(18) Atkinson, R. J.; Posner, A. M.; Quirk, J. P. J. Inorg. Nucl. Chem. 1972.34 (71,2201-11. (19) Bowden, J. W.; Bolland, M. D. A.; Posner, A. M.; Quirk, J. P. Nature (London) 1973,245, 81-3. (20) Russell, J. D.; Patterson, E.; Fraser, E. R.; Farmer, V. C. J. Chem. Soc., Faraday Trans. 1 1975,71, 1623-30. (21) Parfitt, R. L.; Atkinson, R. J.; Smart, R. S. C. Soil Sci. Soc. A m . Proc. 1975,39 (5), 837-41. (22) Breeuwsma, A,; Lyklema, J. J . Colloid Interface Sci. 1973,43 (2), 437-48. (23) Atkinson, R. J.; Parfitt, R. L.; Smart, R. S. C. J. Chem. Soc., Faraday Trans. 1 1974, 70 (8),1472-9. (24) Parfitt. R. L.: Russell. J. D.: Farmer. V. C. J . Chem. Soc.. Faraday Trans. 1 1976, 72 (4), 1082-7. (25) Murphy, J.; Riley, J. P. Anal. Chirn. Acta 1962,27, 31-6. (26) Dousma, J.; de Bruyn, P. L. J . Colloid Interface Sei. 1976,56 (31, 527-39.
(27) Russell, J. D.; Parfitt, R. L.; Fraser, A. R.; Farmer, V. C. Nature (Lortdon) 1974,248, 220-1. (28) , , Breeuwsma. A. Dissertation. Amicultural Universitv. Waeeningen, The Netherlands, 1973. (29) Hingston, F. J.; Posner, A. M.; Quirk, J. P. J . Soil Sci. 1974,25, 16-26. (30) Sigg, L. Dissertation, ETH, Zurich, 1979. (31) Stumm, W.; Sigg, L. Symposium: Begrenzung des PhosphorI
-
I ,
Y
eintrages aus diffusen Quellen in stehenden Gewasser; Wahnbachtalsperrenverband, Sept 1978; Siegburg, BRD. (32) Roefs, W. A. J. HzO 1972,5 (23), 546-8. (33) Lerk, C. F. Dissertation, THD, Delft, 1965. (34) Fillos, J.; Swanson, W. R. J . Water Pollut. Control Fed 1975, 47, 1032-41.
Received for review M a y 14, 1979. Accepted December 26, 1979.
Gas Chromatographic Determination of Selenium( -11, -(VI) in Natural Waters
0), -( IV), and
Hirofumi Uchida, Yasuaki Shimoishi, and Kyoji Tdei' Department of Chemistry, Faculty of Science, Okayama University, 3-1-1 Tsushima-naka, Okayama-shi 700, Japan
Selenium in river water and seawater was determined with electron capture detection (ECD) gas chromatography using 1,2-diamino-3,5-dibromobenzene withodt preconcentration. The oxidation numbers of selenium are -11, 0, IV, and VI. The reagent can react only with selenium(1V) to form 4,6-dibromopiazselenol, which is extracted into toluene and determined by gas chromatography. After selenium(-11,O) is oxidized by bromine and selenium(V1) is reduced by a bromine-bromide solution to the quadrivalent state, the selenium is determined by the same method, and, thus, the contents of selenium(-II,O), -(IV), and -(VI) are calculated separately. The limit of the determination was 2 ng L-I. Also, the preservation of the sample water is discussed. Selenium(V1)was found to be abundant in seawater.
