The analysis of these geological samples is an indication of the possibilities for analyzing complex samples. A total of 28 elements are measured simultaneously. With longer irradiations and a better Si(Li) detector, more elements could be measured. The technique is most sensitive for the analysis of solutions nebulized and evaporated onto thin Formvar films or extracted onto ion exchange resin which is formed into a pellet. Thus, it has applications in the trace analysis of environmental samples. As discussed above, the X-ray intensities can be converted to concentrations using the internal standard or comparator method. The y-rays from resonant reactions or Coulomb excitation can be measured at the same time as the X-rays, but quantitative analysis requires a standard containing the elements of interest for comparison. Preparation of thin uniform targets and the limitations in beam current are problems. Although a large particle accelerator is necessary for this type of analysis, time on these machines is becoming increasingly available for analytical work. The calculations can be straightforward if the samples are prepared as described above so that the rather complex Equation 4 can be simplified.
ACKNOWLEDGMENT The authors gratefully acknowledge the technical support provided by the University of Virginia Department of Physics, in particular R. McKnight and D. Hills.
LITERATURE CITED (1) T. 6.Johansson. R. Akselsson, and S. A. E. Johansson, Nucl. Instrum. Methods, 84, 141 (1970). (2) D. A. Leich and T. A. Tombrello, Nucl. lnstrum. Methods, 108, 67 (1973). (3) K. Alder and A. Winther, Ed., "Coulomb Excitation," Academic Press, Inc., New York, NY, 1966. (4) E. Ricci, Nucl. hstrum. Methods, 94, 565 (1971). (5) L. C. Northcliff and R. F. Schilling, Nucl. Data Tables, A7, 233 (1970) (6) R. F. Sippel and E. D. Glover, Nucl. lnstrum. Metbods, 9, 37 (1960). (7) R. K. Jolly and H. B. White, Jr., Nucl. Instrum. Methods, 97, 103 (1971). (8) J. A. Cairns, Nucl. Instrum. Methods, 92, 507 (1971). (9) B. K. Barnes, L. E. Beghian, G. H. R. Kegel, S. C. Mathur, and P. W. Quinn. Meeting of the American Physical Society, Washington, DC, April 1973, No. EO 13. (IO) R. McKnight, S.T. Thornton, and R. R. Karlowicz, Phys. Rev., Sect. A, 9, 267 (1974). (1 1) T. J. Gray, North Texas State university, Denton, TX, personal communication, 1973. (12) W. Bambynek and B. Crassemann, Rev. Mod. Phys., 44, 716 (1972). (13) H. E. Gove, D. Stupin, A. Pape, A. Gallman, G. Guillaume, P. Flntz, and J. C. Sens, Division of Nuclear Physics, Seattle, WA, November 1972. (14) S. D. Rasberry and K. H. F. Heinrich, Anal. Chem., 46, 81 (1974). (15) T. B. Pierce, P. F. Peck, and D. R. A. Cuff, Nucl. hstrum. Methods, 87, 1 (i969). (16) F. J. Flanagan, Geochlm. Cosmochim. Acta, 33, 1189 (1973). (17) G. E. Brown, D. J. Hughes, and J. Esson, Chem. Geol., 11, 223 (1973).
RECEIVEDfor review August 20, 1974. Accepted December 23, 1974. Financial support for this study was provided by the National Aeronautics and Space Administration (NGR-47-005-176). Aspects of this work were presented a t the 8th Middle Atlantic and 26th Southeastern Regional Meeting of the ACS.
