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Determination of bismuth in marine samples by anodic stripping voltammetry
Electroanalytical Chemistry and lnterfacial Electrochemistry, 49 (1974) 255 264 © Elsevier Sequoia S.A., Lausanne
255
Printed in The Netherlands
D E T E R M I N A T I O N O F B I S M U T H IN MARINE SAMPLES BY A N O D I C STRIPPING VOLTAMMETRY
T. M. FLORENCE
Chemical Technology Division, Australian Atomic Energy Commission, Research Establishment, Lucas Heights, N.S.W. (Australia) (Received 24th July 1973)
INTRODUCTION
Very little information is available about the distribution of bismuth in the marine environment. Noddack and Noddack I used a spectrographic method to estimate a value of 0.2 #g Bi 1- j in Norwegian fjord water, while Brooks 2 applied anion exchange separation of bismuth followed by emission spectrography to a sample of South Atlantic water and found 0.017 pg Bi 1-1. More recently, Portmann and Riley 3 used the ion exchange separation procedure of Brooks followed by dithizone extraction and reported bismuth concentrations of 0.025, 0.039 and 0.033 pg 1-1 in surface waters from the Engfish Channel, the Irish Sea, and the North Atlantic, respectively. A deep water (2000 m) sample from the North Atlantic contained 0.015 pg Bi 1-1. Florence 4 determined bismuth in Pacific Ocean water by direct anodic stripping voltammetry and found 0.0434).21 #g 1- j for surface samples filtered through a 0.45 pm Millipore filter. Insoluble bismuth was in the range <0.005 to 0.011 #g 1-1. No results are available in the literature on the concentration of bismuth in marine organisms, although Florence 4 calculated that, using a value of 0.13 /~g 1-1 for bismuth in seawater, the biological concentration factors (p.p.m. Bi in dried organism//~g ml-1 Bi in seawater) in fish, oysters, abalone and seaweed must be less than 80. The scarcity of data on bismuth in environmental materials is no doubt largely due to the lack of sensitive methods for its determination. Table 1 summarises the maximum sensitivity for bismuth attainable under ideal conditions by a variety of analytical techniques, and shows that the recently developed thinfilm anodic stripping method 1° is 200 times more sensitive than any other technique. Of all the metals amenable to analysis by anodic stripping voltammetry 11-13 bismuth is perhaps the one most easily determined. Not only is the stripping wave sharp and highly sensitive, but because bismuth can be deposited at potentials so positive that most other metals are not deposited, inter-element compound formation in the mercury film is avoided and the method is almost specific for bismuth. This paper describes the application of thin-film anodic stripping voltammetry to the determination of bismuth in seawater and a variety of marine organisms. Seawater is analysed directly, no separation or pre-concentration steps being necessary. With some marine organisms a s h e d . a t 450°C a preliminary
256
T.M. FLORENCE
TABLE 1 SENSITIVITY OF SOME TRACE M E T H O D S FOR BISMUTH
Technique
Limit of detection"/ ugl ~
Reference
Molecular spectrophotometry Atomic absorption (flame) Atomic absorption (carbon rod) Flame emission Neutron activation Arc emission spectrography Spark source mass spectrometry Thin-film anodic stripping voltammetry
l0 b 50 7 500 100 c 25 1
5 6 7 8 9 8 8
0.005
This work
Ideal, interference-free conditions. b 4 c m cells, Cary 16 spectrophotometer. 10 ml sample volume.
separation is required to eliminate residual organic matter, and this was accomplished by a rapid anion exchange procedure. EXPERIMENTAL
Apparatus and reagents The anodic stripping technique described previously 4'1°'14 was used. This involves simultaneous deposition of mercury and trace metals onto a highly polished rotated (2000 rev. rain -1) glassy carbon electrode followed by a rapid anodic voltage scan. All measurements were made at 25 +_0.5°C and with a SCE reference electrode. Reagent-grade nitric, hydrochloric and perchloric acids were found to have a very low bismuth content and were used without further purification. Water was demineralised then distilled from permanganate and stored in polythene bottles.
Preparation of samples Seawater. Samples of surface seawater were collected in high-density polythene bottles which had been soaked in 1 M HC1 for two weeks and then rinsed with water. The seawater was filtered on the day of collection through a 0.45 pm Millipore filter (type H A W P 04700) which had previously been washed with 1 M HC1. The first 500 ml of filtrate was rejected and the remainder stored in acidcleaned 1-1 polythene bottles containing 5 ml of 10 M HC1. The Millipore filters were retained for determination of insoluble bismuth. Analyses were normally completed within 48 h of collection. Marine organisms. The collection of samples and the muffle ashing and wet ashing procedures have been described previously 4. Dissolution of samples muffle-ashed at 450°C was accomplished as follows. A 1.0 g portion of finely-ground ash was weighed into a beaker, then 10 ml water, 10 ml 10 M HC1 and 1 ml 15 M H N O 3 added. The beaker was covered and the solution boiled gently on a hot plate for 30 min. The solution was filtered through a 9 cm No. 542 Whatman paper that had previously been washed with 5 M HC1.
A N O D I C S T R I P P I N G VOLTAMMETRY F O R MARINE BISMUTH
257
The small residue was washed with two 5 ml portions of hot 5 M HC1. The filtrate and washings were evaporated to 1 2 ml, then evaporated twice more after the addition of 5 ml of 10 M HC1. The final volume remaining should be about 2 ml. The residue was dissolved by warming with 20 ml of water, and the solution filtered again through a No. 542 W h a t m a n to remove last traces of silica which could block the ion exchange column. The paper was washed with a few ml of water and the filtrate reserved for ion exchange separation. 1on exchange separation of bismuth. The solution of muffle ash was passed through a 6 crux0.5 cm diameter column of Bio-Rad AGI-X8 (100-200 mesh) anion exchange resin (C1- form) at a flow rate of 0.7 ml min-1. The column was first conditioned by washing with water and passing 10 ml of 0.6 M HC1. After the sample solution had passed through, the column was washed with 10 ml of 0.6 M HC1, then the bismuth eluted by passing, successively, 5 ml H 2 0 , 15 ml 0.5 M N a O H , 5 ml H 2 0 , and 10 ml 1 M HC10 4. The eluants were collected in a 50 ml volumetric flask containing 5 ml of 4 M H C 1 0 4 and 3 ml of 0.6 M HC1. After the addition of 0.1 ml of 1 x 10 - 2 M mercuric nitrate the solution was diluted to volume.
