ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
T h e fact that mild eluting conditions are sufficient to remove the metal ions from the Porasil-8OHQ column is an added plus compared t o previous techniques. Should the metal ion need to be recovered after elution from the column, only dilute acid conditions are present, -0.1 M , so that purification is not difficult. In most ion-exchange techniques, selectivity is obtained through the addition of complexing agents to the eluent (27,28),which must be removed in any purification scheme for the eluted ions; or very strong (several molar) acid conditions are used for elution which also complicate post-column metal purification. Thus. the good stability and likelihood of higher capacity and efficiency of silica immobilized chelating agents offer great promise for metal ion chelation chromatography, and work is underway in our laboratories to demonstrate this.
(8) (9) (10) (1 1)
(12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23)
LITERATURE CITED
(24) (25) (26)
(1) G. Nickless, Adv. Chromatogr., 7, 121-159 (1968). (2) J. Michal, "Inorganic Chromatographic Analysis", Van Nostrand-Reinhold. New York, 1970. ( 3 ) F. Helfferich. Adv. Chromatogr., 1, 3-60 (1965). (4) H. F. Watton, Ed., "Ion-Exchange Chromatography", Dowden, Hutchinson and Ross, Stroudsburg, Pa., 1976. (5) T. Braun and G. Ghersini, Ed., "Extraction Chromatography", Elsevier. New York, 1975. (6) E. Cerrai and G. Ghersini, Adv. Chromatogr., 9, 3-189 (1970). (7) F. Helfferich, Adv. Chromatogr., 1, 77-81 (1965).
(27) (28)
373
J. R. Parrish, Anal. Chem., 49, 1189 (1977). E. Grushka and E. J. Kikta, Anal. Chem., 49, 1005A (1977). I . Sebestian and I. Haiasz, Adv. Chromatogr., 14, 347 (1977). E. Grushka, "Bonded Stationary Phases in Chromatography", Ann Arbor Science Publishers, Ann Arbor Mich., 1974. D. W. Lee and M. Halrnann, Anal. Chem., 48, 2214 (1976). J. L. Lundgren and A. A. Schilt, Anal. Chem., 49, 974 (1977). E. M. Moyers and J. S. Fritz, Anal. Chem., 48, 1117 (1976). D. E. Leyden and G. H. Luttrell, Anal. Chem., 47, 1612 (1975). D. E. Leyden, G. H. Luttrell. A. E. Sioan, and N. J. DeAngelio. Anal. Chim. Acta, 84. 97 (1976). I . I. Antokolskaya, G. V. Myaesoedova, L I. Boishakova, and 0. P. Shvoeva. J . Chromatogr.. 102, 287 (1974). J. M. Hill, J . Chromatogr., 76, 455 (1973). K. F. Sugawara, H. H. Weetall, and G. D. Schucker, Anal. Chem., 46, 489 (1974). 2. Slovak, S. Slovakova, and M. Srnrz, Anal. Chlm. Acta, 75, 127 (1975). M. M. Guedes da Mota, F. G. Roemer. and B. Griepink, Fresnius 2. Anal. Chem., 287, 19 (1977). J. R. Parrish, Lab. fract., 2 4 , 399 (1975). G. Schwarzenba& and H. Fhschka, "Comdexorne+i4c Ttrations", Methuen. London, 1969. S. Takamoto, 0.Fernando, and H. Freiser, Anal. Chem., 37, 1249 (1965). J. R. Jezorek and H. Freiser, Anal. Chem.. following paper in this issue. L. R. Snyder, "Principles of Adsorption Chromatography", Marcel Dekker, New York. 1968. DO 21-22. M. D. Arguello and'J. S. Fritz, Anal. Chem., 49, 1595 (1977) J. S. Fritz and J. N. Story, Anal. Chem., 46, 825 (1974).
RECEIVED for review August 24, 1978. Accepted November 9,1978. This work was conducted with the financial assistance of the US.Department of Energy.