Radioactive-Tracer Displacement Method for the Determination of Small Quantities of Nickel Ruth A. German, David L. Hamilton, and M. P. Menon Department of Chemistry, Savannah State College, Savannah, Ga.3 1404
In thls work, the metal chelate dlsplacement reactlon has been shown to have analytical appllcatlon. The dlsplacement reaction between nickel and zlnc ethylenedlamlnetetraacetate was used to develop a radloactlve tracer displacement method of analysis of traces of nlckel. This method Is based on the extractlon of radloactlve zlnc dlsplaced by nlckel from the zlnc chelate whlch Is labeled wlth 245-d 65Zn into a dlthlzone-carbon tetrachlorlde solutlon and the subsequent measurement of the activlty of an allquot of the extract. With thls procedure, nlckel can be measured In concentrations as low as 0.05 ppm and perhaps even lower, dependlng on the specific actlvlty of the radloreagent used. The transltlon metals do Interfere wlth the analysls, but the Interferences of many may be removed by suitable masklng agents. In the presence of slgnlflcant amounts of interferers, nlckel is flrst separated by the dlmethyl glyoxime method and then determlned by the procedure reported here. This method was tested by analyzing several NBS reference standards and was comparable in accuracy wlth other technlques.
Nickel is one of the important transition elements which occurs as an impurity in steel, alloys, high purity metals, and also in food products (1-3). Accurate analysis of these 658
ANALYTICAL CHEMISTRY, VOL. 47, NO. 4 , APRIL 1 9 7 5
samples for the trace quantity of nickel poses a problem for the analyst because of either the lack of sensitivity in most of the detection techniques or interferences from foreign elements in the samples. The quantity of nickel which could be measured without much difficulty by most of the spectrophotometric techniques is, for instance, 5 micrograms ( 4 ) . In this work, an attempt was made to make use of the displacement reaction between the nickel ion and the zinc chelate of ethylenediaminetetraacetic acid, labeled with radioactive 65Zn for the development of a more sensitive method of analysis of nickel. The use of radioreagents for the analysis of trace elements has been shown to give better sensitivities than spectrophotometric techniques by Menon and others (5-8), provided a suitable radioreagent for the desired element can be found. Since chelate stability constants for the nickel salt and zinc salt of ethylenediaminetetraacetic acid differ by more than one hundred ( 9 ) , the following reaction may be used to determine nickel if the zinc released by the former can be estimated accurately: Ni(H20)62++ ZnL2-
--*NiL" +
Zn(H20)G'+
where L2- represents the ethylenediaminetetraacetate ion. The zinc ion in aqueous solution is known to be readily extracted with a solution of dithizone in carbon tetrachloride through the formation of zinc dithizonate (see Ref. 4, p
Table I. Effect of pH on the Blank and the Released Zinc Activities" N e t activity of the released
Blank activity, % Reaction yield,
PH
Zn2+, cpm
CPm
3 .? 4.2 4.7 4.9 5.4
4472 5428 8557 8189 6507
16875 17783 13153 15050 17783
relative
41.5 50.2 79.5 75.8 60.5
0 Reaction conditions: Temp., 40 OC; reaction time, 30 min; C N ~ , 10-5M/l.; CZ~LZ-/CN~, 100; specific activity, 1.37 X lo8 cpm/mmole.
Table 11. Effect of Specific Activity of the Radioreagent on the Activity of the Producta Net activity of
%
Blank activity,
the product
Reaction yield
cpmJmmole
cpm
CPm
relative
1.02 x 108 1.286 X lo8 1.371 x lo8 1.457 X lo8
11444 7200 13153 17973
6589 8240 8918 9381
80.7 80.1 81.8 80.7
Specific activity of radioreagent,
5.0 5
10
15
20
25
33
35
40
45
Tina (minutas)
Figure 1. Dependence of the product activity on the time after mixing the reactants
Reaction conditions: pH, 4.7; temp., 40 OC; reaction time, 30 min; CN~Z+: w 5 M / l . ; CZ"LZ-/CN~Z+: 100.
