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Fluorimetric determination of chromium at low levels with 3-hydroxyflavone and application of the method to steels
Analytica Chimica Acta, 183 (1986) 263-267 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Short Communication
FLUORIMETRIC DETERMINATION OF CHROMIUM AT LOW LEVELS WITH 3-HYDROXYFLAVONE AND APPLICATION OF THE METHOD TO STEELS
A. CABRERA-MARTfN*a,
J. S. DURANDb and S. RUBIO-BARROSO
Departamento de Quimica Analitica, C.S.I.C. and Facultad de Cienciaa Q&micas, Universidad Complutense de Madrid, 28040 Madrid (Spain) (Received 27th July 1985)
Summary. The method is based on a 1:l Cr(VI)/3-hydroxyflavone complex which is extracted into benzene at H,, = -0.6. The complex has a pK* value of 7.8 f 0.1. Chromium is determined in the range 10-650 ng ml-’ with excitation at 368 nm and emission measurement at 530 nm. The relative standard deviation is 6.1% for 332 ng ml-’ chromium (R = 10). The method is applied to the determination of chromium in steels.
Flavonols containing hydroxyl groups at positions 3 and/or 5 are useful for analytical purposes. The relations between the structures of hydroxyflavones, their metal reactivity and the fluorescence of the products have often been studied. Present knowledge indicates that metals (including Zr, Th, Ga, Al, SC, Ge, Nb and Ta) are chelated by adjacent hydroxyl and carbonyl groups and hydroxyflavones not having this configuration are unreactive. In neutral or alkaline solution, the reactive group can be perihydroxycarbonyl or vicinal hydroxyl [l] ; in acidic solution, metals react with a hydroxyl group and an adjacent carbonyl group [2]. The o-hydroxyl group is believed to be responsible for fluorescence, i.e., bonding of metal to oxygen attached to the carbon in the 3- and $-positions produces fluorescence [3]. The ratio of metal to hydroxyflavone in the chelate species is often 1:l or 1:2, but can be higher, and, for a particular metal, can vary with acidity. Some of the species are charged and can be extracted into oxygenated solvents. Even if uncharged, some chelates of polyhydroxyflavones cannot be extracted into solvents such as chloroform [4]. However, the gallium and indium complexes of kaempferol can be extracted into chloroform from perchlorate medium [ 51. 3-Hydroxyflavone has been used in acidic media for the fluorimetric determination of Al, Hf(IV), Sn( IV), W( VI) [ 31, Be [S] , Sb( III) [7] and *Present address: Departamento de Qufmica II, E.T.S. de Ingenieros Industriales, Universidad PolitQnica de Madrid, Jo& Gutierrez Abascal 2, 28006 Madrid, Spain. bPresent address: Departamento de Qufmica Analftica, Facultad de Ciencias Qufmicas, Universidad de1 Pafs Vasco, 20017 San Sebastian, Guipuzcoa, Spain. 0003-2670/86/$03.50
0 1986 Elsevier Science Publishers B.V.
