(27)A. W. Walde, J. Phys. Chem., 43, 431 (1939). (28)H. C.Brown, D. H. McDaniel, and 0. Hafliger in "Determination of Organic Structures by Physical Methods", Academic Press, New York, 1955,p 567 1 . 291, V. P. Vasii'ev and L. A. Kocheraina. Russ. J. fhvs. Chem.. 41. 1149 (1967). (30)J. J. Christensen, D. P. Wrathall, R . M. Izatt, and D. 0. Tolman, J. Phys. Chem., 71,3001 (1967). (31)A. L. McClellan, "Tables of Experimental Dipole Moments", Vol. 2, Rahara Enterprises, El Cerrito, Calif., 1974. (32)A. L. McClellan, "Tables of Experimental Dipole Moments", W. H. Freeman, San Francisco, 1963. (33)J. P. Greenstein and M. Winitz, "Chemistry of the Amino Acids", Vol. 1, Wiley, New York, 1961,p 471.
-
1
(34)J. T. Edsall and J. Wyrnan, "Biophysical Chemistry", Academic Press, New York, 1958,pp 371-372. (35) W. Melander and C. Horvath, Biochemistry, submitted for publication.
,
RECEIVEDfor review August 16, 1976. Accepted October 8, 1976. This work was supported by a research grant No. GM 20993 from NIH, by grant No. 17245from the National Cancer Institute, DHEW. The chromatographic equipment was purchased with the Of a grant No*ENG 74-23124 from the National Science Foundation.
Liquid Chromatographic Determination of Residual Acrylamide Monomer in Aqueous and Nonaqueous Dispersed Phase Polymeric Systems E. R. Husser," R. H. Stehl, D. R. Price, and R. A. DeLap Analytical Laboratories, Dow Chemical U S A . Midland, Mich. 48640
Two analytlcal procedures are described for the determination of residual levels of acrylamide monomer in aqueous and nonaqueous "dispersed phase" polymeric systems. The procedures consist of extractlon-precipltatlon of the polymer to remove the monomer, followed by separation and quantitation using high performance liquid chromatography with UV detection. For nonaqueous dlspersions, a detection limit of 10 ppm acrylamide has been achieved using normal phase partition llquld chromatography. A detection limit of 0.1 ppm acrylamide has been achieved for aqueous dlspersions using ion-exclusion liquid chromatography.
Acrylamide is used as a reactive monomer in many polymer systems. Being a toxic material, the ability to accurately measure the levels of this chemical which might be present in the work place or in the environment is necessary. Numerous analytical procedures for the determination of residual levels of this monomer have been developed (1-5). Polarography is reported in the literature (6-13) as a simple and sensitive method for the determination of acrylamide. It, however, lacks the specificity needed for some polymer systems. Some substituted acrylamides and alkyl esters of acrylic acid are polarographically indistinguishable from acrylamide. A gas-liquid chromatographic procedure for the trace level determination of acrylamide in river water is reported in the literature by Croll et al. (14, 1 5 ) . It involves bromination of the acrylamide to form a,P-dibromopropionamidewhich is separated and quantitated using electron-capture gas-liquid chromatography (EC-GC). It is a specific and highly sensitive procedure, which with some modification can be used for polymer systems. However, it has some serious limitations. The work presented in this paper is the result of a need for the determination of residual acrylamide monomer in both aqueous and nonaqueous dispersed phase polymeric systems. Nonaqueous polyacrylamide dispersions have found numerous applications as flocculating agents in water treatment facilities, and more recently as an aid in secondary oil recovery. In order to ensure the safe handling of this product, and to control the release of acrylamide monomer into the environ154
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
ment, a fast, simple, and precise internal standard liquid chromatography procedure has been developed. It can be used to determine residual acrylamide in nonaqueous dispersions down to the 10 ppm level. Acrylamide monomer is used as a modifier in certain aqueous polyacrylic dispersed phase systems. Because of the potential for human exposure from certain uses, it is necessary to ensure low levels of residual monomer. Therefore, an ionexclusion liquid chromatography procedure with a detection limit of 0.1 ppm has been developed which permits close monitoring of trace levels of acrylamide in aqueous or semiaqueous solutions.