Analytical procedures Seawater. To 50 ml of filtered, acidified (0.05 M HC1) seawater add 0.1 ml of 1 x 10 -2 M mercuric nitrate. Rinse the voltammetric cell and electrodes with some of the sample solution then add 10 ml of sample solution to the cell. Deaerate with helium for 10 min, electrolyse at - 0 . 2 5 V for 5 min and follow with a 5 V min-1 voltage scan to - 0 . 0 3 V. Repeat this deposition and stripping procedure. Finally electrolyse at - 0 . 2 5 V for 30 min and strip as before using a full scale sensitivity of 2 #A. The third stripping peak is used for measurement. The bismuth wave occurs at - 0 . 1 0 V and its height is proportional to concentration. Prepare and analyse a standard in the same way by spiking 50 ml of seawater with 0.1 ml of a 1 x 10 -5 M standard bismuth solution. Muffle-ashed samples. Rinse the cell and electrodes with some of the solution from the ion exchange column and transfer 10 ml to the cell. Analyse as for seawater but use a deposition potential of - 0 . 2 0 V and a deposition time of 10 min for the third scan. The peak occurs at - 0 . 0 5 V. Wet-ashed samples. Finely-ground samples (2.0 g) dried at 105°C were wet ashed with H N O 3 + HC10 4 as described previously 4. Evaporate the H C 1 0 4 to a volume of about 1 ml, cool, add 15 ml water and filter through an acid-washed 9 cm No. 542 W h a t m a n paper. Wash the paper with a few ml of water and collect the filtrate and washings in a 25 ml volumetric flask. Add 2.5 ml of 0.6 M HC1 and 0.05 ml of 1 x 10-2 M mercuric nitrate and dilute to volume. Analyse as described for muffle-ashed samples. Bismuth retained on Millipore filters was determined as described above after wet ashing with 2 ml of 15 M H N O 3 and 2 ml of 72~o HC104. RESULTS AND DISCUSSION
Anodic stripping voltammetry of bismuth Bismuth was found to yield sharp anodic stripping peaks from a variety of
258
T.M. FLORENCE
supporting electrolytes, although maximum sensitivity was obtained in acidic chloride media. In 0.4 M HCIO 4 peak height was independent of chloride concentration in the range 0.02~3.2 M HC1, while at constant hydrochloric acid concentration the sensitivity was little affected by changes in perchlorate concentration. Peak height was directly proportional to bismuth concentration, deposition time and anodic scan rate. The effect of deposition potential on the bismuth stripping peak was investigated in a 0.4 M HC10 4 + 0.06 M HC1 supporting electrolyte. The sensitivity was found to increase by 30% when the deposition potential was decreased from - 0 . 5 V to - 0 . 2 V, which was the optimum potential for maximum sensitivity. In this medium the bismuth peak potential occurs at - 0 . 0 5 V, but in seawater samples acidified with 0.05 M HC1 the peak is at - 0 . 1 0 V so a deposition potential of - 0 . 2 5 V is used. The relatively positive deposition potential used for bismuth ensures that the method will be highly selective. Very few metals can be deposited at - 0 . 2 V, so the possibility of inter-element compound formation in the mercury film is very slight. The following elements gave less than 2% error in the determination of 0.01 p.p.m. Bi when tested at the concentrations shown (in p.p.m.): Ag + (0.1); Cu 2+ (2.5); Ve 3+ (5.0); Pb 2+ (5.0); Sb 3+ (0.05); Sb 5+ (0.1); B r - (100); P O ] (1000). Antimony(III) produces a sharp wave at - 0 . 1 9 V when a deposition potential of - 0 . 2 5 V is used, but very little antimony is accumulated on the electrode if the deposition potential is - 0 . 2 0 V. In any case, antimony in environmental materials is invariably present in the (V) state, which is electrochemically inactive. Very high concentrations of bromide and iodide interfere via their oxidation waves which occur adjacent to the bismuth peak. The only serious interference in the bismuth determination was encountered from tungsten(VI) and molybdenum(VI). These two elements interfere with the determination by anodic stripping voltammetry of many metals, although the interference mechanism is obscure. In the case of bismuth low results are obtained in the presence of molybdenum and tungsten, although the shape of the peak is unchanged (Table 2). Molybdenum and tungsten are unlikely to be present in marine samples at concentrations high enough to have a significant effect on the TABLE 2 INTERFERENCE OF Mo(VI) AND W(VI) IN THE DETERMINATION OF BISMUTH Supporting electrolyteof 0.4 M HC104+0.06 M HCI Concentration/p.p.m.
Error in determination of 0.01 p.p.m. Bi/%
Mo
W
30 10 2
--
--
2
- 30 -15 nil -35
1
-27
--
0.1
-3
ANODIC STRIPPING VOLTAMMETRY FOR MARINE BISMUTH
259
bismuth wave 15, and in any case, spiking the sample with a standard bismuth solution would detect any depression of the peak. With constant tungsten or m o l y b d e n u m concentration the height of the peak is still proportional to bismuth concentration. The recommended anion exchange separation procedure eliminates molybdenum and tungsten. The determination of bismuth by the in s i t u deposited mercury film method was compared using rapid d.c. anodic stripping and differential pulse stripping (Princeton Applied Research 174 polarograph 16). With the hanging mercury drop electrode the differential pulse technique is 5 10 times more sensitive than d.c., but using the ultra-thin mercury film electrode the sensitivities were very similar. In addition, the pulse technique was more affected by traces of surfactants and the stripping peak was broader. There appears to be no advantage in using the instrumentally more complicated pulse method in favour of a simple d.c. voltammeter. Seawa ter
The data obtained for bismuth in New South Wales and Queensland coastal waters are collected in Table 3, and a typical stripping peak for bismuth in TABLE3 BISMUTH IN SURFACE SEAWATER FROM THE PACIFIC OCEAN Sample no.
