(4) E. P. Parry and D. P. Anderson, Anal. Chem., 45, 458 (1973). (5) D. J. Fisher. W. E. Belew. and M. T. Kellev in “Polaroaraohv 1964”. G. J. Hills, Ed., Interscience, New York, 1966,‘pp 1043-1659 (6) G. L. Kirschner and S. P. Perone, Anal. Chem., 44, 443 (1972). ( 7 ) H. S. Carslaw and J. C. Jaeger, “Conduction of Heat in Solids”, 2nd ed., Clarendon Press, Oxford, 1959, p 100. (8)W. H. Reinmuth and C. E. Wilson, Anal. Chem., 34, 1159 (1962). (9) P. Delahay, Anal. Chem., 34, 1161 (1962). (IO) J. Bonastre, M. Astruc, and J. L. Bentata, Chim. Anal. (Paris), 50, 113 (1968).
(1 1) D. J. Myers, R. A. Osteryoung, and J. Osteryoung, Anal. Chem., 46, 2089 (1974). (12) P. Delahay, “New Instrumental Methods in Electrochemistry”, Interscience, New York, 1954, p 43ff.
RECEIVEDfor review October 12,1976. Accepted December 13,1976’ We are grateful to the Science Foundation for supporting this work through Grant MPS-75-05361.
Coprecipitation and Electrodeposition of Polonium from Sea Water J. P. Cowen, V. F. Hodge,” and T. R. Folsom Scripps Institution of Oceanography, L a Jolla, Calif.
92093
Polonium can be electrodeposited onto carbon rods dlrectly from acidified sea water, stripped from the rods, and autoplated onto silver counting disks with an overall recovery of tracer of 40 f 2% for an electrodepositiontime of 16 h, 61 f 6 % in 24 h, and 85 f 4% In 48 h. Coprecipitation of polonium from sea water, previously acidified then neutrallred, results In recoveries of 80 f 8 % (including autoplating) when 6 mL 1 N NaOH/L are used in the precipitation. Absolute *loPo activities in sea water by the two methods agree. Filtration of coastal sea water through a glass wool plug removes 32% of the *loPo activity.
Similar affinity of polonium and plutonium for marine surfaces implies that studies of the more easily measured polonium might be valuable in predicting some consequences of plutonium disposal in the oceans (1-4). Rates a t which 239,240Pu and 210Podeposit out of sea water onto surfaces of giant brown algae and “inert” surfaces, such as glass and cellulose, suggest that both nuclides are associated in coastal sea water with colloidal sized species having diffusivities of about 3X cm2/s. This parallel behavior possibly represents an initial step in the incorporation of both a-radioactive heavy elements into marine food webs and/or their transport t o coastal sediments. T h e study of zlOPo is facilitated by the greater activity concentrations found on marine surfaces and in sea water, about 200 times that of plutonium. Such studies require many determinations of the natural a-radioactive zlOPoin sea water. Thus a simple, high recovery procedure was sought. Tsunogai and Nozaki analyzed Pacific Ocean surface water by consecutive coprecipitations of polonium with calcium carbonate and bismuth oxychloride after addition of lead and bismuth carriers t o acidified sea water samples ( 5 ) . After concentration, polonium was spontaneously deposited onto silver planchets. Quantitative recoveries of polonium were assumed a t the extraction steps and plating step. Shannon, Cherry, and Orren, who analyzed surface water from the Atlantic Ocean near the tip of South Africa, extracted polonium from acidified samples as the ammonium pyrrolidine dithiocarbamate complex into methyl isobutyl ketone (6). They also autoplated polonium onto silver counting disks. An average efficiency of 92% was assigned to their procedure after calibration with 210Po-210Pbtracer experiments. Our experience with autoplating polonium from several thousand biological and sea water samples is t h a t yields are often about 85% but it is not uncommon to get yields of from 50-100%. Likewise it appears risky to assume that removal of 494
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
polonium from sea water by coprecipitation will always be nearly quantitative because of its high adsorbability. Two simple procedures for concentrating 210Po from sea water are compared here: coprecipitation upon partial precipitation of the natural calcium and magnesium with sodium hydroxide (7) and a new method, electrodeposition of polonium directly from acidified sea water onto carbon rods. Polonium thus concentrated, was autoplated onto silver counting disks held in spinning Teflon holders. Recoveries of zlOPowere monitored by the addition of zOsPo.
