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Boron
Chapter 11. Boron Boron (B, at. mass 10.81) is a metalloid with properties somewhat similar to those of silicon. In chemical analysis, only boron(III) compounds are of importance. Boron forms complexes with fluoride and polyalcohols (e.g., mannitol and glycerol). In an anhydrous medium, boric acid reacts with methanol to form the volatile trimethyl borate.
11.1. Methods of separation and preconcentration 11.1.1. Distillation Distillation of boron as the volatile trimethyl borate (b.p. 65~ is the most common method of isolating boron before its spectrophotometric determination [ 1-4]. When separating small amounts of boron, a quartz distillation apparatus should be used since laboratory glassware contains boron. An anhydrous medium promotes the quantitative formation and distillation of methyl borate (water hydrolyses the ester). The usual procedure is to add methanol and concentrated sulphuric acid to a dried sample, and heat the still in a glycerol- or oil-bath, gradually raising the temperature to about 120~ The distillate is collected in a quartz or platinum receiver containing dilute NaOH solution (see Section 11.2.1). If the sample solution contains fluoride, boron partly distils as volatile BF3. This is prevented by masking fluoride as the stable aluminium complex. Colloidal silica partially traps boron, thus inhibiting its quantitative distillation. Traces of boron have been separated as trimethyl borate by microdiffusion methods [5]. Volatilization of trace amounts of boron as fluoride BF3 is also utilized before its determination [6,7]. Pyrohydrolysis at 900-1,200~ in a stream of superheated steam, decomposes metal borides and enables the separation of boron as the fairly volatile boric acid [8,9].
11.1.2. Extraction With the tetraphenylarsonium cation, the fluoroborate anion forms an ion-pair, which can be extracted with chloroform. Quantitative conversion of boron into BF4-, and subsequent quantitative extraction are ensured by using an excess of fluoride (at pH 2-3), and by leaving the solution to react for sufficient time before extraction [7]. Boric acid is often extracted from acidic solutions (1-6 M HC1) with 2-ethyl-l,3-hexanediol in CHC13 [10-14], with 2,2,4-trimethyl- 1,3-pentanediol in CHC13 [ 15], or with 2-methyl-2,4-pentanediol in CHC13 [16] or in MIBK [17].
11.1.3 Ion-exchange and other methods From a weakly acid solution A1F63-, 8042-, PO4 3-, and NO3- can be retained on anionexchangers, while boric acid, being only very slightly dissociated, is eluted. Borate ions can be absorbed on anion-exchangers only in neutral or alkaline media. Strongly basic anionexchangers retain traces of boron as BF4- from dilute solutions of hydrofluoric acid [18]. The
122
11. Boron
boric acid-mannitol complex can also be retained on anion-exchangers [19]. In the analysis of a number of materials (e.g., silicon- and germanium compounds and volatile reagents) traces of boron are pre-concentrated and boron is then separated by volatilization of the matrix. Mannitol, which forms a non-volatile complex with boric acid, is added to retain all the boron present in the residue [20]. Boron is fairly volatile in acidic media. While boron traces are determined in chlorosilanes, it is advisable to add some chlorotriphenylmethane [21], which forms a non-volatile compound with boron thus preventing its volatilization, when the matrix is evaporated. Ref. 21 is not cited. Trace amounts of boron can be concentrated by specific adsorption on a column of Sephadex G-25 [22].
11.2.
Methods of determination
The very sensitive curcumin method is often used for determining trace amounts of boron. The carmine-acid method is much less sensitive. Extraction-spectrophotometric methods based on ion-associates of BF4- with Methylene Blue and other basic dyes are of importance in the determination of boron.
11.2.1. Curcumin method Curcumin is a natural compound extracted from the curcuma root and purified by crystallisation. It has the formula:
HO~CH---CH
\""/ / H3CO
CH
\c / ~ c / II 0
.CH=CH~OH
IH O
\"J/
(11.1)
kOCH,
The reagent, classified as a [3-diketone dissolves, giving a yellow colour, in methanol, ethanol, acetone, and glacial acetic acid. In acid media, curcumin and boron form a violetred 2:1 complex called rosocyanin. The sensitivity of the method and the reproducibility of the results obtained depend on the quality of the curcumin reagent, and on rigorous observance of the reaction conditions (temperature, time, reagent quantities) [23-25]. Commercial curcumin samples differ considerably in quality. Under the most favourable conditions, the molar absorptivity of rosocyanin is 1.8.105 at ~max = 550 nm (a = 16.6). In a modification of the curcumin method, a ternary complex is formed between curcumin, boron, and oxalic acid [26]. The method is more rapid, but it is only about half as sensitive. The ternary complex formed (rubrocurcumin) contains curcumin, boron, and oxalate in the ratio 1:1:1. Numerous elements (e.g., Be, Fe, Ge, Mo, Ti) form coloured complexes with curcumin, and interfere in the determination of boron. Oxidants (e.g., HNO3) , and substances forming stable complexes with boron (e.g., HF), also interfere. In general, therefore, boron is first separated by distillation as trimethyl borate.