Experimental conditions: pH 4.7, Gi = 10-5M. CZnLz-I&,= 100, temp. = 26
OC
941). If the zinc chelate used for the reaction is labeled with radioactive 65Zn(T1/2 = 245 d) and the displaced zinc is separated from the mixture by a dithizone-CC14 extraction, the activity in the extract may be related to the concentration of nickel in the sample. The kinetic study of the above reaction (to be published later) revealed that, at the trace level of nickel, the above reaction will go to completion only if the ratio of zinc chelate to nickel is about 50 or more. It is, therefore, essential that the product activity be isolated from the unreacted radioreagent as purely as possible to eliminate contamination from chelate activity. It has been found from repeated experiments that none of the radioreagent, *ZnL2-, is extracted with the dithizone-CC14 solution, although the presence of uncomplexed zinc can give rise to some activity in the blank.
EXPERIMENTAL Apparatus. A single-channel gamma ray spectrometer using a 3- X 3-in. NaI(T1) well detector was used to measure the radioac-
tivity of 65Zn. Reagents. Reagent grade disodium ethylenediaminetetraacetate (NaZEDTA), dithizone, and dimethyl glyoxime were used in this work. All other chemicals used in this study were also of reagent grade purity. Stock solutions of nickel sulfate and zinc sulfate (-0.1 M ) were standardized by titration with a primary standard solution of disodium ethylenediaminetetraacetate (0.10M) a t a p H of 4.7 using the potentiometric end-point detection. From the standardized stock solution of nickel sulfate, a 5 X 10-5M solution was prepared by successive dilution by volume, with water. The error introduced in this step is estimated to be less than 1%. Preparation of the Radioreagent. To prepare the radioreagent, the appropriate quantity of zinc sulfate solution was mixed with 0.3 mCi of 65Zn ( 5 mCi of 65Zn in the form of ZnClz was supplied by Union Carbide Corp.) solution and 10 ml of acetic acid-acetate buffer (pH 4.7) and the resulting solution was reacted with 5 ml of 0.10M Naz EDTA. This mixture was diluted to 50 ml to have a final concentration of 10-2M of the radioreagent. The labeled radioreagent may contain a slight excess of,zinc but not any uncomplexed ligand other than what is permitted by the equilihrium function. If it does, nickel will react with the ligand rather than
displacing zinc from the chelate. The activity of the blank depends upon the amount of the unreacted zinc in the reagent solution. However, the blank activity can be reduced by extracting the entire radioreagent solution with 10-3M dithizone solution in CC14. Reaction Conditions for Maximum Product Yield. Although the stability or formation constant of NiEDTA (log K f= 18.4) differs by more than two orders of magnitude from the corresponding ZnEDTA (log K f= 16.1) (9),the displacement reaction was extremely slow in dilute solutions when the mole ratio of nickel to zinc chelate is 1:l. However, our kinetic study revealed that the reaction can be brought to near equilibrium in about 30 minutes if the concentration of zinc chelate is at least 50 times greater than that of nickel ion. The product yield was also dependent on the pH of the reaction mixture and also the temperature. Figure 1 shows the dependence of the product yield on the reaction time. At room temperature, the reaction seems to reach equilibrium in about 35 to 40 minutes. The effect of pH on the blank and the displaced zinc activities is shown in Table I. It is obvious from this table that the maximum yield and the minimum blank are obtained a t a p H of 4.7. As expected, the product activity also increased with temperature. Unlike other techniques, the magnitude of the measurable quantity can be enhanced, in a radioreagent method, by increasing the specific activity of the radioreagent. This is demonstrated in Table 11. The sensitivity of this method, therefore, depends on the specific activity of the reagent used. It is interesting to note from Table I1 that the blank activity is not constant. This is attributed to the different concentrations of uncomplexed zinc present in the radioreagent solutions. The percent reaction yield is calculated from the specific activity of