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).
(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). (12) Moore, G. Allison, H. W.,. Phus. ” 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). . 31, (19) Thompson, B. A,, A s a ~ CHEW 1492 (1959).
e.,
~I
, - - - - I .
(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).
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
droplets of water. Measure the absorbance a t 520 mp against a reagent blank that has been carried along with the samples. The cadmium content is read from a calibration curve prepared by running appropriate volumes of the 1-pg./ml. standard cadmium solution through the entire procedure.
0. 5
0. 4
DISCUSSION
~
0. 3
c
U
P Ln 0
2
0.2
0. I
0.0
5
IO
Cadmium IMgl Figure 1 . wash
Loss of cadmium dithizonate with N a O H
5 pg. of Cd extracted from 2M N a O H with 15 ml. of 0.005% dithizone in CCId No wash X, 0, A, Washed with two 5 4 . volumes of 2% aqueous N a O H
quart of reagent grade carbon tetrachloride. Shake occasionally during a period of several hours and store in a refrigerator. STANDARD CADMIUM SOLUTION, 1 pg. of Cd/ml. Dissolve 0.100 gram of highpurity cadmium metal in 5 ml. of concentrated HC1 plus 1 ml. of concentrated HNO, and dilute with water to 1 liter. Dilute this stock solution by a factor of 100 to produce the 1 pg./ml. standard solution and store in a polyethylene container. LA-2 IN XYLENE,1%. Preparea 1% solution (by volume) of Amberlite LA-2 (Rohm 8: Haas, Philadelphia, Pa.) in xylene (neutral histological). POTASSIUM IODIDE WASH SOLUTION. Dilute 50 ml. of 1M K I to about 450 ml. with water, add 1.0 ml. of concentrated H2S04,and dilute to 500 ml. with water. OTHERREAGENTS. All other reagents should meet the ACS reagent specifications. High-purity demineralized water should be used throughout the procedure. Procedure. Dissolve samples containing 2 to 10 pg. of Cd in approximately 9 V H2S04 (use 5 ml. for zinc samples weighing 100 mg. or less; proportionally more acid t o maintain the proper excess for larger samples). Transfer the sample to 125-ml. separatory funnels and add 5 ml. of 1X K I solution. Dilute to a total volume of about 50 ml. If the p H a t this point is not below 3.0, acidify with dilute HzS04. Add 10 ml. of 1% LA-2 in xylene, shake vigorously for 20 seconds, and allow the layers to separate about 16 minutes. Discard the lower (aqueous) layer even though it remains opalescent.
Add 50 mi. of potassium iodide wash solution, shake for 20 seconds, and discard the aqueous phase as before. Strip the cadmium from the organic phase by shaking it 20 seconds with two separate 10-mi. portions of 1M Na2C03. Combine the carbonate solutions with 5 ml. of a 20y0 aqueous solution of sodium potassium tartrate (NaKC4H406.4Hz0)and 5 ml. of a 5% aqueous solution of hydroxylamine hydrochloride in 125-ml. separatory funnels. Swirl to mix and allow the solution to stand 2 minutes. Add 15 ml. of 6M NaOH and exactly 15.0 ml. of dithizone solution. Shake for 30 seconds. Transfer the carbon tetrachloride (lower) layer to 1-em. spectrophotometer cells using cotton plugs in the stems of the separatory funnels to remove
Effects of Variables. PRELIMINARY WORK. During preliminary studies on samples having Zn to Cd ratios greater than 1000, the method of Fischer and Leopoldi (9) appeared to have sufficient sensitivity but poor reproducibility, particularly from one set of samples to the next. This method ,in common with the other direct dithizone procedures, depends upon removing coextracted zinc dithizonate by washing the organic phase with dilute base. The washing procedure leads to an unreproducible partial decomposition of the cadmium dithizonate as shown in Figure 1. The lack of reproducibility of the direct methods can thus be traced directly to slight variations in the washing procedure that are difficult to avoid in routine work. Thus the prior separation of cadmium from zinc improves the reproducibility of the dithizone cadmium determination by eliminating the necessity for the basic washing step. The materials selected for the caamium separation were the high molecular weight, water insoluble secondary amines Amberlite LA-1 and L.4-2 and the tertiary amine XE-204 supplied by Rohm 8: Haas (Figure 2). Preliminary extractions of solutions of zinc and cadmium were carried out with 5061, LA-1 in xylene. All of the cadmium and a portion of the zinc was extracted from aqueous solutions 0.1 to 1.OM in K I and 0 to 0.0551 in H2S04. Decreasing the concentration of L.4-l in the organic phase and increasing the H2S04 in the aqueous phase reduced the extraction of zinc without affecting that of the cadmium.
