standards of even the lowest concentrations can be prepared accurately and quickly. Also, the errors introduced by mixing high buffer-to-sample ratios in powdered form are almost completely eliminated in solution. This method was developed with two major considerations in mind: High sensitivity had to be attained for all seven elements being determined, and the matrix, or interelement effect, had to be suppressed to achieve optimum precision and accuracy. The second consideration was particularly critical because seven elements had to be determined simultaneously over unusually wide concentration ranges (in the case of barium and stron-
0.20
0.10
2 ,o
1.0
0.40 0.60
4.0
6.0
IO
MANGANESE-MICROGRAMS Figure 5.
Analytical curve for manganese Range: 0.1 0 to 10 pg.
an internal standard and buffer, the concentration of this element is maintained a t an identical value in both the samples and the standards. I t has already been shown in Table IV that the cobalt buffer minimizes differences in spectral response caused by matrix variations. However, another related variable is the amount of sample residue deposited on the electrodes, as pointed out by Yachtrieb ( I d ) . (Nachtrieb is one of the originators of the copper-spark method ( 8 ) , the basis for the graphite-spark technique.) Kachtrieb’s method was used to analyze only very dilute solutions in which the amount of electrode residue was no greater than several micrograms. If the amount were significantly larger, the spark could possibly eject the residue before it could be uniformly and reproducibly vaporized and excited. The accuracy data in Table I11 show that the 60 pg. of cobalt deposited
tium, more than two full orders of magnitude). This constitutes the variable matrix previously mentioned. To achieve both these goals, a technique was chosen which possessed much greater inherent sensitivity than was required. The sample could then be diluted in solution and matrix effects thereby minimized. Adequate sensitivity was achieved by the technique generally known as the graphite-spark ( I S ) with different excitation conditions. The excitation conditions were changed mainly to lower the limit of detection for silicon. The solution used to dissolve the sublimate also serves to dilute it homogeneously with a relatively high concentration of cobalt which serves as a spectrometric buffer. The cobalt also serves as an internal standard because it has suitable lines of medium excitation potential over the wide spectral range of interest. Because cobalt is used as
Table 1V.
Effect of Bo and Sr Variations on the Analysis of Si, Ca, Mg, Mn, and Ni
Element
Standard 1
Ba Sr
1 5 25
Ca
m M
11
Si Ni
(Results given in pg.) Standard 3 Standard 2 1.5 1.0
150 1 .o
Standard 4 150 25
Added
Found
Added
Found
Added
Found
ldded
Found
1.0 1.5 1.0 1.5 5.0
0.97
1.0 1.5 1.0 1.5 5.0
0.98
1.0 1.5 1.0 1.5 5.0
0.94
1.0 1.5 1.0 1.5 5.0
0.98 1.5
1.4 1.0 1.5 4.8
1.3 1.0 1.5 4.9
1.2 1.0 1.5 4.7
VOL. 34. NO. 1 1 , OCTOBER 1962
0.98 1.6
5.4
0
1373
on the electrodes in this procedure aid in suppressing variations in spectral response caused by a variation in total sample impurity residue on the electrodes from 0.7 pg. (standard 5) to 49 pg. (standard 9). Accordingly, within the specified analytical ranges covered, this method is independent of matrix variations due to the total quantity of all elements as well as to the relative proportions of each element in sublimed deposits. Even though this method was developed primarily for the determination of deposits on electron-tube parts, it has great potential in the analysis of deposits on other substrates because of its insensitivity to interelement effects and wide ranges of concentration for the elements studied. Extension of the method t o other substrates would require only minor changes in aliquoting and acid concentration of the solution used to remove the sample deposit. Adjustment of the acidity is necessary to ensure complete removal of the deposit n-hile attacking the substrate as little as possible.