EXPERIMENTAL Prior to collection, enough reagent grade 12 M HCl was added to acid-washed polyethylene sample bottles to bring the acidity of the sea water to about 0.5 M. Four to 7 L of sea water were found to give a 1u counting error of 10%or less with counting times of 1000 to 2500 min. The time between collection and analysis was usually less than 1week, preferably 1or 2 days. In the laboratory, samples were accurately weighed into 11-L polyethylene buckets (remembering to subtract the weight of the added HC1) and about 0.5 pCi of *08Po (z08Potracer solution, E.R.D.A. Health and Safety Laboratory, New York, N.Y. 10014) was mixed into the sample by means of a magnetic stirrer. Partial Precipitation Procedure. The sample was neutralized with 50% (20 M) NaOH (Baker “Analyzed Regent”) until a slightly cloudy precipitate persisted. Then 12 M HC1 (Baker “Analyzed Regent”) was added dropwise until the precipitate just dissolved. (The addition of a few drops of a 0.04% solution of bromothymol blue facilitated observation of the disappearance of the precipitate.) Six mL of 1 M NaOH/L of sea water was then added, giving a milky precipitate. After 2 h of stirring, the precipitate was allowed to settle overnight. The liquid was siphoned off and the remaining precipitate transferred with several small deionized water rinses to a 1-L polyethylene bottle whose top had been cut off and centrifuged. The precipitate was dissolved with 1 2 M HC1 and the resulting neutral solution (about 40 mL) adjusted to 0.5 M with 12 M HC1. This solution was transferred to a 100-mL glass beaker with three 0.5 M HCl rinses, bringing the final volume to about 50 mL. Polonium was autoplated onto silver. Electrodeposition. Carbon rods (Ultra Carbon, P.O. Box 747, Bay City, Mich.) appropriately wired to a constant voltage power supply were inserted into 6 to 7 L of sea water which had been acidified to 0.5 M with 1 2 M HCl and spiked with zOsPo. One set of four 30 cm X 0.30 cm rods, held securely in a polyethylene holder a t the corners of a 1.5-cm square, acted as the cathode and another functioned as the anode. The anode and cathode holders were held approximately 7 cm apart. Polonium deposited onto the cathode rods. Stirring was accomplished by means of a 6-cm Teflon-coated magnetic bar. At a constant voltage of 2.3 V, vigorous stirring, and an acid concentration of 0.5 M, the set-up was allowed to plate for 16-48 h. (The whole apparatus should be located in a fume hood to remove corrosive gases.) After plating, the cathode rods were withdrawn from the sea water with the voltage still applied, and broken off above their entrance into the water (about 15 cm exposed). The rods were further broken into
~~
Table I. Comparison of Sodium Hydroxide Precipitation and Carbon Rod Plating Methods for Concentrating zlOPo from Aliquots of Acidified Sea Water Removed from 50-L Parent Samples Date 210Poactivity, sampled pCilLa Recovery, %
Method Sample 1 (0.5 M HCl) NaOH NaOH Carbon Rod Carbon Rod NaOH NaOH Sample 2d (0.5 M HC1) NaOH NaOH Carbon rod Carbon rod Carbon rod Carbon rod NaOH NaOH
5/12/75 5/12/75 5/12/75 5/12/75 5/17/75 5/17/75
0.115 f 0.009 0.113 f 0.009 0.116 f 0.007 0.110 f 0.005 0.104 f 0.008 0.097 f 0.008
71 66 81' 89' 74 80
5120175 5120175 5120175 5/20/75 5/27/75 5/27/75 5/27/75 5/27/75
0.031 f 0.002 0.025 f 0.002 0.034 f 0.002 0.035 f 0.003 0.034 f 0.003 0.028 f 0.002 0.040 f 0.003 0.034 f 0.002
86 74 63' 65' 381 411 73 90
Collected on 4/25/75 at Scripps a f l standard counting error. Pier. Electroplating time, 48 h. Collected on 5/20/75 a t Scripps Pier. e Electroplating time, 24 h. f Electroplating time, 16 h.
