(3) Boyd, G . E., Larson, Q.V., Zbid., 64, 988 ( 1960). (4) Boyd, G . E., Lamon, V., Motta, E. E.,J . Am. Chem. oc. 82, 813, (1960). ( 6 ) Boyd, 0. E., Lareon, Q. V., Motta,
s.
E. E., U. S. At. Energy Comm., Rept. AECD-2151 (June 1948). (0) Cobble, J. W., Nelson, C . M., Parker, G.W., Smith, W. T., Jr., Boyd, 0. E., J . Am. Chem. SOC. 74, 1852 (1952). (7) Colemnn, C . F., Brown, K. B., Moore, J. C..Allen. K. A.. Proc. Intern. Conf. Yencrful Uers Atomic Energy, Geneva, Paper €’/510 U.S.A. (1058). (8) Faddeeva. M. S.. Pavlov. 0. N.. . ,Bakuniiia, V. V., Zhur. Neoig. Khim:
3, !65 (1958). (9) Fisher, 8.A., Meloche, V. W., ANAL. CAEM.24, 1100 (1952).
(10) Gerlit, J. B., Proc. Intern. Conf. PeRccful Uses Atomic Energy, Geneva
1965, VO~. 7, pp. 145-51 (1956). (11) Goishi, W., Libby, W. F., J . Am. Chem. SOC.74, 6109 (1952). (12) Meloche, V. W., Preuas, A. F., ANAL. CHEM.26, 1911 (1954). (13) Miller, H. H., Kclloy, M. T., Thoma-
son, P. F., Second International Polarographic Conference, Cambridge, England, 1959. (14) Moore, F. L., ANAL.CHEM.29, 1660 (1957). (15) Morgan, F., Sizelmd, M. L., At.
Energy Research Establ., Gt. Britain, Rept. AERE-C/M-96, Harweli, England (1957). (16) Rickard, R. R., Wyatt, E. I., ORNL Mmter Anal tical Manual, Sect. 2. TID-7015. Lethod No. 2 21831 ( h n e 1960). ’ (17 Smith, E. L., Page, J. E., J . SOC. dhm. Znd. (Lon$n) 67, 48 1948). (18). Tribalat, S., Monograp on Rhenium and Technetium,” p. 9, GauthierVillare, Parie, France, 1957. RECEIVED for review September 23, 1960. Accepted February 9, 1961. Oak Ridge National Laboratory is operated by Union Carbide Corp. for the U. 5. Atomic Energy Commiesion.
6
Determination of Small Amounts of Cobalt Using Isotope-Dilution with Cobalt-60 K. F. SPOREK‘ Bioferm Corp., Wasco, Calif.
b An isotope-dilution method for the determinationof cobalt employs cobalt60 as the tracer and a spectrophotometric procedure based on the extraction of cobalt thiocyanate with methyl isobutyl ketone. The extraction is carried out under neutral or slightly basic pH conditions and this makes the procedure virtually speciflc for cobalt. The method is suitable for the determination of cobalt at concentrations from a few parts per million upwards in biological materials, vitamin Bll, salts, metals, etc. When a well-type scintillation detector is used for measuring the radiation of cobalt-60 in liquid samples, the procedure is relatively simple and rapid. The results obtained in testing the behavior of a number of metals and the various stages of the extraction of cobalt thiocyanate with methyl isobutyl ketone, in the presence of cobalt-60 as tracer, are also presented.
T
HE
determination of small amounts
of cobalt is not simple, because of
lock of specific methods. With most existing procedures prior separation of cobalt or removal of interfering ions from thc tested sample is necessary. Even such traditional methods as those using 1-nitroso-2-naphthol and nitrosoR salt suffcr from interference by iron, chroniiuni, copper, nickel, manganese, and nitratc (2, 4 ) and many modifications of thesc methods, some rather laborious, have been developed to make thcrn suitable for specific purposes. 1 Present address, Owens-Illinois Technical Center, 1700 North Westwood, Toledo, Ohio.
