Correction for the cyanamide monoanion fraction yields a value of 58.4 1. mol-' s-l which agrees to within 20% of the value determined at pH 10.5. Using inverse prediction methods (16),the least-squares parameters indicate a relative precision of about f3.5% at the 95% confidence level using this calibration curve to determine cyanamide concentrations from measurements of the initial rate. In order to determine whether the manual data collection procedure used in this study contributed any imprecision to the measurements, an automated data collection system employing a P D P 8/e minicomputer and A/D converter input was also used for one series of measurements. Over the entire useful range of cyanamide concentrations, a comparison of the precisions of the two collection procedures showed no significant difference. Thus computerized data collection can be used for convenience, but is not necessary for good accuracy. CONCLUSION The preceding evaluation has shown that sensitive and fast determinations of cyanamide can be made by measuring the initial rate of the complexation reaction between cyanamide and S P F in basic solution. At pH 9.0 the limit of detection is 13 ppb H2NCN. The total time for each determination is less than 1 min. Since the method requires a calibration curve, it is necessary to use the traditional silver precipitation to standardize a stock solution of cyanamide, but this needs to be done only infrequently (once a month). Due to the equilibrium between S P F and ammonia, this method would require extensive modification to be applicable to samples containing ammonia (like urine).
ACKNOWLEDGMENT We are indebted to S. R. Crouch for the use of his stopped flow mixing apparatus. LITERATURE CITED (1) D. A. Buyske and V. Downing, Anal. Chern., 32, 1798 (1960). (2) D. J. Donaldson, F. J. Normand, G. L. Drake, and W. A. Reeves, U.S. Depf. Agric., Agric. Res. Serv., [Rep.], 68, 72 (1972); Cbern. Abstr., 80, 28245 (1974). (3) V. I. Lukin, Tr., Sakbalin. Ob/. Stn. Zasbcb. Rast., No. 1, 19 (1970); Chern. Abstr., 78, 1019 (1973). (4) V. Tupalov, Rasfenievcld. Naclki, 9 (5). 145 (1972): Cbern. Absfr.. 78. 12560 (1973) S. Fukuda and H. Kunisha, Jpn. Kokai, 66, 590 (1974); Chem. Abstr., 82, 32714 (1975). N. I. Sax, "Dangerous Properties of Industrial Materials", 2nd ed, Reinhold, New York, N.Y., 1963, p 648. "Cyanamide", American Cyanamide Co., Wayne, N.J. Y. I. Mushkin, Z a v d . Lab., 33, 296 (1967); Cbern. Abstr.. 67, 39959 (1967). F. J. Holler, M.S. Thesis, Ball State University, Muncie, Ind., 1972. T. A. Niernan, Ph.D. Thesis, Michigan State University, East Lansing, Mich., 1975. (1 1) T. A. Nieman and C. G. Enke, Anal. Cbern., 48, 619 (1976). (12) H. A. Laitinen. "Chemical Analysis". McGraw-Hill. New York. N.Y., 1960, p 214-216. (13) P. M. Beckwith and S.R. Crouch, Anal. Chern., 44, 221 (1972). (14) D. D. Perrin, "Dissociation Constants of Organic Bases in Aqueous Solution", Butterworths, London, 1965, p 442. (15) W. R. Fearon, Analyst(London), 71, 562 (1946). (16) E. Ostle. "Statistics in Research", 2nd ed, Iowa State University Press, Ames, Iowa, 1963, p 176.
RECEIVEDfor review November 17, 1975. Accepted January 26, 1976. The authors gratefully acknowledge a fellowship received by one of us (T.A.N.) from the Analytical Division of the ACS (sponsored by the Procter and Gamble CO.).
Radiochemical Extraction of Copper with MetalDiethyldithiocarbamates Sixto Bajo and Armin Wyttenbach' Swiss Federal lnstitute for Reactor Research, 5303 Wurenlingen, Switzerland
Cu can be extracted with Bi(DDC)3 or Zn(DDC)2 in chloroform from aqueous solutions up to 5 N in HF, HF4B, H&04, H3P04, HCIO4, up to 3 N in HCI, and up to 1 N in "OB. In most cases, extraction times of 1 min are sufficlent for quantitative extraction. Washing of the organic phase without loss of Cu is possible with NaOH or with water. Cu can KMn04 be back-extracted into an aqueous phase by "03, or TI3+. Extractions from 0.1 N H&04 or from a citrate buffer with Bi(DDC)3 are selective except for Au3+, TI3+, Hg2+, and Ag+; these elements can be eliminated by first extracting the sample with Ni( DDC)2. Appllcation of the Bi(DDC)3 reagent to solutions of neutron activated geological or plant material yielded samples Containing 64Cu in excellent radiochemical purity with one single extraction. Detailed procedures and results of the analysis of standard materials are given.