Table IV. Effect of Acidity and Time on zlOPo Concentrations in Aliquots Removed from Single 100-L Parent Sample" Date sampled
Method Raw sea water NaOH Carbon rodC NaOH Carbon rodC 0,l M HCl Carbon rodC Carbon rod' 0.5 M HCl NaOH Carbon rodC NaOH Carbon rodC
pCi/L
5/29/75 5/29/75 6/2/75 6/2/75
0.097 f 0.006 0.044 f 0.003 0.028 f 0.002 0.047 f 0.003
62 74 86 53
6/2/75 6/4/75
0.053 f 0.003 0.050 f 0.004
64 50
6110175 6/10/75 6/23/75 6/23/75
0.052 f 0.005 0.060 f 0.006 0.050 f 0.002 0.058 f 0.002
72 55 82 60
fl standard counting a Collected on 5/28/75 a t Scripps Pier. error. c Electroplating time, approximately 24 h.
Table V. Effect of Ascorbic Acid on Autoplating Yields of 208Po Tracer Ascorbic acid
~
Recovery, %
Recovery,
~~
Dissolved sea water precipitateb None None 0.25 g 0.50 g Deionized waterC None None None 0.25 g 0.50 g 0.50 g
Table 11. Effect of Added Sodium Hydroxide on the Recovery of 20*PoTracer from Sea Water" Volume of 1 N NaOH, mL/L
Recovery, %
3 6 10
86 83 86
a After neutralization of acid used to stabilize polonium in solution (0.5 M HC1).
Table 111. Change in zlOPoActivity Concentrations with Time in Acidified and Unacidified Sea Water" Acidified (0.5M HC1) Date sampled
6/23/75c 0.042 f 0.002 6/26/75 0.045 f 0.002 6130175 0.044 f 0.002
107 f 5 84 f 4 96 f 4 80 f 4 75 f 4 75 f 4
" fl standard counting error. 0.5 M in HCl, tracer added prior to autoplating; not before initial sea water precipitation. 0.5 M in HC1.
Unacidified
Recovery, pCi/L
96 f 4 102 f 4 81 f 5 81 f 4
Recovery,
%
pCi/Lb
%
104 100 78
0.037 f 0.002 0.023 f 0.002 0.014 f 0.001
77 82 77
fl standard counting a NaOH precipitation method used. error. Collected on 6/23/75 a t Scripps Pier.
the spinning rod. It was convenient t o plate 4 at a time on a 15 cm X 15 cm hot plate. Radioactive Counting. All silver planchets were counted for 1000-2500 min beneath 350-450 mmz silicon surface harrier detectors (Ortec) which were monitored by pulse height analyzers (TMC). Reagent blanks amounted to less than 0.001 pCi/L compared to typical sea water activities ranging from 0.01 t o 0.1 pCi/L.
RESULTS A N D DISCUSSION small pieces to fit on the bottom of a 100-mL beaker by inserting the rods between a watch glass and the lip of the beaker and snapping off the end of the rods inch by inch. Fifty mL of 0.5 M HC1 was poured over the rods and the polonium autoplated directly from this solution containing the carbon rods. Autoplating of Polonium.Spinning silver disks were chosen after variable plating yields were encountered using a non-stirred silver bottom apparatus or with stationary hanging silver disks in stirred solutions. A 2.5-cm silver disk was held in a thin Teflon cap having a 1.6-cm center hole. The cap was snapped onto an inverted "T" shaped Teflon holder. Thus, only a 1.6-cm circle on one side of the silver disk was exposed. The holder was slipped onto a glass or Teflon rod which could be rotated at 4 rps by a small electric motor. The assembled rotor was lowered into the 0.5 M HCl sample solution contained in a 100-mL beaker sitting on a hot plate. Autoplating was complete (90%or better recovery of zo8Pofrom spiked samples) after spinning 1.5 h at 85 "C. Periodic additions of deionized water were required to compensate volume loss during plating or the sample beaker was covered with a Teflon watch glass drilled to accommodate
An initial comparison of t h e two methods is found in Table
I. Recoveries of 208Potracer in t h e precipitation method were 77 f 7% ( n = 8) compared t o 40 f 2% ( n = 2) for t h e electrodeposition method with 16 h plating time, 64 f 1%( n = 2 ) in 24 h a n d 85 f 4% ( n = 2) in 48 h. Even though t h e electrodeposition method requires less attention, i t requires long plating times for high recoveries. Thus t h e recovery of 210Po by direct plating appears t o be rate limited by diffusion of polonium t o t h e cathodes since t h e applied potential difference is far in excess of that required to reduce Po(1V) t o
Po. Recoveries are based on the added 208Potracer, presumably P O C I ~ ~Equilibrium -. was assumed t o have occurred during t h e 2-h mixing of spike and sample which was always 0.5 M in acid at this stage. Table I1 shows that recovery of 2osPofrom sea water was insensitive t o t h e amount of 1 N NaOH used in precipitation. ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
495
Table VI. Removal of 210Pofrom Sea Water by Glass Wool Filter Plug" Fraction analyzed Case 1: 10.9-L sea water Filtered twiceC Filtered sea water, 1st pass Filtered sea water, 2nd pass Retained on 1st filter Retained on 2nd filter Sum ZlOPo :. 34% zlOPoretained on filter Case 11: 11.3-L sea water Filtered onced Filtered sea water Retained on filter Sum zlOPo :. 30% zlOPoretained on filter
pCi/Lb
0.028 f 0.002 0.026 f 0.002 0.010 f 0.001 0.004 f 0.001 0.041 f 0.002
0.026 f 0.002 0.011 f 0.001 0.037 f 0.002
a 1inch X 2 inch PVC plastic pipe coupling stuffed with glass wool held in by Nytex-90 nets held in both ends by pressed-in PVC rings. b fl standard counting error. Collected at Scripps Pier on 7/10/75 and filtered same day. Collected at Scripps Pier on 7/10/75 and filtered same day.
