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Selenium
Chapter 44. Selenium Selenium (Se, at. mass 78.96) forms selenide, selenite and selenate ions, in the oxidation states -II, IV, and VI, respectively. Selenium(IV) compounds are the most stable. Selenium dioxide sublimes readily (unlike TeO2). On dissolution in nitric acid, selenium is oxidized to Se(IV). Strong oxidizing agents (e.g., aqua regia) oxidize Se to Se(VI). Moderate reducing agents reduce Se(IV) and Se(VI) to the element. Selenium compounds are more easily reduced and less easily oxidized than the corresponding tellurium compounds.
44.1. Methods of separation and preconcentration 44.1.1. Distillation Selenium is usually separated by distillation as the volatile selenium bromide (SeBr4) or chloride (SeCI4) [1,2]. Selenium is distilled from concentrated HBr medium [in the presence of bromine to prevent the reduction of Se(IV)] and from concentrated HC1. Perchloric or sulphuric acid is added to the still, and distillation is continued until white fumes of H2SO4 or HC104 appear. Passage of nitrogen through the liquid promotes the distillation. During the distillation, Te remains quantitatively in the still, but As, Ge, and Sb are distilled with the selenium. Selenium may be separated from various non-volatile materials as the volatile SeO2, which forms when a stream of oxygen is passed over the sample in a tube heated to 1,000~ The SeO2 sublimed onto the cold part of the tube is dissolved and determined [3,4]. A special apparatus has been proposed for the separation of selenium as SeO2 [5]. The volatile hydrogen selenide, HzSe, is also used for separation of selenium [6].
44.1.2. Precipitation Selenium is readily separated by reduction to the element with SnC12, $02, hypophosphite, or hydrazine [7]. Arsenic [8,9] and tellurium [10] are suitable collectors for traces of Se. When selenium is precipitated from 1-8 M HC1 with SO2, the following are wholly or partly reduced to the element: Te, Au, Pt, Pd, Hg, Bi, Sb, Sn, and Cu. Traces of selenium can be co-precipitated with Fe(III), La, Mn, or aluminium hydroxide [11-13]. Se(VI) has been co-precipitated as PbSeO4 together with PbSO4 [14]. When samples mixed with NazCO3 and MgO are ignited at 800~ a sinter is formed, from which water leaches Se(VI), while sparingly soluble MgTeO4 remains in the solid residue [ 15].
44.1.3. Extraction and ion-exchange The selenium(W) chloride complex in 6-7 M HC1 reacts with methyl ethyl ketone to form a compound which can be extracted with chloroform. Such extraction separates Se from Te [16]. Complexes of selenium with xanthate [17], 4-nitro-o-phenylenediamine (toluene) [18], and DDTC [19,20] have also been used in extractive separation of Se from many elements.
380
44. Selenium
Selenium(IV) has been selectively retained on an anion-exchange column [21 ], and on a cation-exchanger modified with Bismuthiol II [22] or with Bismuthiol(II)-sulphonic acid [23]. Sorption of Se (along with Te) on a polyurethane foam has also been applied [24]. The ion-exchange methods for separating Se and Te are discussed in Chapter 49.
44.2. Methods of determination The sensitive method based on 3,3'-diaminobenzidine is widely used. Selenium is determined either in the aqueous medium or after extraction with toluene. The method based on the coloured sol of elemental selenium is much less sensitive. Spectrophotometric methods of determining selenium have been reviewed [25,26].
44.2.1.3,3'-Diaminobenzidine method Selenium(W) reacts with 3,3'-diaminobenzidine (DAB) in acid medium to form the yellow piazselenol, which is sparingly soluble in water and which is utilized for spectrophotometric determination of Se [27-29] (formula 44.1). HzN
NH~
H~N
~Se
The colour reaction is carried out in 0.1 M HC1, and the time necessary for colour development in the aqueous pseudo-solution is 50 min. In the extractive spectrophotometric method [36], the time for reaction at pH 2-3 (in the presence of formic acid) is 30 min, after which the solution is neutralized to pH 6-7, and the piazselenol is extracted into toluene. The colour reaction may be accelerated by heating the solution. Within the pH range 5-10, the distribution coefficient of piazselenol between toluene and water is high, and one portion of toluene extracts practically all the selenium complex into the organic phase. The free reagent (DAB) is also extracted. Related solvents such as benzene and xylene may be substituted for toluene. The two absorption maxima of piazselenol occur at 340 and 420 nm. Since DAB absorbs strongly at 340 nm but negligibly at 420 nm, absorbances are measured at 420 nm. The molar absorptivity of the toluene solution of piazselenol at 420 nm is 1.02.104 (sp. abs. 0.13). This method is specific for selenium. Tellurium does not react with DAB, but V(V) and Fe(III) oxidize DAB to give coloured oxidation products. Iron(III) can be masked with fluoride or phosphate. EDTA is used as masking agent to prevent the precipitation of metals in the neutral medium. Substances capable of reducing selenium to the element interfere in the determination of selenium by the 3,3'-diaminobenzidine method.