EXPERIMENTAL Nonaqueous Dispersed-Phase Polymer Systems. Znstrumen-
tation. A Perkin-Elmer Model LC-55 variable wavelength liquid chromatography detector operating a t 240 nm was used for this analysis. It is a single beam spectrophotometer equipped with a 8+1, 6-mm path length flow-through cell. The mobile phase was pumped a t 1.0 ml/min, -500 psi, with a Waters Associates Model 6000A constant volume solvent delivery system. A Valco six-port valve with a 10+1 external sample loop was used to inject the sample onto the analytical column. The detector response was recorded on a Sargent-Welch Model DSRG multiple millivolt span recorder. Reagents. All standards and solvents were of reagent grade and distilled in glass quality, and were used without further purification. The mobile phase consisted of 15%methanol, 85%methylene chloride (volume/volume). The analytical column was a Partisill0 PAC (250 X 4.6 mm) available from Whatman, Inc. I t is a prepacked microparticle (10 p ) column containing a permanently bonded cyano stationary phase on a porous silica support. Procedure. The residual acrylamide monomer is extracted by adding 10 g (It0.l mg) of the nonaqueous dispersed polymer dropwise to 50 ml of methanol containing the internal standard benzamide. The mixture is stirred for 2 h on a magnetic stirrer. I t is then centrifuged for 15-20 min to isolate the polymer from solution. A 10-pl aliquot of the supernate is injected directly onto the column under the following chromatographic conditions (Figure 1): Column: Mobile phase: Flow rate: Pressure: Detector:
Partisil 10 PAC (250 X 4.6 mm) 15% methanol, 85% methylene chloride 1 ml/min 500 psi 240 nm
Quantitation is achieved by peak height measurements and calculations are based on the internal standard benzamide. A response
Standard
I 2
I 0
I 4
Sample
I 6
I
I
I
I
0
2
4
6
--
Time, minutes
Figure 1.
Sample analysis for nonaqueous dispersed polymer sys-
0
tems Column: Partisil 10 PAC (250 X 4.6 rnm). Mobile phase: 15% CH30H/85% CH2C12.Flow: 1 ml/rnin. Wavelength: 240 nrn at 0.1 aufs. Injection: 5 pl
factor is determined from similarly chromatographed standards of benzamide and acrylamide in methanol a t known concentrations. Aqueous Dispersed Phase Polymer Systems. Instrumentation. The Perkin-Elmer Model LC-55 detector was used for this analysis also. It was operated a t 225 nm. The mobile phase was pumped a t 0.7 ml/min, -100 psi, by a Brinkmann Instruments Model Labotron dual syringe pump. A Chromatronix six-port valve with a 0.5-ml sample loop was used to inject the sample onto the analytical column. T h e detector response was recorded on a Sargent-Welch recorder. Reagents. All standards and solvents were of reagent grade and distilled in glass quality, and were used without further purification. The mobile phase of 0.01 N HZS04 was made with de-ionized water. The analytical column was packed with Dowex 50W-X4 ion exchange resin, (30-35 p ) available from Bio-Rad Laboratories. Column Preparation. A 250 mm X 9 mm i.d. glass column was packed with the Dowex 50W-X4 resin using a slurry technique (16). The resin was slurried with water and added to the top of the column while applying a vacuum to the bottom. The column was conditioned by pumping 0.1 N hydrochloric acid for a t least 2 h followed by deionized water for l h. Procedure. T h e acrylamide is extracted from the aqueous dispersion by the following “extraction-coagulation” procedure. Ten ml of methylene chloride, 40 ml of water, and 0.5 ml of concentrated hydrochloric acid are placed into the 4-02 bottle and stirred vigorously on a magnetic stirrer. Five g of the dispersed polymer is added dropwise and allow to continue stirring for -30 min. The samples are then centrifuged at 10 OOO rpm and 0 “C for 1h in a refrigerated centrifuge. In order to achieve the detection limit of 0.1 ppm, decant off the supernate (-40 ml) and concentrate to 10 ml by evaporation with filtered air. If the concentrated extracts are not clear, filter through a 10-p filter. The extracts are chromatographed by ion exclusion liquid chromatography under the following conditions (Figure 2): Column: Mobile phase: Injection: Flow Rate: Pressure: Detector:
Dowex resin 50W-X4 (250 X 9 mm) 0.01 N HzS04 0.5 ml 0.7 ml/min -100 psi 225 nm
Identification by retention time and quantitation by peak height measurement is achieved by comparison with similarly chromatographed standards of acrylamide in water.
