Radiochemical Determination of Sulfur-35 in Large Samples of Vegetation Conrad P. Willis, Dale G . Olson, and Claude W. Sill Health Seroices Laboratory, U. S . Atomic Energy Commission,Idaho Falls. Idaho 83401 SULFUR DIOXIDE is one of the major industrial pollutants having a deleterious effect on both plants and animals. To measure the deposition velocity of sulfur dioxide on plants, sulfur dioxide tagged with sulfur-35 was released over a test field and samples of alfalfa were taken for analysis. Large samples were required because of the low levels of activity and relatively high detection limit for sulfur-35. Liquid scintillation counting is the preferred method for measuring the low energy (168-kev) beta particles (1-3). The sulfur can be easily prepared for counting by oxidation to sulfate with perchloric acid and then precipitated with barium. The insoluble barium sulfate is suspended in a scintillator solution gelled with Cab-0-Si1 (Godfrey L. Cabot, Inc., 77 Franklin St., Boston, Mass.) for counting (4, 5). Gordon and Curtis (6) have shown that a white insoluble material in the scintillation mixture increases the counting efficiency by reflecting additional. light out of the counting vial. Preparation of samples for the counting of sulfur-35 is difficult because of the volatility of both sulfur and some of its compounds and the low energy of the beta particles emitted. Present methods of sample preparation such as the direct addition of a powdered sample to the scintillator or the solubilization of samples with alcoholic hyamine hydroxide or potassium hydroxide before addition to the scintillator (7) are obviously not applicable to large samples of vegetation. Drying and muffling in dishes of stainless steel, Vycor, silica, or porcelain in the presence of an oxidizing agent such as magnesium nitrate (8) corrode the container, leave a large amount of insoluble material, and are very time-consuming. The large container required for muffling large samples and the small size of most laboratory muffle furnaces limit the ashing to one or two samples at a time. Wet ashing with nitric and perchloric acids is rapid and large numbers of samples can be accommodated simultaneously in conventional borosilicate glassware. However, because of the volatility of sulfuric acid, large losses will occur in an open system unless large quantities of perchloric acid are used to moderate the reaction. This paper describes a method of decomposing 30-gram samples of alfalfa by wet-ashing with less than a 4$ loss of the sulfur. Twelve samples can be decomposed at a time by one operator in less than 2 hours. EXPERIMENTAL.
Reagents. Prepare a scintillator solution by dissolving 10 grams of PPO (2,5-diphenyloxazole), 150 mg of POPOP { 1,4-bis[-2(5-phenyloxazole)-benzene]], and 120 grams of
naphthalene in 1 liter of dioxane. Filter the scintillator solution to remove any particles and store in a brown bottle to prevent deterioration by light. Prepare a 4z Cab-0-Si1 suspension each day by weighing sufficient Cab-0-Si1 into a bottle and adding the appropriate volume of scintillator solution. All other reagents used in this study are standard reagent grade materials. Procedure. Weigh 30 grams of green alfalfa into a 1-liter Erlenmeyer flask, add 30 ml each of nitric and perchloric acids, and place the flask on a medium temperature hot plate. The sample will bump and must be swirled periodically to prevent loss until the solids have dissolved in the hot acid. If the solution turns black in the hot perchloric acid, add a few drops of additional nitric acid to clear the charred organic material. After the vigorous reaction between perchloric acid and the organic material has subsided, add 2 or 3 ml of nitric acid to