Reagents 3,3'-Diaminobenzidine hydrochloride (DAB), 0.5% solution in boiled and cooled water. The solution is stable for a few hours, then turns brown under the influence of atmospheric oxygen. Standard selenium solution: 1 mg/ml. Dissolve in water 1.4050 g of SeO2 (resublimed and stored over P205), and dilute the solution with water in a volumetric flask to 1 litre.
44.2. Methods of determination
381
Procedure To the sample solution containing not more than 100 ~tg of Se in a volume o f - 2 0 ml, add 2 ml of 10% formic acid solution and 2.5 ml of DAB, and then adjust the pH to 2.0-2.5. Let the solution stand for 30 min, then neutralize with ammonia to pH 6-7. Extract the piazselenol by shaking for 1 min with two portions of toluene. Dilute the extract with toluene to the mark in a 25-ml standard flask, and measure the absorbance at 420 nm against water or a reagent blank solution.
Note. If metal ions are present (A1, Bi, Cu, Ni, etc.), add at the start of the procedure 1-5 ml of 5% EDTA solution as masking agent. To mask Fe(III), add also NaF (0.05-0.2 g, depending on the amount of Fe). 44.2.2. Selenium sol m e t h o d Reduction to a brown-yellow sol of elemental selenium in acid medium containing a protective colloid has been made the basis of a simple but rather insensitive method for determining selenium. Suitable protective colloids for preventing coagulation of the selenium sol are gum arabic, gelatine, and poly(vinyl alcohol). Tin(II) chloride, ascorbic acid, thiourea, or hydrazine are used to reduce selenium(W). In 3-4 M HC1 solutions, SnCI2 rapidly reduces Se(IV) in the cold. Depending on the reducing agent and acid strength, pseudo-solutions of different colour are obtained. The molar absorptivity of the selenium sol obtained with SnCI2 in 3 M HC1 containing poly(vinyl alcohol) is 1.7.103 at 400 nm (a = 0.022). Towards longer wavelengths, the absorptivity of the sol decreases; in the ultraviolet it increases. At 325 nm the absorptivity is twice as great, and at 450 nm half as great, as that at 400 nm. Interference in this method for determining selenium comes from Te, Hg, Au, and platinum metals, all of which are easily reduced to the element.
Reagents Tin(H) chloride, SnC12.2H20, 20% solution in 3 M HC1. Standard selenium solution: 1 mg/ml. Preparation as in Section 44.2.1. Poly(vinyl alcohol), 2% solution.
Procedure Distillation separation of Se. To the sample solution of selenium(iv) in a 50-ml still, add just enough conc. HC1 to bring its concentration to 7 M. Add 10 ml of H2SO4 (1+1) and a few fragments of porous porcelain, and connect the still to a condenser, the end of which is immersed in a small amount of 2 M HC1 in the receiver. Distil until white fumes of H2804 appear in the still. To the cooled still, add 5 ml of conc. HC1 and distil again until white fumes appear. The receiver should be cooled with ice-water. Determination of Se. To the sample solution (in -3 M HC1) containing not more than 0.5 mg of Se in a volume of--15 ml, add 3 ml of poly(vinyl alcohol) solution, and mix well. Add 1.0 ml of the SnCI2 solution with stirring. Measure the absorbance of the coloured pseudo-solution at 400 nm, vs. water as the reference.