Coastal source
Bi/#g I 1 Soluble a
1 2 2 3 4 4 5
Botany Bay, NSW Jervis Bay, NSW Jervis Bay, NSW Jibbon Beach, NSW Jibbon Beach, NSW Jibbon Beach, NSW Queensland
0.11, 0.ll 0.045, 0.043 0.045~ 0.018, 0.021 0.041 _+0.003~ 0.038c 0.12, 0.10
Insoluble b
0.002 0.002 <0.001 ~).005 -<0.001
" Duplicate results shown. b Retained by 0.45 #m Millipore filter. c After anion-exchange separation. Corrected for 90% recovery of Bi. a Mean and standard deviation of 7 determinations. seawater is shown in Fig. 1. The results were obtained by direct anodic stripping voltammetry, although two samples were also analysed after separation of the bismuth by anion exchange. The ion exchange procedure was that described earlier with the exception that 50 ml of seawater was adjusted to an acidity of 0.1 M HC1 before passage through the column 3. The relative standard deviation of the direct method was determined by analysing seven aliquots of a seawater containing 0.041 ILg Bi 1 1, and was found to be 7% (Table 3). Anodic stripping voltammetry has the property of measuring only free metal ion plus labile metal complexes, i.e. complexes which will dissociate under the influence of the electrode potential during the time the complex species is near the
260
T.M. FLORENCE
oioov-oiTo . -o¢2o-q-osv -Ts
•
--
"
-
-~.2s
0
OiOOV-0ii0 -0i20
Fig. 1. Anodic stripping voltammetry of bismuth. (1) Seawater containing 0.11 /~g Bi 1-1. Direct determination of bismuth; 15 min deposition time. (2) Standard 4 x 10 -s M Bi, 5 min deposition time. (3) Determination of bismuth in catfish ash (1.0 g) after anion exchange separation; 15 rain deposition time. (4) Determination of bismuth in cunjevoi ash (1.0 g) after anion exchange separation; 10 min deposition time.
electrode surface. To determine whether particulate or inert organic complexes of bismuth which do not contribute to the stripping current are present in seawater, two seawater samples (nos. 3 and 4, Table 3) were oxidised to destroy organic matter before measurement of the bismuth concentration by anodic stripping voltammetry. Oxidation was carried out by addition of hydrogen peroxide and exposure to u.v. light, and also by fuming 50 ml of seawater with 1 ml of H N O 3 plus 1 ml of HC10 4. No increase in the bismuth peak height was observed after either of these oxidation procedures, so it can be assumed that the analytical procedure used measures total bismuth. Seawater acidified with 0.05 M HC1 immediately after filtration and stored in polythene bottles showed no decrease in bismuth concentration for at least three months. The reagent grade hydrochloric acid used in this work had a bismuth content of only 0.05/~g Bi 1-1
Marine organisms The bismuth contents of several marine organisms collected on the New South Wales coast are given in Table 4, together with ashing data and biological concentration factors calculated using a value of 0.05/~g Bi 1 1 for seawater. The bismuth stripping peaks recorded for catfish and cunjevoi are shown in Fig. 1. The relative standard deviation of the determination of bismuth in cunjevoi flesh was 3.8~o (Table 4). To check that bismuth was not lost during dry ashing at 450°C some samples were also wet ashed with HC10 4 plus H N O 3. Table 4 shows that there was no significant difference between samples ashed by the two methods. As a further check on volatility losses of bismuth, an oyster sample which had been
ANODIC STRIPPING VOLTAMMETRY
261
FOR MARINE BISMUTH
TABLE 4 BISMUTH IN MARINE ORGANISMS
Sample
°/o ash/fresh weight ~
Bismuth/#g kg- 1 fresh weight b
Biological concn. factor e
Muffle ashing a'c Seaweed, Ecklonia radiata Seaweed, Hormosira banksii Fish muscle, luderick, Girella tricuspidata Whole fish, bream, Mylio australis Fish muscle, catfish, Plotosus anguillaris Fish skeleton, luderick Abalone flesh, Haliotis ruber Oyster flesh, Crassostrea commereialis (1) Oyster flesh (2) Cunjevoi flesh, Pyura stolonifera (Ascidian)
Wet ashing d
4.5 _+0.3 2.43 _+0.07
---
90 49
3.33
.3.6 _+0.0
3.2
72
3.95
1.25 + 0.05
2.00 7.13
1.54_+0.09 <0.1
<0.1
72 <2
1.72
1.03_+0.1
--
21
5.09 1.70
3.7 _+0.2 4.8 _+0.5
3.9 --
74 96
4.69
8.0 +0.31
7.9
160
10.4 9.2
25
" Ashing temperature of 450°C.
b Results are mean and difference of two separate weighings. c After anion exchange separation of bismuth. d H N O 3 + HC10~" e p.p.b. Bi in fresh sample/#g 1-1 Bi in seawater. I Mean and standard deviation of five separate weighings.
ashed at 450°C for 3 days was analysed, then heated at 450°C for another 3 days. No decrease in bismuth content was observed after the additional ashing time. Interference studies indicated that no inorganic ion, or combination of ions, present at the concentration expected in marine samples should interfere with the determination of bismuth. However, with most samples which had been dry ashed at 450°C an unusual interference effect was present which prevented accurate determination of bismuth by the direct method. With these samples the mercury deposition current decreased with electrolysis time, and successive anodic scans yielded bismuth peaks of diminishing height. This effect could be caused by some substance in solution fouling the electrode surface and decreasing the available electrode area. This substance is probably organic in nature, because the interference does not occur in samples wet ashed by prolonged fuming with nitric and perchloric acids. Dry ashing, however, is the preferred method because of the saving in operator time and the ability to use larger samples. The anion exchange procedure provided a simple and rapid method for separating bismuth from the residual organic matter in dry ashed samples. Recoveries of bismuth spikes added at the start of analysis were 88-95~o for dry ashed samples (including ion exchange separation) and 95-100~o for wet ashing.