754
0
ANALYTICAL CHEMISTRY
The recently introduced technique which employs tetraphenylarsonium chloride as reagent (1, IO), although better than the above, still requires separation or masking of iron, copper, molybdenum, vanadium, chromium, and nitrate. Other recently described methods with smaller or larger degree of specificity for cobalt use diethylenetriamine (7), oxamidoxime (9),and acetylacetone (8). However, they involve so many manipulatory steps that the possibility of errors and msociated lowering of precision is serious. The availability of radioactive cobalt60 and the ease with which its gamma radiation can be accurately measured suggested its use as a tracer in a chemical method which would be highly specific for the metal, and simple and rapid in operation, though not necessarily giving complete recovery from the tested samples. Work involving radiocobalt has been reported in connection with testing of recoveries of the metal following different oxidation procedures (6). Several analytical procedures using cobalt-60 as tracer have also been described; these, however, employed rather lengthy and laborious gravimetric trchniques (8, 11-19) and mere not suitable for the determination of cobalt in the 100- t o 500-pg. range. A method was required for the general determination of cobalt in vitamin Bl2 and in biological materials. Of several existing methods examined, the extraction of cobalt thiocyanate with a watcr-immiscible ketone undoubtedly provided a simple and fast procedure. However, many metals interfered (6) and it was necessary to find a means of overcoming this dif-
ficulty. A certain amount of experimentation was carried out and, under weakly alkaline conditions, cobalt W&B found to be the only metal extractable with a water-immiscible ketone [methyl isobutyl ketone (4-methyl-%pentanone), ethyl amyl ketone]. Based on this a spectrophotometric method was developed with radiocobalt used as tracer, thus making quantitative recovery of cobalt unnecessary in the extraction step and so simplifying the procedure further. Detailed investigation of the factors involved showed that the procedure described below is satisfactory for use with vitamin B12 and biological materials, and should be generally applicable for a large variety of other materials. EXPERIMENTAL
Apparatus. Scaling Unit, NuclearChicago Corp., Model 161A, operated at 1200 volts with the scale selection knob a t “256” value. This setting gave 1 unit on t h e mechanical register of t h e instrument for each 256 actual counts (actual counts X 256-l) ; because the calculations were simpler and the aecurac of results was not involved, the meclanical register units rather than the actual counts were used throughout this work. Well-type scintillation detector, Nuclear-Chicago Corp., Model DS-3, used with 16 x 150 mm. glass test tubes for counting liquid samples. Dual Timer, Nuclear-Chicago Corp., Model T1. Spectrophotometer, Beckman Rilodel DU, operated a t 620 mp with 1-cm. glass cells. Reagents. Radioactive Cobalt Solution. Cobalt-60 in t h e form of
cobaltous sulfate, c:irrier-free, was dissolvcd in dilute sulfuric acid t o give a filial solutinn containing about 0.02 pc. per ml. (equivalcnt to about 1 mpg. of Cow per ml.). I n cach experiment 5 inl. of the above solution was uscd, equivalent to 0.1 pc., and the radioactivity of the final methyl isobutyl kstone extract was determined by counting 5-ml. volumes ovcr 5-minute periods. The total number of counts usually obtained was around 300,000, which was considered sufficient. Standard Cobalt Solution. Reagent grade cobaltous chloride hexahydrate was dissolved in dilute sulfuric acid and diluted so that 1 ml. contained 10 mg. of Co. The solution was then standnrdizcd by thr gravimetric procedure of Lnmbie (6) using cobalt-60 and the “rcbidue” counting technique and then i t was diluted to contain 50 pg. of Co per ml. Ammonium Thiocyanate Solution. Approximately 5070 solution was preared and adjusted with strong sodium ydroxide solution to pH 7.5. Methyl isobutyl ketone (MIBK), reagent grade, was satisfactory.