RESULTS A N D DISCUSSION Several of the problems typically associated with the chromatographic separation of amides have been overcome by the application of high performance liquid chromatography. The use of microparticle high efficiency LC columns has significantly reduced the analysis time, and eliminated ex-
Figure 2.
10
20
30 0 Time, minutes
10
20
30
Sample analysis for aqueous dispersed polymer systems
Column: Dowex 50W-X4 (30-35 p).Mobile phase’:0.01 N H2S04. Flow rate: 0.7 ml/min. UV detection: 225 nm at 0.02 fs. Injection: 0.5 ml
cessive peak broadening and tailing due to adsorption phenomena. The greater surface area and smaller particle diameters of these packings allow for faster mass transfer between the mobile and stationary phases, resulting in improved column efficiency and resolution. The polar bonded phase packing used in the nonaqueous LC system demonstrated excellent stability even under somewhat severe conditions, where large aqueous injections were made. A similar pellicular type LC column which does not have the bonded functional groups but rather coated glass beads, was also investigated for this analysis. It demonstrated the same high efficiency and good resolution, but soon showed signs of deterioration due to bleeding of the stationary phase. A limiting factor for higher sensitivity was the UV cutoff of the mobile phase. The detector was operated at 240 nm for the nonaqueous LC system to avoid absorbance by methylene chloride. The variable wavelength capability of the detector was essential in obtaining sub-part-per-million sensitivity for the aqueous system. Acrylamide absorbs UV radiation about 100 times stronger at 225 nm than at 254 nm, the normal operating wavelength for a single wavelength LC detector. The simplicity and short analysis times of the nonaqueous LC system have made it ideally suited for “in-plant’’ operation by nontechnical personnel. The use of an internal standard simplifies the calculations and reduces error due to sample preparation and injection. The UV response with respect to acrylamide concentration is demonstrated to be linear in the ppm region as well as the low percent levels. This broad linearity range makes the nonaqueous LC analysis applicable to both “in-process” monitoring, as well as final product quality control. The average response factor between the internal standard benzamide and acrylamide is 3.97 with a relative standard deviation of f0.04.A precision study was conducted by analyzing a sample ten times. The results of this study, summarized in Table I, indicate that the error limits for the analysis at the 0.6% acrylamide level (95%confidence) are f 2 % relative. In order to determine the optimum extraction time for the nonaqueous system, a sample was analyzed at 30-min intervals during a 2-h extraction and then a t the end of 19-h total extraction time. The results (Table 11) indicate that the acrylamide is readily extracted, and that a 1-2 h extraction is adequate. A recovery study demonstrated average recoveries of ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
155
Table I Precision Data For Non-Aqueous LC System
Sample
-Sample
Weight of Dispersed Polymer
Conc. of Internal Std.
Peak Height AA Benzarnide
136 rnm 107 mm 139 109 107 133 103 133 110 142.5 106 191 110.5 105.5 107 80.5 106 134.5 104.5 130 Avg. = 0.563 S.D. = 0.005 Error 95% confidence i2% Relative 0.1 mgiml 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
4.1067gram 4.0531 3.9584 4.0444 4.0791 5.7399 3.0057 2.3591 3.9976 3.9118
1 2 3 4 5 6 7 8 9 10
Table I V Precision Data For Aqueous LGSystem
1
% Acrylarnide*
% Residual Acryla-
0.96% 0.97 0.97 0.98 0.97
Sample
No. mg AArn Sample
1 blank 2 3 4 5 6 7 8 9 10
54.6 rng 54.2 55.4 54.2 54.9 43.2 32.4 22.8 56.2 55.9
rng AArn added
Total mg AArn Rec.