4-(Pyridylazo)resorcinol-Based Continuous Detection System for Trace Levels of Metal Ions John R. Jezorek' and Henry Freiser" Department of Chemistry, University of Arizona, Tucson, Arizona 8572 1
A sensitive continuous metal ion detection system based on the use of PAR-ZnEDTA has been developed and characterized. Sensitivities of the order of 0.1 to 1.0 nmol, which represents a thousand-fold improvement over previously reported systems, were observed for 13 metal ions. The system has been employed in a liquid chromatographic separation study.
In the course of our liquid chromatographic study of metal ions using immobilized chelating agents ( I ) , we considered it desirable to develop a spectrophotometric detection scheme. Inasmuch as the metal ions themselves are not highly colored, the scheme would have to involve mixing the eluate with a suitable chromophore. We investigated a number of metallochromic indicators, and finally settled on 4-(2-pyridylazo)resorcinol (PAR) as the optimum choice. Fritz and coworkers (2, 3) had previously used this reagent and also found it to be superior to other dyes they studied. PAR forms water-soluble complexes with a larger number of metal ions than any other commonly available indicator. These complexes have large molar absorptivities ( lo4) ( 4 ) a t about 500 nm and hence exhibit high sensitivity for photometric detection.
-
'On leave during the 1977-78 academic year from the Department of Chemistry, University of North Carolina at Greensboro, Greensboro, N.C. 27412. 0003-2700/79/035 1-0373$01 OO/O
Although PAR has been extensively used as a reagent for spectrophotometric analysis of metal ions ( 4 ) and, as noted above, in several chromatographic studies, apparently no formal investigation or characterization of its use in a chromatographic context has been published. We wish to report here our findings on the stability of various types of PAR solutions, its suitability as an indicator for various metal ions, and some difficulties encountered due to metal ion contamination of chromatographic elements.
EXPERIMENTAL The liquid chromatography apparatus, detection hardware, and immobilized-oxine support have been described elsewhere ( I ) . The water used for PAR and metal-ion solutions was house distilled and deionized, or was obtained from a Corning Megapure still (Corning Glass Works, Corning, N.Y. 14830). The preparation of water used for elution solvents has been previously discussed (1). PAR was used as received from Eastman Organic Chemicals (Rochester, N.Y. 14650). Metal-ion solutions were prepared from the analytical reagent grade perchlorate salts (G. F. Smith Chemical Co., Columbus, Ohio 43223) except for lanthanum(II1) which was the 99.99% pure nitrate salt from MC & B (The Matheson Co., East Rutherford, N.J.). PAR solutions at pH 9 and 11 were 0.05 M in NH4C1, to which enough concentrated NH3 was added to obtain the desired pH (pH meter). In addition the pH 11 PAR solutions were equimolar in ZnEDTA. PAR solutions at pH 7 were 0.1 M in Tham (tris(hydroxymethylamino)methane), with HCI added to reach the desired pH value. ZnEDTA reagent was prepared by mixing equivalent amounts of standardized zinc perchlorate and lo-*M EDTA solutions. All F 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