K
PROCEDURE
Determination of Correction Factor. Measure various volumes of t h e standard inactive cobalt solution containing between 100 and 500 pg. of cobalt into a series of small separatory funnels and add to each accurately 5.0 ml. of the radioactive cobalt solution containing about 0.1 pc. of Cow. Add 10 ml. of the 50% ammonium thiocyanate, p H 7.5, solution and adjust the volume in the separatory funncl with water to about 25 ml. Add about 8 ml. of methyl isobutyl ketone and shake strongly for a few minutes. Allow the layers to separate and reject thc lower aqueous phase. Add to the funnel 10 ml. of the ammonium thiocyanate solution rtnd 15 ml. of water, and shake again. The extract should be pure blue a t this stage in the presence of cobalt. Reject the aqueous solution; if necessary, centrifuge the extract and measure 5 ml. of i t accurately into a counting test tube. Measure the counts over 5-minute periods, then use the same extract for determining absorbance at 620 mp against water. Prepare a blank following the above procedure exactly but omitting the radioactive and the inactive cobalt solutions and using water instead. Use the blank counts (background) and blank absorbance values for correcting those obtained with the standard cobalt solutions. Calculate the correction factor as follows: Micrograms of Co taken x counts (corrected for background) -F Absorbance at 620 mp Determination of Cobalt. Measure accurately into a small separatory funnel not more than about 10 ml. of t h e sample solution which should contain not more than about 500 pg. of cobalt and should be weakly acidic, and add 5.0 ml. of the radioactive cobalt solution accurately measured.
Follow the procedure exactly ~9 described for correction. Calculate the amount of cobalt present in the aliquot taken as follows: Factor X absorbance at 620 mp Counts (corrected for background) pg. of cobalt found Ashing of Biological Materials. Measure a suitable quantity of t h e sample into a small beaker and evaporate t o dryness at 100’ to 150’ C. If t h e total quantity of t h e sample is intended for one test, add t h e radioactive cobalt solution (5.0 ml.) at this stage and nllow i t to evaporate with t h e sample. Add to t h e residue 5 ml. of concentrated sulfuric acid and about 0.1 gram of solid potassium perchlorate and heat slowly with constant swirling of the beaker, then boil. If the hot digest is not colorless, add a few small crystals of potassium perchlorate and boil. Finally volatilize the bulk of sulfuric acid. At this stage all organic matter should have been completely oxidized; a blue tint will be noticeable in the resence of about 100 pg. of Co in the got digest and on cooling this will turn pale pink. Dissolve the cold digest in an appropriate quantity of water and use for the determination of cobalt as per method. Removal of Copper. Make t h e sample solution (which should contain t h e radiocobalt tracer) slightly acid and add a few grams of acidwashed iron filings or coarse powder. Shake t h e mixture for 5 t o 10 minutes t o ensure complete reduction t o copper metal, then filter or centrifuge. Use t h e clear solution for cobalt determination as per method.
-
DEVELOPMENT OF METHOD
Ashing of Biological Samples. Prior to the determination of cobalt in bio-
Table II.
pH of A UeOUe MIBK Extract in Presence of ?haw 50 mg. Fe 50 mg. Cu 4.70 Dark brick-red Dark yellowbrown 6.10 Red Dark yellowbrown 6.30 Light red Dark yellowbrown 6.75 Pale pink Dark yellowbrown 6.80 Faint pink Dark yellowbrown 6.85 Very faint pink Dark yellowbrown 6.90 Nearly colorless Dark yellowbrown 7.00 Colorless exYellow-brown, aqueous pale tract, aqueous blue turbid yellow Light yellow, 7.30 aq. deep blue Pale yellow 7.60 7.70 7.90
Very pale yellow,a very dee 8 u e Faint& yellow Nearly colorless Colorless
8.30 8.50 8.70
logical materials i t is necessary t o destroy t h e organic matter. I n t h e case of vitamin B1,t h e cobalt ion is held so strongly by t h e organic matrix t h a t unless t h e matrix is entirely destroyed, either none or low recoveries of t h e metal are obtained and this will not be corrected by t h e presence of radiocobalt tracer; also the oxidation mixture may be discolored by the presence of the partially disrupted molecule. This difficulty is undoubtedly
Effect of pH on Extraction of Cobalt Thiocyanate with Methyl Isobutyl Ketone
Composition of Aqueous Soln. Subetance yo Sulfuric acid 4 2 0.4 Water (sulfuric ... acid) Sodium carbonate 0.2 0.4 1.0 2.0 3.0 4:O Sodium hydroxide 0.02 0.8 8.0 a
Table 1. Effect of pH on Extraction of Iron and Copper Thiocyanates with Methyl Isobutyl Ketone
(125 pg. of Co present in each test) No. of Counta Co’o Absorbance x 256-l Found Extracted, at 620 pH Aqueous MIBK yo Mp Extract
... ... ...
