then affect the cerium determination. Although the existence of the isomeric states Sm1433m and ?Jd141m as almost identical to Ce139min their nuclear characteristicb has not been reported in a more recently published decay scheme ( I ) , and is thus somewhat doubtful, alternate procedures have been employed to check whether the cerium measurement is valid. Significant amounts of l.&hour Nd149 and 9.0minute SmI43 should be produced by the (n, an) reactions of Ndl" and Sm'44, respectively, upon irradiation with 14-m.e.1~.neutrons for a relatively long time--Le., 20 minutes. One- to two-gram samples of monazite, gadolinite, and allanite were activated for 20 minutes and counted after a delay of 15 minutes, but no peaks characteristic of Sd149 and Sm143, decaying with the respective half lives, were observed. This ruled out the presence in the above minerals of significant amounts of neodymium and samarium which could affect the cerium determination. Other minerals which have a relatively low cerium content were not investigated, because it would be impossible to detect such small amounts of' neodymium and samarium even if they were present. The analyses of geological minerals for the rare earth elements are too limited in the literature (8) for a comparison to be made with our results.
Rengan and Meinke (9) have reported the analysis of monazite for praseodymium and their average value is 28.4 pg./mg., which is in agreement with our result (31.6 pg./mg.). However, the value reported for the cerium content in both monazite and allanite is 947 pg./gram ( 8 ) ,which varies considerably from our values (225 pg.1 mg. and 103 pg./mg., respectively). Although monazite samples obtained from different sources may vary in the elemental composition, this disagreement seems large. The cerium content of the rare earth minerals is usually higher than that of praseodymium. Because the value of 947 pg./gram for cerium concentration in monazite is less than that of praseodymium obtained in two independent investigations [ ( 9 ) , present study], it is assumed that our results are correct within the experimental errors.
ACKNOWLEDGMENT
We are grateful to Richard E. Wainerdi, head of this laboratory, for his encouragement and valuable suggestions in connection with this work, and also to Lloyd E. Fite and other staff members for their help in the use of various equipment.
LITERATURE CITED
( I ) Biryuskov, E. I., Shinanskaya, Isu. Akad. S a u k U.S.S.R. Sev. 1963, p. 1402 (1963) (in Russian); .\'ucl. Sci. Abstr. Yo. 18, 9339. (2) Cornish, F. W., Brit. Atoniic Energy
Research Establishment R e p f . AERE C/R 1224 (1956). ( 3 ) Covell, D. F., ANAL. CHEM. 31, 1785 (1959). ( 4 ) Gibbons, I)., Wantage Laboratories,
Berkshire, England, private communication, 1964. ( 5 ) Goldman, I>. T., "Chart of Suclides," 6th ed., 1962, distributed by Edncational Relations, Department MWH, General Electric Co., Schenectady, s.Y. ( 6 ) Hillebrand, W. F., Lundell, G. E. F. "Applied Inorganic Anal" Wiley, Sew York, 1953. ( 7 ) Kramer, H. H., Molinsky, \-, J., Tilkury, R. S.,Wahl, W.H., Stier, P. ll,, R e p t . NYO-10, 174, p. 80, (April 23, 1962, to J ~ l y31, 1963). (8),Lyon, W. S., Jr., "Guide to Artivation Analysis," p . 547, Wiley, Sew York, 1953. (9) Rengan, K., Weinke, W. W., Asar,. CHEM.36, 157 (1964). (10) Wille, R . G., Fink, R . W., P h p . Rec. 118, 242 (1960). hZ. P. ~ I E S O N 11.Y. CL-YPERS Activation Analysis Research Laboratory Texas A & hl University College Station, Texas WORK supported by the Division of Isotopes Development, I'SAEC, and by the office of Lunar and Planetary Programs, SASA.