264
boron [8]. It is shown here that chromium(V1) reacts with 3-hydroxyflavone in a 24% (v/v) ethanol/water medium giving a 1:2 non-fluorescent complex with a global dissociation constant (pK) of 10.6 f 0.2. The related pK, values in that medium are pKa2 = -2.4 + 0.1 and pKal = 9.0 + 0.1. The spectrofluorimetric behaviour of 3-hydroxyflavone in the ethanolic medium is studied as well as the reaction in benzene medium at H, = -0.6. A fluorimetric method based on quenching of the reagent fluorescence is proposed to determine chromium in steels. Experimental Reagents. 3-Hydroxyflavone, 5 X lo4 M, was prepared by dissolving 11.9 mg of the solid (Eastman-Kodak) in 100 ml of absolute ethanol. Standard solutions of Cr(VI), 5 X lo4 M, were prepared by dissolving 18.38 mg of potassium dichromate (Merck p.a.) in 250 ml of distilled water. Solutions of cations and anions were prepared from appropriate soluble salts (Merck p.a.), generally nitrates or sodium salts, with initial concentration of 0.1 M. Apparatus. Fluorescence intensities were measured with a Perkin-Elmer model MPF-44A spectrofluorimeter, with l-cm quartz cells and a 150-W xenon-arc source. Standard fluorescent samples no. 1 and no. 2 (Perkin-Elmer) were used to standardize the source intensity daily. The pH values were measured with a Metrohm E-516 meter provided with a combined glass/calomel electrode (EA-120). Procedure. In an extraction tube, 8 ~1 of 5 X lo4 M 3-hydroxyflavone and the appropriate volume of 5 X lo4 M chromium(V1) were mixed with enough perchloric acid to reach Ho = -0.6 in the final volume of 4 ml. For calibration, 0.8 ml of 60% perchloric acid was added. Then, the chromium complex was extracted into 4 ml of benzene, after shaking during 1 min. The fluorescence intensity at h,, = 358 nm and A,, = 530 nm, with both slits at 4 nm and the sensitivity at S X 10, was measured against a blank. The measurements are stable during 72 h if no irradiation occurs. Results and discussion Acid-base characteristics of 3-hydroxyflavone in the excited state. In a preliminary study, the fluorescence intensity of the reagent (in 24% ethanol/ water) was found to increase swiftly as the concentration was increased from 10” to lo4 M. At concentrations exceeding 5 X lo4 M, the reagent precipitated, therefore the concentration chosen for the recommended procedure was lo* M. Absorption and emission spectra were recorded at several pH values. In acidic media, the wavelength excitation maximum occurs at 354 nm (HL species) but in alkaline media, this is shifted to 410 nm because of dissociation of the hydroxyl group to give the unprotonated ligand. The emission maximum occurs at 525 nm in acidic media with excitation at 354 nm, and at 516 nm in alkaline media with excitation at 410 nm. In strongly acidic media, maximum emission occurs at 435 nm with excitation at 383 nm because the HzL+ species is formed.
265
Quenching of the fluorescence intensity with time was observed in all media, the decrease being 11.6% after 60 min of irradiation. The temperature has a great influence; above 25°C the intensity decreases, below 15°C it increases. The intensity remains constant in the range 15-25%. Ionic strength did not affect the fluorescence intensity up to 0.5 M in sodium perchlorate or nitrate. Determination of the apparent pK, of 3-hydroxyflavone. The related acidity constants were calculated from plots of the variation of fluorescence intensity vs. the concentration of proton in the medium [9], The fluorescence intensities were measured at A,, = 354, 410 and 383 nm (characteristic of HL, L- and HzL+ species, respectively) and A,, = 520 nm for pH l-13 and 435 nm for the Hammett acidity zone (Fig. 1). For the equilibria HzL+ * HL + H+ (K&) and HL + L- + H+ (K$), the values obtained were pK$ = -3.45 k 0.05 and pKzl = 8.85 k 0.05 at 20°C and ionic strength 0.1 M. These pK,* values in the excited state are different from those found for the ground state, which were pKaz = -2.40 f 0.10 and pKal = 9.00 * 0.10 because the acid-base properties of HL are different in the excited and ground states, particularly in strongly acidic media. Spectrofluorimetric study of the chromium(IV) system in benzene medium. In a 24% (v/v) ethanol/water medium, chromium(V1) did not affect the fluorescence of the neutral species of 3-hydroxyflavone. Less polar media were therefore tested and benzene proved to be the most suitable. The efficiency of extraction of 3-hydroxyflavone and its Cr(V1) complex was studied by determining the ligand and Cr(V1) concentrations remaining in the aqueous layer. The extraction efficiency was found to be almost 100% at
lr
0 -Ho
I
’
0
4
6
l2
pH
Fig. 1. Variation of the fluorescence intensity of 3-hydroxyflavone (HL) as a function of [H+]. [HL] = 1.0 x 10” M. Medium: 24% ethanol/water (v/v). Curves: (1) Aex = 383 nm, hem= 435 nm; (2) A, = 354 nm, Aem = 520 nm; (3) h, = 410 nm, A, = 520 nm.
