Radiotracer Method for Determination of Cadmium LOUIS E. MATTISON and JAMES C. WOLFORD, II Department o f Chemistry, King College, Bristol, Tenn.
b A new, rapid, and sensitive method is presented for the separation and determination of cadmium. Cadmium is extracted quantitatively (9970) from dilute (0.02M) potassium iodide/ iodine-1 3 1 solution into 5% pyridine/ benzene. The amount of cadmium extracted is proportional to the iodine1 3 1 radioactivity in the organic phase; the curve is linear from 10 through 2000 pg. of the cation. The extracted species was isolated and identified as bis(pyridine)cadmium diiodide. Ascorbic acid (1%) is present in the aqueous phase to prevent the formation and extraction of free iodine. Many metal ions do not extract; a few extract partially. Among the ions tested, the two major interferences are Cu(ll) and Hg(ll), both of which extract quantitatively.
M
is being devoted to the determination of trace quantities of metals in biological systems, in toxicologic and geochemical studies, and in nuclear reactor research. These methods involve a variety of techniques for separation and quantitative evaluation. Cadmium is frequently separated by dithizone extraction from strongly alkaline aqueous solution (2, 10, 11). This method applies best to relatively small amounts of cadmium, often less than 20 pg. The extracted cadmium dithizonate is determined by spectrophotometric analysis of the organic phase; the measured absorbance is sensitive to impurities, sunlight, and time of standiag. Cadmium also extracts quantitatively from acidic potassium iodide solution into ethyl ether or ketone solvents. Under these conditions, the following elements are reported to extract well as iodide complexes ( 8 ) : Sb(III), Cd(II), Au(III), In(III), Pb(II), Hg(II), Sn(II), Tl(III), and small amounts of Tl(1). Free iodine often accompanies the complex. In a few cases the extracted iodide complex can be determined spectrophotometrically, as in the case of Sb(111) and Bi(II1). Usually, however, the iodide extraction serves only to isolate a given metal from a group of metal ions. The metal extracted can then be measured with an organic reagent, often dithizone. An indirect precipitation method describes the determination of Cd(I1) and Hg(I1) using iodine-131 and a copper complex (9). UCH ATTESTIOK
A few radiometric methods have been developed in recent years. Among these are a method for the determination of fluoride ion using tantalum-182 (Y), a method for the determination of mercury using mercury-203 (S), and a method for lead using lead-210 (12). These studies indicate a variety of useful applications for radiometric methods of analysis. EXPERIMENTAL
Reagents. Standard cadmium solu-
tion, 1 mg. per ml., was prepared by dissolving 1.0000 gram of reagent grade cadmium metal (99.9yo). in nitric acid and diluting to 1 liter. The final solution was 0.1M in nitric acid. This general procedure was used in preparing other metal ion solutions. Dilutions were made as required. Iodine-131 was obtained from Radioisotopes Division of the Oak Ridge National Laboratory, Oak Ridge, Tenn., and from Abbott Laboratories, North Chicago, Ill. The potassium iodide (O.O2ill)/ascorbic acid (1%) solution was prepared by dissolving 0.855 gram of potassium iodide and 2.5 grams of ascorbic acid in water and diluting to 250 ml. The solution was used within 24 hours of preparation. Enough iodine-131 was added to give a final solution radioactivity of 3.33 pCi/ml. Ten milliliters oi this solution were used for each deterinination. The 5% pyridine/benzene was prepared on a volume-volume basis and stored in the refrigerator prior to use. All reagents were checked for interfering metal ion contamination; the level of such contamination was less than 0.5 pg. ,411 glassware was cleaned with 1 :3 nitric acid and distilled deionized water. Only reagent grade chemicals were used. Procedure. Pipet a 10-ml. aliquot of potassium iodide (0.02M)/ascorbic acid (l%)/iodine-131 solution (3.33 pC.i/ml.) into a 125-ml. separatory funnel. Pipet a 1-ml. aliquot of sample solution containing 10 to 2000 pg. of cadmium. This solution should be 0.02 to 0.3M in hydrochloric or nitric acid. Swirl the solution gently to mix. Pipet 10 ml. of 5% pyridine/benzene to complete the mixture. Extract manually for 2 minutes. Allow the layers to separate, run off the lower layer, and decant the organic phase into a centrifuge tube: Centrifuge for 5 minutes, then carefully pipet 5.0 ml. of the organic phase into a small screw-top sample bottle. Count the gamma radioactivity. After correcting for the blank
Table 1.