. indicated 0.56% acrylamide 29.6 82.7 30.2 85.0 29.6 83.4 30.0 84.0 66.6 23.6 17.7 50.2 12.5 36.4 30.7 85.7 30.6 86.3
% of Spike Recovered ____
Avg. Recovery = 99.4% Standard Deviation 3.88
*
greater than 99%. A sample with a known concentration of residual acrylamide was spiked with an additional amount of monomer. The recovery data for nine spiked samples is summarized in Table 111. The detection limit for acrylamide on the nonaqueous LC system a t 240 nm was 20 ng injected on the column. For a 5-pl injectian and a 10-g sample, this corresponds to 10 ppm. In order to achieve the sub-part-per-million detection level necessary for the aqueous dispersed phase polymer systems, the ion-exclusion liquid chromatography procedure was employed. Because of the aqueous mobile phase used, the detector could be operated a t 225 nm. With this added sensitivity, concentration of the extract, and by injecting 0.5 ml of extract onto the analytical column, a detection level of 0.1 ppm was attained. Ion-exclusion chromatography can be described as an aqueous form of partition chromatography (16, 17). The separations are obtained by combining partitioning effects with ionic interactions due to the pK, of the solute. If a solute species has a pK, less than the pH of the mobile phase, it is disassociated and excluded from the porous polymer support by ionic interactions with its sulfonated surface. A species with 156
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
__
15.4 16 7
15 5 16.1 15.6 15.8 16 1 15 8 160 16.5
ug A A m Sample
ug A A m Added
ug
Total A A m Rec.
%
Of
Splke Rec.
83 82 82 82 80 80 120 92 76 86
98.6 24 3 ug 25.8 1128 34.0 118.5 31 0 1 1 2.2 989 3117 11.1 25.9 2580 1849.0 Avg = 101 6 R.S D. = i 12.4 Error 195% confidence) = i 24 8 fluty1 Acrylate Eased Acrylonitrile Modified
89 118 99 106 97 86
1
96.4 98.0 98.7 96.7 99.3 101.0 108.8 96.1 99.3
i o.1 o .
5.0 4.7 5.0 4.6 50 4.6 5.0 46 50 4.5 6 ND" 50 40 7 20.5 5.0 26.5 8 15 50 6.1 9 N.D * 5.0 3.8 10 N.D." 50 43 Avg = 86 3 R S.D. = i 12 6 Error 195% confldsnce) = i 25 2 Ethyl Acrylate based Acrylonitrile Modified
2 3 4 5 6 7
Table I l l Recovery Data For Non-Aqueous LC System
ppm Acrylamide
105.5 rnm
Table v Recovery Dara For Aqueous LC System
1
Sample
Peah Helght Am
53461 gr 5.1933 5 4354 5 2636 5 4805 5 3529 5 3337 5 3943 5 3834 5 3747
Ethyl Acrylate Based Polymer Dispersion 1 05 2 05 3 0.5 4 0.5 5 0.5
Table / I Optimum Extraction Time Data For Non-Aqueous LC System
19 hrs
Weight
107.5 108.5 108.5 108.0 109.5 109.0 110.0 112.0 Avg = 15.8 S D. = 0.395 Error 95% Confidence t 5% Relatlve
0.552 0.561 0.560 0.569 0.566 0.560 0.566 0.569 0.566 0.567
"The values o f 90 acrylamide i n this table were listed w i t h three significant figures t o indicate the differences and do n o t represent the accuracy o f the method.
ET 30 min 60 min 90 min 120 min
2
2
.