Amberlite LA-1
R CH,-C( CHs)z-CH2-C(
I
CHa)~--CHrCH=CH-CH~-NH--CR' !
fi
I,
Amberlite LA-2 R i
CH~-(CH~),-CH~--NFI-C-R'
I
R"
Amberlite XE-204 [ C H r C (CH~)~-CHZ-C( CH~)-CHz-CH=CH-C€12] Figure 2.
zN-( CHz)a-CH*
Amine structures
Sum of the side chains R, R', and R" equals approximately 11 to I 4 carbon atoms. VOL. 34, NO. 1 1 , OCTOBER 1962
e
1375
AMINE FORM ASD PH. Iodide and sulfate forms of the amine were prepared by pre-equilibration of 5% LA-1 in xylene with 47y0 HI and 6-11 H2SOa, respectively. Extraction of cadmium in the presence of zinc by the free base, iodide, and sulfate forms a t various aqueous pH levels is shown in Table I. It is virtually complete at a n equilibrium p H of 3 or lower regardless of the initial form of the amine. Only traces of zinc were extracted under these conditions. TYPE OF AMINE. hmberlite LA-2 and Amberlite XE-204 were employed to determine the effect of the amine structure on the extraction of traces of iodocadmate from large amounts of zinc. No significant decrease in the extraction of 50 pg. of cadmium was observed with either amine (1% solutions in xylene) from 0.03 to 0.3M K I solutions at pH's between 0 and 5.0. Zinc extraction with LA-2 is shown in Figure 3. Under comparable conditions, XE-204 extracted about twice as much zinc. For this reason, no additional work was done with XE-204. Amberlite LA-2 was used for all subsequent extractions. SOLUTION CONDITIOXS. Figure 3 also shows that the presence of chloride enhances the extraction of zinc and reduces the separation achieved. I n the absence of chloride, zinc extraction is minimized and is independent of both iodide concentration and p H throughout the ranges tested. Other tests have established that there is no change in cadmium extraction over ranges of iodide concentration from 0.03 to 0.3M
Table I.
Efiect of Amine Form and pH on Cadmium Extraction
Extraction with 10 nil. of 5 s LA-1 in xylene from solutions containing 1.0 mg. of Cd and 100 nig. of Zn in 50 nil. of 0.1.11 KI Cd - ~.
Amine form Free base
Iodide
Equi-
ex-
1.0 3.0 5.0 8.5 9.5 1.0 3.0 5.0 8.5 9.5
1.2 5.6 6.3 8.55
. ..c
100 100 84 0 0 100 100 100 19 2
9.5
. , . c
20
Original librium tracted,n PH pH 76
9.25 1.0
2.8 3.2 . , . c
Sulfate
Q
Based on dithizone determination of
cadmium in aqueous phase. Faint precipitate formed in aqueous
phase. Heavy precipitate formed in aqueous phase.