11.2. Methods of determination
123
Reagents Curcumin, 0.1% solution in glacial acetic acid (25 mg of curcumin in 25 ml of the solvent). The solution is prepared on the day of use. Standard boron solution: 1 mg/ml. Dissolve 0.5716 g of H3BO3 in water, and dilute the solution with water to 100 ml in a volumetric flask. Mixture of conc. H2804 and glacial CH3COOH (1 + 1). Mix equal volumes of the two acids immediately before use. Alkaline solution. Dissolve 1 g of NaOH in 100 ml water, add 3 g of glycerol. Store the solution in a polyethylene bottle. Methanol. Purify by distillation from solid NaOH in a quartz still. Store in a polyethylene bottle. Quartz distillation apparatus. Distillation flask of capacity 50-75 ml. The distance between the bulb of the distillation flask and the side-arm should be at least 10 cm.
Procedure
Separation of boron by distillation. Evaporate an alkaline sample solution containing a microgram quantity of boron to dryness. If it is necessary to ignite and fuse the residue (e.g., if mannitol is present), a platinum vessel should be used. Add to the solid residue 1-2 ml of conc. H2804, stir with a glass rod, and wash the contents of the vessel with 25 ml of methanol into a distilling flask fitted with a condenser. Immerse the condenser tip in a trapping solution (2 ml of the alkaline solution and 18 ml of H20) contained in a platinum dish. Distil the contents of the still by heating on a glycerol bath; at the end of distillation the temperature should be 120~ After all the methanol has been distilled off, cool the still, add 10 ml of methanol, and repeat the distillation. Determination of boron. Take an aliquot (containing < 1 ~tg B) of the distillate, and evaporate it to dryness in a platinum crucible. Ignite the residue until all the organic matter has been burned off and the mineral residue has been melted. Place the crucible in a water bath at exactly 60~ add from a pipette exactly 2.5 ml of curcumin solution, and keep the vessel on the bath for about 3 min with occasional stirring. To the cooled vessel, add 1 ml of the HzSO4-CH3COOH mixture, and mix thoroughly by swirling the vessel. After 20 min wash the contents with ethanol ( 7 0 % ) into a 25-ml volumetric flask and dilute to the mark with ethanol. Mix the solution, and measure its absorbance at 550 nm, using the blank solution as a reference.
Notes. 1. The blank solution must be prepared very carefully. 2. If boron is determined without distillation as trimethyl borate, any traces of HF or HNO3 must be carefully removed before the addition of curcumin (e.g., by evaporating the solution 2 or 3 times with dilute HC1 in the presence of mannitol).
11.2.2. Carminic acid m e t h o d Carminic acid belongs to the group of boron reagents derived from a-hydroxyanthraquinone. These reagents give coloured complexes with boron in conc. sulphuric acid medium. Boron occurs as the B 3+ cation in conc. H2804, and as BO + in less concentrated H2804. Carminic acid (also called carmine or Carmine Red) is a natural product obtained from cochineal. In concentrated sulphuric acid medium carminic acid reacts with boron(III):
124
11. Boron j B z~ CH3
0
OH
CH3 COICHOHII'CH3
O
"~
B=,
conc.s,s0,
.o- T HOOC
Y 0
T
-o.
OH
.o- T
T
HOOC
0
T OH
-o. (11.2)
The reagent is red (~max -- 520 nm), whereas the boron complex is violet-blue [27]. The molar absorptivity of the complex at 615 nm (vs. reagent solution as the reference) is 5.5.103 (a = 0.51). Boron reacts slowly with carminic acid. The reaction may be accelerated by diluting the H2804 (e.g., to -~92%), but the acid concentration should not be lower than 90%. The absorbance reaches a maximum within 45-60 min, after which it remains constant for a few hours. Oxidising agents and fluoride interfere in the carminic acid method [28-31 ]. Before its determination, boron is normally separated either as volatile trimethyl borate or by other methods. Carminic acid solutions in conc. H2SO4 and glacial CH3COOH were added directly to the chloroform extract of boron with 2,2,4-trimethyl- 1,3-pentanediol [ 15].
Reagents Carminic acid solution: Dissolve 25 mg of the reagent in 100 ml of conc. sulphuric acid. Standard boron solution: 1 mg/ml. Preparation as in Section 11.2.1.
Procedure Place 2.5 ml of acidic solution (-~4 M H2804), containing not more than 25 lag of B, in a 25ml standard flask. Add 5 drops of conc. HC1, 12.5 ml of conc. H2804, and carmine solution up to the mark. Mix the solution thoroughly and set aside for 1 h. Measure the absorbance of the solution at 650 nm, using a blank solution as reference.