77.2 ug 82.2 850 79.0 2160 16.4 1550.0
10,000 ug 900
258 ug 258
10,240 1,163 ug
116
93 102
'lndicaler none defected w t h a detectcon l8rnlf 01 0 1 porn
a pK, greater than the pH of the mobile phase will be allowed to partition into the polymer support because of its non-ionic character, and thereby be retained. The aqueous LC procedure demonstrated linear response for acrylamide in the sub-part-per-millionregion, and a precision of f 5 % relative error at the 16-ppm level (95% confidence), Table IV. The recovery study involving spiked samples, indicated that the recovery depends on the type of polymer dispersion analyzed and, to some extent, the amount of residual acrylamide monomer present. Three different types of aqueo,us polymer dispersions were investigated. Two of the dispersions were ethyl acrylate-based polymers, one of which was modified with acrylonitrile. The third dispersion was an acrylonitrile-modified butyl acrylate polymer system. The behaviors of these three dispersed polymer systems during the extraction-coagulation step of the analysis were different. Although all three eventually yielded clear extracts which could be chromatographed free of suspended polymer, they differed in the ease with which this separation of the polymer was achieved. Table V summarizes the recovery data obtained for the three types of dispersions. I t shows average recoveries in the 80-100% range. Although the spread of these recoveries appears to be large, it can be explained statistically based on the way the recovery experiments were designed (18, 19). Most of the dispersions contained a detectable level of residual acrylamide. In actuality, therefore, we were observing a dual recovery: the recovery of the acrylamide already present, plus the recovery of that which we spiked into the sample. To calculate the recovery of the spike, we subtracted
n
n-methylol acrylamide
I\I,N'methylene-bis-acrylamldt
1
Acrvlic Acid n.Methylol Acrylainide Acrylohitrite
I
Acry lam i d
Acrylamide
c
Acrylic Acid
N.N'.Methylene.bis-Acrylamide
Ethyl Acrylate
L 0
I
I
0
10
i 20
I 30
i 40
Time, minutes
Figure 3. Separation of acrylamide from expected interferences by
ion-exclusion chromatography Column: Dowex 50W-X4 (30-35 p). Mobile phase: 0.01 N H2S04. Flow rate: 0.7 ml/min. Detection: 225 nm. Sensitivity: 0.1 aufs. Injection: 0.5ml
the acrylamide already present from the total acrylamide observed. Because both of these values had uncertainties of f 5 % relative, the uncertainty in the difference is expressed by Equation 1,
where ei and ef are the absolute uncertainties of the individual values. Therefore, statistically the expected error in recovery for a 31.0-pg spike to a sample already containing 79.0 pg acrylamide, is f 2 2 % which in fact is what we obtained experimentally. The high recovery possible with the aqeuous LC analysis is the major advantage it has over the gas chromatographic approach. A modification of the Croll EC-GC procedure to analyze the aqeuous dispersed phase polymer systems was investigated in this laboratory. Excellent specificity and sensitivity were demonstrated; however, the average recovery of spiked samples, although reproducible, was very low. Because of the large correction factor this represents, and the error already demonstrated for a recovery study such as this, the procedure was abandoned in favor of the aqueous LC method. Figure 3 is a chromatograph showing the separation of acrylamide from expected interferences on the aqueous LC system using the ion-exclusion column. By modifying the mobile phase of the nonaqueous LC system to 5%methanol, 95% methylene chloride, separation of the same expected interferences was achieved on the normal phase partition column (Figure 4). This capability makes the two LC systems interchangeable in certain cases. The nonaqueous LC system has the advantage of speed, but is limited by its higher detection limit. Analysis of the aqueous dispersed phase polymers on this system results in a detection limit of 50 ppm acrylamide due to the incompatibility of large aqueous injections into the nonaqueous mobile phase. Specificity for acrylamide is the major advantage of these LC procedures over polarography. Some alkyl esters of acrylic acid are not polarographically distinguishable from acrylamide; therefore, one could expect interferences from the ethyl acrylate and butyl acrylate based aqueous dispersions. Sub-
2
4
L 6
Time. minutes
E
10
Figure 4. Separation of acrylamide from additional impurities by norm1
partition chromatography
Column: Partisil 10 PAC (250 X 4.6 mm). Mobile phase: 5% MeOH/95% CHpC12. Flow: 1 ml/min. Wavelength: 240 nm at 0.1 aufs
stituted acrylamides such as N,N'-methylenebis(acrylamide), and n-methylol acrylamide may be present in the aqueous dispersed polymer systems as added cross-linkers. These acrylamide derivatives are also indistinguishable from acrylamide by polarography (13). The authors would like to reemphasize at this point that acrylamide is a toxic substance by any means of exposure. We strongly recommend that rubber gloves and eye goggles be worn while preparing and handling concentrated acrylamide solutions. The use of a laboratory hood, while preparing these solutions, is suggested as an added precaution. If exposure to this chemical occurs, wash immediately with an abundance of water. If any wearing apparel is contaminated, remove it immediately and launder before reuse.