other reagents were analytical reagent grade, obtained from Departmental supplies, and used as received. R E S U L T S AND D I S C U S S I O N PAR Solution Stability. We found that storage of approximately 3 x M PAR solutions either in the light or in glass bottles caused deterioration overnight so that little or no detector response was obtained upon injection of a metal sample. This situation did not occur if plastic containers were used and the solutions stored in the dark. Attempts to prepare loe4M, or higher, unbuffered PAR stock solutions for subsequent dilution resulted also in significant overnight deterioration, whether stored in glass or plastic, as evidenced by a 50% decrease overnight in the absorbance a t 411 nm (A,, for PAR), and the appearance of a new band a t 340 nm. A M solution prepared independently or immediately diluted from the above stock reagent after preparation, and buffered at p H 9, showed good stability over several days. A M stock solution buffered with NH4’/NH3 at pH 9 was found to be more stable than the unbuffered stock; and that buffered at p H 11 even more so. These solutions could be kept for several days without significant deterioration. Tham buffered PAR solutions, lo4 M or higher, were found to be adsorbed on the walls of the plastic container after a day or so. This was also true of pH 9 solutions at 5 X M and higher, but not at p H 11 a t any concentration studied. T h e PAR-ZnEDTA reagent buffered with NH4+/NH3at pH 11 (discussed below) was found to deteriorate rapidly if glass-bottle storage was employed. A drop in absorbance a t 411 nm from 0.88 to 0.62 for a 3 X M solution occurred in 2 days, and an increase in absorbance from 0.01 to 0.12 occurred at about 495 nm. It is likely that either PAR or ZnEDTA abstracted metal ions from the glass, in the former case forming PAR-metal complexes or, in the latter, releasing Zn(I1) from the Zn(PAR)2complex which absorbs at 495 nm. However, when these PAR-ZnEDTA solutions were stored in clean plastic containers, there was no change in either the absorbance or chromatographic response for up to 5 weeks, a t which time we halted our monitoring of the solutions. Even though, , ,A for PAR is 411 nm, there was some residual absorbance at the monitoring wavelength, 510 nm, due to some contaminant metal-PAR complexes and to the high-pH PAR band (Arnm 500 nm) from its dianion ( 4 ) . This, along with the fact that two liquid streams were being mixed using peristaltic pump pressure for one stream led to some detector noise at higher sensitivities. Use of two 1-m mixing coils helped to reduce the noise significantly at the 0.4, 0.2, and 0.1 AUFS settings commonly used. I t is of interest that use of acid eluents (0.1 M e.g.) resulted in less noise than if solutions closer to neutral were used. This may be a result of better mixing of the two eluent streams due t o the heat released in neutralization of the acid by the p H 9 or 11 NH4’/NH, PAR solutions. Metal-PAR Chromatographic Interactions. Metal-ion sample solutions were typically from 1-5 X lo-*M, ( - 10-50 ppm) while the PAR detector solution was about 3 X lo-’M. M metal, which spreads For an injection of 10 pL of 5 X out into a volume of 1 mL (the narrowest chromatographic peak obtained) or more at the detector, the chromophore concentration was present at about a 100-fold excess over the metal. Indeed, as expected, we found no increase in peak size for 10 ppm Zn solution or increasing the indicator concenM. tration 10-fold, to 3 X Because previous work by Munshi and De (5)recommended a p H between 6 and 7 for rare-earth analysis using PAR, and because of the ease of hydroxide precipitation (Ksp= lo-”) (6) we decided to use pH 7 , Tham-buffered PAR solutions for initial lanthanide work. Subsequent studies with La(III), however, revealed that the optimum detector response with
Table I. Relative Responses for Some Metals with PAR and PAR-ZnEDTAO peak height, in peak height, in PARPARmetala PAR ZnEDTA metal PAR ZnEDTA Zn2’ 3.0 2.1 Caz+ 0.1 2.5 0.1 1.8 Ni2+ 3.0 2.3 Mg2+ 0.2 0.1
Sr2 BaZ+ PbZ +
2.4 3.2
Cu2+
1.7
1.6
Co2+
2.0
2.0
0.7 0.2 0.3 1.2 ~ 1 3 + 1.5 0.2 0.3 2.1 Fe3Mn2+ Hg2 4 0.8 (see text) La3+ a Detector at 510 nm 0.4 AUFS; (PAR-ZnEDTA) and M dummy column, 1 0 (PAR) = 3 x 10-jM (Metal = M )injected; dummy column. pL metal ( = +
+