266
pH <9 for the ligand; at pH >9 the extraction decreased because of ionization of the ligand. The extraction efficiency for the chromium(V1) complex was found to be near 100% when H,, = -0.6. A shaking time of 1 min sufficed to reach equilibrium between the aqueous and organic layers. The fluorescence emission of the complex in the benzene layer was measured at 530 nm with excitation at 358 nm (4-nm slit-widths) after extraction from solution at H, = -0.6 (in perchloric acid). The fluorescence intensities after 60 min of irradiation were stable for <5 min. The fluorescence quenching observed was 5% after 5 and 12% after 30 min. The recorded spectra of the complex at different acidities were similar to those for the ligand itself in the same media; they differed only in the intensities measured, the fluorescence being quenched when the complex was formed. The most suitable acidity was H, = -0.6 in perchloric acid. At lower acidities, the complex seemed not to be formed. Stoichiometry and stability constant. The stoichiometry of the Cr(V1) complex was established by the mole ratio and continuous variations methods, under the experimental conditions giving maximum emission. The composition of the complex was 1:l. The apparent dissociation constant in the excited state ~was calculated as pK * = 7.8 f 0.1, at 20°C and p = 0.1. In other work, it was found that the complex in the ground state had a stoichiometry (1:2 Cr(VI)/L) and infrared spectroscopy indicated that the uncharged ligand present in the ground state becomes a zwitterion in the excited state. This is in agreement with reported data on 4-methyl-7-hydroxycoumarin [lo]. It seems likely that the zwitterionic species reacts with HCrOi to form a sevenmembered ring complex. Calibration graph. A linear relationship between the fluorescence intensity and the Cr(V1) concentration was found for the range 10-650 nm ml-‘; the correlation coefficient was 0.991. The detection limit for a signal/noise ratio of 3 [ll] was 10 ng ml-’ Cr(V1). To examine the statistics of the method, a 325 ng ml-’ Cr(V1) solution was taken through the procedure 10 times; the mean result was 332 + 45.6 ng ml-’ at the 95% probability level, the relative error being 4.4%, and the relative standard deviation 6.1%. Interferences. The interferences of foreign substances were studied by mixing 335 ng 1-l Cr(VI), the foreign substance and enough perchloric acid Ions or sodium hydroxide to give H,, = -0.6 before the benzene extraction. which did not interfere at loo-fold molar concentrations compared to Cr(V1) were BO;, SO;-, SO’,-, SCN-, PO:-, F-, ClO;, ClO;, Cl-, NO;, tartrate, Li, Na, K, NH:, Al, As(V), Pb(II), Cd, Zn, W(VI), Th(IV), Os(VIII), Se(V1) or Te(V1). There was no interference at lo-fold molar levels from Cu(II), Be, Sr, Ni(II), Co(II), Ga, Mn(II), Ce(III), U(VI), Tl(1) or oxalate. Equimolar concentrations of Zr, Mo(VI), La, Bi, Fe(II1) and Pd(I1) were tolerated. Excess of fluoride masked the interference of lOO-fold molar amounts of Fe(III), Sn(IV), Zr, U(V1) and Ga. Iron(II1) could also be masked by 15% phosphoric acid. Interference was defined as any variation in the fluorescence
267 TABLE 1 Determination of chromium in steels Sample
lb 2c 3e
Error
Chromium content (%) Certified
Founda
2.72 0.70 1.18
2.60 * 0.3 0.62 * 0.2 1.15 + 0.1
-0.12 -0.08 -0.03
aAverage of five determinations with standard deviation. bChromium-nickel steel (Hoepfner Gebr., Hamburg) containing 0.29% MO and 0.66% Ni. CSteel F-154 (CENIM, Madrid) containing 0.11% C, 0.415% Mn, 0.108% Si, 0.0141% P, 0.0129% S and 2.889% Ni. dSteel F-122 (CENIM, Madrid) containing 0.33% C, 0.57% Mn, 0.26% Si, 0.015% P, 0.012% S and 4.18% Ni.