Six mL of 1 N NaOH/L was chosen because it gave an easily manipulated volume of precipitate. T h e need to acidify sea water to be used for polonium analysis was investigated by periodically subsampling two parent samples over a week. Two 50-L polyethylene carboys were filled with sea water on the same date at the same location. One was acidified with 1 2 M HC1 t o 0.5 M. T h e 50-L carboys were shaken thoroughly before water was removed. The 210Poconcentration in the unacidified sea water showed a dramatic decrease to less than half in seven days (Table 111), the time sometimes lapsing between field sampling and analysis in the laboratory. Since the unacidified sea water sample was acidified to 0.5 M on addition of the 208Potracer, Z1OPowas either lost t o the walls of the carboy or converted to a suspended species not leachable in the 0.5 M acid. Another approach t o the effect of acidification was to subsample a large bottle of sea water, first untreated with acid and then progressively made more acidic, allowing two or more
496
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
days between treatments. T h e results of such a test are summarized in Table IV. T h e need for acidification on collection is again demonstrated and the large variations encountered in analyzing raw sea water by the NaOH precipitation method are evident. Table V shows results of a test designed to see if ascorbic acid would improve the autoplating yields as reported for various environmental samples (8).If any effect was observed, it was a slight decrease in overall yield on the addition of ascorbic acid. A cursory experiment was performed to determine whether polonium was largely carried on particulate matter which would most likely account for the large variations encountered when analyzing raw sea water. Fresh, untreated sea water was passed through a 1-inch i.d., 2-inch long PVC pipe coupling which was firmly packed with glass wool. T h e filtered sea water was analyzed by the partial precipitation method and the glass wool by refluxing with hot concentrated nitrichydrochloric acid mixture (1:2) after addition of zosPo tracer. Table VI indicates t h a t 30 t o 34% of the zloPowas collected by this filter. In coastal waters the amount of particulate material suspended in the water column is quite variable, depending on such factors as land runoff, upwelling, and biologic activity. The significant particulate zlOPoactivity found by these cursory experiments emphasizes the need for precisely describing sampling techniques.
LITERATURE C I T E D (1) V. F. Hodge, F. L. Hoffman, and T. R. Folsom, Health Phys., 27, 29-35 (1974). (2) T. R. Folsom and V. F. Hodge, Mar. Sci. Commun., 1, 213-247 (1975). (3) T. R. Folsom, V. F. Hodge, and M. Gurney, Mar. Sci. Cornmun., 1, 39-49 (1975). (4) E. D.Goldberg, M. Koide, and V. F. Hodge, unpublished work, Scripps Institution of Oceanography, La Jolla, Calif., 1976. (5) 5. Tsunogai and Y . Nozaki, Geochem. J., 5, 165-173 (1971). (6) L. V. Shannon, R. D. Cherry, and M. J. Orren, Geochim. Cosmochim. Acta, 34, 701-711 (1970). (7) V. F. Hodge, F. L. Hoffman, R. L. Foreman, and R. T. Folsom, Anal. Chem., 46, 1334-1336 (1974). (8) R. L. Blanchard, Anal. Chem., 38, 189-197 (1966).
RECEIVEDfor review October 12,1976. Accepted December 13,1976. T h e work was supported by the U.S. E.R.D.A. and T h e Office of Naval Research.