382
44. Selenium
44.2.3. Other methods Similarly to 3,3'-diaminobenzidine, other aromatic o-diamines also react with selenium(IV) in HC1 medium. The o-phenylenediamine method [30,31] is more sensitive than the DAB method. The following reagents have been proposed for selenium: N-methyl-o-phenylenediamine (e = 1.9-104 at 346 rim) [32], 2-aminodiphenylenediamine [33], 4-nitro1,2-diaminobenzene [34], 4,5-diamino-2,6-dimercaptopyrimidine [35], 1,2-diamino-4chlorobenzene [36], 4,5,6-triaminopyrimidine [37], 3,4-diaminobenzoic acid [38], and 2,3diaminonaphthalene [39,40]. Among the organosulphur compounds proposed for the spectrophotometric determination of selenium, are: Bismuthiol II (known as a reagent for Te) [41], 1,4diphenylthiosemicarbazide [42], 2-mercaptoethanol [43], dithio-oxamide (rubeanic acid) [44], and DDTC [45]. Dithizone has been a basis for a sensitive method of determining Se (~ - 7.4.104) [46]. The mechanism of the reaction between Se(IV) and dithizone is still the subject of contradictory opinions [47,48]. In a sensitive, indirect method Se(IV, VI) is reduced by Cr(II) to hydrogen selenide which, on passing in a stream of nitrogen through an alkaline solution of Fe(CN)63-, reduces the latter to Fe(CN)64- [Se(-II) --+ Se(IV)]. The ferrocyanide formed reacts with the 1,10phenanthroline complex of Fe(III) to give an equivalent amount of the complex Fe(phen)32+. The absorbance of this complex is measured at 508 nm (e = 6.8.104) [49]. The molar absorptivity increases to ~ - 1.4.105 0Vmax= 535), if bathophenanthroline is used instead of 1,10-phenanthroline. In another indirect method Se(IV) oxidizes ferrocene to the ferricenium ion which is oxidized to Fe(III), then reduced to Fe(II), to be determined finally by the colour reaction with 1,10-phenanthroline (~ = 4.2-104) [50]. Selenium has been determined with 5,5-dimethyl-l,3-cyclohexanedione [51], 6-amino1-hydroxynaphthalenesulphonic acid [52,53], and 1-aminonaphthalene-7-sulphonic acid [54]. Determinations of Se involved also the following dyes: Rhodamine B [55], Methylene Blue [56], Xylenol Orange [57], and Rhodamine 6G (by the amplification method, in iodide medium, after oxidation of iodide to iodine, and reaction of the IO3- with the dye) [58]. Some spectrophotometric methods for Se determination are based on its catalytic effect on the redox reactions of various organic compounds [59-65].
44.3. Analytical applications The 3,3'-diaminobenzidine method has been applied for determination of Se in biological materials [28,66], soils [67], air [68], silicates [11], sulphide ores [1], copper [8,14,18], organic substances [69], lead [8,14], steel [29], antimony and bismuth tellurides [70], thin Cd-Se films [71], silver chloride and uranium oxide [12]. Aromatic diamines were used for determining selenium in biological materials [35,36], environmental samples [72], sewage [73], copper [38], non-ferrous metals alloys [31], semiconductors [35], sulphur [38], commercial sulphuric acid [30]. Selenium has been determined in tellurium with the aid of dithizone [46]. Selenium present in anodic slimes has been determined after the reaction with iodide and extraction of the liberated iodine [74]. In pharmaceutical products selenium was determined with the use of 1-aminonaphthalene-7-sulphonic acid [54].