262
T.M. FLORENCE
Ion exchange separation of bismuth Brooks 2 and P o r t m a n n and Riley 3 investigated the conditions necc~bary for the separation of bismuth from seawater by anion exchange chromatography. Riley and Taylor 17 also described the quantitative retention of bismuth from seawater at p H 9.0 on the chelating resin Chelex-100. A possible source of error in these studies is that the retention of bismuth on the columns was checked by spiking seawater with a standard solution of ionic bismuth and measuring the recovery of the spike. The natural bismuth in seawater is unlikely to exist entirely in a simple ionic f o r m and may show quite different behaviour to an ionic standard Is'19 Piro et al. as found that even after one year, radioactive ionic zinc added to seawater at its natural p H had not come to equilibrium with the zinc complexes present in seawater. To check retention on an anion exchange resin of bismuth naturally present in seawater, 50 ml of filtered seawater adjusted to an acidity of 0.1 M HC1 was passed through a 6 c m x 0.5 cm diameter column of Bio Rad AGI-X8 (100-200 mesh) at 0.7 ml m i n - 1 and the effluent analysed for bismuth by anodic stripping voltammetry. The seawater originally gave a figure of 0.11 ~g Bi 1 1, and the effluent analysed as 0.010_+0.002 /~g Bi 1-1. The effluent from the first column was then passed through a second anion exchange column of similar size, and the new effluent was again found to contain 0.010+0.002 pg Bi 1-1. It appears then that the sample of seawater chosen for this experiment contained 9~'/o of the total bismuth in a form which is not absorbed by the resin at an acidity of 0.1 M HCl. This result can be compared with the > 99~o retention of a bismuth-207 spike reported by P o r t m a n n and Riley 3 when using the same experimental conditions. Inorganic species in seawater are engaged in the most complex equilibria, and careful investigation of any separation method should be made before quantitative recovery of the element sought is assumed. Methods were investigated for eluting bismuth absorbed on the resin from 0.6 M HC1. Bismuth is difficult to remove from anion exchange resins, and Brooks 2 used 1 1 of 0.25 M H N O 3 to elute a 7 ml resin column, while P o r t m a n n and Riley 3 found that 285 ml of 1 M H N O 3 was necessary for a 2 ml column. These amounts of acid are inconveniently large for routine analysis, and alternative eluants were sought. The efficiencies of various eluants are shown in Table 5. TABLE 5 E L U T I O N O F BISMUTH F R O M AN A N I O N E X C H A N G E RESIN 6 cm x 0.5 cm diameter column of Bio Rad AGI-X8 (100-200 mesh) containing 0.104/~g Bi
Eluant 10ml 1 M H N O 3 10 ml 1 M HC10 4 10 ml 3 M HC10¢ 25 ml 3 M HC10 4 20 ml 4 M HC10 4 5 ml H 2 0 , 15 ml 0.5 M N a O H 5 ml H20, 15 ml 0.5 M NaOH, 5 ml HzO , 10 ml 1 M HC10 4
Recover}' of bismuth/°/£ 32 40 69 76 95 I0 100
ANODIC STRIPPING VOLTAMMETRY FOR MARINE BISMUTH
263
Recovery of bismuth was almost complete with 20 ml of 4 M HC104, but a small bismuth blank was introduced by this relatively large amount of perchloric acid. Elution with 15 ml of 0.5 M N a O H followed by 10 ml of 1 M HC104 with a water wash in between was more efficient and gave a negligible blank. Anion exchange separation of bismuth from solutions of dry-ashed marine samples was carried out at a nominal acidity of 0.6 M HC1, although essentially complete retention of bismuth on the resin was achieved for 25 ml samples containing 0.2-1 M HC1. Nitrate and perchlorate ions must be absent from the solution because they prevent the absorption of bismuth on the resin. Addition of 0.5 g NaC10 4 or 0.5 g N a N O 3 per 20 ml of 0.6 M HC1 led to a leakage of 65~o and 46Vo, respectively, of bismuth from the column. SUMMARY
Anodic stripping voltammetry with a polished glassy carbon electrode mercury-plated in situ was used for the determination of bismuth in seawater and a variety of marine organisms. Seawater was found to contain 0.02-0.11 #g Bi 1-1 in surface samples, and biological concentration factors for the organisms studied were in the range < 2 for fish skeleton to 160 for the flesh of the ascidian cunjevoi. Detailed analytical procedures are given and a comprehensive study of analytical procedures and interferences has been made. Anodic stripping voltammetry has unrivalled sensitivity for bismuth and allows this metal to be determined in seawater without any prior separation or concentration. Anion exchange from 0.6 M HC1 was used to separate bismuth from marine organism solutions. Possible errors in the application of ion exchange techniques to seawater analysis are discussed.
REFERENCES I. Noddack and W. Noddack, Ark. Zool., 1 (1940) 32. R. R. Brooks, Analyst, 85 (1960) 745. J. E. Portmann and J. P. Riley, Anal. Chim. Acta, 34 (1966) 201. T. 1~I. Florence, J. Electroanal. Chem., 35 (1972) 237. K. N. Bagdasarov, P. N. Kovalenko and M. A. Shemyakina, Zh. Anal. Khim., 23 (1968) 515. E. E. Pickett and S. R. Koirtyohann, Anal. Chem., 41 (1969) 28A. G. F. Kirkbright, Analyst, 96 (1971) 609. G. H. Morrison (Ed.), Trace Analysis, Interscience, New York, 1965. Gulf General Atomic Inc., Neutron activation analysis sensitivity data sheet. T. M. Florence, J. Electroanal. Chem., 27 (1970) 273. E. Barendrecht in A. J. Bard (Ed.), Electroanalytical Chemistry, Vol. 2, Marcel Dekker, New York, 1967, p. 53. 12 Kh. Z. Brainina, Talanta, 18 (1971) 513. 13 T. M. Florence, Proc. Roy. Aust. Chem. Inst., 39 (1972) 211. 14 T. M. Florence, J. Electroanal. Chem., 26 (1970)293. 15 E. D. Goldberg, W. S. Broecker, M. G. Gross and K. K. Turekian, Radioactivity in the Marine Environment, National Academy of Sciences, Washington, 1971. 16 J. B. Flato, Anal. Chem., 44 (1972) 75A. 17 J. P. Riley and D. Taylor, Anal. Chim. Acta, 40 (1968) 479. 1 2 3 4 5 6 7 8 9 10 11
264
T.M. FLORENCE
18 A. Piro, M. Bernhard, M. Branica and M. Verzi, Proceedings o f a S)wtposium on the Interaction o f Radioactive Contaminants with the Constituents of the Marine Environment, Seattle, 1972 (Int. Atomic Energy Agency Report IAEA-SM-158). 19 M. Branica in Reference Methods for Marine Radioactivity Studies, Int. Atomic Energy Agency Tech. Rept. Series 118, Vienna, 1970.