Note. If boron is separated by distillation, evaporate the distillate to dryness, and dissolve the residue in 2.5 ml of 4 M H2804. 11.2.3. Methylene Blue method The sensitive method for determination of boron has been based on an extractable ionassociate of the anionic complex BF4-with Methylene Blue (MB) (formula 48.1) [32-35]. Formation of the boron complex in acidic (H2804) solution is not very rapid after the addition of fluoride in excess; it requires some time at room temperature. The sulphuric acid concentration can be 0.2-0.8 M , and concentrations of -~0.4 M for fluoride and --2.10 -4 M for Methylene Blue are suitable. Under these conditions, BF4- is formed in --30 min. Before extraction of the ion-pair, the sample solution should be diluted to reduce the acidity. 1,2Dichloroethane is the most recommended solvent for the extraction of the boron ion-pair; a good quality solvent is important for good extraction. Polyethylene separating funnels must be used because of the hydrofluoric acid medium. The molar absorptivity of the 1,2-dichloroethane solution of the MB-BF4- ion-pair is 8.2.104 (a = 7.6) at 665 nm. Ions giving extractable ion-pairs with MB interfere (e.g., C104-, SCN-). Usually the method is applied after a distillative separation of boron.
11.2. Methods of determination
125
Reagents Methylene Blue (MB), 1.2-10 -3 M (--0.05%) solution. Standard boron solution: 1 mg/ml. Preparation as in Section 11.2.1.
Procedure To a 100-ml separating funnel, add sample solution containing not more than 2.5 /ag of B, then 2.5 ml of 10% NaF solution, and enough H2SO4 to give, finally, 8 ml of the solution, at concentration of about 0.4 M. After 30 min, add 40 ml of water, 10 ml of the MB solution, and shake the solution with two 10-ml portions of 1,2-dichloroethane (shaking time 1 min). Dilute the clear extract to volume with solvent in a 25-ml standard flask, mix well, and measure the absorbance of the solution at 665 nm, using the blank solution as a reference.
11.2.4 Other Methods Besides the Methylene Blue, other spectrophotometric methods, based on ion-associates of anionic boron complexes with basic dyes are used. Extractable associates with BF4- are obtained with Nile Blue A (formula 4.32) [7,36,37], Capri Blue (formula 4.31) [38], Malachite Green (formula 4.26, with Me instead of Et), Chrompyrazole II (CHC13, e = 6.7.104 at 595 nm) [40], etc. In addition to BF4-, other anionic boron complexes, also forming ion-associates with basic dyes, have been applied to determine boron, namely: 2,4-dinitro-l,8-naphthalenediol and Brilliant Green (formula 4.26) (toluene, e - 1.0.105 at 637 nm [41-43], 2,6-dihydroxybenzoic acid, and Malachite Green (chlorobenzene, e - 9.5.104 [5], 2,3-dihydroxynaphthalene and Crystal Violet (benzene, e = 8.8-104 [44]), mandelic acid, and Malachite Green (benzene, e = 6.5.104 ) [45,46], pyrocatechol derivatives, and Ethyl Violet (toluene, e = 1.05-104 [47,48]. The ion-pair of the salicylate complex of boron with ferroin has also been proposed (CHC13) [49]. 1,1'-Dianthrimide (1,1'-dianthraquinonyl amine) reacts with boron in a conc. H2SO4 medium, after heating at 70-90~ for 2-4 h. A complex with the following structure is obtained:
2+
0
O'-'i'-O
0
(11.3)
The colour of 1,1'-dianthrimide in conc. H2804 is olive-green, whereas that of the boron complex is dark blue (e - 1.9.104 at 630 nm) [50]. Azomethine H, the product of the condensation of H-acid (1-amino-8-naphthol-3,6disulphonic acid) and salicylaldehyde (formula 11.4) is often used for determining boron. The colour reaction is carried out in acetate buffer (pH --5.2) in the presence of ascorbic acid. The absorbance is measured at 415 nm after 2 h.
126
11. Boron
HO3S N=CH~ (11.4) OH
HO/
HO3S/
11.3. Analytical applications The curcumin method (in either the rosocyanin or rubrocurcumin version) has been applied for determining trace amounts of boron in: biological materials [10], soils and plants [ 17], waters [51 ], silicon [52], chlorosilanes [20], uranium [1,53], zirconium and its alloys [53,54], nickel [55,56], copper alloys [56], cast iron and steel [12,57-59], beryllium and magnesium [53], and phosphates [2]. This method was also used for determining boric acid admixtures (about 0.05%) in powdered boron [11]. Some synthetic compounds having the structure similar to that of curcumin, were used in determining boron in water [60]. The earminie acid method has been applied for determining boron in geological materials [6,13,61], silicon [62], nickel and cobalt [63], magnesite [64], glass [65], and fertilizers [66]. An automatic method has been applied for determining boron in sewage and in river water [67]. The Methylene Blue method has been utilized in determinations of boron in biological materials [33], soils and rocks [68], steels [32], silicon [7,69], copper and its alloys [34,35], and various chemical materials [70,71 ]. Boron has been determined by the 1,1'-dianthrimide method in biological materials [72], dairy products [73], cast iron and steel [44], and nickel alloys [74]. The method with azomethine H has been used for determining boron in plant materials [75], biological samples [76], plants [77], soils [77-79], water [80], sewage [4,81 ], rocks and bituminous [22,55,82], steel [47], copper, nickel, and cobalt alloys [9], boron nitride [83], and fertilisers [84]. Azomethine H has been utilized in automatic determination of boron [81] and in flow injection analysis (FIA) [75]. Boron has also been determined in plants and soils with the use of 4-methoxyazomethine H [85].