References
References 1. Barcza L., Zsindely S., Z. Anal. Chem., 199, 10,117 (1964). 2. Rybakov A.A., Ostroumov E.A., Zh. Anal. Khim., 38, 446 (1983). 3. R62afiska B., R62owski J., Mikrochim. Acta, 1984 II, 481. 4. Meyer A., Hofer Ch., T61g G., Z. Anal. Chem., 290, 292 (1978). 5. Han H.B., Kaiser G., T61g G., Anal. Chim. Acta, 128, 9 (1981). 6. Remer D.C., Veillon C., Tokonsbalides P.T., Anal. Chem., 53, 245 (1981). 7. Bye R., Talanta, 30, 993 (1983). 8. Luke C.L., Anal. Chem., 31, 572 (1959). 9. Kujirai O. et al., Talanta, 30, 9 (1983). 10. Voronkova M.A., Sidorenko G.A., Zh. Anal. Khim., 22, 1085 (1967). 11. Chau Y.K., Riley J.P., Anal. Chim. Acta, 33, 36 (1965). 12. Russell B.G. et al., Talanta, 14, 957 (1967). 13. Reichel W., Bleakley B.G.,Anal. Chem., 46, 59 (1974). 14. Jackwerth E., Z. Anal. Chem., 235, 235 (1968). 15. Kniazeva R.N., Kleiman V.Ya., Zavod. Lab., 31,410 (1965). 16. Jordanov N., Futekov L., Talanta, 12, 371 (1965); 13, 163 (1966); 15, 850 (1968). 17. Donaldson E.M., Talanta, 24, 441 (1977). 18. Donaldson E.M., Talanta, 35, 633 (1988). 19. Desai G.R., Paul J., Microchem. J., 22, 176 (1977). 20. Lo J.M., Lin C.C., Yeh S.J.,Anal. Chim. Acta, 272, 169 (1993). 21. Nakayama M., Chikuma M., Tanaka H., Talanta, 30, 455 (1983). 22. Nakayama M. et al., Talanta, 31, 269 (1984). 23. Nakayama M. et al., Talanta, 34, 435 (1987). 24. Stewart I.I., Chow A., Talanta, 40, 1345 (1993). 25. Murashova V.I., Sushkova S.G., Zh. Anal. Khim., 24, 729 (1969). 26. Raptis S.E., Kaiser G., T61g G., Z. Anal. Chem., 316, 105 (1983). 27. Barcza L., Mikrochim. Acta, 1964, 967. 28. Cummins L.M., Martin J.L., Maag D.D., Anal. Chem., 37, 430 (1965). 29. Nivibre P., Chim. Anal., 47, 125 (1965). 30. T6ei K., Ito K., Talanta, 12, 773 (1965). 31. Shkrobot E.P., Shebarshina N.I., Zavod. Lab., 35, 417 (1969). 32. Kasterka B., Chem. Anal. (Warsaw), 25, 215 (1980). 33. Kasterka B., Mikrochim. Acta, 1989 I, 337. 34. Stibilj V., Dermelj M., Franko M., Byrne A.R., Anal. Sci., 10, 789 (1994). 35. Izquierdo A., Prat M.D., Aragones L., Analyst, 106, 720 (1981). 36. Nbve J., Hanocq M., Molle L., Mikrochim. Acta, 1980 I, 41,259. 37. Bodini M.E., Alzamora O.E., Talanta, 30, 409 (1983). 38. Kasterka B., Dobrowolski J., Chem. Anal. (Warsaw), 32, 749 (1987). 39. Huang X.R. et al., Fresenius'J. Anal. Chem., 354, 195 (1996). 40. Ramachandran K.N., Kumar G.S., Talanta, 43, 1711 (1996). 41. Navratil O., Sorfa J., Coil. Czech. Chem. Comm., 34, 975 (1969). 42. Sushkova S.G., Murashova V.I., Zh. Anal. Khim., 21, 1475 (1966). 43. Afsar H., Apak R., Tor I., Analyst, 114, 1319 (1989). 44. Lebed' N.B., Pantaler R.P., Zh. Anal. Khim., 41, 2224 (1986). 45. Warner D.A., Paul J., Microchem. J., 20, 292 (1975). 46. Kasterka B., Dobrowolski J., Chem. Anal. (Warsaw), 15, 303 (1970). 47. Stary J. et al., Anal. Chim. Acta, 57, 393 (1971).