0.738 Green turbid 0.723 Blue-green0.865 Purple-bluea
118.3 55.0 10.0
372.7 464.2 509.5
4
10.0
552.5
98
0.925
Purple-blue”
6.9
15.0 6.3 21.3 18.7 28.8 22.5
629.5 545.8 559.3 538.0 525.5 505.3
98 99 96 97 95 96
0.925 0.905 0.900 0.865 0.845 0.825
Deep blue Deep blue Pure blue Pure blue Pure blue Pure blue
33.0 72.8 504.7
487.6 456.3 24.7
93 86 5
0.800 Pure blue 0.715 Pure hlue Nil Colorless
...
7.4
...
7.7 7.8 * .
.
... ...
76 90 98
Interference of iron noticeable.
VOL 33, NO. 6, MAY 1961
0
755
Table 111.
Form Used Metal Al Mn Sn HI3 Pb
AN!Th
Mg. 100
AlCI,
100 100
MnS04.Ha0 SnC14.5HaO SnCl?.2Ha0 Hg(NOJr. Ha0 Pb( NOah AgNO; 'NISOc.6HaO Th(N0:)(.4HsO
Effect on Extraction of Cobalt with MIBK None; satisfactory extraction, aqueous clear, colorlW
100 100 100 100
600 10
100 K&r& reduced with h droxylamine K&&, reduced with
sox
UwN03r.6HaO
100
Aqueous deep yellow, extract pale yellow Aqueous brown, extract yellow A ueous green, extract colorless Aqueous yellow, extract deep yellow None
W
Yellow precipitate, extract colorlea None
Mo
NhWO, reduced with hydroxylamine or SOX ( N H d & f ~ d h4Ha0 .
Fe
Above reduced with hydroxylamine Above reduced with stannous chloride or araeni011s acid or SO, FQ(Sod:. 9H20
10
Aqueous dee orange, extract deep trick-red
cu
CUSO~. 5HaO
10
A iieous dee yellow, extract Brown-ye&w
100
AE above but aqueous green
None; satiRfactory extraction, aqucous turbid None; satisfactory extraction None None; satisfactor extracExtract decolorized tion, aqueous eiiow, clear No change Interference wit{ c o extraction None; satisfactory extracNone tion, aqueous green, clear Aqueous pale yellow, extract None; satisfactory extraction, aqueous pale yellow, wlorleae clear None; satisfactory extracNone tion, aqueous colorlesa, clear Aqueous clear colorless, ex- None; satisfactory extraction tract colorlese None; satisfactory extracNone tion, aqueous colorless, clear Aqueous colorleas, extract Interference with Co extraction orange Aqueous colorless, extract None: satisfactory extraction; aqueous colorless, colorless clear Aqueous develops yellow None; satisfactory extra+ tion; aqueous turbid, yelrecipitate, extract colorlow ess, clear Aqueous blue, extract nearly Slight interference; satisfactory extraction, aqueou8 colorlese blue, clear Aqueous develops yellow None; satisfactory extraction; aqueous turbid, yelrecipitate, extract colorION ese, clear White precipitate formed
10
Ce Cr
U
Effect of Metals on Extraction of Cobalt Reaction with CNS- in Effoct of Buffering Acid Solution to pH 7-7.5 Nono None
~
Aqueous deep orange, extract deep orange Aqueous orange, extract deop orange
Aqueous deep orange, extract red
Above reduced with HCI and Fe metal
Table IV.
s.
125
150 150 50
None 125
(Co@used aa tracer) co Found,
Contaminants Added NiS04.6H10, 2 grams As above
MB.
4 130
Fer(S04),.9H20, 25 mg. CuS04.5H10, 10 mg. 100ma. -
149
100 mg. NaCl, 10 grams As above
50.1 None
150.5 152.0
ANALYnCAL CHEMISTRY
Remarh
Value corrected for blank (4 becomes 126 pg.
pg.
Co)
Cu removed with Fe filinge MIBK extract reshaken with fresh thiocyanate aoln.