intensity measured exceeding 2.5 times the standard deviation obtained for the measurement of Cr(V1) alone. Application to the determination of chromium in steels. The proposed method was applied to the determination of chromium in various steels (Table 1). The samples (0.5-1.0 g) were dissolved in 50 ml of acid mixture (3:l HCL/HNOB for sample 1 and 4:3:1 HN03/HC104/HC1 for samples 2 and 3). The solutions obtained were diluted to exactly 250 ml and aliquots were treated with 0.1 N permanganate in 50% phosphoric acid in order to oxidize chromium to Cr(V1) and to mask iron. The excess of permanganate was then reduced by addition of 1% sodium azide. These solutions were suitable for application of the method proposed above. The results for chromium are shown in Table 1. REFERENCES 1 M. KatyaI, TaIanta, 15 (1968) 95. 2 T. Kanno, Japan Analyst (London), 10 (1961) 8. 3 M. Katyai and S. Prakash, TaIanta, 24 (1977) 367. 4 E. B. Sandell and H. Onishi, Photometric Determination of Traces of Metals, Part 1, 4th edn., InterscienceWiIey, New York, 1978, p. 325. 5 B. S. Garg and R. P. Singh, TaIanta, 18 (1971) 761. 6 T. Hayashi, K. Hara, S. Kawai and T. Ohno, Chem. Pharm. Bull., 18 (1970) 1112. 7 A. Mukara, E. Omae and T. Suzuki, Bunseki Kagaku, 29 (1980) 780; Chem. Abs., 94 (1981) 95139t. 8 W. Tkacz and L. Pszonicki, Chem. Anal., 22 (1977) 1013. 9 T. Forster, Z. Elektrochem., 54 (1950) 42. 10 G. Takatan, R. J. Juneau and S. G. Schuhnan, Anal. Chim. Acta, 64 (1972) 1044. 11 J. D. Winefordner, W. J. McCarthy and P. A. St. John, J. Chem. Educ., 44 (1967) 80.
Short Communication
FLUORIMETRIC DETERMINATION OF CHROMIUM AT LOW LEVELS WITH 3-HYDROXYFLAVONE AND APPLICATION OF THE METHOD TO STEELS
A. CABRERA-MARTfN*a,
J. S. DURANDb and S. RUBIO-BARROSO
Departamento de Quimica Analitica, C.S.I.C. and Facultad de Cienciaa Q&micas, Universidad Complutense de Madrid, 28040 Madrid (Spain) (Received 27th July 1985)
Summary. The method is based on a 1:l Cr(VI)/3-hydroxyflavone complex which is extracted into benzene at H,, = -0.6. The complex has a pK* value of 7.8 f 0.1. Chromium is determined in the range 10-650 ng ml-’ with excitation at 368 nm and emission measurement at 530 nm. The relative standard deviation is 6.1% for 332 ng ml-’ chromium (R = 10). The method is applied to the determination of chromium in steels.