PAR occurs at p H 9. This is in agreement with the report that the rare earth-PAR molar absorptivities increase as pH increases ( 4 ) . Even though La(II1) normally precipitates as the hydroxide at pH 8 or higher, reaction with PAR apparently is sufficiently rapid, even up to pH 11,to prevent or minimize hydroxide-complex formation. I t should be noted that a somewhat more concentrated PAR solution (1 X M PAR) is necessary to get a detector response of the same magnitude as with transition metals, possibly because of the necessity of overcoming hydroxide complex formation. As the alkaline earths, Mn(II), Pb(II), and other metals which we wanted to study chromatographically did not give a suitable response with the PAR indicator solution, we decided to use the PAR-ZnEDTA system described by Fritz and Arguello ( 7 ) . Typically we prepared this reagent to be about 3 x M in both PAR and ZnEDTA. The detection of the alkaline earth metals and others which do not react with PAR alone depends on the displacement of Zn from the ZnEDTA complex and its subsequent coordination with PAR. For example, Ca 2PAR ZnEDTA * CaEDTA Zn(PARI2 (1)
+
+
+
The overall reaction is composed of three simpler ones. Ca E D T A CaEDTA log K = 10.7 (2)
+
+ PAR + Zn(PAR)2 log K = 23.5 (4) Zn + EDTA ZnEDTA log K = 16.5
Zn
(3) (4)
ZnEDTA
since log K’caEDTA at p H 11 = 10.6, and log K ’ ‘ z n at ~~ pH~ ~ 11 and -0.5 M NH3 = 8.5. Under NH,/NHJ buffer conditions a t p H 11, this displacement reaction is quite rapid, and good response was found for all the metals tested [except, as discussed below, for Fe(III), Hg(II), and Al(III)]. If these metals are of interest, rather than the alkaline earths, Mn(II), and Pb(II), then PAR by itself at pH 9 would probably be the better choice. In Table 1are compared relative responses for some metals with PAR and PAR-ZnEDTA. I t is not known why Mn(I1) and Pb(I1) gave such small responses with PAR alone, but for these and a number of other ions, significant improvement in sensitivity was realized with the PAR-ZnEDTA system. However, as seen in Table I, Fe(III), Al(III), and Hg(I1) could not be detected a t the p H 11 conditions used with the PAR-ZnEDTA. Presumably, rapid formation of the metal oxides in these cases prevented formation of the metal-PAR complex. The chromatographic detector characteristics of the PAR-ZnEDTA system were further investigated by preparing a calibration curve with Cd(I1) from 1 x to -1 x 10-5 M. The Porasil-oxine column previously described ( I ) was used
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
in the isocratic mode, elution of the Cd peak being effected with p H 2.7 “0,. Except for the 1 X M Cd solution, M to minimize detector when it was reduced to 0.75 x noise, the PAR-ZnEDTA concentration was 3 X 10” M. Both peak height and area were examined as a function of Cd concentration, and straight line response was obtained for each over the entire concentration range studied. The linear-regression equations were H = 1.037 X lo4 [Cd] + 0.1859 and A = 1.825 X l o 4 [Cd] + 0.2803 for peak height and area, respectively, with correlation coefficients of 0.9987 and 0.9990, where [Cd] is from 1-100 X 10-5 M. It should be noted that a 5 x 10-5M Cd solution, for which a significant response was easily obtained, represents an approximately 1000-fold reduction in amount of metal ion over that used in previous published studies. Contamination Problems. As a rule, in any method for trace analysis, much time will be spent wrestling with contaminated reagents, cleaning glassware, etc. This investigation was no exception, and we would like to comment briefly on some of our experiences. As we have earlier shown ( I ) , the Porasil-oxine column material is a fairly efficient substrate for separating metal ions in a sample mixture. This was a mixed blessing, however, in that the column material also is a good