124.3
due to the high nitrogen content and the presence of phosphorus in vitamin Btt. The tested procedures included ignition in a glass beaker and in a platinum crucible, boiling with mixtures of sulfuric-nitric acids, sulfuricnitric-hydrochloric acids, nitric acidpotassium perchlorate, sulfuric acidpotassium permanganate, sulfuric acidhydrogen peroxide, and sulfuric acidpotassium perchlorate. Only the lastmentioned mixture gavQ complete and
756
P
Determination of Cobalt in Presence of Other Metals
co Added, None
P
rapid destruction of organic matter with satisfactory recovery of cobalt. This procedure was therefore used throughout this work. Extraction of Cobalt Thiocyanate. T h e formrttion of a blue cobalt thiocyanate complex and its extraction with water-immiscible ketones under acidic conditions are generally known. Under the above conditions, however, many other metals interfere because of the formation of colored and ex-
tractable thiocyanates. I n this work methyl isobutyl ketone was chosen as the extractant and the extraction wm subjected to critical testing with the view of finding conditions which would allow simple and complete suppression of the interferences. Preliminary testa indicated that while most other metal thiocyanates were either not formed or were not extractable under neutral or weakly alkaline conditions, that of cobalt was stable and the extraction was satisfactory. More detailed testing of the p H effect on the extraction, using the procedure described as per method, showed that cobalt could be recovered quantitatively under high enough p H to prevent the formation of thiocyanates of iron, nickel, copper, and other metals (Tables I, 11, and 111). The extraction efficiency of cobalt was also satisfactory and, although this was relatively unimportant in the presence of cobalb60, the values indicated that in many cases accurate results could be expected even without the internal standard. As iron m d copper would generally be expected to be prcsent in most tested samples, special emphasis was put on testing their effects on the mtithod.
I n preliminary experiments, ferric thiocyanate waa not formed in solutions at p H 6.9 or higher (Table I) and when the samples were shaken with methyl isobutyl ketone colorless extracts were obtained. With copper, high p H of solution also suppressed the formation of the thiocyanate; at pH 7.6 or higher aa much as 50 mg. of Cu could be tolerated. Cobalt, on the other hand, formed the colored thiocyanate a t pH 8 and higher and the complex was extractable. Results obtained in quantitative tests are shown in Table IV. When the tested solutions were first adjusted t o a high p H and then shaken with MIBK, low and variable recoveries of cobalt were usually obtained; when, however, the original extraction waa performed from an acid solution and the extract was reshaken with a n alkaline thiocyanate solution, the recoveriea of cobalt were satisfactory (Table V) and the interference due to the originally extracted iron (or other metals) was suppressed. This technique was adopted for the final procedure. In further tests the optimum concentration of ammonium thiocyanate for the extraction of cobalt waa checked. The results showed that satisfactory recoveries were obtained when the aqueous solution contained over 10% of ammonium thiocyanate (Table VI). Finally a series of experiments was run to provide data for correction (Table VII). The procedure was then applied to testing a sample of crystalline vitamin BIZ for its cobalt content. For this purpose a sample of 98% pure cyanocobalamin was crystallized from acetone and water, the crystals were dried in vacuum, and a solution was then prepared; the vitamin Blt content of the solution was determined by ultraviolet absorption (14). The values obtained (Table VIII) gave a mean of 4.38% cobalt in the sample; the theoretical value, assuming molecular weight of cyanocobalamin as 1355.42 ( I 4 ) , is 4.35%.
The stability of the cobalt thiocyanate extract was also tested and found satisfactory; there was no difference in absorbance after 24 hours’ standing of either the test or blank extracts. DISCUSSION OF RESULTS
The described method can be considered practically specific for cobalt, and, particularly in work with biological matenals, the only necessary precaution against possible interferences is the adjustment of pH during the extraction step. The method is also suitable for testing materials containing substantial excess of other metals, including iron and copper, as may be the case in testing alloys and metal salts. Then, only copper would be expected to cause difficulty when present in
Table
V.
Recovery of Cobalt by Extraction from Alkaline and Acid Solutions
No.of Counte
pH of Extraction Procedure Aqueous Aqueoue soh. ad’ueted to pH indicated, tfien extd. 7.35 with MIBK Aqueous soh. made slight1 acid and extd. wit[ MIBK; extract reshaken with pH-adjusted thiocyanate eolution indicated
Table VI.