Chau and Riley (1) maintained t h a t selenium in seawater existed predominantly in the form of selenite ion; meanwhile, S i l l h (2) indicated that the selenium was mostly selenate ion. Recently, Sugimura and Suzuki ( 3 )determined the presence of Se(1V) and Se(V1) in seawater in almost equal proportions. On the other hand, Yoshii e t al. ( 4 ) stated t h a t Se(V1) in natural waters was 1.5-20 times as prevalent as Se(1V). In a previous paper ( 5 ) ,total selenium and Se(IV),but not Se(-I1,O) or Se(V1). in natural waters were determined by ECD gas chromatography. The oxidation numbers of selenium are -11, 0, IV, and VI. This paper will report the individual amounts of Se(-II,O), Se(IV),and Se(V1) in natural waters. 1,2-Diamino-3,5-dibromobenzenecan react only with selenium(1V) t o form 4,6-dibromopiazselenol, which can be extracted quantitatively into 1 mL of toluene from 500 mL of sample water ( 5 ) .The selenium(1V) is determined from the peak height of the piazselenol in the gas chromatogram. Selenium(-I1,O) is oxidized to Se(1V) quantitatively by bromine solution and selenium(-II,O,IV) is determined by the same procedure. Selenium(-11,O) is calculated from the difference between Se(1V) and Se(-II,O,IV). Selenium(V1) is reduced to Se(1V) by bromine-bromide redox buffer solution, and, a t the same time, selenium(-I1,O) is oxidized t o the quadrivalent state. Thus, the total selenium can be deter0013-936X/80/0914-0541$01.00/0
mined, and Se(V1) is calculated by deducting selenium(-II,O,IV) from the total selenium. Experimental
Apparatus. A Shimdazu Model GC-5A gas chromatograph, equipped with a "3Ni ECD, was used. A glass column (1m X 3 m m i.d.) was packed with 15% SE-30 on 60-80 mesh Chromo-orb W. The column and detector temperatures were maintained a t 200 and 280 "C, respectively. The nitrogen flow rate was 28 mL min-l. A Shimadzu Model 101 recorder was used at a chart speed of 5 m m min-l. Reagents. 1,2-Diamino-3,5-dibromobenzene Monohydrochloride. The synthesis of the reagent was reported elsewhere ( 5 ) . The reagent (0.6 g) was dissolved in 500 mL of concentrated hydrochloric acid and washed with toluene (25 mL) to remove toluene-soluble matter. The solution was stored in a brown glass bottle and could be used safely for 1 month. S e ( 0 ) Stock Solution ( I mg of Se in 1 mL of CS2) Elemental selenium (500 mg) was dissolved in 500 mL of carbon disulfide. Working solutions were prepared by appropriate dilution. The concentration of the working solution was determined from the calibration graph by gas chromatography after the conversion of Se(0) to Se(1V) by bromine-bromide redox buffer (6). The solution could be used safely for 1 week. Se(ZV) Stock Solution (0 975 mg of Se m L - l ) Selenium dioxide (1.433 g) was dissolved in 1000 mL of distilled water and standardized gravimetrically. The solution was stable for 6 months. Se(V1)Stock Solutmn (1 178 mg of Se m L - ' ) Selenic acid monohydrate (220 mg) was dissolved in 100 mL of distilled water, and working solutions were prepared by appropriate dilution. T h e concentration was determined by gas chromatography, after Se(V1) was reduced to Se(0) and then oxidized t o Se(1V) (6). Procedure. Pretreatment of Sample Waters Concentrated hydrochloric acid should be added to the sample water immediately after sampling (1 mL L-l) and filtered by a membrane filter (pore size 0.45 pm) to remove suspended matter as soon as possible.