the radioreagent on the assumption that the displaced zinc is quantitatively extracted into the dithizone-CC14 solution. The efficiency of extraction of labeled zinc into dithizone-CC14 mixture was found, by another experiment, to be better than 99%. It was also observed that the reaction yield increases from about 68% a t room temperature (26 "C) to about 82% a t 45 "C. Considering the low boiling point of carbon tetrachloride (76.7 "C), the maximum temperature for the reaction was arbitrarily chosen as 40 OC. Procedure for the Analysis of the Interference-Free Samples. Into a 25-ml test tube, add 3-4 ml of the standard solution which contains 5 fig of nickel, or 3-4 ml of an interference-free sample solution which contains no more than 10 fig of nickel; add 5 ml of the buffer (pH 4.7) and dilute to 9 ml with water. Add 1 ml of 10-2M solution of the radioreagent and keep the reaction mixture a t 40 "C for 30 min. For samples containing 10-100 fig, the concentration of the radioreagent will have to be increased by the appropriate order of magnitude. At the end of the reaction time, transfer ANALYTICAL CHEMISTRY, VOL. 47, NO. 4, APRIL 1975
659
Table 111. Results of Interference Studies" Net a c t i v i v of Interferer
Amount and nature of
studied
masking agent used
... 1 mg 1 mg 1 mg 1 mg 1 mg 1 mg
None
Fe3'd 0.4 g N a F ~ 1 ~ 0.2' g N a F Mn2' 0.2 g N a F Mn2' 0.4 g N a F C U ~ +0.2~ g T h i o u r e a S n 2 + d 1 g Sod. Pot. tartrate 1 mg CO" 0.2 g Sod. thio-
the released Log ~y~
zinc, cpm
18.6' 25.1 16.1 13.8 13.8 18.8 not known
8931 3417 8182 30775 24800 6326 11783
16.3 31042 sulfate 1 mg Cd" 16.5 20746 0.5 g NaI 1 mg Mg2' None 8.7 8550 15.5 1 mg ~ a 0.4 ~ g N+a F ~ 8887 4750 1 mg C r 3 + d 0.5 g a s c o r b i c a c i d not known Amount of nickel added to each solution containing the interferer, 5.87 pg. These values represent the log of the stability constants of the chelates of the respective metal ions with ethylenediaminetetraacetate. This is the log of the stability constant of nickel ethylenediaminetetraacetate. It appears that the interference of these metals may be removed by complexing them with the appropriate amount of the indicated masking agents.
:I 10.0
d
/
/ 1I
/
9 .2
.4
Cmcentratlon
.6
.8
1.0
12
of Wlok*l I n th. R.astlm niItur.
1.4 (ur/nl)
Figure 2. Calibration curve for the analysis of nickel using the radioactive-tracer displacement method
Specific activity of the radioreagent used: 1.37 X lo8 cpm/mmole the solution to a separatory funnel containing 5 ml of 2 X 10-4M solution of dithizone in CCl4 and shake for two min. The dithizone extraction of zinc is slow and it takes about 2 minutes for completion (10). Collect 4 ml of the extract in a counting tube and measure the activity of the sample resulting from 65Zn gamma rays with energy above 0.2 MeV using the gamma ray spectrometer. The prominent gamma rays emitted by 65Zn have energies 1.115 MeV (49%) and 0.511 MeV (3.8%), respectively. Prepare a blank using the same procedure but containing no nickel and measure its activity in a 4-ml extract. Subtract the blank activity from the gross activities of the samples to obtain their net activities. Two one-minute counts were made for each sample and the average counting rate was recorded as gross activity of the sample. The counting rate was always greater than 10,000 cpm and the counting error (deviation from the mean) was less than 0.5%. The concentration of the nickel in the sample taken for analysis is determined either by comparing the net activity of the sample with that of the standard, or from the calibration curve. Several metal ions, mostly transition elements, will interfere with the analysis of nickel with the above procedure as is the case with the photometric methods ( 4 ) . However, the interference of Some of the metals may be removed by complexing them with suitable masking agents as was demonstrated in our interference studies. For the analysis of samples containing Cu2+, Mn2+ Cd2+, Hg2+,etc., nickel will have to be separated from the interfekrs by the dimethyl glyoxime method before applying the above procedure. Interference Studies. The ethylenediaminetetraacetate ion forms chelates with a large number of metal ions, but their stability