scavenger for trace impurities in eluent solvents. When the column is conditioned with eluent at a particular pH prior to sample injection, any metal impurities in the eluent which can complex with the oxine at that pH will be retained. These impurities may cause insignificant interference with sample peaks upon subsequent elution of the column. The PAR or PAR-ZnEDTA metallochromic indicator was chosen for the detector system in this chromatographic work because of its sensitivity. I t also provides a convenient and sensitive test for reagent Contamination. Singly ionized PAR (pH 5.5) has an absorption maximum at 411 nm (our value) as mentioned earlier and doubly ionized PAR (pK 12.3) a t about 490-500 nm ( 4 ) . Using a solution of PAR-ZnEDTA (pH 11, 3 X M) and diluting it in half with the solvents or solutions in question allows trace contaminants to be easily seen. If the diluting medium is totally pure, dilution should yield an absorbance, Ad, one-half that of the original PAR solution, A,. If metal contaminants are present in significant amounts, Ad a t 510 nm will be greater than one-half A,, because of the contribution a t 510 nm of the metal-PAR complexes. A drop in absorbance at 411 nm will also be noted as there is less free PAR present when some is complexed by the contaminants. I t should also be noted that an increase in acidity (lower pH) causes the free PAR band at 411 nm to increase and the 500-nm band to decrease because of a shift in the PAR-’ e PAR-’ equilibrium to the left. Data for some reagents are presented in Table 11. Looking particularly at the Ad,5lOvalues, it can be seen that even with rather extensive precautions such as doubly distilling and triply deionizing, cleaning all glass and plasticware with 1:l H N 0 3 and EDTA, contamination is still present. For completely pure diluents, the Ad,510values should be 0.114 and 0.117 AU for Table I1 (a) and (b), respectively. The redistilled Corning water appears to be the most pure. Of the solutions tested, the 1 M Na2S04can be seen to be highly contaminated; the 0.01 M Na2S04and NaHSO, somewhat contaminated, although ion-exchange treatment helped considerably, as shown for the 0.01 M Na2S04. Contamination of the pH 1 H N 0 3 is masked in that Ad,510decreases and the Ad,411increases because of the shift in the PAR-’ + PA4R-’equilibrium due to the lower p H after dilution. No attempt was made to positively identify these contaminants. However, about 700 mL of dd3 water was run through a column of immobilized ozine, the metal contaminants collected by eluting with 1 M HCl, and atomic ab-
Table 11. Dilution in Half of a 3 x Solution by Selected Reagents
375
M PAR-ZnEDTA
( a ) A , , , , , = 1 . 0 4 1 ; A 0 ~ 5=, 00.228 absorbance, A d , nm diluting solution 411 510 d d 3 H,Oa 0.499 0.155 dd3d H,Ob 0.519 0.130 Corning H,OC 0.506 1.128 pH 2 HC10, in dd3 H,O 0.483 0.172 0.505 0.137 pH 2 HNO, in d d 3 d H,O ( b ) A , , , , , = 1.002;A,,j,, = 0.233 redistilled Corning H,0 0.495 0.129 1 M Na,SO, 0.300 0.391 0.01 M Na:SO,, dil. from above 0.476 0.146 0.01 M Na,SO,, after ion-exchange 0.497 0.138 treatmentd 0.01 M NaHSO, 0.476 0.146 pH 1 HNO, 0.515 0.100 pH 1 HNO,, same as above except pumped 0.426 0.199 through the liquid chromatograph House distilled, triply deionized. “ a ” followed b y distillation. From Corning Megapure still. 