255
Printed in The Netherlands
D E T E R M I N A T I O N O F B I S M U T H IN MARINE SAMPLES BY A N O D I C STRIPPING VOLTAMMETRY
T. M. FLORENCE
Chemical Technology Division, Australian Atomic Energy Commission, Research Establishment, Lucas Heights, N.S.W. (Australia) (Received 24th July 1973)
INTRODUCTION
Very little information is available about the distribution of bismuth in the marine environment. Noddack and Noddack I used a spectrographic method to estimate a value of 0.2 #g Bi 1- j in Norwegian fjord water, while Brooks 2 applied anion exchange separation of bismuth followed by emission spectrography to a sample of South Atlantic water and found 0.017 pg Bi 1-1. More recently, Portmann and Riley 3 used the ion exchange separation procedure of Brooks followed by dithizone extraction and reported bismuth concentrations of 0.025, 0.039 and 0.033 pg 1-1 in surface waters from the Engfish Channel, the Irish Sea, and the North Atlantic, respectively. A deep water (2000 m) sample from the North Atlantic contained 0.015 pg Bi 1-1. Florence 4 determined bismuth in Pacific Ocean water by direct anodic stripping voltammetry and found 0.0434).21 #g 1- j for surface samples filtered through a 0.45 pm Millipore filter. Insoluble bismuth was in the range <0.005 to 0.011 #g 1-1. No results are available in the literature on the concentration of bismuth in marine organisms, although Florence 4 calculated that, using a value of 0.13 /~g 1-1 for bismuth in seawater, the biological concentration factors (p.p.m. Bi in dried organism//~g ml-1 Bi in seawater) in fish, oysters, abalone and seaweed must be less than 80. The scarcity of data on bismuth in environmental materials is no doubt largely due to the lack of sensitive methods for its determination. Table 1 summarises the maximum sensitivity for bismuth attainable under ideal conditions by a variety of analytical techniques, and shows that the recently developed thinfilm anodic stripping method 1° is 200 times more sensitive than any other technique. Of all the metals amenable to analysis by anodic stripping voltammetry 11-13 bismuth is perhaps the one most easily determined. Not only is the stripping wave sharp and highly sensitive, but because bismuth can be deposited at potentials so positive that most other metals are not deposited, inter-element compound formation in the mercury film is avoided and the method is almost specific for bismuth. This paper describes the application of thin-film anodic stripping voltammetry to the determination of bismuth in seawater and a variety of marine organisms. Seawater is analysed directly, no separation or pre-concentration steps being necessary. With some marine organisms a s h e d . a t 450°C a preliminary
256
T.M. FLORENCE
TABLE 1 SENSITIVITY OF SOME TRACE M E T H O D S FOR BISMUTH
Technique
Limit of detection"/ ugl ~
Reference
Molecular spectrophotometry Atomic absorption (flame) Atomic absorption (carbon rod) Flame emission Neutron activation Arc emission spectrography Spark source mass spectrometry Thin-film anodic stripping voltammetry
l0 b 50 7 500 100 c 25 1
5 6 7 8 9 8 8
0.005
This work
Ideal, interference-free conditions. b 4 c m cells, Cary 16 spectrophotometer. 10 ml sample volume.
separation is required to eliminate residual organic matter, and this was accomplished by a rapid anion exchange procedure. EXPERIMENTAL
Apparatus and reagents The anodic stripping technique described previously 4'1°'14 was used. This involves simultaneous deposition of mercury and trace metals onto a highly polished rotated (2000 rev. rain -1) glassy carbon electrode followed by a rapid anodic voltage scan. All measurements were made at 25 +_0.5°C and with a SCE reference electrode. Reagent-grade nitric, hydrochloric and perchloric acids were found to have a very low bismuth content and were used without further purification. Water was demineralised then distilled from permanganate and stored in polythene bottles.
Preparation of samples Seawater. Samples of surface seawater were collected in high-density polythene bottles which had been soaked in 1 M HC1 for two weeks and then rinsed with water. The seawater was filtered on the day of collection through a 0.45 pm Millipore filter (type H A W P 04700) which had previously been washed with 1 M HC1. The first 500 ml of filtrate was rejected and the remainder stored in acidcleaned 1-1 polythene bottles containing 5 ml of 10 M HC1. The Millipore filters were retained for determination of insoluble bismuth. Analyses were normally completed within 48 h of collection. Marine organisms. The collection of samples and the muffle ashing and wet ashing procedures have been described previously 4. Dissolution of samples muffle-ashed at 450°C was accomplished as follows. A 1.0 g portion of finely-ground ash was weighed into a beaker, then 10 ml water, 10 ml 10 M HC1 and 1 ml 15 M H N O 3 added. The beaker was covered and the solution boiled gently on a hot plate for 30 min. The solution was filtered through a 9 cm No. 542 Whatman paper that had previously been washed with 5 M HC1.
A N O D I C S T R I P P I N G VOLTAMMETRY F O R MARINE BISMUTH
257
The small residue was washed with two 5 ml portions of hot 5 M HC1. The filtrate and washings were evaporated to 1 2 ml, then evaporated twice more after the addition of 5 ml of 10 M HC1. The final volume remaining should be about 2 ml. The residue was dissolved by warming with 20 ml of water, and the solution filtered again through a No. 542 W h a t m a n to remove last traces of silica which could block the ion exchange column. The paper was washed with a few ml of water and the filtrate reserved for ion exchange separation. 1on exchange separation of bismuth. The solution of muffle ash was passed through a 6 crux0.5 cm diameter column of Bio-Rad AGI-X8 (100-200 mesh) anion exchange resin (C1- form) at a flow rate of 0.7 ml min-1. The column was first conditioned by washing with water and passing 10 ml of 0.6 M HC1. After the sample solution had passed through, the column was washed with 10 ml of 0.6 M HC1, then the bismuth eluted by passing, successively, 5 ml H 2 0 , 15 ml 0.5 M N a O H , 5 ml H 2 0 , and 10 ml 1 M HC10 4. The eluants were collected in a 50 ml volumetric flask containing 5 ml of 4 M H C 1 0 4 and 3 ml of 0.6 M HC1. After the addition of 0.1 ml of 1 x 10 - 2 M mercuric nitrate the solution was diluted to volume.