@ 1980 American Chemical Society
Volume 14,Number 5, May 1980
541
0
I
1
'
'
4 6 8 10 Volume of bromine solution(m1)
2
Flgure 1. Effect of the volume of bromine solution (3%) on the oxidation of Se(0) to Se(lV): (0)12.5 ng of Se(0) in 500 mL of distilled water plus 25 mL of concentrated hydrochloric acid; ( 0 )blank of distilled water
'
0
°
I
7
02 014 016 018 Volume of brwnine solution(rn1) Figure 3. Effect of the volume of bromine solution (3 %) on the oxidation of Se(0) to Se(lV) coexisting with hydrobromic acid (25 mL): (0)12.5 ng of Se(0) in 500 mL of distilled water plus 50 mL of concentrated hydrochloric acid; ( 0 )blank of distilled water
Y
O
'
7
1'0 15 20 25 30 Volume of hydrobromic acid (ml)
'
Figure 2. Effect of the volume of hydrobromic acid (47%) on the reduction of Se(VI) to Se(lV) after boiling (15 min): (0)23.6 ng of Se(Vl) in 500 mL of distilled water plus 50 mL of concentrated hydrochloric acid; ( 0 )blank of distilled water
S e ( I V )Determination (Procedure 1 ) . In a 500-mL separating funnel, 500 mL of the pretreated sample water and 20 mL of concentrated hydrochloric acid were added and mixed well. The solution was shaken vigorously with 25 mL of toluene to saturate it with toluene and to remove toluene-soluble matter. The aqueous phase was transferred into another 500-mL separating funnel after the phase separation. To the solution 0.12% 1,2-diamino-3,5-dibromobenzene solution (10 mL) was added; the solution was allowed to stand for 2 h. After one milliliter of toluene was added, the solution was vigorously shaken for 5 min, and the 4,6-dibromopiazselenol which formed was extracted. The extract was washed twice with 3 mL of perchloric acid (2 + l),2 pL of the extract was injected into the gas chromatograph, and the peak height of 4,6-dibromopiazselenol was measured. S e (-ZZ,O,IV) Determination (Procedure 2 ) . T o the pretreated sample solution (500 mL) 25 mL of concentrated hydrochloric acid and 2 mL of bromine solution (3%)were added and mixed for 5 min a t room temperature. By this treatment Se(-II,O) is oxidized completely to the quadrivalent state, but Se(V1) is not reduced. Two milliliters of 1M NHpOH-HC1was added to the solution to reduce the excess bromine to bromide. The solution was washed with 25 mL of toluene as mentioned in procedure 1. The determination of Se(-II,O,IV) was the same as for procedure 1. Selenium(-I1,O) is calculated from the difference between the peak heights of the piazselenol by procedures 1 and 2. Se(-ZZ,O,IV,VI) Determination (Procedure 3 ) . T o the pretreated sample water (500 mL), concentrated hydrochloric acid (50 mL) ,47% hydrobromic acid (25 mL), and 3% bromine solution (0.5 mL) were added and boiled gently for 15 min. By this treatment Se(-II,O) is oxidized to Se(IV) and Se(V1) is 542
Environmental Science & Technology
Storage time(days1 Figure 4. Effects of filtration and HCI on sample storage: ( 0 )total selenium content (filtered): (B) total selenium content (not filtered); (0) selenium(1V) content (filtered); (U)selenium(lV) content (not filtered). Sample contains no hydrochloric acid
60h
,8
0
e
xi
F 40.