25-cm column of Dowex 50W-X8, on SO,*- cycle. sorbance analysis for Cu(I1) made. A value of about 5 ppb was obtained. So, we are really dealing with rather small amounts of contaminants which are magnified by being scavenged by the immobilized 8 0 H Q during column conditioning. Blank elutions run after conditioning the column with about 20 mL of water normally produced peaks a t two locations on the blank chromatogram if the column was eluted with pH 1 or 2 acid. Usually a single peak came out early in the run and this is believed to be Ca(I1) or a Ca(I1)-Mg(I1) mixture. One or more later peaks are probably various transition metals which came from the water, reagents, glassware, and/or the stainless steel of the liquid chromatograph itself. The size of the first peak increases as the volume of water (solvent A) used to condition the column increases. The size of the second peak is independent of the volume of conditioning solvent, implying that it originates primarily from impurities already in the acid eluent (solvent B) or produced on passage through the LC System. T h a t the latter is a major source of contamination, especially where acidic eluents are involved, can be seen from the data for pH 1 “0, in Table II(b). Even thought the p H drop of the PAR-ZnEDTA upon dilution counteracts the effect of metal contaminants upon dilution, Ad.510 for the HNO, which was passed through the LC System shows that the extent of contamination has increased dramatically upon traversing the LC (pump, tubing injection valve, etc.). No matter how pure the aqueous reagents are, they will cause corrosion of the stainless steel during passage through the LC and become more or less contaminated themselves. Interference by alkaline earths can be avoided by conditioning the column a t a slightly acid pH (for example, 3 or below) which prevents their retention. However, there is really no good way to completely eliminate contamination from stainless steel corrosion, and at best what one obtains is a base line which rises during a gradient run due to the increased extent of metal contamination in the more acidic solution used to elute the column. If the gradient is a sharp one, however, that is, there exists a rapid pH rise during a gradient so that a more or less diffuse pH “front” obtains somewhere in the column, and because the contaminants in the acid “pile up” at the edge of this “front”, a peak will emerge simultaneously with the base-line rise. The same situation is seen if a column
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
conditioned with water or a solution rather more basic than solvent B is eluted isocratically with solvent B. If one of the sample peaks is found a t this position, interference will, of course, occur. Corrosion of the stainless steel appears t o be a limiting factor in trace metal analysis by HPLC, given the equipment available a t this time. What is needed besides very pure reagents and solutions is an all-Teflon system or a t least one where the amount of stainless steel is severely limited (8). T h e detection system described here is ideally suited for use in conjunction with an HPLC investigation involving a wide variety of polyvalent metal ions. We have found it more than adequate for use in applying chemically bonded chelating agents as immobile phases ( I ) , but it should be readily adaptable for ion exchange and liquid-liquid reverse phase partition separations as well.
LITERATURE CITED (1) J. R. Jezorek and Henry Freiser, Anal. Chem., preceding paper in this issue. (2) K. Kawazu and J. S. Fritz, J . Chromatogr., 77, 397 (1973). (3) J. S. Fritz and J. N. Stary, Anal. Chem., 46, 825 (1974). (4) S. Shibata, in "Chelates in Analytical Chemistry", H. Flaschka and A. J. Barnard, Ed., Vol. 4, Marcel Dekker, New York, 1972, pp 116-165. (5) K. N. Munshi and A. K . De, Anal. Ctem., 36, 2003 (1964). (6) L. G. Sillen and A. E. Martell, Ed., Stability Constants of Metal-Ion Complexes", Suppl. No. 1 , Special Pub. No. 25, The Chemical Society, London, 1971. (7) M. D. Arguello and J. S. Fritz, Anal. Chem., 49, 1595 (1977). (8) W. A. MacCrehan, R. A. Durst, and J. M. Beliama, Anal. Left., 10, 1175 (1977).
RECEIVED for review August 24, 1978. Accepted November 9, 1978. This work was conducted with the financial assistance of the U.S. Department of Energy.