ACKNOWLEDGMENT The authors acknowledgethe assistance of B. F. Waling and W. E. Kester. Their advice on sample preparation and extraction procedures was an invaluable contribution to the work presented in this paper. LITERATURE CITED (1) G. Schmoetzer, Chromatographia, 4 (9), 391 (1971) (Liquid Chromatographic Determination). (2) H. Seeboth. H. Goersch, and W. Buettner, Monatsber. Deut. Akad. Wiss. Berlin, 8 (6-7), 439 (1966) (Thin Layer Chromatographic Determination). (3) M. Bachmann and R. Dagon, Chimia, 26 ( 5 ) ,264 (1972) (Spectrometric Determination). (4) L. I . Rapaport and N. G. Ledovskikh, Gig. Sanit., 37 (3), 74 (1972) (Spectrometric Determination). (5) A. R. Matlocks, Anal. Chem., 40, 1347 (1968) (Spectrometric Determination). (6) V. F. Kurenkov, E. V. Kuznetsov, and V. A. Myagchenkov, Zh. Anal. Khim., 28, 1236 (1973) (Polarographic Determination). (7) A. S. Gorokhovskaya and L. F. Markova, Zh. Obshch. Khim., 38,967 (1968) (Polarographic Determination). (8) V. A. Myagchenkov, V. F. Kurenkov, A. 8. Dushechkin,and E. V. Kugnetsov, Zh. Anal. Khim., 22, 1272 (1967) (Polarographic Determination). (9) Ferenc Vaida, Acta Chim. Acad. Sci. Hung.,53.241 (1967) (PoiaroaraDhic - . Determination). 10 (1-2), 221 (1965-66) (10) Ferenc Vajda, Banyasz, Kut. lntez. Kozlem., (Polarographic Determination). (11) D. C. MacWilliams, D. C. Kaufman, and B. F. Waling, Anal. Chem., 37, 1546 (1965) (Polarographic Determination). (12) E. M. Skobets, G. S. Nestyuk, and V. I. Shapoval, Ukr. Khim. Zh., 28, 72 (1962) (Polarographic Determination). (13) S. R. Betso and J. D. McLean, Anal. Chem., 48, 766 (1976)(Polarographic Determination).
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
157
(14) B. T. CrOll and G. M. Simkins, Analvst(London), 97 (1 153h 281 (1972) (Gas
Chromatographic Determination), (15) B. T. Croll, Analyst(London), 96 (1138),67 (1971) (Gas Chromatographic Determination). (16) M. Richards, J. Chromatogr., 115,259-261 (1975) (Ion Exclusion Determination). (17) D. J. Patel. R. A. Bhatt, and S.L. Bafna, Chem. hd. (London),21 10, (1967) (ion Exclusion Determination).
(18) H. A. Laitinen, "Chemical Analysis", McGraw-Hill, New York, 1960,Chap. 26. (19) "Guide for Measures of Precision and Accuracy", Anal. Chem., 47,2527 ( 1975).
RECEIVED for review July 9, 1976. Accepted September 23,
1976.
Liquid-Li quid Extraction of Cadmiurn with DiethyIdithiocarbamic Acid Sixto Bajo and Armin Wyttenbach' Swiss Federal Institute for Reactor Research, 5303 Wiirenlingen, Switzerland
The extractlonof Cd2+ with zinc bis(diethyldlthiocarbamate), Zn[(C2H5)2NCS2]2, from acid solutlons and wlth HDDC from alkaline solutions Is investigated. It Is shown that extractlon with HDDC from 5 N NaOH and back-extraction with 2 N HCI offers a fast and selective isolation of Cd2+ from a variety of matrices; the only Interference found was TI1+. Applicatlons of thls separation scheme to the isolation of Cd from flssion products and to the determination of Cd by neutron activation analysis in metallic zinc and in geological and blologlcal materlals are glven. The recovery of Cd is quantitative and there is therefore no need to determine the chemical yield of the separation. The recovefed Cd is of high radiochemical purlty.