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44. Selenium
48. Campbell A.D., Yahaya A.H., Anal. Chim. Acta, 119, 171 (1980). 49. Bode H., Schulze K., Z. Anal. Chem., 327, 154 (1987). 50. Kamaya M., Murakami T., Ishii E., Talanta, 34, 664 (1987). 51. Bodini M.E. et al., Talanta, 37, 439 (1990). 52. Ramachandran K.N., Kaveeshwar R., Gupta V.K., Talanta, 40, 781 (1993). 53. Manish R., Ramachandran K.N., Gupta V.K., Talanta, 41, 1623 (1994). 54. Pyrzyfiska K., Anal. Sci., 13, 629 (1997). 55. Liu S., Zhou G., Huang Z., Talanta, 37, 749 (1990). 56. Bernal J.L. et al., Talanta, 37, 931 (1990). 57. Amin A.S., Zareh M.N., Anal. Lett., 29, 2177 (1996). 58. Ramesh A., Ramakrishna T.V., Subramanian M.S., Bull. Chem. Soc. Jpn., 67, 2121 (1994). 59. Ensafi A.A., Dehaghi G.B., Anal. Lett., 28, 335 (1995). 60. Ensafi A.A., Afldaami A., Massoumi A., Anal. Chim. Acta, 232, 351 (1990). 61. Safavi A., Afldaami A., Anal. Lett., 28, 1095 (1995). 61a. Afkhami A., Mosaed F., Zh. Anal. Khim., 54, 1271 (1999). 62. Shiundu P.M., Wade A.P., Anal. Chem., 63, 692 (1991). 63. Sanchez-Pedreno C., Albero M.I., Garcia M.S., Saez A., Talanta, 38, 677 (1991). 64. Parham H., Shamsipur M., Bull. Chem. Soc. Jpn., 64, 3067 (1991). 65. Gokmen I.G., Abdelqader E., Analyst, 119, 703 (1994). 66. Cummins L.M. et al., Anal. Chem., 36, 382 (1964). 67. Stanton R.E., McDonald A.J., Analyst, 90, 497 (1965). 68. West P.W., Cimeoman C., Anal. Chem., 36, 2013 (1964). 69. Domenech R., Chim. Anal., 51,440 (1969). 70. Cheng K.L., Goydish B.L.,Anal. Chem., 35, 1965 (1963). 71. Marczenko Z., Mojski M., Czarnecka I., Chem. Anal. (Warsaw), 18, 189 (1973). 72. Ramachandran K.N., Kumar G.S., Talanta, 43, 1711 (1996). 73. Kasterka B., Chem. Anal. (Warsaw), 37, 361 (1992). 74. Somer G., Ekmekci G., Anal. Sci., 13, 205 (1997).
44.1. Methods of separation and preconcentration 44.1.1. Distillation Selenium is usually separated by distillation as the volatile selenium bromide (SeBr4) or chloride (SeCI4) [1,2]. Selenium is distilled from concentrated HBr medium [in the presence of bromine to prevent the reduction of Se(IV)] and from concentrated HC1. Perchloric or sulphuric acid is added to the still, and distillation is continued until white fumes of H2SO4 or HC104 appear. Passage of nitrogen through the liquid promotes the distillation. During the distillation, Te remains quantitatively in the still, but As, Ge, and Sb are distilled with the selenium. Selenium may be separated from various non-volatile materials as the volatile SeO2, which forms when a stream of oxygen is passed over the sample in a tube heated to 1,000~ The SeO2 sublimed onto the cold part of the tube is dissolved and determined [3,4]. A special apparatus has been proposed for the separation of selenium as SeO2 [5]. The volatile hydrogen selenide, HzSe, is also used for separation of selenium [6].
44.1.2. Precipitation Selenium is readily separated by reduction to the element with SnC12, $02, hypophosphite, or hydrazine [7]. Arsenic [8,9] and tellurium [10] are suitable collectors for traces of Se. When selenium is precipitated from 1-8 M HC1 with SO2, the following are wholly or partly reduced to the element: Te, Au, Pt, Pd, Hg, Bi, Sb, Sn, and Cu. Traces of selenium can be co-precipitated with Fe(III), La, Mn, or aluminium hydroxide [11-13]. Se(VI) has been co-precipitated as PbSeO4 together with PbSO4 [14]. When samples mixed with NazCO3 and MgO are ignited at 800~ a sinter is formed, from which water leaches Se(VI), while sparingly soluble MgTeO4 remains in the solid residue [ 15].
44.1.3. Extraction and ion-exchange The selenium(W) chloride complex in 6-7 M HC1 reacts with methyl ethyl ketone to form a compound which can be extracted with chloroform. Such extraction separates Se from Te [16]. Complexes of selenium with xanthate [17], 4-nitro-o-phenylenediamine (toluene) [18], and DDTC [19,20] have also been used in extractive separation of Se from many elements.
380
44. Selenium
Selenium(IV) has been selectively retained on an anion-exchange column [21 ], and on a cation-exchanger modified with Bismuthiol II [22] or with Bismuthiol(II)-sulphonic acid [23]. Sorption of Se (along with Te) on a polyurethane foam has also been applied [24]. The ion-exchange methods for separating Se and Te are discussed in Chapter 49.