Analytical procedures Seawater. To 50 ml of filtered, acidified (0.05 M HC1) seawater add 0.1 ml of 1 x 10 -2 M mercuric nitrate. Rinse the voltammetric cell and electrodes with some of the sample solution then add 10 ml of sample solution to the cell. Deaerate with helium for 10 min, electrolyse at - 0 . 2 5 V for 5 min and follow with a 5 V min-1 voltage scan to - 0 . 0 3 V. Repeat this deposition and stripping procedure. Finally electrolyse at - 0 . 2 5 V for 30 min and strip as before using a full scale sensitivity of 2 #A. The third stripping peak is used for measurement. The bismuth wave occurs at - 0 . 1 0 V and its height is proportional to concentration. Prepare and analyse a standard in the same way by spiking 50 ml of seawater with 0.1 ml of a 1 x 10 -5 M standard bismuth solution. Muffle-ashed samples. Rinse the cell and electrodes with some of the solution from the ion exchange column and transfer 10 ml to the cell. Analyse as for seawater but use a deposition potential of - 0 . 2 0 V and a deposition time of 10 min for the third scan. The peak occurs at - 0 . 0 5 V. Wet-ashed samples. Finely-ground samples (2.0 g) dried at 105°C were wet ashed with H N O 3 + HC10 4 as described previously 4. Evaporate the H C 1 0 4 to a volume of about 1 ml, cool, add 15 ml water and filter through an acid-washed 9 cm No. 542 W h a t m a n paper. Wash the paper with a few ml of water and collect the filtrate and washings in a 25 ml volumetric flask. Add 2.5 ml of 0.6 M HC1 and 0.05 ml of 1 x 10-2 M mercuric nitrate and dilute to volume. Analyse as described for muffle-ashed samples. Bismuth retained on Millipore filters was determined as described above after wet ashing with 2 ml of 15 M H N O 3 and 2 ml of 72~o HC104. RESULTS AND DISCUSSION
Anodic stripping voltammetry of bismuth Bismuth was found to yield sharp anodic stripping peaks from a variety of
258
T.M. FLORENCE
supporting electrolytes, although maximum sensitivity was obtained in acidic chloride media. In 0.4 M HCIO 4 peak height was independent of chloride concentration in the range 0.02~3.2 M HC1, while at constant hydrochloric acid concentration the sensitivity was little affected by changes in perchlorate concentration. Peak height was directly proportional to bismuth concentration, deposition time and anodic scan rate. The effect of deposition potential on the bismuth stripping peak was investigated in a 0.4 M HC10 4 + 0.06 M HC1 supporting electrolyte. The sensitivity was found to increase by 30% when the deposition potential was decreased from - 0 . 5 V to - 0 . 2 V, which was the optimum potential for maximum sensitivity. In this medium the bismuth peak potential occurs at - 0 . 0 5 V, but in seawater samples acidified with 0.05 M HC1 the peak is at - 0 . 1 0 V so a deposition potential of - 0 . 2 5 V is used. The relatively positive deposition potential used for bismuth ensures that the method will be highly selective. Very few metals can be deposited at - 0 . 2 V, so the possibility of inter-element compound formation in the mercury film is very slight. The following elements gave less than 2% error in the determination of 0.01 p.p.m. Bi when tested at the concentrations shown (in p.p.m.): Ag + (0.1); Cu 2+ (2.5); Ve 3+ (5.0); Pb 2+ (5.0); Sb 3+ (0.05); Sb 5+ (0.1); B r - (100); P O ] (1000). Antimony(III) produces a sharp wave at - 0 . 1 9 V when a deposition potential of - 0 . 2 5 V is used, but very little antimony is accumulated on the electrode if the deposition potential is - 0 . 2 0 V. In any case, antimony in environmental materials is invariably present in the (V) state, which is electrochemically inactive. Very high concentrations of bromide and iodide interfere via their oxidation waves which occur adjacent to the bismuth peak. The only serious interference in the bismuth determination was encountered from tungsten(VI) and molybdenum(VI). These two elements interfere with the determination by anodic stripping voltammetry of many metals, although the interference mechanism is obscure. In the case of bismuth low results are obtained in the presence of molybdenum and tungsten, although the shape of the peak is unchanged (Table 2). Molybdenum and tungsten are unlikely to be present in marine samples at concentrations high enough to have a significant effect on the TABLE 2 INTERFERENCE OF Mo(VI) AND W(VI) IN THE DETERMINATION OF BISMUTH Supporting electrolyteof 0.4 M HC104+0.06 M HCI Concentration/p.p.m.
Error in determination of 0.01 p.p.m. Bi/%
Mo
W
30 10 2
--
--
2
- 30 -15 nil -35
1
-27
--
0.1
-3
ANODIC STRIPPING VOLTAMMETRY FOR MARINE BISMUTH
259
bismuth wave 15, and in any case, spiking the sample with a standard bismuth solution would detect any depression of the peak. With constant tungsten or m o l y b d e n u m concentration the height of the peak is still proportional to bismuth concentration. The recommended anion exchange separation procedure eliminates molybdenum and tungsten. The determination of bismuth by the in s i t u deposited mercury film method was compared using rapid d.c. anodic stripping and differential pulse stripping (Princeton Applied Research 174 polarograph 16). With the hanging mercury drop electrode the differential pulse technique is 5 10 times more sensitive than d.c., but using the ultra-thin mercury film electrode the sensitivities were very similar. In addition, the pulse technique was more affected by traces of surfactants and the stripping peak was broader. There appears to be no advantage in using the instrumentally more complicated pulse method in favour of a simple d.c. voltammeter. Seawa ter
The data obtained for bismuth in New South Wales and Queensland coastal waters are collected in Table 3, and a typical stripping peak for bismuth in TABLE3 BISMUTH IN SURFACE SEAWATER FROM THE PACIFIC OCEAN Sample no.