Y
I%
20. I)
u
=
G-&
0-
reduced to Se(1V). Thus, all selenium is now in the quadrivalent state. After the solution was cooled to room,temperature, 2 mL of 1 M NHZOH.HC1 was added to the solution to reduce the excess bromine. The solution was washed with 25 mL of toluene, and the total selenium was determined by the same method as procedure 1. Selenium(V1) is calculated by deducting Se(-II,O,IV) from Se(-II,O,IV,Vl). Results and Discussion Oxidation of Se( -11,O) to Se(1V). Since 1,2-diamino3,5-dibromobenzene can react only with Se(IV), Se(1V) in natural waters can be determined directly as it stands (procedure 1). Since se(-I1,O) should be oxidized to Se(1V) without reduction of Se(VI),many kinds of oxidizing agents were examined and a bromine solution (3%) was chosen. The
Table 1. Analysis of Simulated Mixtures of 4.6 ng of Se(O), 4.8 ng of Se(IV), and 5.9 ng of Se(VI) peak helght
dlstllled water (500 mL) amount recovered, ng
16.3 15.0 31.2 30.5 51.9 52.2
procedure 1 (Se(lv)) procedure 2 (Se(0) Se(lV)) procedure 3 (total Se)
+
4.9 4.5 9.5 9.3 15.9 16.0
seawater, mL
Se(O),
500 500 500 500 500 500 100 100 100 100 100 100
none 10.8 10.8 10.8 10.8 10.8 none 5.4 5.4 5.4 5.4 5.4
ng
Added Se(0) and Se(0) respectively. a
added SeWI), ng
Se found, a ng
none 5.9 5.9 5.9 5.9 5.9
5.2 14.8 15.7 15.4 15.0 16.0 5.1 15.5 16.1 16.3 16.5 15.4
Oh
16.3 15.7 30.3 29.7 50.5 50.9
102 94 101 99 104 105
Table II. Recovery of Added Selenium added
peak height
recovery,
artificial seawater (500 mL) amount recovered, ng
recovery, %
4.9 4.7 9.2 9.0 15.5 15.6
102 98 98 96 101 102
Table 111. Determinationof Selenium in River Water Se
recovered, ng
recovery,
9.6 10.5 10.2 9.8 10.8
89 97 94 91 100
10.4 11.0 11.2 11.4 10.3
92 97 99 101 91
Yo
+ Se(VI)were determined by procedures 2 and 3,
volume of bromine solution was examined as follows. Carbon disulfide containing 12.5 ng of Se(0) was put into a beaker and dried by heating. Distilled water (500 mL) and concentrated hydrochloric acid (25 mL) were poured into the beaker and a 3% bromine solution was added to oxidi7~Se(0) to Se(1V). The excess bromine should be reduced to bromide by the addition of 1 M NHZOH-HCl (2 mL), because the bromine disturbs the determination by oxidizing the reagent. By this treatment, the oxidation is complete with 1-10 mL of bromine solution (Figure 1) because the peak height coincides with the amount of Se (12.5 ng) on the calibration line within experimental error. For complete oxidation of Se(0) to Se(IV), concentrated hydrochloric acid is necessary (more than 15 mL for 500 mL of water, and it takes more than 30 s a t room temperature). From these results, the determination procedure for Se(-II,O,IV) is decided, as shown in procedure 2. Though the oxidation of Se(-11) by bromine is not carried out, Se(-11) would be converted to Se(1V) by this treatment. Reduction of Se(V1) to Se(1V). In a previous paper (51, Se(V1)was reduced to Se(0) by selenium-free sulfuric acid and titanium trichloride, and then the Se(0) was oxidized to Se(1V) by bromine-bromide redox buffer. The Se(1V) formed was determined by gas chromatography, but the blank value tended to be a little high. Of the many agents used to directly reduce Se(V1) to Se(IV), hydrobromic acid was determined to be the best. T o a mixture of 500 mL of distilled water containing 23.6 ng of Se(V1) and 50 mL of concentrated hydrochloric acid, 10-30 mL of 47% hydrobromic acid was added and boiled for 15 min (Figure 2). Selenium(V1) is reduced quantitatively to Se(1V) by more than 20 mL of hydrobromic acid because the peak height corresponds to 23.6 ng of Se(1V). A volume of more than 40 mL of concentrated hydrochloric acid is necessary for the reduction, and boiling should be for more than 10 min.
sample a
Yoshii River, Okayama Pref., Feb 5, 1979 Takahashi River, Okayama Pref., Feb 5, 1979 Asahi River, Okayama Pref., Feb 5, 1979 Asahi River, Okayama Pref., Feb 17, 1979 a
Se(-ll,O)
ng L-1 Se(lY) Se(Vl)
total Se
12
16
202
231
5
4
7
16
11