Estimation of Antimony in Biological Materials by Neutron Activation Analysis SIR: Thermal neutron activation combined with a chemical separation is a rapid, accurate method for estimating antimony in small samples of biological materials. Using the following method fifty samples may be analyzed in one day. Sulfuric acid digestion of the activated samples is fo'lowed by a precipitation separation of the antimony, which is combined with a gravimetric yield determination. The act,ivity is detected using a n end window Geiger-Xluller counter. T o separate the antimony, the samples are dissolved in hot concentrated sulfuric acid, and the antimony is precipitated as the metal using chromous chloride ( I ) . The precipitate is dissolved in a mixture of hydrochloric acid and hydrogen peroxide and any arsenic 01'mercury which is coprecipitated with the antimony is eliminated by precipitation with ammonium hypophosphite ($1. Copper is precipitated as cuprous fprricyanide and the excess reagent is i,ernoved as the cobaltous compound. The antimony is finally precipitated with
chromous chloride, and a comparison of the chemical yields and activities is made with those of a standard. EXPERIMENTAL
Preparation and Irradiation of Samples. T h e samples, preferably about 20 mg., are weighed out and wrapped in aluminum foil. A sample of high purity antimony potassium tartrate (about 0.5 mg.) is weighed into a silica tube, which is then sealed. T h e samples and standard are packed in a standard aluminum can and irradiated for three days a t a thermal neutron flux of 1012n/cm.2,/sec.,returned, and processed. The standard is dissolved in hydrochloric acid and diluted as necessary. Antimony Isotopes. Naturally occurring antimony produces two active species by thermal neutron capture, SblZ2and SblZ4,with half-lives of 2.74 and 60.0 days. The contributions of these to the activity induced in natural antimony after irradiation for three days a t a thermal neutron flux of 1012n/cm.2/sec. are 270 and 4.5 n x / gram respectively. Thus activation of
antimony under these conditions produces principally Sblz2which emits 1.42 and 1.99 hl.e.v. 8-particles and 0.57 14.e.v. y-rays. Possible interfering reactions are the formation of Sbl2j from Sn124, and the fast neutron reactions giving rise to SblZ2 and Sbl24-i.e., Te122(n,p)Sb122 and 1127(n,~)Sb124. S o n e of these reactions are significant in biological material, with the exception that the relatively high concentration of iodine in the thyroid may cause an increase of up to 10% in results of antimony analysis. Reagents. All the reagents used are of AnalaR grade, except the chromous chloride, which is prepared in the laboratory by reduction of chromic chloride, with either zinc and hydrochloric acid or a Jones reductor (3). The solution is stored under hydrogen to prevent air oxidation. This reagent is available commercially, stored under amyl alcohol, but is unsuitable because the amyl alcohol inevitably reaches the reaction tube and prevents efficient packing of the antimony precipitate when centrifuging. Digestion of Samples. Three milliliters of 18M sulfuric acid and 1 ml. VOL. 37, NO. 8, JULY 1965
1059
Table I.
HC1, molarity
Conditions for Chromous Chloride Precipitation of Antimony 16M Heating HzSOd, "03, time, CrC12, Sb precipi-
molarity
0.5 1.1 1.6 2.7 0 5 0.5 0.5 0.5 0.5 0.5
n_
3.6 3.6 3.6 3.6 0.0 3.6 7.2 14.4 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6
_
F,
0.5 0.5 0.5 0.5 0.5
0.5
0.5 0.5 0.5 0.5
drops
1 2 4 8
Table It. Conditions for Precipitation of Arsenic with Hypophosphite
Ammonium hypo: phosphite (satd. As Heating solution HC1, time, at 20" C.) precptd., molarity min. ml. % 2.2 3.3 4.4 6.6 6.6 6.6 6.6 6.6 6.6 6 6
6.6 6.6
5 5 5 5 5 10 15 20 20 20 ~. 20 20
2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 0.25 0.5 1.0 2.0
17.4 29.0 54.5 59.7 59.7 75.5 87.6 91.5 77.8 91.5 97.1 99.0
of antimony carrier (54.9 gram of antimony potassium tartrate in 1 liter of water; 1 ml. equivalent to 20 mg. of antimony) are added to the activated tissue sample in a 125-ml. conical beaker. T h e beaker is heated until fumes of sulfur trioxide are observed, and a further 2 or 3 minutes to complete the digestion. The charred solution is cleared by adding solid sodium nitrate in small portions (about 0.5 gram total) until the solution is pale yellow. Heating is continued to drive off all the nitric acid. A little sodium sulfite (about 0.3 gram) is added to keep the antimony in the trivalent state and so minimize losses of the more volatile pentavalent compounds. Sulfuric acid alone is chosen as the digestion medium, because it gives low losses, is efficient in keeping the antimony in solution, and does not interfere in the next separation step. The addition of nitrate is necessary to clear the charred products of the digestion, but all traces must be removed: otherwise there is a significant loss during the 1060
ANALYTICAL CHEMISTRY
min. 2
2 2 2 2 2 2 2 1 2 5 2 2 2 2 2 2
ml.