Coastal source
Bi/#g I 1 Soluble a
1 2 2 3 4 4 5
Botany Bay, NSW Jervis Bay, NSW Jervis Bay, NSW Jibbon Beach, NSW Jibbon Beach, NSW Jibbon Beach, NSW Queensland
0.11, 0.ll 0.045, 0.043 0.045~ 0.018, 0.021 0.041 _+0.003~ 0.038c 0.12, 0.10
Insoluble b
0.002 0.002 <0.001 ~).005 -<0.001
" Duplicate results shown. b Retained by 0.45 #m Millipore filter. c After anion-exchange separation. Corrected for 90% recovery of Bi. a Mean and standard deviation of 7 determinations. seawater is shown in Fig. 1. The results were obtained by direct anodic stripping voltammetry, although two samples were also analysed after separation of the bismuth by anion exchange. The ion exchange procedure was that described earlier with the exception that 50 ml of seawater was adjusted to an acidity of 0.1 M HC1 before passage through the column 3. The relative standard deviation of the direct method was determined by analysing seven aliquots of a seawater containing 0.041 ILg Bi 1 1, and was found to be 7% (Table 3). Anodic stripping voltammetry has the property of measuring only free metal ion plus labile metal complexes, i.e. complexes which will dissociate under the influence of the electrode potential during the time the complex species is near the
260
T.M. FLORENCE
oioov-oiTo . -o¢2o-q-osv -Ts
•
--
"
-
-~.2s
0
OiOOV-0ii0 -0i20
Fig. 1. Anodic stripping voltammetry of bismuth. (1) Seawater containing 0.11 /~g Bi 1-1. Direct determination of bismuth; 15 min deposition time. (2) Standard 4 x 10 -s M Bi, 5 min deposition time. (3) Determination of bismuth in catfish ash (1.0 g) after anion exchange separation; 15 rain deposition time. (4) Determination of bismuth in cunjevoi ash (1.0 g) after anion exchange separation; 10 min deposition time.
electrode surface. To determine whether particulate or inert organic complexes of bismuth which do not contribute to the stripping current are present in seawater, two seawater samples (nos. 3 and 4, Table 3) were oxidised to destroy organic matter before measurement of the bismuth concentration by anodic stripping voltammetry. Oxidation was carried out by addition of hydrogen peroxide and exposure to u.v. light, and also by fuming 50 ml of seawater with 1 ml of H N O 3 plus 1 ml of HC10 4. No increase in the bismuth peak height was observed after either of these oxidation procedures, so it can be assumed that the analytical procedure used measures total bismuth. Seawater acidified with 0.05 M HC1 immediately after filtration and stored in polythene bottles showed no decrease in bismuth concentration for at least three months. The reagent grade hydrochloric acid used in this work had a bismuth content of only 0.05/~g Bi 1-1
Marine organisms The bismuth contents of several marine organisms collected on the New South Wales coast are given in Table 4, together with ashing data and biological concentration factors calculated using a value of 0.05/~g Bi 1 1 for seawater. The bismuth stripping peaks recorded for catfish and cunjevoi are shown in Fig. 1. The relative standard deviation of the determination of bismuth in cunjevoi flesh was 3.8~o (Table 4). To check that bismuth was not lost during dry ashing at 450°C some samples were also wet ashed with HC10 4 plus H N O 3. Table 4 shows that there was no significant difference between samples ashed by the two methods. As a further check on volatility losses of bismuth, an oyster sample which had been
ANODIC STRIPPING VOLTAMMETRY
261
FOR MARINE BISMUTH
TABLE 4 BISMUTH IN MARINE ORGANISMS
Sample
°/o ash/fresh weight ~
Bismuth/#g kg- 1 fresh weight b
Biological concn. factor e
Muffle ashing a'c Seaweed, Ecklonia radiata Seaweed, Hormosira banksii Fish muscle, luderick, Girella tricuspidata Whole fish, bream, Mylio australis Fish muscle, catfish, Plotosus anguillaris Fish skeleton, luderick Abalone flesh, Haliotis ruber Oyster flesh, Crassostrea commereialis (1) Oyster flesh (2) Cunjevoi flesh, Pyura stolonifera (Ascidian)
Wet ashing d
4.5 _+0.3 2.43 _+0.07
---
90 49
3.33
.3.6 _+0.0
3.2
72
3.95
1.25 + 0.05
2.00 7.13
1.54_+0.09 <0.1
<0.1
72 <2
1.72
1.03_+0.1
--
21
5.09 1.70
3.7 _+0.2 4.8 _+0.5
3.9 --
74 96
4.69
8.0 +0.31
7.9
160
10.4 9.2
25
" Ashing temperature of 450°C.
b Results are mean and difference of two separate weighings. c After anion exchange separation of bismuth. d H N O 3 + HC10~" e p.p.b. Bi in fresh sample/#g 1-1 Bi in seawater. I Mean and standard deviation of five separate weighings.
ashed at 450°C for 3 days was analysed, then heated at 450°C for another 3 days. No decrease in bismuth content was observed after the additional ashing time. Interference studies indicated that no inorganic ion, or combination of ions, present at the concentration expected in marine samples should interfere with the determination of bismuth. However, with most samples which had been dry ashed at 450°C an unusual interference effect was present which prevented accurate determination of bismuth by the direct method. With these samples the mercury deposition current decreased with electrolysis time, and successive anodic scans yielded bismuth peaks of diminishing height. This effect could be caused by some substance in solution fouling the electrode surface and decreasing the available electrode area. This substance is probably organic in nature, because the interference does not occur in samples wet ashed by prolonged fuming with nitric and perchloric acids. Dry ashing, however, is the preferred method because of the saving in operator time and the ability to use larger samples. The anion exchange procedure provided a simple and rapid method for separating bismuth from the residual organic matter in dry ashed samples. Recoveries of bismuth spikes added at the start of analysis were 88-95~o for dry ashed samples (including ion exchange separation) and 95-100~o for wet ashing.