Lead, Cadmium, and Zinc Bis(diethy1dithiocarbamate) and Diethyldithiocarbamic Acid as Reagents for Liquid-Liquid Extraction Sixto Bajo" and Armin Wyttenbach Swiss Federal Institute for Reactor Research, 5303 Wurenlingen, Switzerland
The extraction constants of Pb(DDC),, Cd(DDC),, and Zn(DDC),-where DDC is used to denote the diethyldithiocarbamate anion-were determined for the system H,O/CHCI,; log K,, was found to be 7.94 f 0.09, 5.77 f 0.05, and 2.39 f 0.02. Solutions of Cd(DDC)2 and Zn(DDC), In CHCI, were shown to be stable for at least 40 days. When solutions of Cd(DDC),, Zn(DDC),, or HDDC as reagents in CHCI, were used to extract acid aqueous solutions, their titer deteriorated as a function of pH and time; however, this deterioration is much less rapid than the destruction of a solution of NaDDC. The title compounds therefore can be considered suitable reagents for extractions from acid aqueous solutions.
Extraction of metals as their diethyldithiocarbamates (the anion (C2H5)2NCS2- will in the following be denoted as DDC) is becoming more and more popular. T h e reagent is traditionally introduced into the aqueous phase as NaDDC or into the organic phase as NH2(C2H5)DDC.Substitution of these reagents by metal diethyldithiocarbamates (dissolved in an organic diluent) results in an improved stability in contact with acid aqueous solutions and in an increased specificity of the extraction. Examples for the application of metal dithiocarbamates as reagents can be found in many publications (1-11). Selectivity of these extractions is governed by the extraction constants of the reagents (11,which unfortunately are not well known for the practically important system CHC13/H20. Furthermore, safe application of these reagents requires some knowledge about the stability of their titer. T h e present work was undertaken in order to contribute to the knowledge of metal dithiocarbamates as reagents by determining the extraction constants of Pb(DDC)2,Cd(DDQ2, and Zn(DDC)2and by measuring the stability of their solutions (as well as of HDDC) in CHC13 alone and in contact with 0003-2700/79/0351-0376$01 0010
various acids. Both points are prerequisites to the appropriate use of these reagents.
EXPERIMENTAL Extractions. Extractions were done in 250-mL separatory funnels at room temperature (21 "C) on a shaking machine with a capacity of 6 cm and a frequency of 6.6 s-l. The phases were then separated by decantation. Reagents. The solid products Pb(DDC)*, Cd(DDCI2, and Zn(DDC& were prepared as described before ( I ) . They were weighed to prepare solutions in CHC1, of known content, which were stored in dark bottles. Solutions of HDDC in CHC1, were prepared by shaking equal volumes of Zn(DDC),/CHCl, solutions and 2 M HCl for 30 s. Determination of the Extraction Constants. Solutions of the different metal diethyldithiocarbamates in CHC1, (30 mL) were shaken with aqueous solutions of HC104 (100 mL). The concentration of the M(DDQ2 as well as the acidity was varied within the limits given in Table I, whereas the concentration of C 1 0 in ~ the aqueous phase was kept constant at 1 M with NaC104. The concentration of the metals in the organic phase both before and after the experiment was determined by evaporating and wet ashing aliquots which were then analyzed by complexometry (Zn, Cd) or by atomic absorption (Pb). Control of the Titer of Solutions of Cd(DDC),, Zn(DDC)*, and HDDC in CHC1,. The variation of the titer with time was checked by periodically extracting a radioactive aqueous metal solution (100 mL) with aliquots of the organic solutions (30 mL) under conditions that were substoichiometric with respect to the reagent. The activity extracted was measured and taken to be proportional to the titer. The following systems were used: (a) for Cd(DDCI2and Zn(DDC)*in CHCl,: Hg2+('03Hg) in 1 M HCl, extraction time was 2 min; (b) for HDDC in CHCl,: Cu2+(62Cu) in 0.1 M HC104, and Zn2+ (65Zn)in a citrate buffer of pH 5, extraction time was 4 min. Control of the Titer of Organic Solutions in Contact with Acid Aqueous Solutions. Aliquots of the organic solutions (30 mL) were shaken for various times with 100 mL " 0 3 , HC104, HCl, or H2S04of different concentrations. Thereafter an excess of CuL+(64Cu)was added t o the aqueous phase and shaking was C 1979 American Chemical Society