Standard Calibration Curve Data
Cd(I1) added, 1131,
1000 750 500 250 100 75 50 25 10
c.p.m.
195,100 142.500 99;300 48,000 19,600 14,800 9;800 4,900 2,100
Mean = 197 Std. dev. = 5 . 4 Rel. std. dev., %
=
Deviation
Cd(I1) counts min.-1
from
mean
pg.-1
-
195 190 i99 192 196 198 194 196 210
2
-7
+i +-
5 1 1 3 1 +13
2.7
and decay, determine the cadmium ion by referring to a standard calibration curve. The calibration curves were prepared using 1000-, 750-, 500-, 250-, loo-, 50-, 25-, and 10-pg. portions of the standard cadmium solution through the above procedure. A blank is always included. The statistics of this calibration curve are reported in Table I. This calibration should be repeated with each new shipment of the radioreagent. Apparatus. Gamma radioactivity was measured with a General Electric single-channel gamma ray spectrometer using a 2- X 2-inch NaI(T1) detector. Measurements were made a t 0.36 m.e.v. using a 100-k.e.v. window. RESULTS AND DISCUSSION
The method depends on the use of a dilute (0.02M), active (3.3 pCi/ml.) iodide solution, one in which the ratio of active to inactive iodine is relatively high. Since the extracted species is nonvolatile, samples can be counted hours or even days after preparation. Many metal ions do not extract; a few extract partially. The two major interferences are Cu(I1) and Hg(I1). In fact, the method has been applied successfully to the quantitative determination of Cu(I1). I n the microgram region, the method compares favorably with the dithizone method. It does not have the sensitivity of neutron activation analysis (6). To determine cadmium at the nanogram level, the extracted species might be analyzed by neutron activation of cadmium or iodine. This technique was successful VOL. 38, NO. 12, NOVEMBER 1966
0
1675
in the derivative activation analysis of Table II. Determination of Cadmium in Presence of Other Metal lons
Metal ion(s) added, pg.
Cd(I1) added,
Cd(I1) found,
rg.
fig.
100 100
97 99 97 102 100
Co(I1) Ni(I1) Al(II1) 1000 Fe(II1) 1000 UOZ(I1) 1000 Th(1V) Bi(II1) Mn(I1)
100
100 100
100
100
100
100 100 101 102 99 98 105
100
103
100
100
3 M TlU)
100
100 100 100
1000 La(I11) 1 mg. NBS 12c, steel 1 mg. NBS 432, tin alloy Mean = 100 Std. dev. = 2.3 Rel. std. dev., %
=
2.3
Table 111.
Determination of Cadmium in Presence of interfering Metal Ions
Metal ion(s) added, pg.
Cd(I1) added, pg.
None 1000 Pb(I1) 1000 Zn(I1) 1000 Ag(1) 100 Cu(I1) 100 H d I I ) 1 mg. fiBS'87a, aluminum alloy 5
Cd(I1) found, Pg." 100
100 100
100 100 100
100
125 200 160 185 155
100
108
Mean value of three determinations.
Table IV. Determination of Copper in National Bureau of Standards Sample
54 D Metal ion(s) present, fig,
+ +
241 Cu(I1) 41 Pb(I1) 241 Cu(I1) 41 PbfII) 289 Cu(II)'+ 50 Pb(I1)
Cu(I1) Cu(I1) added, found,
Cu,
fig.
fig.