clean the sides of the flask and to facilitate oxidation of the last trace of organic matter. Heat the sample for about 15 minutes after the last addition to eliminate all nitric acid. Because loss of sulfuric acid increases with increasing perchloric acid volatilized, do not heat longer than necessary to remove the nitric acid. Cool the solution, add 50 ml of water, and continue cooling in a cold water bath for several minutes to precipitate potassium and ammonium perchlorates. Transfer to a 90-ml centrifuge tube and centrifuge at 2000 rpm (1500 rcf) for 5 minutes to remove silica and the insoluble perchlorates. Decant the supernate into a 250-ml Erlenmeyer flask, add a boiling chip, and boil the solution until the volume has been reduced to 60 ml. Add 10 ml of a 20% solution of barium chloride dihydrate and continue boiling for 5 minutes. Transfer the solution to a 90-ml Centrifuge tube and centrifuge at 2000 rpm for 5 minutes. Decant and discard the supernate, making sure that none of the barium sulfate is lost. Transfer the barium sulfate to a 22-ml counting vial using a fine jet of water to facilitate the transfer and centrifuge at 1800 rpm (800 rcf) for 10 minutes. By placing a I-inch cork on top of the inverted rubber cushion, the counting vials can be centrifuged directly in a 50-ml conical centrifuge tube shield (Catalog No. 320, International Equipment Co., Needham Heights, Mass.). Draw off the supernate through a finetipped pipet, using a water aspirator to remove as much water as possible. Add 18 ml of Cab-0-Si1 scintillator suspension to the vial and shake vigorouslv to susuend the barium sulfate. Placing the vial in anultras6nic bath containing about 1 inch of water for a few seconds loosens the cake and markedly facilitates uniform dispersion of the barium sulfate. Add 2 drops of a 0.25N sodium hydroxide solution and shake to mix. Store the samples in the dark for 1 hour to allow any initial fluorescence to decay and count for 20 minutes in a suitable lowbackground scintillation counter (Beckman Model LS 11, Beckman Instruments, Inc., Fullerton, Calif.). DISCUSSION AND RESULTS
~~
(1) N. S. Radin and R. Fried, ANAL.CHEM., 30,1926 (1958). (2) H. Jeffay, F. 0. Oiubajo, and W. R. Jewell, Zbid., 32,306 (1960). ( 3 ) G. J. Blair and F. C. Crofts, Soil Sci., 107,277 (1969). (4) C. F. Gordon and A. L.Wolfe, ANAL.CHEM., 32, 574 (1960). (5) D. G . Ott, C. R. Richmond, T. T. Trujillo, and H. Foreman, Nucleonics, 17, No. 9, 106 (1959). (6) B. E. Gordon and R. M. Curtis, ANAL.CHEM., 40, 1486 (1968). (7) D. L. Hansen and E. T. Bush, Anal. Biochem., 18, 320 (1967). (8) H. C. Chapman and P. F. Pratt, “Methods of Analysis for Soil, Plants and Waters,” Division of Agricultural Science, University of California, 1961, p 195. 124
Decomposition. Although not violent, the reaction of green alfalfa with perchloric acid is very vigorous and care must be exercised during the decomposition to prevent loss of sulfur as sulfuric acid. As much as 80z of the sulfur has been lost while decomposing 50 grams of alfalfa with 10 ml of perchloric acid, even though large quantities of nitric acid were used. Sufficient perchloric acid must be present to prevent significant loss of sulfuric acid during the decomposition but not enough to prevent subsequent complete
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
Table I. Recovery of a% from Green Alfalfa added, 35.9 recovered, dpm(X IO3) From 30 g alfalfa From 100 mg SO4 12.3 24.6 49.3 61.6 98.6 123.2
95.2 95.2 93.5 96.5 94.0 93.5
98.4 98.0 96.4 95.8 95.9 96.6
Blank on 30 grams of alfalfa 28.7 f 15% cpm. Blank on 100 mg of sulfate ion 23.4 15% cpm.
*
Time
(hGuiS)
Figure 1. Adsorption of sulfur-35 on glass and nylon vials 1. Glass vial with scintillator 2.