tated, %
2.0
93.0
2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 0.5 0.75 1.o 1.5 2.0 3.0
93.0 94.0 93.0 72.0 70.0 70.0 69.0 93.0 93.0 92.0 10.0 30.0 45.0 63.0 93.0 98.0
chromous chloride precipitation of antimony. Xormally a digestion followed by a precipitation with chromous chloride gives yields in the range of 92-7%. If even one drop of 16M nitric acid is present, the recovery falls to about 75% (Table I). Hydrogen peroxide (30y0w./v.) may be used in place of the nitrate. The addition of sulfite is not then necessary. After digestion the solution is diluted with 3 to 4 ml. of water and 1 ml. of 1 1 X hydrochloric acid. If any precipitate is present, the mixture is warmed until solution is complete. If addition of the hydrochloric acid is delayed, the precipitate may be difficult to redissolve. Precipitation of Antimony with Chromous Chloride. The solution obtained after digestion is washed into a 50-ml. centrifuge tube, using enough water t o give a final volume of about 25 ml. An excess of chromous chloride (3 ml. of a 0.75-11 solution) is added. T h e precipitated antimony is spun down, the supernatant discarded, and the precipitate washed twice with water. Completeness of precipitation requires the addition of a sufficiently large excess of the reagent to a warm, acid solution of antimony (Table I). If the solution is too hot, evolution of gas prevents successful centrifugation. The precipitation is not specific for antimony. Cnder the conditions used As, Bi, Cu, P t , Au, and Hg are precipitated. Tests show that Ba, Ca, Cd, Co, Cr, Fe, I, K, Li, Mg, Mn, S i , P, Pb, Sr, Sn, V, and Zn are not precipitated. Tracer experiments show that, in the absence of added carrier, only a fraction of the present (1 to 5%) is precipitated with the antimony by chromous chloride. Possible interference b y in the activity estimation of the final antimony precipitate is prevented by the precipitation of arsenic using hypophosphite (see below). Solution of Antimony Precipitate. One milliliter of arsenic carrier (13.6
grams of As203 in 1 liter of water with the minimum hydrochloric acid; equivalent to 10 mg. of ;Is per ml.), 7 drops (0.35 ml.) of 300/, w./v. hydrogen peroxide, and 6 ml. of 1131 hydrochloric acid, are added to t h e washed precipitate of antimony. T h e sample is shaken until all the antimony has dissolved, and immediately solution is complete, 2 ml. of saturated ammonium hypophosphite solution (about 7M) is added. Arsenic carrier is added before solution of the antimony to ensure equilibration with any active arsenic. There are some anomalous results in tracer studies of the arsenic precipitation when the arsenic is added after solution of the antimony. The antimony precipitate is dissolved in the cold, and the ammonium hypophosphite is added as soon as possible, to minimize losses of volatile pentavalent antimony chloride. Even with these precautions, tracer experiments show that about 10% of the antimony originally present is lost. During solution of the antimony, As, Bi, Hg, Cu, and Au dissolve, and Pt dissolves partially. Precipitation of Arsenic and Mercury with Hypophosphite. T h e solution from the previous step is heated in a boiling water bath. After 2 to 3 minutes a brown precipitate appears. This is heated for a further 20 minutes, until the original precipitate has become black and granular. For complete elimination of arsenic and mercury, the acidity and hypophosphite concentration should not be less than t'he above amounts (Table 11). The time of heating should not be shortened. This reagent also accomplishes the precipitation of platinum and gold. Precipitation of Copper as Cuprous Ferricyanide. The solution from the arsenic precipitation is diluted to 30 ml. and cooled. Five drops ( 0 . 2 5 ml.) of copper sulfate solution (lOyc w./v.) are added and the solution is well shaken. The blue color is discharged due to the reduction of the cupric ions to the cuprous state by excess hypophosphite from the previous step. excess of potassium ferricyanide solution (0.5 ml. of 10% w./v.) is added, which precipitates copper ferricyanide. The excess ferricyanide is precipitated in turn by the addition of cobaltous chloride solution (0.5 ml. of 10% W . / : V . ) ~ the excess cobaltous ions remaining in solution. This step is not, successful if the ferricyanide is added to a solution which is too hot, as the hypophosphite present reacts with the ferricyanide (Table 111). Failure to remove the excess ferricyanide results in a low chemical recovery from the final precipitat'ion of antimony with chromous chloride. The arsenic and ferricyanide precipitates are centrifuged out and the supernatant' is filtered in preparation for the final precipitation. Final Precipitation of Antimony with Chromous Chloride. The solution is heated to 90' C. and the antimony is precipitated as before by Y
~~
the addition of excess chromous chloride (3 ml. of 0 . i 5 N solution). T h e precipitate is spun down and washed twice with water and once with acetone, and slurried with acetone onto a weighed aluminum planchet. The precipitate is dried under an infrared lamp a t 90-5” C., weighed, and counted, using an end window Geiger counter. Results are calculated by comparison of the weight recovered and the activity with that of a standard sample. The standard is processed and mounted in the same manner as the samples, so that the comparison is exact. Recoveries. This separation scheme gives chemical recoveries of io70 of the antimony originally present. I n addition to t h e 10% loss occurring in the solution of t h e initial antimony precipitate, losses, each of about 5 y G , are encountered in the digestion step, a n d with t h e combined arsenic and ferricyanide precipitate. T h e remainder of the loss occurs in the transference of the final precipitate to the planchet. These were determined by tracer experiments. Similar experiments have been made to show the satisfactorv elimination of interfering elements. Radiochemical Purity of Final Antimonv PreciDitate. T h e radiochemical purity of &the final precipitate is demonstrated by y scintillation spectrometry and half-life considerations. The separation does not remove bismuth, but bismuth activated by neutron capture to give Bizlowill give only 0.3% of the activity of an equal amount of antimony activated to give Sblz2. I n samples with large amounts of bismuth, a circumstance rare in biological samples, counting may be effected by a y scintillation technique, using the 0.57 hl.e.v. y ray of SblZ2. As Bizlo is a pure p emitter, its activity is not detected.