References 1. Onishi H., Ishiwatari N., Nagai H., Bull. Chem. Soc. Jpn, 33, 830 (1960). 2. Kocher J., Bull. Soc. Chim. France, 1962, 1247. 3. Landry J.C., Landry M.F., Monnier D., Anal. Chim. Acta, 62, 177 (1972). 4. Monzo J., Pomares F., de la Guardia M., Analyst, 113, 1069 (1988). 5. Shida J., Suzuki H., Abe S.,Anal. Chim. Acta, 169, 349 (1985). 6. Farzaneh A., Troll G., Neubauer W., Z. Anal. Chem., 296, 383 (1979). 7.Grallath E., Tsch6pel P., K61blin G., Stix U., T61g G., Z. Anal. Chem., 302, 40 (1980). 8. McKinley G.J., Wendt H.F., Anal. Chem., 37, 947 (1965). 9. Ciba J., Stankiewicz W., Matusiak H., Z. Anal. Chem., 321, 592 (1985). 10. Mair J. W., Day H.G., Anal. Chem., 44, 2015 (1972). 11. Grotheer E. W., Anal. Chem., 51, 2402 (1979).
References 12. Donaldson E.M., Talanta, 28, 825 (1981). 13. Troll G., Sauerer A.,Analyst, 110, 283 (1985). 14. Betty K.R., Day G.T.,Analyst, 111,455 (1986). 15. Aznarez J., Ferrer A., Rabadan J.M., Marco L., Talanta, 32, 1156 (1985). 16. Aznarez J., Mir J. M.,Analyst., 109, 183 (1984). 17. Aznarez J., Bonilla A., Vidal J.C., Analyst, 108, 368 (1983). 18. Semov M.P., Zh. Anal. Khim., 23, 245 (1968). 19. Barbier Y., Rosset R., Bull. Soc. Chim. France, 1968, 5072. 20. Marczenko Z., Mojski M., Kasiura K., Chem. Anal. (Warsaw), 14, 1331 (1969). 21. Kawasaki K., Higo M., Anal. Chim. Acta, 33, 497 (1965). 22. Yoshimura K., Kariya R., Tarutani T.,Anal. Chim. Acta, 109, I15 (1979). 23. Uppstr6m L.R., Anal. Chim. Acta, 43, 475 (1968). 24. Dyrssen D. W., Novikov Y.P., Uppstr6m L. R., Anal. Chim. Acta, 60, 139 (1972). 25. Quint P., Umland F., Z. Anal. Chem., 295, 269 (1979). 26. Quint P., Umland F., Sommer H.D., Z. Anal. Chem., 285, 356 (1977). 27. Schwing-Weill M.J.,Analusis, 14, 290 (1986). 28. Gupta H.K., Boltz D.F., Mikrochim. Acta, 1974, 415. 29. F16chon J., Kuhnast F.A., Bull. Soc. Chim. France, 1976, 739. 30. Rosenfeld H.J., Selmer-Olsen A.R., Analyst, 104, 983 (1979). 31. Samsoni Z., Szeleczky M.A., Mikrochim. Acta ,1980 I, 445. 32. Blazejak-Ditges D., Z. Anal. Chem., 247, 20 (1969). 33. Yoshino K. et al., Anal. Chem., 56, 839 (1984). 34. Ci~ek Z., Studlarov~ V., Talanta, 31,547 (1984). 35. Kleimenova O.K., Dedkov Yu D., Zavod. Lab., 55, 3 (1989). 36. Gagliardi E., Wolf E., Mikrochim. Acta, 1968, 140. 37. Nicholson R.A., Anal. Chim. Acta, 56, 147 (1971). 38. Buldini P.L., Anal. Chim. Acta, 82, 187 (1976). 39. Xing-Chu Q., Ying-Quan Z., Analusis, 14, 46 (1986). 40. Busev A.I., Yakovlev P. Ya., Kozina G. V., Zh. Anal. Khim., 22, 1227 (1967). 41. Kuwada K., Motomizu S., T6ei K., Anal. Chem., 50, 1788 (1978). 42. T6ei K., Motomizu S., Oshima M., Watari H., Analyst, 106, 776 (1981). 