Glass vial with scintillator gelled with Cab-0-Sil
3. Nylon vial with scintillator 4. Nylon vial with scintillator gelled with Cab-0-Sil
precipitation of barium sulfate. In general, 1 ml of perchloric acid should be used for each gram of green alfalfa. The results in Table I show the recovery of sulfur-35 under the recommended conditions. Samples as large as 120 grsms have been decomposed successfully using a proportionately larger volume of acid. The quantity of nitric acid used initially makes little difference in the organic-perchloric acid reaction as long as sufficient is present to oxidize the easily oxidizable material. If insufficient nitric acid is added, the organic matter will char in the hot perchloric acid and turn black. Additional nitric acid should be added when this occurs, to initiate the reaction between perchloric acid and organic matter. Perchloric acid oxidizes the last of the organic material very slowly. Two or 3 ml of nitric acid will aid in the oxidation and speed up the decomposition considerably. Because nitric acid retards precipitation of barium sulfate, the excess acid must be evaporated before addition of barium. Large quantities of ammonium and potassium perchlorates precipitate when the acid is diluted and cooled, but no sulfur-35 was detected in the precipitate. If barium is present in the plant, some sulfur will be lost in the insoluble material. However, unless unusual levels of barium are present, the loss of sulfur-35 will be very small with the relatively large quantities of sulfur normally present. Effect of Sulfur on Counting Rate. Chapman and Pratt (8) report that the natural sulfur content of vegetation commonly ranges from less than 0.1 to over 1%. Accordingly, the effect of such variations on the counting rate was determined. One-milliliter aliquots of sulfur-35 as sulfate were measured into a series of centrifuge tubes containing from 20 to 200 mg of sulfate ion. Perchloric acid was added and the sulfate precipitated with barium and counted. The results were linear and showed a decrease of only 6 z in the counting rate over the 20- to 200-mg range. Since the sulfur content of the alfalfa used in this study was found to be 0.1 1 30-gram samples were taken so that the sulfate present would be about 100 mg. Under these conditions the counting rate was proportional to the activity up to at least 2 X lo4 dpm. Barium sulfate can be easily transferred quantitatively from a centrifuge tube to a counting vial by washing with water
z,
and centrifuging the counting vial. The precipitate must be washed to remove the remaining acid before the scintillator solution is added. Glass counting vials can be centrifuged safely if a proper shield is prepared. Although barium sulfate is peptized somewhat by water and is not removed completely from the suspension by centrifuging the counting vial, less than 3 of the activity is lost if care is used in removing the wash. Interferences. Sill and Willis (9) have shown that probably all positive ions having a charge greater than 2 and an ionic radius larger than 1.1 A are carried efficiently on barium sulfate from a strong sulfuric acid-sulfate ion system. However, in the absence of a large excess of sulfate ions, the ability of barium sulfate to carry foreign ions is decreased markedly. The conditions used in this study eliminate all interferences except radioactive barium, radium, and strontium, which are diluted many orders of magnitude by the large excess of barium added to precipitate the sulfur. No interference from other radionuclides was detected in this study, even though the test field is directly downwind from a chemical plant reprocessing spent nuclear fuels. Counting Efficiency. The counting efficiency for sulfur-35 was estimated from a certified carbon-14 standard. Since the beta energies of the two radionuclides are nearly the same, their counting efficiencies should be very similar. The counting efficiency was measured to be 62% for carbon14 under the conditions used, from which the counting efficiency for sulfur-35 was estimated to be 6 5 z . The counting rate of soluble sulfur-35 in a scintillator solution decreases with time much more rapidly than can be attributed t o radioactive decay. Radin and Fried ( I ) suggest that this decrease is due to adsorption on the walls of the counting vial, thereby changing the counting efficiency. This suggestion was confirmed in the present work, as shown in Figure I. One milliliter of tracer containing 8.8 X l o 4 dpm of sulfur-35 was added to 18 ml of scintillator. Curves 1 and 3 show the changes in counting rates that occurred as a function of time in glass and nylon vials, respectively. Curves 2 and 4 show the corresponding counting rates resulting when a 4 % Cab-0-Si1 gel is formed to prevent the migration of sulfate ions t o the sides of the vial. The slight decrease in counting rate shown in curves 2 and 4 is due to the theoretical radioactive decay of sulfur-35. The vials used for curves 1 and 3 were emptied, rinsed twice with scintillator solution, refilled, and counted. The activity remaining in the vials was 18.5% in the glass and 2 0 z in the nylon, showing that the activity was on the walls of the vial. Surprisingly, Cab-0-Si1 did not give a firm gel with the scintillator solution used in this work. As a consequence, (9) C. W. Sill and C. P. Willis, ANAL.CHEM., 38, 97 (1966).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
a
125
both the barium sulfate and the Cab-0-Si1 slowly settle out, leaving a clear solution of scintillator at the top of the vial. This results in a change in the counting rate similar to that shown in Figure 1. The addition of sufficient sodium hydroxide to render the suspension slightly alkaline causes the suspension to set to a thixatropic gel that remains firm for at least several weeks. As shown in curves 2 and 4 of Figure 1,
no decrease beyond that due to radiological decay occurs from
a rigid gel. An over-all recovery of 93 with this procedure.
or better is obtained routinely
RECEIVED for review September 15, 1969. Accepted October 30,1969.