44.2. Methods of determination The sensitive method based on 3,3'-diaminobenzidine is widely used. Selenium is determined either in the aqueous medium or after extraction with toluene. The method based on the coloured sol of elemental selenium is much less sensitive. Spectrophotometric methods of determining selenium have been reviewed [25,26].
44.2.1.3,3'-Diaminobenzidine method Selenium(W) reacts with 3,3'-diaminobenzidine (DAB) in acid medium to form the yellow piazselenol, which is sparingly soluble in water and which is utilized for spectrophotometric determination of Se [27-29] (formula 44.1). HzN
NH~
H~N
~Se
The colour reaction is carried out in 0.1 M HC1, and the time necessary for colour development in the aqueous pseudo-solution is 50 min. In the extractive spectrophotometric method [36], the time for reaction at pH 2-3 (in the presence of formic acid) is 30 min, after which the solution is neutralized to pH 6-7, and the piazselenol is extracted into toluene. The colour reaction may be accelerated by heating the solution. Within the pH range 5-10, the distribution coefficient of piazselenol between toluene and water is high, and one portion of toluene extracts practically all the selenium complex into the organic phase. The free reagent (DAB) is also extracted. Related solvents such as benzene and xylene may be substituted for toluene. The two absorption maxima of piazselenol occur at 340 and 420 nm. Since DAB absorbs strongly at 340 nm but negligibly at 420 nm, absorbances are measured at 420 nm. The molar absorptivity of the toluene solution of piazselenol at 420 nm is 1.02.104 (sp. abs. 0.13). This method is specific for selenium. Tellurium does not react with DAB, but V(V) and Fe(III) oxidize DAB to give coloured oxidation products. Iron(III) can be masked with fluoride or phosphate. EDTA is used as masking agent to prevent the precipitation of metals in the neutral medium. Substances capable of reducing selenium to the element interfere in the determination of selenium by the 3,3'-diaminobenzidine method.
Reagents 3,3'-Diaminobenzidine hydrochloride (DAB), 0.5% solution in boiled and cooled water. The solution is stable for a few hours, then turns brown under the influence of atmospheric oxygen. Standard selenium solution: 1 mg/ml. Dissolve in water 1.4050 g of SeO2 (resublimed and stored over P205), and dilute the solution with water in a volumetric flask to 1 litre.
44.2. Methods of determination
381
Procedure To the sample solution containing not more than 100 ~tg of Se in a volume o f - 2 0 ml, add 2 ml of 10% formic acid solution and 2.5 ml of DAB, and then adjust the pH to 2.0-2.5. Let the solution stand for 30 min, then neutralize with ammonia to pH 6-7. Extract the piazselenol by shaking for 1 min with two portions of toluene. Dilute the extract with toluene to the mark in a 25-ml standard flask, and measure the absorbance at 420 nm against water or a reagent blank solution.
Note. If metal ions are present (A1, Bi, Cu, Ni, etc.), add at the start of the procedure 1-5 ml of 5% EDTA solution as masking agent. To mask Fe(III), add also NaF (0.05-0.2 g, depending on the amount of Fe). 44.2.2. Selenium sol m e t h o d Reduction to a brown-yellow sol of elemental selenium in acid medium containing a protective colloid has been made the basis of a simple but rather insensitive method for determining selenium. Suitable protective colloids for preventing coagulation of the selenium sol are gum arabic, gelatine, and poly(vinyl alcohol). Tin(II) chloride, ascorbic acid, thiourea, or hydrazine are used to reduce selenium(W). In 3-4 M HC1 solutions, SnCI2 rapidly reduces Se(IV) in the cold. Depending on the reducing agent and acid strength, pseudo-solutions of different colour are obtained. The molar absorptivity of the selenium sol obtained with SnCI2 in 3 M HC1 containing poly(vinyl alcohol) is 1.7.103 at 400 nm (a = 0.022). Towards longer wavelengths, the absorptivity of the sol decreases; in the ultraviolet it increases. At 325 nm the absorptivity is twice as great, and at 450 nm half as great, as that at 400 nm. Interference in this method for determining selenium comes from Te, Hg, Au, and platinum metals, all of which are easily reduced to the element.
Reagents Tin(H) chloride, SnC12.2H20, 20% solution in 3 M HC1. Standard selenium solution: 1 mg/ml. Preparation as in Section 44.2.1. Poly(vinyl alcohol), 2% solution.