43. Fogg T.R., Duce R.A., Fashing J.L.,Anal. Chem., 55, 2179 (1983). 44. Sato S., Uchikawa S., Anal. Chim. Acta, 143, 283 (1982). 45. Sato S.,Anal. Chim. Acta, 151,465 (1983). 46. Sato S., Talanta, 32, 447 (1985). 47.0shima M., Motomizu S., T6ei K., Anal. Chem., 56, 948 (1984). 48. Oshima M., Shibata K., Gyouten T., Motomizu S., T6ei, Talanta, 35, 351 (1988). 49. Bassett J., Matthews P.J., Analyst, 99, 1 (1974). 50. Gupta H.K., Boltz D.F., Anal. Lett., 4,161 (1971). 51.0stling G.,Anal. Chim. Acta, 78, 507 (1975). 52. Parashar D.C., Sarkar A.K., Singh N., Anal. Lett., 22, 1961 (1989). 53. Hayes M.R., Metcalfe J., Analyst, 87, 956 (1962). 54. Ishikuro M., Kimura J., Bunseki Kaguku, 37, 498 (1988). 55. Andrew T.R., Nichols P.N., Analyst, 91, 664 (1966). 56. Pakalns P., Analyst, 94, 1130 (1969). 57. Tolk A., Tap W.A., Lingerak W.A., Talanta, 16,111 (1969). 58. Thierig D., Z. Anal. Chem. 310, 154 (1982). 59. Vialatte A., A16v~que J., Guinot H., Taupe A.,Analusis, 11,446 (1983). 60. Lambert I.L., Paukstelis J. V., Bruckdorfer R.A., Anal. Chem., 50, 820 (1978).
127
128
11. Boron
61. Kiss E.,Anal. Chim. Acta, 211,243 (1988). 62. Chen J.S., Lin H.M., Yang M.H., Fresenius'J. Anal. Chem., 340, 357 (1991), 63. Norwitz G., Gordon H., Anal. Chim. Acta, 94,175 (1977). 64. Shelton N.F., Reed R.A., Analyst, 101, 396 (1976). 65. Reed R.A., Analyst, 102, 831 (1977). 66. Peterson H.P., Zoromski D.W., Anal. Chem., 44, 1291 (1972). 67. Lionnel L.J., Analyst, 95, 194 (1970). 68. Stanton R.E., McDonald A.J., Analyst, 91, 775 (1966). 69. Lanza P., Buldini P.L., Anal. Chim. Acta, 70, 341 (1974). 70. Beskova E.S., Zhuravlev G.I., Zh. Anal. Khim., 28,1411 (1973). 71. K6the J., Ackerman G., Z. Anal. Chem., 320, 545 (1985). 72. Kaczmarczyk A., Messer J.R., Peirce C.E., Anal. Chem., 43,271 (1971). 73. Raber H., Likussar W., Mikrochim. Acta, 1970, 577. 74. Burke K.E., Albright C.H., Talanta, 13, 49 (1966). 75. Krug F.J. et al., Anal. Chim. Acta, 125, 29 (1981). 76. Ciba J., Chru~ciel A., Fresenius'J. Anal. Chem., 342, 147 (1992). 77. Carrero P., Burguera J.L., Burguera M., Rivas C., Talanta, 40, 1967 (1993). 78. Chen D. et al., Anal. Chim. Acta, 226, 221 (1989). 79. Kaplan D.I., et al., Soil Sci. Soc. Am. J., 54, 708 (1990). 80. Lopez F.J., Gimenez E., Hernandez F., Fresenius' J. Anal. Chem., 346, 984 (1993). 81. Edwards R.A.,Analyst, 105, 139 (1980). 82. Schucker G.D., Magliocca T.S., Yao-Sin S.,Anal. Chim. Acta, 75, 95 (1975). 83. Mikhailovskaya V.S., Martunova L.M., Buyanovskaya A.G., Sinenko Yu.A., Zh. Anal. Khim., 47, 1331 (1992). 84. Hofer A., Brosche E., Heidinger R., Z. Anal. Chem., 253,117 (1971). 85. Zaijun L,. Zhu Z., Jan T.,Anal. Chim. Acta, 402, 253 (1999).