%
None
233
3.50
200
440
3.60
None
276
3.45
Mean = 3.52 N. B. S. 54D, % Cu = 3.62 Rel. error, % = 2.8
100 Cd(I1)
+ 1000 Fe(II1) 100 CdfII) + 1000 Pb(I1) io0 C d i I i j + iooo ZniIIj 100 Cd(I1) + 100 Hg(I1) 100 Cd(I1) + 100 Cu(I1) 100 Cd(I1)
0
Cd(I1) found, fig." 100 100
107
108
165
200
Mean value of three determinations.
1676
0
Element C H N
ANALYTICAL CHEMISTRY
Calculated,
Found,
22.90 1.92 5.34
23.11 1.85 5.27
%
5%
The infrared spectrum and melting point of this solid were identical with that of Cdpy212synthesized from cadmium nitrate, pyridine, and potassium iodide in aqueous solution. The two spectra were quite similar to that of pyridine alone. CdpynIz is defined as a distorted, polymeric octahedral structure
(0.
Table V. Extraction from 0.002M Potassium iodide Solution
Metal ion(s) added, pg.
lead iodine in an extracted lead iodide complex. Derivative activation analysis was the subject of a recent contribution (4) which involved making the zinc chelate of 5,7-dibromo-8-hydroxyquinoline; zinc was determined from the bromine-82 radioactivity. Standard calibration curve data, 10 to 2000 pg., are shown in Table I. The cadmium level (micrograms) is linear with the iodine-131 activity in the organic phase. It is linear to 2000 pg., or as far as it has been tested. The background count averaged 310 f 10 c.p.m.; the blank, which includes background, averaged 500 =t50 c.p.m. In general, the data show 197 counts minutes-1 micrograms-', with a relative standard deviation of 2.7%. With a well-type detector the counting efficiency would be higher, perhaps a third for the same iodine-131 level. Using the dithizone method the per cent extraction for cadmium was 99%. Cadmium extracts as bis(pyridine)cadmium diiodide, a white crystalline solid. The mixed ligand complex was isolated from a large-scale extraction, followed by evaporation of the organic phase under reduced pressure. The crystalline solid separated from the last few milliliters of liquid, was collected by filtration, washed with 5% pyridine/benzene, and dried in a vacuum desiccator. The solid melts with decomposition in the range of 153' to 167" C. It was sent to Galbraith Laboratories, Inc., for elemental analysis. The data below support the proposed formula.
Ascorbic acid serves several purposes. Its main function is to keep free iodine from forming and being extracted. Air oxidation of the iodide ion in acid solution is a problem which must be controlled if the blank is to be kept low. One reference states: "The iodides seem to be somewhat unpopular among experimental chemists, they have a most remarkable tendency to photochemical or spontaneous oxidation to small amounts of brown 13-. In not too acidic solution, ascorbic acid can make the visible region free for spectrophotometric study but destroys, of course,
the ultraviolet region beyond 28kK" (6). Other oxidizing agents such as ferric ion are reduced and thereby prevented from converting iodide into iodine. The extraction of Bi(II1) is completely blocked in the presence of ascorbic acid. In the absence of ascorbic acid the extraction of Bi(II1) is quantitative. Finally, the ascorbic acid keeps the solution a t a convenient and reproduceable pH of 2.5 initially; the pH is about 5.7 following extraction. Cadmium is quantitatively extracted in the equilibrium pH range 5.3 to 6.1. Table I1 lists a number of elements that do not interfere with the extraction of cadmium. The data indicate a relative standard deviation of 2.3% in the determination of cadmium. Other ions that do not extract are As(III), Sb(III), Na(I), In(III), Sn(II), and K(1). The data in Table I11 illustrate the interferences from Pb(II), Zn(II), Hg (11),Ag(I), and Cu(I1). One interesting aspect is that lead does not greatly affect the cadmium determination if present in small amounts. The other is that the Cd(I1) and Cu(I1) levels are nearly additive. Indeed, it has been observed that the Cu(I1) level us. iodine-131 activity is linear in the region 100 through 1000 pg. of Cu(I1). The data in Table IV illustrate the analysis of Cu(I1) in the National Bureau of Standards sample 54D. Sample 54D is a tin alloy which contains antimony, copper and lead, and smaller amounts (