Although the reaction 6 3 C ~ ( n , y ) 6 4offers C ~ an excellent sensitivity for the determination of copper by activation analysis, counting of samples by nondestructive y-ray spectrometry is very often not possible. This is especially true 902
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, M A Y 1976
for samples of biological or geological interest and is due to the interferences caused by 24Na: the y-line a t 1345 keV (64Cu)is emitted with a very low probability of 0.5% and is hard to measure in the presence of a large activity a t 1369 keV (24Na). The annihilation radiation of 64Cu is abundantly emitted, but is not specific enough; in particular, this line will also be generated by 24Na.In view of the relative sensitivities and contents of Cu and Na (Table I), the necessity for chemical separation of Cu is evident. Another situation where the separation of Cu is mandatory is when examining metallic samples (e.g., coins and other samples of archeological interest), where a great activity of 64Cu will preclude the determination of minor activities with half-lives smaller or similar to the 12.7-h halflife of e4Cu. Many different procedures have been used to isolate Cu from an activated sample, among which we will note electrolysis ( 1 ) ; amalgamation with mercury (2, 3 ) ; exchange with solid CuCNS ( 4 , 5 ) ; and extraction with various chelating agents, with an excess (6, 7) or with a substoichiometric amount of the reagent (8-13). Although all these procedures can yield radiochemically pure "Cu; they usually involve many steps (which makes
them lengthy); they call for the addition of masking agents (which makes the subsequent isolation of other activities from the same solution difficult); and they usually do not work quantitatively (which necessitates the determination of the yield of the separation). Furthermore, extraction of Cu complexes with a small extraction constant can be carried out only from nearly neutral or basic solutions, which will lead to difficulties if the sample contains macroquantities of easily hydrolyzable metals. Since we have demonstrated ( 1 4 ) that extractions with metal-diethyldithiocarbamates as reagent are simple, fast, quantitative, and show good discriminating possibilities, we would like to present the application of these systems to the neutron activation analysis of copper.
dictated by the value of K,, of the metal to be separated. Although the values for the extraction constant K,, for certain metals with DDC are known (16), these values are not conveniently applied to a real problem, because the conditional extraction constant Kh, can be much smaller than K,, and can sometimes not be deduced easily from K e x .We therefore determined experimentally the descending order of l / n log Kk, of 14 metals ( 1 4 ) ; this order was found, for elements of interest in the present context, to be Au3+, Hg2+,T13+, Ag+, Ni2+, Cu2+, Bi3+, Zn2+,and is valid for 0.1 N HzS04 as well as for a citrate buffer of pH 5. I t thus follows that Bi(DDC)3 or Zn(DDC)2 will extract Cu2+ together with A d + , Hg2+, T13+,and Ag+. Ni2+ will not be appreciably extracted for kinetic reasons, the formation of the Ni(DDC)2 complex being extremely slow. Evidently Bi(DDC)3 will be a better choice than Zn(DDC)2, because the latter reagent will also extract some six elements situated in the order of l / n log Kkx between Bi and Zn. If the coextraction of Hg2+,T13+,and Ag+ presents difficulties, these elements can be removed by an extraction with Ni(DDC)2 prior to the extraction of Cu with Bi(DDC)3. However, this will normally not be necessary because T1 forms no y-emitting activation product, and Hg and Ag have sensitivities much smaller than Cu and thus will not impede its determination by y spectrometry. Finally, a word should be said on the possible use of Cu(DDC)2 as extractant for Cu. If the quantity of Cu(DDC)2 used is large compared to the mass of Cu in the aqueous sample, Cu could, in principle, be quantitatively extracted into the organic phase (again together with Au3+, Hg2+, T13+,and Ag+). However, this reaction is not fast enough to be of practical use.