11.1. Methods of separation and preconcentration 11.1.1. Distillation Distillation of boron as the volatile trimethyl borate (b.p. 65~ is the most common method of isolating boron before its spectrophotometric determination [ 1-4]. When separating small amounts of boron, a quartz distillation apparatus should be used since laboratory glassware contains boron. An anhydrous medium promotes the quantitative formation and distillation of methyl borate (water hydrolyses the ester). The usual procedure is to add methanol and concentrated sulphuric acid to a dried sample, and heat the still in a glycerol- or oil-bath, gradually raising the temperature to about 120~ The distillate is collected in a quartz or platinum receiver containing dilute NaOH solution (see Section 11.2.1). If the sample solution contains fluoride, boron partly distils as volatile BF3. This is prevented by masking fluoride as the stable aluminium complex. Colloidal silica partially traps boron, thus inhibiting its quantitative distillation. Traces of boron have been separated as trimethyl borate by microdiffusion methods [5]. Volatilization of trace amounts of boron as fluoride BF3 is also utilized before its determination [6,7]. Pyrohydrolysis at 900-1,200~ in a stream of superheated steam, decomposes metal borides and enables the separation of boron as the fairly volatile boric acid [8,9].
11.1.2. Extraction With the tetraphenylarsonium cation, the fluoroborate anion forms an ion-pair, which can be extracted with chloroform. Quantitative conversion of boron into BF4-, and subsequent quantitative extraction are ensured by using an excess of fluoride (at pH 2-3), and by leaving the solution to react for sufficient time before extraction [7]. Boric acid is often extracted from acidic solutions (1-6 M HC1) with 2-ethyl-l,3-hexanediol in CHC13 [10-14], with 2,2,4-trimethyl- 1,3-pentanediol in CHC13 [ 15], or with 2-methyl-2,4-pentanediol in CHC13 [16] or in MIBK [17].
11.1.3 Ion-exchange and other methods From a weakly acid solution A1F63-, 8042-, PO4 3-, and NO3- can be retained on anionexchangers, while boric acid, being only very slightly dissociated, is eluted. Borate ions can be absorbed on anion-exchangers only in neutral or alkaline media. Strongly basic anionexchangers retain traces of boron as BF4- from dilute solutions of hydrofluoric acid [18]. The
122
11. Boron
boric acid-mannitol complex can also be retained on anion-exchangers [19]. In the analysis of a number of materials (e.g., silicon- and germanium compounds and volatile reagents) traces of boron are pre-concentrated and boron is then separated by volatilization of the matrix. Mannitol, which forms a non-volatile complex with boric acid, is added to retain all the boron present in the residue [20]. Boron is fairly volatile in acidic media. While boron traces are determined in chlorosilanes, it is advisable to add some chlorotriphenylmethane [21], which forms a non-volatile compound with boron thus preventing its volatilization, when the matrix is evaporated. Ref. 21 is not cited. Trace amounts of boron can be concentrated by specific adsorption on a column of Sephadex G-25 [22].
11.2.
Methods of determination
The very sensitive curcumin method is often used for determining trace amounts of boron. The carmine-acid method is much less sensitive. Extraction-spectrophotometric methods based on ion-associates of BF4- with Methylene Blue and other basic dyes are of importance in the determination of boron.
11.2.1. Curcumin method Curcumin is a natural compound extracted from the curcuma root and purified by crystallisation. It has the formula:
HO~CH---CH
\""/ / H3CO
CH
\c / ~ c / II 0
.CH=CH~OH
IH O
\"J/
(11.1)
kOCH,
The reagent, classified as a [3-diketone dissolves, giving a yellow colour, in methanol, ethanol, acetone, and glacial acetic acid. In acid media, curcumin and boron form a violetred 2:1 complex called rosocyanin. The sensitivity of the method and the reproducibility of the results obtained depend on the quality of the curcumin reagent, and on rigorous observance of the reaction conditions (temperature, time, reagent quantities) [23-25]. Commercial curcumin samples differ considerably in quality. Under the most favourable conditions, the molar absorptivity of rosocyanin is 1.8.105 at ~max = 550 nm (a = 16.6). In a modification of the curcumin method, a ternary complex is formed between curcumin, boron, and oxalic acid [26]. The method is more rapid, but it is only about half as sensitive. The ternary complex formed (rubrocurcumin) contains curcumin, boron, and oxalate in the ratio 1:1:1. Numerous elements (e.g., Be, Fe, Ge, Mo, Ti) form coloured complexes with curcumin, and interfere in the determination of boron. Oxidants (e.g., HNO3) , and substances forming stable complexes with boron (e.g., HF), also interfere. In general, therefore, boron is first separated by distillation as trimethyl borate.
11.2. Methods of determination
123
Reagents Curcumin, 0.1% solution in glacial acetic acid (25 mg of curcumin in 25 ml of the solvent). The solution is prepared on the day of use. Standard boron solution: 1 mg/ml. Dissolve 0.5716 g of H3BO3 in water, and dilute the solution with water to 100 ml in a volumetric flask. Mixture of conc. H2804 and glacial CH3COOH (1 + 1). Mix equal volumes of the two acids immediately before use. Alkaline solution. Dissolve 1 g of NaOH in 100 ml water, add 3 g of glycerol. Store the solution in a polyethylene bottle. Methanol. Purify by distillation from solid NaOH in a quartz still. Store in a polyethylene bottle. Quartz distillation apparatus. Distillation flask of capacity 50-75 ml. The distance between the bulb of the distillation flask and the side-arm should be at least 10 cm.