262
T.M. FLORENCE
Ion exchange separation of bismuth Brooks 2 and P o r t m a n n and Riley 3 investigated the conditions necc~bary for the separation of bismuth from seawater by anion exchange chromatography. Riley and Taylor 17 also described the quantitative retention of bismuth from seawater at p H 9.0 on the chelating resin Chelex-100. A possible source of error in these studies is that the retention of bismuth on the columns was checked by spiking seawater with a standard solution of ionic bismuth and measuring the recovery of the spike. The natural bismuth in seawater is unlikely to exist entirely in a simple ionic f o r m and may show quite different behaviour to an ionic standard Is'19 Piro et al. as found that even after one year, radioactive ionic zinc added to seawater at its natural p H had not come to equilibrium with the zinc complexes present in seawater. To check retention on an anion exchange resin of bismuth naturally present in seawater, 50 ml of filtered seawater adjusted to an acidity of 0.1 M HC1 was passed through a 6 c m x 0.5 cm diameter column of Bio Rad AGI-X8 (100-200 mesh) at 0.7 ml m i n - 1 and the effluent analysed for bismuth by anodic stripping voltammetry. The seawater originally gave a figure of 0.11 ~g Bi 1 1, and the effluent analysed as 0.010_+0.002 /~g Bi 1-1. The effluent from the first column was then passed through a second anion exchange column of similar size, and the new effluent was again found to contain 0.010+0.002 pg Bi 1-1. It appears then that the sample of seawater chosen for this experiment contained 9~'/o of the total bismuth in a form which is not absorbed by the resin at an acidity of 0.1 M HCl. This result can be compared with the > 99~o retention of a bismuth-207 spike reported by P o r t m a n n and Riley 3 when using the same experimental conditions. Inorganic species in seawater are engaged in the most complex equilibria, and careful investigation of any separation method should be made before quantitative recovery of the element sought is assumed. Methods were investigated for eluting bismuth absorbed on the resin from 0.6 M HC1. Bismuth is difficult to remove from anion exchange resins, and Brooks 2 used 1 1 of 0.25 M H N O 3 to elute a 7 ml resin column, while P o r t m a n n and Riley 3 found that 285 ml of 1 M H N O 3 was necessary for a 2 ml column. These amounts of acid are inconveniently large for routine analysis, and alternative eluants were sought. The efficiencies of various eluants are shown in Table 5. TABLE 5 E L U T I O N O F BISMUTH F R O M AN A N I O N E X C H A N G E RESIN 6 cm x 0.5 cm diameter column of Bio Rad AGI-X8 (100-200 mesh) containing 0.104/~g Bi
Eluant 10ml 1 M H N O 3 10 ml 1 M HC10 4 10 ml 3 M HC10¢ 25 ml 3 M HC10 4 20 ml 4 M HC10 4 5 ml H 2 0 , 15 ml 0.5 M N a O H 5 ml H20, 15 ml 0.5 M NaOH, 5 ml HzO , 10 ml 1 M HC10 4
Recover}' of bismuth/°/£ 32 40 69 76 95 I0 100
ANODIC STRIPPING VOLTAMMETRY FOR MARINE BISMUTH
263
Recovery of bismuth was almost complete with 20 ml of 4 M HC104, but a small bismuth blank was introduced by this relatively large amount of perchloric acid. Elution with 15 ml of 0.5 M N a O H followed by 10 ml of 1 M HC104 with a water wash in between was more efficient and gave a negligible blank. Anion exchange separation of bismuth from solutions of dry-ashed marine samples was carried out at a nominal acidity of 0.6 M HC1, although essentially complete retention of bismuth on the resin was achieved for 25 ml samples containing 0.2-1 M HC1. Nitrate and perchlorate ions must be absent from the solution because they prevent the absorption of bismuth on the resin. Addition of 0.5 g NaC10 4 or 0.5 g N a N O 3 per 20 ml of 0.6 M HC1 led to a leakage of 65~o and 46Vo, respectively, of bismuth from the column. SUMMARY
Anodic stripping voltammetry with a polished glassy carbon electrode mercury-plated in situ was used for the determination of bismuth in seawater and a variety of marine organisms. Seawater was found to contain 0.02-0.11 #g Bi 1-1 in surface samples, and biological concentration factors for the organisms studied were in the range < 2 for fish skeleton to 160 for the flesh of the ascidian cunjevoi. Detailed analytical procedures are given and a comprehensive study of analytical procedures and interferences has been made. Anodic stripping voltammetry has unrivalled sensitivity for bismuth and allows this metal to be determined in seawater without any prior separation or concentration. Anion exchange from 0.6 M HC1 was used to separate bismuth from marine organism solutions. Possible errors in the application of ion exchange techniques to seawater analysis are discussed.
REFERENCES I. Noddack and W. Noddack, Ark. Zool., 1 (1940) 32. R. R. Brooks, Analyst, 85 (1960) 745. J. E. Portmann and J. P. Riley, Anal. Chim. Acta, 34 (1966) 201. T. 1~I. Florence, J. Electroanal. Chem., 35 (1972) 237. K. N. Bagdasarov, P. N. Kovalenko and M. A. Shemyakina, Zh. Anal. Khim., 23 (1968) 515. E. E. Pickett and S. R. Koirtyohann, Anal. Chem., 41 (1969) 28A. G. F. Kirkbright, Analyst, 96 (1971) 609. G. H. Morrison (Ed.), Trace Analysis, Interscience, New York, 1965. Gulf General Atomic Inc., Neutron activation analysis sensitivity data sheet. T. M. Florence, J. Electroanal. Chem., 27 (1970) 273. E. Barendrecht in A. J. Bard (Ed.), Electroanalytical Chemistry, Vol. 2, Marcel Dekker, New York, 1967, p. 53. 12 Kh. Z. Brainina, Talanta, 18 (1971) 513. 13 T. M. Florence, Proc. Roy. Aust. Chem. Inst., 39 (1972) 211. 14 T. M. Florence, J. Electroanal. Chem., 26 (1970)293. 15 E. D. Goldberg, W. S. Broecker, M. G. Gross and K. K. Turekian, Radioactivity in the Marine Environment, National Academy of Sciences, Washington, 1971. 16 J. B. Flato, Anal. Chem., 44 (1972) 75A. 17 J. P. Riley and D. Taylor, Anal. Chim. Acta, 40 (1968) 479. 1 2 3 4 5 6 7 8 9 10 11
264
T.M. FLORENCE
18 A. Piro, M. Bernhard, M. Branica and M. Verzi, Proceedings o f a S)wtposium on the Interaction o f Radioactive Contaminants with the Constituents of the Marine Environment, Seattle, 1972 (Int. Atomic Energy Agency Report IAEA-SM-158). 19 M. Branica in Reference Methods for Marine Radioactivity Studies, Int. Atomic Energy Agency Tech. Rept. Series 118, Vienna, 1970.