P R I N C I P L E OF THE METHOD
EXPERIMENTAL
NaDDC (DDC is defined as diethyldithiocarbamate anion, (C2H&NCS2-) is often used to chelate metals. NaDDC is water-soluble and can thus be added to the aqueous phase; the metal chelates formed are subsequently extracted into an organic solvent such as CHC13 or CC14. The fast decomposition of the acid HDDC in aqueous solutions (15) limits the application of NaDDC to solutions of very low acidities. This inconvenience can be circumvented by using the reagent in a form solubIe in the organic phase (such as diethylammonium diethyldithiocarbamate, Zn(DDC)2, or Pb(DDC)Z), in which case destruction is very much slower and allows higher acidities of the aqueous phase. However, in all cases, many metals will be extracted, and selectivity is usually sought-sometimes with modest success-by variation of the pH and/or by the use of masking agents. Contrary to this indifferent use of DDC we have shown ( 1 4 ) that metal-diethyldithiocarbamates exhibit an inherent selectivity which rests on the fact that a given metaldiethyldithiocarbamate (denoted in the following by BDDC) will only extract a certain group of metals (denoted by A), whereas all other metals (denoted by C) are not extracted. The borderline between the two groups, A and C, is shifted according to the choice of BDDC, and is given by the following condition:
Preparation of Ni(DDC)2, Bi(DDC)3, and Zn(DDC)2. The crystallized products were prepared as described before (14). They were checked for their metal content by complexometry; the following values were found (values in parentheses are the content expected for the given formula): Ni(DDC)Z (green): 16.3% (16.5%); Bi(DDC)3 (yellow): 31.9% (32.0%); Zn(DDC)Z (colorless): 18.1%
Table I. Typical Concentrations and Sensitivities of Na and Cu Sensitivities, cps/& Typical concentration, ppm Na
Cu
cu
Na Weight ratio, Na/Cu
~
511 1369
511
1335
keV
keV
keV
keV
Earth 2 4 000 55 4.4 x l o 2 crustb Sea 11 000 0.006 1.8 x l o 6 1.1 5.1 11.6 0.07 waterc Human 1 5 0 0 0.14 1.1 X l o 4 bodyc a Approximate values for an irradiation of 1 0 min at 1013 n/cm2 sec and a measurement with a 50-cm3Ge(Li) detector. b S . R. Taylor, Geochim. Cosmochim. Acta, 28, 1 2 7 3 (1964). c F. Kieffer, Chimia (Anrau), 2 7 , 596 (1973).
1 1 >log K b x ,>~ - log K;,,c n m n where K,, is the conditional extraction constant of the respective metals, and n and m the number of DDC they bind into the complex. This relation holds for a wide range of pH, although it should not be overlooked that KIx itself can be a function of the acidity and of the concentration of certain anions in the aqueous phase. The choice of the reagent BDDC for a given separation problem will thus be 1 - log K &
(18.1%).
These products were used t o prepare 5 X M solutions in CHC13. In the case of Zn(DDC)2, it was verified that the titer of this solution was stable for at least 5 months if kept in a dark bottle. Extractions. Unless otherwise stated, extractions of 64Cu were done from 100 ml of aqueous solutions containing 100 pg of Cu2+ (and varying quantities of acids) with 30 ml CHC13 1.7 X M in metal-DDC. This amount of reagent corresponds t o roughly 30 times the quantity necessary to completely complex the Cu present. The extractions were done in 250-ml separatory funnels on a shaking machine with an amplitude of 6 cm and a frequency of 6 s-l. The organic phase was drained after waiting a few minutes. Analysis of Silicate Rocks. Three hundred mg of irradiated material was placed together with 100 pg of Cu-carrier into a Teflon beaker and dissolved by heating with 5 ml of 23 M HF. Any precipitated fluorides were dissolved by addition of 1.5 ml of concentrated H2S04, and fluoride was expelled by heating to 220 OC. Boiling with 100 ml of H20 and 0.5 g of ascorbic acid for a few minutes yielded a clear solution. This solution was extracted for 5 min with 30 ml of Ni(DDC)2 1.7 X M in CHC13, the organic phase was discarded, and Cu was extracted from the aqueous phase for 10 min with 30 ml Bi(DDC)31.7 X M in CHC13. Analysis of Biological Materials. Then 300 to 500 mg of irradiated material was placed together with 100 gg of Cu carrier in a glass beaker and heated with 5 ml of concentrated "03, 1 ml of concentrated H2S04, and 1 ml of 60% HClOd to a final volume of 0.5 ml. The solution was diluted with 100 ml of HzO and extracted as in the case of rocks.
RESULTS AND DISCUSSION Extraction with Zn(DDC)Z. Extraction of Cu was checked from aqueous solutions containing HF, HFdB, ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
903
Table 11. Extraction of Cu with Ni(DDC), % 64Cuextracted into the organic phase
Normality of acid
1st extraction, 2 niin 0.1-2
3
0.1 0.1 H,SO, 0.04 0.04 HW, HF 0.5 0.5 HF,B 0.1 0.2 HC10, 0.2 1.2 HC1 0.2 0.7 HNO, 0.5 a Citrate buffer