Procedure Distillation separation of Se. To the sample solution of selenium(iv) in a 50-ml still, add just enough conc. HC1 to bring its concentration to 7 M. Add 10 ml of H2SO4 (1+1) and a few fragments of porous porcelain, and connect the still to a condenser, the end of which is immersed in a small amount of 2 M HC1 in the receiver. Distil until white fumes of H2804 appear in the still. To the cooled still, add 5 ml of conc. HC1 and distil again until white fumes appear. The receiver should be cooled with ice-water. Determination of Se. To the sample solution (in -3 M HC1) containing not more than 0.5 mg of Se in a volume of--15 ml, add 3 ml of poly(vinyl alcohol) solution, and mix well. Add 1.0 ml of the SnCI2 solution with stirring. Measure the absorbance of the coloured pseudo-solution at 400 nm, vs. water as the reference.
382
44. Selenium
44.2.3. Other methods Similarly to 3,3'-diaminobenzidine, other aromatic o-diamines also react with selenium(IV) in HC1 medium. The o-phenylenediamine method [30,31] is more sensitive than the DAB method. The following reagents have been proposed for selenium: N-methyl-o-phenylenediamine (e = 1.9-104 at 346 rim) [32], 2-aminodiphenylenediamine [33], 4-nitro1,2-diaminobenzene [34], 4,5-diamino-2,6-dimercaptopyrimidine [35], 1,2-diamino-4chlorobenzene [36], 4,5,6-triaminopyrimidine [37], 3,4-diaminobenzoic acid [38], and 2,3diaminonaphthalene [39,40]. Among the organosulphur compounds proposed for the spectrophotometric determination of selenium, are: Bismuthiol II (known as a reagent for Te) [41], 1,4diphenylthiosemicarbazide [42], 2-mercaptoethanol [43], dithio-oxamide (rubeanic acid) [44], and DDTC [45]. Dithizone has been a basis for a sensitive method of determining Se (~ - 7.4.104) [46]. The mechanism of the reaction between Se(IV) and dithizone is still the subject of contradictory opinions [47,48]. In a sensitive, indirect method Se(IV, VI) is reduced by Cr(II) to hydrogen selenide which, on passing in a stream of nitrogen through an alkaline solution of Fe(CN)63-, reduces the latter to Fe(CN)64- [Se(-II) --+ Se(IV)]. The ferrocyanide formed reacts with the 1,10phenanthroline complex of Fe(III) to give an equivalent amount of the complex Fe(phen)32+. The absorbance of this complex is measured at 508 nm (e = 6.8.104) [49]. The molar absorptivity increases to ~ - 1.4.105 0Vmax= 535), if bathophenanthroline is used instead of 1,10-phenanthroline. In another indirect method Se(IV) oxidizes ferrocene to the ferricenium ion which is oxidized to Fe(III), then reduced to Fe(II), to be determined finally by the colour reaction with 1,10-phenanthroline (~ = 4.2-104) [50]. Selenium has been determined with 5,5-dimethyl-l,3-cyclohexanedione [51], 6-amino1-hydroxynaphthalenesulphonic acid [52,53], and 1-aminonaphthalene-7-sulphonic acid [54]. Determinations of Se involved also the following dyes: Rhodamine B [55], Methylene Blue [56], Xylenol Orange [57], and Rhodamine 6G (by the amplification method, in iodide medium, after oxidation of iodide to iodine, and reaction of the IO3- with the dye) [58]. Some spectrophotometric methods for Se determination are based on its catalytic effect on the redox reactions of various organic compounds [59-65].
44.3. Analytical applications The 3,3'-diaminobenzidine method has been applied for determination of Se in biological materials [28,66], soils [67], air [68], silicates [11], sulphide ores [1], copper [8,14,18], organic substances [69], lead [8,14], steel [29], antimony and bismuth tellurides [70], thin Cd-Se films [71], silver chloride and uranium oxide [12]. Aromatic diamines were used for determining selenium in biological materials [35,36], environmental samples [72], sewage [73], copper [38], non-ferrous metals alloys [31], semiconductors [35], sulphur [38], commercial sulphuric acid [30]. Selenium has been determined in tellurium with the aid of dithizone [46]. Selenium present in anodic slimes has been determined after the reaction with iodide and extraction of the liberated iodine [74]. In pharmaceutical products selenium was determined with the use of 1-aminonaphthalene-7-sulphonic acid [54].
References
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