Second Harmonic Alternating Current Polarography. Some Experimental Observations with Zinc Ion-Zinc Amalgam System in Chloride Media Thomas G. McCord‘ and Donald E. Smith2 Department of Chemistry, Northwestern University, Euanston, Ill. 60201
RECENTLY, second harmonic ac polarographic studies of processes following the simple quasi-reversible electrode reaction mechanism ka,u
O+ne+R
have yielded excellent agreement between theory and experiment ( I , 2). Rate parameters obtained ( k , and a) were consistent with results of other methods. Measurements under conditions of the classical faradiac impedance experiment (without dc polarization) ( I ) and under ac polarographic conditions (with dc polarization) ( 2 ) were represented in these recent data. The most difficult aspect associated with developing a rigorous formulation of the second harmonic response involves predicting effects of spherical diffusion which originate in the dc polarization process (3-6). This is particularly true in the case of amalgam-forming processes where such effects are relatively large (5). The aforementioned agreement between theory and experiment under ac polarographic conditions ( 2 ) was observed for systems characterized by nernstian (diffusion-controlled) dc processes, a situation in which theoretical accommodation of the spherical diffusion effect is not particularly difficult. These studies did not encompass the more severe situation which arises when charge transfer kinetics are sufficiently slow that the dc process is controlled by charge transfer kinetics and diffusion (non-nernstian or quasireversible dc processes) (3, 4 ) . In this particular case the spherical diffusion correction provided by existing second harmonic theory ( 2 ) is expected to be rather inexact because it is based on the assumption of nernstian behavior in the dc 1
sense. Thus, the possibility exists that its use in analysis of second harmonic data might yield inaccurate charge transfer rate parameters whenever the dc process is quasi-reversible. In order to assess the seriousness of the foregoing problem, we carried out a comparison of existing second harmonic theory with experimental results obtained with one example of the “worst case” in which both amalgam formation and a quasi-reversible dc polarization step are operative. The system selected was Zn2+/Zn(Hg)in 1 M KCl, HCl. A detailed fundamental harmonic investigation of this process has been effected by Sluyters and coworkers ((7-10) whose data indicated that the kinetic parameters, k, 3.8 X cm sec-l, 01 G 0.30 (9) [ain this work is equivalent to in References (8) and (9)], were appropriate for our purposes. The results of our second harmonic ac polarographic investigation are presented here. EXPERIMENTAL
The experimental procedures are identical to those described previously (2). Polarograms were examined at the frequencies; 23, 332, and 1110 Hz. The polarographic solution, 2.0 x 10-3M Zn2+ in 1.OM KCl, 1.0 X 10-3M HCI matches one of those reported by Timmer, SluytersReybach, and Sluyters (9). Theoretical equations and data treatment procedures employed in this work are given in Reference (2). Explicit comparison of theoretical and experimental polarograms 5 k , 5 1.2 X involved rate parameters in the ranges, 10-2 cm sec-1 and 0.2 5 a _< 0.5. Other parameters used in theoretical calculations were: IZ = 2, C,* = 2.0 X 10-3M, Do = 0.7 x 10-5 cm2 sec-’ ( I I ) , DR = 2.0 X cm2 sec-1 (12), and A = 0.035 cm2.
Present address, General Electric Corp., Materials and Pro-
cesses Laboratory, Schenectady, N. Y . , 12305 2
To whom correspondence should be addressed.
(1) J. E. B. Randles and D. R. Whitehouse, Trans. Faraday Soc., 64, 1376 (1968). (2) T. G. McCord and D. E. Smith, ANAL.CHEM., 41,131 (1969). (3) J. R. Delmastro and D. E. Smith. J. Electroanal. Chem.,. 9,. 192 (1965). (4) J. R. Delmastro and D. E. Smith. ANAL.CHEM.. 38. 169 (1966). { 5 j T . G. McCord, E. R. Brown, and D. E. Smith,.ibid., p 1615. (6) T. G. McCord and D. E. Smith, ibid., 40, 289 (1968). \-,
126
a
(7) M. Sluyters-Rehbach, A. B. Ijzermans, B. Timmer, J. B. Griffioen, and J. H. Sluyters, Electrochim. Acta, 11, 483 (1966). (8) B. Timmer, M. Sluyters-Rehbach, and J. H. Sluyters, J. Electroanal. Chem., 14, 169 (1967). (9) Ibid.,p 181. (10) Zbid., 19, 85 (1968). (11) I. M. Kolthoff and J. J. Lingane, “Polarography,” Vol. 1, 2nd Ed., Interscience Publishers, New York, 1952, pp 52, 95. (12) W. C. Cooper and N. H. Furman, J. Amer. Clzem. SOC.,74, 6183 (1952).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