In recent years there is growing concern about the role of Cd as a poison in food, water, and air. In fact, Cd can be found consistently in human tissues, although it is not considered to be an essential element. This situation calls for ever more sensitive determinations of Cd. Most methods which have the necessarysensitivity require an isolation of Cd from the matrix prior to its determination; hence, it is of great importance to have methods which can isolate Cd quantitatively, simply, and reasonably fast from a variety of matrices. This is especially true for the determination of Cd by neutron activation, where it is hardly ever possible to measure Cd without chemical separations. Customary separation schemes make use of separations with an anion exchanger in HC1 or HBr, or of liquid-liquid extractions with a variety of complexants; for a review of separation methods, see (1-3). Most of these methods are lengthy and do not work quantitatively. This is one of the reasons why activation analysis is seldom used for Cd (4).The situation is perhaps best characterized by the work of some authors who developed a separation scheme for Cd, only to find that it was not specific enough to be applied (5).The most promising approach seems to be extraction of Cd from alkaline solutions by dithizone, but there is conflicting evidence as to the completeness of extraction as well as to the purity of the extracted Cd (6-8). The present work deals with the extraction by diethyldithiocarbamate, (CzH&NCSZ-, (in the following denoted as DDC), which proved to be fast, quantitative, and specific. Although applications only to fission product solutions and to neutron activation samples are given, application to nonradioactive samples seems feasible. EXPERIMENTAL Reagents. Zn(DDC)Z was prepared and analyzed as described previously (9). The solid product was dissolved in CHC13 to give a 5 158
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
X 10-3 M solution, the titer of which was stable for a t least 5 months if kept in a dark bottle. HDDC was prepared for every analysis by shaking equal volumes of Zn(DDC)Z/CHC13and H C l 1 N. Because of the fast decomposition of HDDC in aqueous acid solutions (IO),shaking times must not be longer than 20 s. The resulting organic phase contains HDDC and is free from Zn. Extractions. Unless otherwise noted, the aqueous phase had a volume of 100 ml and contained 100 pg Cd2+. The organic phase M in Zn(DDC)2 or (CHCI,J had a volume of 30 ml and was 1.7 X 3.4 X IO+j M in HDDC; this quantity of reagent is roughly 60 times the amount theoretically necessary to complex 100 pg of Cd. Extractions were done on a shaking machine with a frequency of 6 s-l and an amplitude of 6 cm. Counting. Counting in experiments with only one tracer was done in a NaI well-type crystal; in all other cases, a well-type Ge(Li) detector was used. '
RESULTS AND DISCUSSION Extraction of Cd2+from Acid Solutions by Zn(DDC)2. The extraction of Cd2+ by Zn(DDC)2 from various acids is given in Figure 1. Extraction time was 2 min, which is long enough to reach equilibrium. For (H) 2 0.2 the reagent Zn(DDC)Z is for practical purposes completely protonated; it can then be shown that the experimentally found extraction of Cd2+ from HC104 and "03 corresponds to an extraction constant K e x , C d of 105.6. The solid line I in Figure 1was calculated using this value. It is seen that there is reasonable agreement between the calculated and the experimentally found extraction. Extraction from HC1 is obviously repressed in comparison to extraction from HC104 or " 0 3 . This is due to the formation of chloro complexes of cadmium with a corresponding decrease in the conditional extraction constant. Taking into account the appropriate a values for the chloro complexes and using again a K e x , C d of 105.6results in the solid line 11in Figure 1,which is seen to be in excellent agreement with the experimental values. The a values used in this calculation were calculated from data in reference (11)for an ionic strength of 0.1. The apparent enhancement of extraction from H3POd is due to the incomplete dissociation of this acid. Extraction from HzSO4 (not shown in Figure 1) is slightly higher than from HC104. The value of 105.6for the extraction constant K e x , C d which fits our results is roughly the same as the value of lo5.*found for CC14 (12),but is a t variance with the value of 104.4given for CHC13 (13). Extraction of Cd2+ by HDDC. The extraction of Cd2+by HDDC, from aqueous solutions of pH ranging from 2 to 11, was evaluated. The aqueous solutions contained a NaOH/ citrate or a NaOH/NaHzP04 buffer. Extractions lasted for 2