constants are different. Those whose formation constants for the chelate are higher than that of nickel ion will more readily displace zinc from labeled chelate than the nickel ion, provided the bonding is similar. This will result in the release of higher activities of zinc which are unrepresentative of the quantity of nickel present. All of the metal ions except Mg2+ listed in Table I11 interfered seriously with the determination of nickel by the above procedure. It is interesting to note that even Mn2+ whose reported stability constant is much lower than those for Zn2+ and Ni2+ ions does interfere with the analysis of nickel. Relative stability constants were taken from Reference (11). Attempts were made to remove the interference of the above metals by adding several masking agents in various amounts to the reaction mixture containing 5.87 Fg of nickel, one a t a time. Each of the samples was processed using the above procedure and the net activity of the 4-ml extract was measured. The results of these studies are presented in Table 111. Analysis of NBS Reference Standards. Four reference stan660
ANALYTICAL CHEMISTRY, VOL. 47, NO. 4 , APRIL 1975
dards of different chemical composition supplied by the National Bureau of Standards were used to test the new procedure for the analysis of nickel. All of the standards contain some of the interfering metals such as Fe, Al, Mn, Cu, Sn, etc. in relatively larger concentrations than nickel. Direct determination of nickel using some of the masking agents included in Table I11 failed to produce good results. lsolation of nickel by the following procedure and subsequent reaction with the zinc chelate, however, proved to be successful. Appropriate quantities of the standards are dissolved in HCl-HNOS or HNOz-HF mixtures and diluted to 100 ml in volumetric flasks. A one-milliliter aliquot of these samples is taken for analysis. Precipitate the hydroxides of the interfering metals that are insoluble with excess concentrated ammonia. Centrifuge, filter, and collect the filtrate in a 25-ml test tube. Wash the precipitate once with 0.5M NH40H and add the washing to the original filtrate. Slightly acidify the filtrate and add 5 ml of 10%ammonium citrate. Neutralize with concentrated ammonia, add a few drops in excess to make pH greater than 7.5, and dilute to 20 ml. Add 2 ml of 1% solution of dimethyl glyoxime in ethanol, mix, and extract with two 3-ml portions of chloroform, shaking for 30 sec each time. Wash the combined extract two times with 5 ml of 0.5M ammonia ( 4 ) . Return nickel to the ionic state by shaking the chloroform extract with 5 ml of 6M HC1. Evaporate the back-extract to dryness and mix the residue with 3 ml of water including washing. Determine the nickel in the separated sample by the procedure previously outlined. A blank is prepared using the above separation procedure. The activity of the blank prepared by employing this procedure was always higher than the one prepared without prior separation. This is attributed to the cumulative effect of the impurities present in the reagents used for separation.
RESULTS AND DISCUSSION The calibration graph relating the net activity of zinc in a 4-ml extract and the concentration of the nickel in the standards, as given in Figure 2, shows that a t least 0.05 ppm or 0.50 pg of nickel can be determined by this method. The sensitivity of the new method is, therefore, at least ten times higher than that for spectrophotometric methods, and perhaps higher than the sensitivity normally obtained for neutron activation analysis of nickel. The reported detection limit for the activation analysis of nickel (product isotope 2.5-hr 65Ni) using a thermal neutron flux of 4.3 X
Table IV. Results of the Analysis of the NBS Reference Standards Using the Radioreagent Method Amount of the
Standards used
sample in an
N e t activity
Amount of nickel
% Nickel in the
% Nickel
aliquot, g
observed, cpm
measured, u g
s ample (this work)
NBS certified
Lab. standard ... 8870" Ferrosilicon 0.01516 6306 SRM 59a 0.01516 6925 Aluminum Alloy 0.00603 7300 SRM 85b 0.00603 7440 Cast Bronze 0.00066 8378 SRM 52C 0.00066 7983 Tin -base 0.125 8180 bearing metal 0.125 6740 SRM 54D Average of two measurements. Average value.