ACKNOWLEDGMENT
The author thanks Lucille Tissot, who performed most of the routine analyses used in assessing the precision of this method, and John Zuber, who assayed the silicon reagent. LITERATURE CITED
(1) Aldrich, L. T., J . A p p l . Phys. 22,
1168-74 (1951). (2) Berth, E. P., Longobucco, R. J., Raag, V., t o be presented at the 6th K‘atl. Conf. on Tube Techniques, New York, September 1962, and to be published in the Proceedings of this Conference. ~.~ (3) Blewett, J. P., Liebhafsky, H. A,, Hennelly, E. F., J . Chem. Phys. 7 , 481 (1939). (4) Churchill, J. R., IND.ENG.CHEM., ANAL.ED.16. 653 11944). (5) Debiesse, J.; L’Onde EIectriqite 30, 351 (iwin). (6) Dehm, R. L., paper prcsented at the Pittsburgh Conference for Analytical Chemistry and Applied Spectroscopy, March 8, 1962. (\ 7. ,) Fowler. R. G.. Wolfe. R. A,. J . O D ~ . ~S O ~Am.’35, . 170-4 (1045). ’ (8) Fred, M., Nachtrieb, N. H., Tomkins, F. S., J . Opt. SOC.A m . 37, 279 (1947). ~
(9) Jaycox, E. K., ANAL. CHEM. 22, 1115-18 (1950). (10) Kaiaer, H., Speclrochim. Acta 2, 1 I1941). (li) Lekerton, W. F., Shepherd, W. G., L’Onde Electri ue 23,.787-93 (1952). Allison, H. W.,. Phus. (12) Moore, G. ” Rev. 77, 246 (1950). I 13) Morris. J. 34.. Pink. F. X.. “Svm. posium on Spectrochemical Analysis for Trace Elemenb,” ASTM Special Technical Publication Yo. 221, 39-46 (1957). (14) Nachtrieb, N. H., “Principles and Practice of Spectrochemical Analysis,” 275-7, McGraw-Hill, New York, 1950. (15) Plumlee, R. H., Smith, L. P., J . A p p l . Phys. 21, 811-19 (1950). (16) Reynolds, F. H., Rogers, M. W., Proc. Inst. Elec. Eng. (London) Pt. B 104, 337-40 (1957). (17) Ruehle, A. E., Jaycox, E. K., ANAL. CHEM.12. 260 (1940). (18) Schmidt, R., Rec. Trav. Chim. 67, 737-45 (1948). (19) Thompson, B. A,, A s a ~ CHEW . 31, 1492 (1959).
e.,
~I
, - - - - I .
RECEIVEDfor review June 11, 1962. Accepted August 6, 1962. Preaented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1962. Part of this work done under Contract NObsr 77637 with the U. S.Navy, Bureau of Ships.
Determination of Traces of Cadmium in Zinc-Rich Materials J. R. KNAPP, R. E. VAN AMAN, and J. H. KANZELMEYER St. Joseph l e a d Co., Zinc Smelting Division, Monaca, Pa.
b A method for the separation of traces of cadmium from zinc-rich materials has been developed. The iodocadmate ion in aqueous 0.1M KI a t pH 3 i s extracted with a 1% solution of a high molecular weight secondary amine (Amberlite LA-2) in xylene. It i s stripped from the organic phase with 1M NaZC03. A conventional dithizone extraction into carbon tetrachloride completes the determination. Only TI and Cu offer interference but neither ordinarily occurs in interfering amounts in zinc metal or zinc oxide. The method i s capable of determining as little as 0.00005~0Cd in zinc metal and has demonstrated a precision of 3.5% relative error in the range of 0.0002 to 0.01 0% Cd.
E
appears to be the most convenient method for the quantitative determination of traces of cadmium in zinc-rich materials. The procedures for direct extraction without prior separation (2, 9) have been investigated and are not sufficiently selective to be used for the determination of concentrations of cadmium below 0.01%. XTRACTION BY DITHIZONE
1374
ANALYTICAL CHEMISTRY
Because pure cadmium solutions can be analyzed by a simple, direct dithizone extraction with excellent sensitivity and accuracy, all that is needed to provide an extremely attractive method for the analysis of traces of cadmium is a rapid and efficient separation from zinc and other dithizone-extractable metals. Kone of the separations currently available for cadmium (3)is satisfactory for this purpose. Controlled-cathode electrodeposition on a mercury cathode is highly selective, but is of limited value for handling large numbers of samples. Cementation by electrolytic replacement using aluminum is inefficient and beset with experimental difficulties when applied to small quantities of cadmium. The classical sulfide precipitation is a rough concentration step rather than an actual separation. It is both inconvenient and subject to large errors unless carried out by experienced analysts. Ion exchange procedures have been reported which employ both chloride (4, 6) and iodide (1, 5 ) media. Baggott and Willcocks (1) stated that if both sulfate and iodide are present, zinc does not form anionic complexes while
cadmium does. Other data on the iodide system (ff) suggested that the stability of iodocadmate ion should be great enough to provide a quantitative separation of cadmium from zinc with a single extraction. Solvent extraction offers the advantages of simpler equipment and greater convenience than the column ion exchange procedures. The success of extractions of the chloroanions of metals by high molecular weight amines (7, 8, 10) suggested their use in the extraction of iodocadmate. Quantitative extractions of cadmium were obtained from 0.1M KI solution into an organic phase consisting of a water-insoluble secondary amine dissolved in xylene. The high degree of separation thus obtained permits the use of a simplified dithizone extraction for the rapid and accurate completion of the determination. EXPERIMENTAL
Apparatus. A Beckman Model B spectrophotometer with I-em. borosilicate cells was used in this work. Reagents. DITHIZOXE SOLUTION. Dissolve 50 mg. of diphenylthiocarbazone in a previously unopened