Procedure
Separation of boron by distillation. Evaporate an alkaline sample solution containing a microgram quantity of boron to dryness. If it is necessary to ignite and fuse the residue (e.g., if mannitol is present), a platinum vessel should be used. Add to the solid residue 1-2 ml of conc. H2804, stir with a glass rod, and wash the contents of the vessel with 25 ml of methanol into a distilling flask fitted with a condenser. Immerse the condenser tip in a trapping solution (2 ml of the alkaline solution and 18 ml of H20) contained in a platinum dish. Distil the contents of the still by heating on a glycerol bath; at the end of distillation the temperature should be 120~ After all the methanol has been distilled off, cool the still, add 10 ml of methanol, and repeat the distillation. Determination of boron. Take an aliquot (containing < 1 ~tg B) of the distillate, and evaporate it to dryness in a platinum crucible. Ignite the residue until all the organic matter has been burned off and the mineral residue has been melted. Place the crucible in a water bath at exactly 60~ add from a pipette exactly 2.5 ml of curcumin solution, and keep the vessel on the bath for about 3 min with occasional stirring. To the cooled vessel, add 1 ml of the HzSO4-CH3COOH mixture, and mix thoroughly by swirling the vessel. After 20 min wash the contents with ethanol ( 7 0 % ) into a 25-ml volumetric flask and dilute to the mark with ethanol. Mix the solution, and measure its absorbance at 550 nm, using the blank solution as a reference.
Notes. 1. The blank solution must be prepared very carefully. 2. If boron is determined without distillation as trimethyl borate, any traces of HF or HNO3 must be carefully removed before the addition of curcumin (e.g., by evaporating the solution 2 or 3 times with dilute HC1 in the presence of mannitol).
11.2.2. Carminic acid m e t h o d Carminic acid belongs to the group of boron reagents derived from a-hydroxyanthraquinone. These reagents give coloured complexes with boron in conc. sulphuric acid medium. Boron occurs as the B 3+ cation in conc. H2804, and as BO + in less concentrated H2804. Carminic acid (also called carmine or Carmine Red) is a natural product obtained from cochineal. In concentrated sulphuric acid medium carminic acid reacts with boron(III):
124
11. Boron j B z~ CH3
0
OH
CH3 COICHOHII'CH3
O
"~
B=,
conc.s,s0,
.o- T HOOC
Y 0
T
-o.
OH
.o- T
T
HOOC
0
T OH
-o. (11.2)
The reagent is red (~max -- 520 nm), whereas the boron complex is violet-blue [27]. The molar absorptivity of the complex at 615 nm (vs. reagent solution as the reference) is 5.5.103 (a = 0.51). Boron reacts slowly with carminic acid. The reaction may be accelerated by diluting the H2804 (e.g., to -~92%), but the acid concentration should not be lower than 90%. The absorbance reaches a maximum within 45-60 min, after which it remains constant for a few hours. Oxidising agents and fluoride interfere in the carminic acid method [28-31 ]. Before its determination, boron is normally separated either as volatile trimethyl borate or by other methods. Carminic acid solutions in conc. H2SO4 and glacial CH3COOH were added directly to the chloroform extract of boron with 2,2,4-trimethyl- 1,3-pentanediol [ 15].
Reagents Carminic acid solution: Dissolve 25 mg of the reagent in 100 ml of conc. sulphuric acid. Standard boron solution: 1 mg/ml. Preparation as in Section 11.2.1.
Procedure Place 2.5 ml of acidic solution (-~4 M H2804), containing not more than 25 lag of B, in a 25ml standard flask. Add 5 drops of conc. HC1, 12.5 ml of conc. H2804, and carmine solution up to the mark. Mix the solution thoroughly and set aside for 1 h. Measure the absorbance of the solution at 650 nm, using a blank solution as reference.
Note. If boron is separated by distillation, evaporate the distillate to dryness, and dissolve the residue in 2.5 ml of 4 M H2804. 11.2.3. Methylene Blue method The sensitive method for determination of boron has been based on an extractable ionassociate of the anionic complex BF4-with Methylene Blue (MB) (formula 48.1) [32-35]. Formation of the boron complex in acidic (H2804) solution is not very rapid after the addition of fluoride in excess; it requires some time at room temperature. The sulphuric acid concentration can be 0.2-0.8 M , and concentrations of -~0.4 M for fluoride and --2.10 -4 M for Methylene Blue are suitable. Under these conditions, BF4- is formed in --30 min. Before extraction of the ion-pair, the sample solution should be diluted to reduce the acidity. 1,2Dichloroethane is the most recommended solvent for the extraction of the boron ion-pair; a good quality solvent is important for good extraction. Polyethylene separating funnels must be used because of the hydrofluoric acid medium. The molar absorptivity of the 1,2-dichloroethane solution of the MB-BF4- ion-pair is 8.2.104 (a = 7.6) at 665 nm. Ions giving extractable ion-pairs with MB interfere (e.g., C104-, SCN-). Usually the method is applied after a distillative separation of boron.