-
5.87 (known) 4.17 4.58 4.83 4.91 5.55 5.30 5.39 4.42
10l2 n/cm2 sec and an irradiation time of one hour is 0.18 gg which was computed on the basis of a detectable photopeak count rate of 10 cpm (12).Our detection limit is based on a net count rate of 530 cpm which is more than three times the standard deviation (U = 140 cpm) in the blank. The specific activity of the radioreagent used in our analysis was 1.37 X lo8 cpm per millimole. The sensitivity of the method may be enhanced, however, using a reagent with higher specific activity as is indicated by the data in Table 11. For samples containing more than 10 gg of nickel, the concentration of the radioreagent should be increased so that the reagent-to-nickel concentration ratio is a t least 50. The results of the interference studies which are presented in Table I11 show that the inkrferences of some of the metals may be removed using certain masking agents in proper amounts, but others like Mn2+, Co2+,Cd2+,etc. will still interfere if the nickel is not isolated from the sample. In the case of lanthanum, LaF3 is precipitated when the fluoride is added. Alkali and alkaline-earth metals do not cause any interference for the analysis. The results of the analysis of the NBS reference standards using this new procedure are presented in Table ZV. Most of our results are in agreement with the values certified by NBS. Our results for the fourth sample, however, differ considerably from the average value reported by NBS. Similar variations also exist in the analytical results reported by different analysts presumably using different techniques. Taking into consideration the errors which might be introduced in the dilution of stock solution, the isolation of nickel, the extraction of the displaced zinc and the counting of the samples, one can expect a cumulative error of about 5% in the final result of each analysis. It is believed that the major source of error is probably the inadequate separation of nickel from the interferers.
...
... 0.028 0.030 0.080 0.082 0.84 0.80 0.0043 0.0035
0.028-0.039
(0.033)*
0.077-0.091 (0.84) 0.76-0.77
(0.76)
0.002-0.003, (0.002,)
CONCLUSION This work shows that the displacement reaction between nickel and zinc ethylenediaminetetraacetate labeled with 65Zn can be used to determine traces of nickel in interference-free samples. For the analysis of samples containing significant concentration of transition metals, nickel has to be separated by the dimethyl glyoxime method prior to the addition of the radioreagent. This method is more sensitive than most of the spectrophotometric methods as well as the neutron activation technique. Since the half-life of 65Zn used for the preparation of the radioreagent is 245 d, a 5mCi supply of the tracer lasts a t least a year and is enough to analyze about 800 samples.
LITERATURE CITED M. D. Cooper, Anal. Chem., 23, 880 (1951). T. E. Green, Anal. Chem., 37, 1595 (1965). M. Kenigsberg and I. Stone, Anal. Chem., 27, 1339 (1955). E. B. Sandell, "Colorimetric Metal Analysis," 3rd ed.,lnterscience Publishers, New York, N.Y., 1961, p 672. J. E. Kenney and M. P. Menon, Anal. Chem., 44, 2093 (1972). M. P. Menon, J. Radioanal. Chem., 14, 63 (1973). M. P. Menon, Anal. Chim. Acta, 64, 151 (1973). F. L. Moore, Anal. Chem., 35, 1032 (1963). A. E. Martell and M. Calvin, "Chemistry of the Metal Chelate Compounds," 5th ed.,Prentice-Hall, Englewood Cliffs, N.J., 1962, p 514. H. Fisher and G. Leopoldi, 2.Anal. Chem., 107, 241 (1937). W. J. Blaedel and V. W. Meloche, "Elementary Quantitative Analysis, Theory and Practice," 2nd ed., Harper & Row, New York. N.Y., 1963, p 576. H. P. Yule, Anal. Chem., 37, 129 (1965).
RECEIVEDfor review July 22, 1974. Accepted November 20, 1974. The authors are grateful to the National Science Foundation for their financial support which made this work possible. The paper was presented in part at the second Rocky Mountain ACS Regional Meeting, Albuquerque, N.M., July 8-9, 1974.
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