Wect
X 266-l Found
cow ExAbsorbance tracted, at 620 % MLL
Aqueous
MIBK
7.45 7.85
997.4 593.0 1336.2
661.2 1119.2 207.1
40 65 13
0.258 0.421 0.082
7.60 7.75 8.30
33.6 57.7 492.2
1723.7 1700.6 1203.2
98 97 71
0.688 0.674 0.455
of Concentration of Thiocyanate on Cobalt Extraction with Methyl Isobutyl Ketone
% NH4CNS in % ‘ Corn Abeorbance Aqueous COunta X 256-l Found EXof Extract Solution Aqueous MIBK tracted at 620 Mp 2
362.5
127.0
26
0.195
5
111.3 18.8
426.8 541.0
79 97
0.70 0.89
5.0
548.3 547.5 527.5 509.8
99 100 99 100
0.93 0.92 0.91 0.92
10 15 20 30
Nil
3.8
Nil
40
Table VII.
Remarke Poor se aration of aqueous and S I B K layers Good separation Fast and complete soparation Fast and complete separation Fad and complete separation Fast and complete separation Fast and complete separation
Determination of Correction Factor Using Cobalt-60
calm. of
Co Added,
a. 0 100
100 200 200 250 250 350 350 0
n 125 250
Absorbance at 620 M r Nil 0.346 0.325 0.524 0.552 0.667 0.621 0.864 0.738 0.001
Nil 0.563 1.107
Extracx (Corr. for Bkgrd.)
Absorbance
Remarks
558.1
Blank
930.7
S o h . of CoCh extracted as Der
269.0 272.0 272.0 267.6 273.4 273.3 269.4 271.1
883.9
712.7 738.8 729.5 678.8 665.0 571.8 1198.1
1207.6 1221.6 1201 .o
x
108
Blank Blank
method
Solns. of CoClt and blanks taken through oxidation proc. with Ha04 - KClOc
271.2 271.2 271 .O
Mean Standard deviation. =k3.22 on mean value of 271.0. Coefficient of variation. f1.2’%.
amounts larger than about 2 mg. per test. A modification to the basic procedure, employing reduction of c o p per with metallic iron, was found satisfactory for overcoming this difficulty. Cobalt is not reduced under those conditions. Common anions did not affect the extraction of cobalt, and sulfate, nitrate, chloride, phosphate, thiosulfate,
-
and citrate had no effect on the results. Hydroxylamine did not &ect the extraction of cobalt, but when present together with iron, chromium, or molybdenum it enhanced their extraction and so indirectly interfered with the test. Hydroxylamine completely suppressed the interference of large quantities of copper, but again iron had to be practically absent. VOL 33, NO. 6, MAY 1961
757
Table VIII.
Determination of Cobalt in Vitamin
Vit. BM Taken, Mg.
Absorbance at 620 Ma
1 012
0.195 -.
2,024 3.036 5.060 5.060 7.084 7.084
0.404 0.586 1.009 1.019 1.458 1.384
___
Bls Using Cobalt-60 as Tracer
a
LITERATURE CITED
for 6 Min. (Corr. for Bkgrd.) Cour 1201.9
i2i6.3
1202 * 8 1242.4 1239.3 1250.6 1215.0
E., Barnes, N. A., €f,A.,H.ANAL. CAEM.23, 1680
(1) Affsprun
43.9 89.4 132.0 220.0 222.8 315.9 308.7
Mean
4.34
4.iz 4.35 4.35 4.40 4.46 4.36 4.38
Standard deviation. 3~0.045on mean value of 4.38. Coefficient of variation. z t l . O % .