11.2. Methods of determination
125
Reagents Methylene Blue (MB), 1.2-10 -3 M (--0.05%) solution. Standard boron solution: 1 mg/ml. Preparation as in Section 11.2.1.
Procedure To a 100-ml separating funnel, add sample solution containing not more than 2.5 /ag of B, then 2.5 ml of 10% NaF solution, and enough H2SO4 to give, finally, 8 ml of the solution, at concentration of about 0.4 M. After 30 min, add 40 ml of water, 10 ml of the MB solution, and shake the solution with two 10-ml portions of 1,2-dichloroethane (shaking time 1 min). Dilute the clear extract to volume with solvent in a 25-ml standard flask, mix well, and measure the absorbance of the solution at 665 nm, using the blank solution as a reference.
11.2.4 Other Methods Besides the Methylene Blue, other spectrophotometric methods, based on ion-associates of anionic boron complexes with basic dyes are used. Extractable associates with BF4- are obtained with Nile Blue A (formula 4.32) [7,36,37], Capri Blue (formula 4.31) [38], Malachite Green (formula 4.26, with Me instead of Et), Chrompyrazole II (CHC13, e = 6.7.104 at 595 nm) [40], etc. In addition to BF4-, other anionic boron complexes, also forming ion-associates with basic dyes, have been applied to determine boron, namely: 2,4-dinitro-l,8-naphthalenediol and Brilliant Green (formula 4.26) (toluene, e - 1.0.105 at 637 nm [41-43], 2,6-dihydroxybenzoic acid, and Malachite Green (chlorobenzene, e - 9.5.104 [5], 2,3-dihydroxynaphthalene and Crystal Violet (benzene, e = 8.8-104 [44]), mandelic acid, and Malachite Green (benzene, e = 6.5.104 ) [45,46], pyrocatechol derivatives, and Ethyl Violet (toluene, e = 1.05-104 [47,48]. The ion-pair of the salicylate complex of boron with ferroin has also been proposed (CHC13) [49]. 1,1'-Dianthrimide (1,1'-dianthraquinonyl amine) reacts with boron in a conc. H2SO4 medium, after heating at 70-90~ for 2-4 h. A complex with the following structure is obtained:
2+
0
O'-'i'-O
0
(11.3)
The colour of 1,1'-dianthrimide in conc. H2804 is olive-green, whereas that of the boron complex is dark blue (e - 1.9.104 at 630 nm) [50]. Azomethine H, the product of the condensation of H-acid (1-amino-8-naphthol-3,6disulphonic acid) and salicylaldehyde (formula 11.4) is often used for determining boron. The colour reaction is carried out in acetate buffer (pH --5.2) in the presence of ascorbic acid. The absorbance is measured at 415 nm after 2 h.
126
11. Boron
HO3S N=CH~ (11.4) OH
HO/
HO3S/
11.3. Analytical applications The curcumin method (in either the rosocyanin or rubrocurcumin version) has been applied for determining trace amounts of boron in: biological materials [10], soils and plants [ 17], waters [51 ], silicon [52], chlorosilanes [20], uranium [1,53], zirconium and its alloys [53,54], nickel [55,56], copper alloys [56], cast iron and steel [12,57-59], beryllium and magnesium [53], and phosphates [2]. This method was also used for determining boric acid admixtures (about 0.05%) in powdered boron [11]. Some synthetic compounds having the structure similar to that of curcumin, were used in determining boron in water [60]. The earminie acid method has been applied for determining boron in geological materials [6,13,61], silicon [62], nickel and cobalt [63], magnesite [64], glass [65], and fertilizers [66]. An automatic method has been applied for determining boron in sewage and in river water [67]. The Methylene Blue method has been utilized in determinations of boron in biological materials [33], soils and rocks [68], steels [32], silicon [7,69], copper and its alloys [34,35], and various chemical materials [70,71 ]. Boron has been determined by the 1,1'-dianthrimide method in biological materials [72], dairy products [73], cast iron and steel [44], and nickel alloys [74]. The method with azomethine H has been used for determining boron in plant materials [75], biological samples [76], plants [77], soils [77-79], water [80], sewage [4,81 ], rocks and bituminous [22,55,82], steel [47], copper, nickel, and cobalt alloys [9], boron nitride [83], and fertilisers [84]. Azomethine H has been utilized in automatic determination of boron [81] and in flow injection analysis (FIA) [75]. Boron has also been determined in plants and soils with the use of 4-methoxyazomethine H [85].
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