1376
ANALYTICAL CHEMISTRY
Table ti. Stripping of lodocadmate 50 fig. of Cd in 10 ml. of 1% LA-2 in xylene stripped with 25 ml. of aqueous
reagent Concentration
Stripping agent KazC03 SaHC03 NHdNOS
(",),SO, 0) 0
I
1
1
I
1
2
3
4
5
HzS0,
PH Figure 3. lite LA-2
Zinc extraction by Amber-
of
Cd in aqueous
1.o 0.1 0.01 1 .o 0.1 0.01 1 .o 0.1 0.01 1.o 0.1 0.01 1 .o 0.1 0.01 1 .o 0.1 0.01
99.6 53.0 53 .O 16.6 22.2 94.8 91.6 34.0 34.0 76.0 49.8 29.0 42.0 48.4 41.6 41.8 17.6 16.2
stripping agent
KC1
%
Extraction from 50 ml. aqueous phase containing 50 pg. of Cd (added os CdCIZ) and 1 gram of Zn (added in form indicated) with 10 rnl. of 1 % LA-2 (iodide form). Polarographic determination after destruction of organic phase with HzSO4 and HNOa 1. 0.01M KI (ZnCI,) 2. 0.03M KI (ZnClz) 3. 0.1 OM KI (ZnCIz) 4. 0.30M KI (ZnClz) 5. 0.01,0.03,0.1, and 0 . 3 M KI (ZnSOd)
nor with acidities from p H 3 to at least 4M H2SOd. STRIPPING OF CADMIUM FROM ORGANICPHASE.The extraction of elemental iodine into the organic layer, combined with a concomitant darkening of the amine has thus far thwarted attempts t o measure the cadmium by extracting dithizone directly into the amine-xylene phase. The cadmium is easily returned t o the aqueous phase by stripping with a number of reagents. Table I1 shows that 1M NatCOs is the most efficient of the reagents tried. It is also suitable as the aqueous phase for the subsequent extraction by dithizone in carbon tetrachloride. CONCENTRATION OF LA-2. When the LA-2 concentration was reduced from 50% to 5y0 and finally to 1%, the extraction of zinc decreased while that of cadmium remained unchanged. Further reduction in amine concentration caused a rapid decrease in cadmium extraction as shown in Figure 4. INTERFERING IONS. The samples prepared for the interference study contained 1 gram of zinc as ZnSOc and 50 pug. of cadmium as CdClz as well as the ions under study. (All of the latter except silver were in the chloride form also). The original aqueous portion was analyzed to give the approximate degree of extraction by the LA-2. The absorbance obtained in the dithizone extrac-
tion was converted to the apparent quantity of cadmium recovered. These data are shown in Table 111. Of the elements tested, only silver was removed to an appreciable extent from the aqueous phase at the same time as the cadmium. This behavior was probably caused by the precipitation of AgCl on addition of the cadmium chloride solution, rather than by extraction into the organic layer. Copper and thallium were partially extracted and gave positive errors in the dithizone extraction. Iron and antimony, when present in d a t i v e l y high concentrations, reacted with the LA-2 and thus reduced the extraction of cadmium. They produced negative errors in the cadmium determination because they were not extracted by dithizone from the basic solution. Table IV gives the amounts of the various metals which 0.%
1
I
3
0.0
0
I
1
I. 0
2.0
3.0
Volume % of LA-2 I n Xylene
Figure 4. Cadmium extraction function of LA-2 Concentration 5 pg. of procedure
Cd
extracted
according
as
to the
--
Table IV.
Interference of Diverse Ions Amounts tested which cause less than 2% relative error in the determination of 5 pg. of cadmium Element Milligrams A1 >I_ . 0. >1 .o
Ag
-
Fe In Mn
Figure 5. Direct reader calibration curve
E V
u m
s
-
0 mol
20
I
I
30
40
50 Clack Divisions
cause less than a 2% relative error in the measurement of 5 pg. of cadmium. The apparent interference of iron reported above is caused by the extraction into LA-2 of the chloroferrate(II1) ion. If chloride is eliminated, even 500 mg. of iron causes no interference (see Table 111). If chloride is introduced during sample preparation, i t can be removed by fuming with sulfuric acid before proceeding with the extraction.
Table 111.
Extractions of Diverse Ions
Extracted Ion added
3?;11
Cu(I1’
Amount, mg. 10 10 10
... ...
5.0b O.5Ob
...
...
Cd measured,a rg.
50 50 71 63 63 50
56 56
100
... ..
10