~~
Table 111.
Conditions for Precipitation of Copper with Ferricyanide
Temp. of solution ( ” C.)
HvDoDhosDhite “I soiutiin (satd. at 20” C.) ml.
HC1, molarity
60 40 20 20 20 20 20 20 20 20 20 20
1 1 1 1 2 5 1 1 1 1 1 1
0.5 0.5 0.5 0.5 0.5 0.5 0.5 1.1 2.7 0.5 0.5 0.5
Sensitivity. T h e method described above permits the determination of gram of antimony. Trials on solutions containing 10-lo to lo-* gram of antimony under standard irradiation and processing conditions give results which agree with the calculated value within 5%. The difference found in two series of duplicate analysis also gives agreement, within 5%. CONCLUSION
This method eliminates blanks other than the standard sample, which gives the specific activity of the antimony. Microseparations are avoided by the addition of inactive antimony carrier in convenient amounts. The yield recovery technique allows rapid working, and removes the need for time-consuming quantitative transfer operations. ACKNOWLEDGMENT
The authors thank Gilbert Forbes and Edgar Rentoul, of the Forensic Medi-
KaFe(CN)e
(10%
W.h.1
ml.
Percentage Cu retained in solution
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.1 0.25 0.5
2.2 1.3 0.4 0.4 1.2 0.8 0.4 7.2 8.6 30.4 0.4
0.0
cine Department of Glasgow University, and J. M . A. Lenihan, of the Western Regional Hospital Board, Regional Physics Department, for support and laboratory facilities during the investigation. LITERATURE CITED
( 1 ) Kennedy, J. H., Bethard, W. F., Schmidt, R. A,, Olehy, D. A. J . Thoracic and Cardiovascular Surg. 44, 570 (1962). (2) Smales, A. A,, Pate, B. D., ANAL. CHEM.24,717 (1952). ( 3 ) ,Yogel, A. J., “Textbook of Quantita-
tive Inorganic Analysis,” 3rd Ed., p.
227, Longmans, Green, London and New York, 1961.
GRANTS supplied by the Medical Research Council and the International Atomic Energy Agency.
R. A. HOWIE M. M. MOLOKHIA HAMILTON SMITH Department of Forensic Medicine The University of Glasgow Glasgow, W. 2, U. K .
Theory of Derivative Voltammetry with Irreversible Systems SIR: Recently, the applications of derivative techniques to stationary electrode polarography (6) a n d to anodic stripping voltammetry (6) have been reported. In each case significant enhancement of analytical sensitivity was obtained over the respective conventional voltammetric techniques. The theoretical behavior for first-, second-, and third-derivative voltammetry was predicted for reversible depositions at plane and spherical electrodes. Experimental correlations were observed a t the hanging drop mercury electrode (6). The quantitative application of derivative voltammetry to irreversible electrode reactions has not been reported yet. This communication describes the development of a theoretical treatment of the derivative voltammetric behavior
for the irreversible case a t a stationary plane electrode. The rigorous expressions for stationary electrode polarography with irreversible systems have been derived previously (1, 7 ) . The solution given by Nicholson and Shain (4) and derived by Reinmuth ( 7 ) has been used in this work to obtain the derivative expressions, using the mathematical approach described previously (6). The final expressions in both the conventional and derivative case are not in closed form, but rather are in the form of infinite series. The series can be evaluated with the aid of a computer, and tables can be prepared for the construction of theoretical curves. Furthermore, the relationship between derivative current-voltage characteristics and kinetic parameters can be described.
THEORY
The expression for current-voltage behavior for the irreversible case with conventional voltammetry at a plane stationary electrode can be given as (4, 7 )
i
=
nFACo*(~Dob)”2x(bt)
(1)
where b is defined as an,Fv/RT, v is the rate of voltage scan in volts/second, and
(V%’
exp( -jFE’/RT)
(2)
d m ! where
E‘
=
an,(E
- EO) + (RT/F)ln(?rDob)i’z/k,
VOL. 37, NO. 8, JULY 1965
(3)
1061