Thc precision of results obtained was consistent with other spectrophotometric methods; the coefficients of variation calculated from values obtained in calibration (Table VII) and determination of cobalt in vitamin Blr (Table VIII) were f 1.2 and f1 .O%, respectively. The accuracy was also satisfactory and in any case would be expected to be high because of the use of
cobalt, depending mainly on the nature of the tested material, presence and amounts of other metals, etc.
cobalt-60 as internal standard. The agreement between thevalueobtained for cobalt content in vitamin Blt (4.38%) and the theoretical value (4.35%) waa considered satisfactory and consistent with results usually obtained in work involving the vitamin. With appropriate minor modifications the method should also give satisfactory results without the use of radio-
Potratz,
(19.51). (2 Bagehawe B., Anal st 84,475 (1959). (3{ Brown, B., Jteinbach, J. F., ANAL.CHEM.31, 1805 (1959). (4) Cogan, E., Zbid., 32, 973 (1960). (5) Crawley, R. H. A,, British Iron and \_.__
6'.
Steel Research Association Sheffield, England, Papers MG/DC/146/57, L.
4438-C/3. (6) Gorsuch, T. T., Analyst84,135 (1959). (7) Howling H. L., Shanley, E. S., Chem. in kana& 12,46 (January 1960). (8) Lambie, D. A., Analyst 84, 173 (1959). (9) Pearse, G . A., Pffaum, R. T., ANAL. CHEM.32, 213 (1960). (10) .Pepkowitz, L. P., Marley, J. L., Zbsd., 27, 1330 1955). (11) Salyer, D., weet, T. R., Zbid., 28, 61 (1956). (12) Zbid., 29, 2 (1957). (13) Thiere, R. E., Williams, J. F., Yoe, J. H., Zbid., 27, 1725 (1955). (14) U . , S. Pharmacopoeia XVI, 16th Revision, 1960, pp. 186-7.
L
l ~ ~ . _ ,
RECEIVED for review September 12, 1960. Accepted January 18, 1961.
Alpha Counter for the Direct Determination of Plutonium in Solution J. T. BYRNE and G. A. ROST' The Dow Chemical Co., Rocky Flats Plant, Denver, Colo.
b The necessity of determining plutonium rapidly in solutions has prompted the design and construction of solution alpha counters in which the sample solutions do not require chemical or physical treatment. The theory, design, advantages, and h i tations of the alpha counters are discussed. The laboratory solution counter can rapidly determine 5 X lo-' to 5 grams of plutonium per liter of solution with a reproducibility of better than 3~5%. The effects of density of the solution and other factors that affect accuracy have been evaluated.
T
HE MOST RAPID and sensitive method for plutonium determination is alpha counting. The usual techniques involve diluting the sample if necessary, precisely transferring a portion to a planchet, evaporating, igniting, counting with either a flow proportional tube or a zinc sulfide (Ag activated) scintillator, and calculating. 1 Present address, Beckman Instruments, Inc., Fullerton, Calif.
758
ANALYTICAL CHEMISTRY
These operations require relatively precise and time-consuming manual manipulation. Machines which can perform all of these operations mechanically have been constructed (1, a) for in-line applications, but they are necessarily bulky and complex. Inline counters have also been constructed (4, 6) which count plutonium solutions without chemical or physical treatment. These instruments use a thin plastic film between the solution and the phosphor but have limited ranges and varying background levels due to contamination of the film. 13y using an air gap rather than a plastic film as a primary means of separating the solution from the phosphor, a wide range of plutonium concentrations can be determined without dilution and a low background count can be maintained. THEORY
In the solution alpha counter, alpha particles emitted by the plutonium ions near the surface of the solution are counted a few millimeters above the surface of the solution using a zinc
sulfide phosphor. To prevent deterioration of the alpha-sensitive zinc sulfide phosphor by acid and organic vapors from the solution, a 0.00025-inch ( I / r mil) Teflon film (Dilectrix Corp., Farmingdale, L. I., N. Y.) is used to cover it. Mylar film has also been used but it is not as chemically resistant as Teflon film. Since the range of an alpha particle is short (3.6 cm. in dry air a t 15' C. and 760 mm. of Hg pressure for a 5.1m.e.v. energy), the depth from which alpha particles may be counted in liquid is limited. Bragg's rule (2) gives a range of 0.0013 inch in water for 5.1m.e.v. plutonium alpha particles. A necessary condition for detection of an alpha particle emitted in solution is that its energy loss from the point of origin to the detector be less than its total energy. This condition requires that the direction of emission (expressed as the angle e, between the emission direction and a perpendicular to the solution surface) must be such that
L + C K eIC -
R, COS e
L1
(1)