Flavonols containing hydroxyl groups at positions 3 and/or 5 are useful for analytical purposes. The relations between the structures of hydroxyflavones, their metal reactivity and the fluorescence of the products have often been studied. Present knowledge indicates that metals (including Zr, Th, Ga, Al, SC, Ge, Nb and Ta) are chelated by adjacent hydroxyl and carbonyl groups and hydroxyflavones not having this configuration are unreactive. In neutral or alkaline solution, the reactive group can be perihydroxycarbonyl or vicinal hydroxyl [l] ; in acidic solution, metals react with a hydroxyl group and an adjacent carbonyl group [2]. The o-hydroxyl group is believed to be responsible for fluorescence, i.e., bonding of metal to oxygen attached to the carbon in the 3- and $-positions produces fluorescence [3]. The ratio of metal to hydroxyflavone in the chelate species is often 1:l or 1:2, but can be higher, and, for a particular metal, can vary with acidity. Some of the species are charged and can be extracted into oxygenated solvents. Even if uncharged, some chelates of polyhydroxyflavones cannot be extracted into solvents such as chloroform [4]. However, the gallium and indium complexes of kaempferol can be extracted into chloroform from perchlorate medium [ 51. 3-Hydroxyflavone has been used in acidic media for the fluorimetric determination of Al, Hf(IV), Sn( IV), W( VI) [ 31, Be [S] , Sb( III) [7] and *Present address: Departamento de Qufmica II, E.T.S. de Ingenieros Industriales, Universidad PolitQnica de Madrid, Jo& Gutierrez Abascal 2, 28006 Madrid, Spain. bPresent address: Departamento de Qufmica Analftica, Facultad de Ciencias Qufmicas, Universidad de1 Pafs Vasco, 20017 San Sebastian, Guipuzcoa, Spain. 0003-2670/86/$03.50
0 1986 Elsevier Science Publishers B.V.
264
boron [8]. It is shown here that chromium(V1) reacts with 3-hydroxyflavone in a 24% (v/v) ethanol/water medium giving a 1:2 non-fluorescent complex with a global dissociation constant (pK) of 10.6 f 0.2. The related pK, values in that medium are pKa2 = -2.4 + 0.1 and pKal = 9.0 + 0.1. The spectrofluorimetric behaviour of 3-hydroxyflavone in the ethanolic medium is studied as well as the reaction in benzene medium at H, = -0.6. A fluorimetric method based on quenching of the reagent fluorescence is proposed to determine chromium in steels. Experimental Reagents. 3-Hydroxyflavone, 5 X lo4 M, was prepared by dissolving 11.9 mg of the solid (Eastman-Kodak) in 100 ml of absolute ethanol. Standard solutions of Cr(VI), 5 X lo4 M, were prepared by dissolving 18.38 mg of potassium dichromate (Merck p.a.) in 250 ml of distilled water. Solutions of cations and anions were prepared from appropriate soluble salts (Merck p.a.), generally nitrates or sodium salts, with initial concentration of 0.1 M. Apparatus. Fluorescence intensities were measured with a Perkin-Elmer model MPF-44A spectrofluorimeter, with l-cm quartz cells and a 150-W xenon-arc source. Standard fluorescent samples no. 1 and no. 2 (Perkin-Elmer) were used to standardize the source intensity daily. The pH values were measured with a Metrohm E-516 meter provided with a combined glass/calomel electrode (EA-120). Procedure. In an extraction tube, 8 ~1 of 5 X lo4 M 3-hydroxyflavone and the appropriate volume of 5 X lo4 M chromium(V1) were mixed with enough perchloric acid to reach Ho = -0.6 in the final volume of 4 ml. For calibration, 0.8 ml of 60% perchloric acid was added. Then, the chromium complex was extracted into 4 ml of benzene, after shaking during 1 min. The fluorescence intensity at h,, = 358 nm and A,, = 530 nm, with both slits at 4 nm and the sensitivity at S X 10, was measured against a blank. The measurements are stable during 72 h if no irradiation occurs. Results and discussion Acid-base characteristics of 3-hydroxyflavone in the excited state. In a preliminary study, the fluorescence intensity of the reagent (in 24% ethanol/ water) was found to increase swiftly as the concentration was increased from 10” to lo4 M. At concentrations exceeding 5 X lo4 M, the reagent precipitated, therefore the concentration chosen for the recommended procedure was lo* M. Absorption and emission spectra were recorded at several pH values. In acidic media, the wavelength excitation maximum occurs at 354 nm (HL species) but in alkaline media, this is shifted to 410 nm because of dissociation of the hydroxyl group to give the unprotonated ligand. The emission maximum occurs at 525 nm in acidic media with excitation at 354 nm, and at 516 nm in alkaline media with excitation at 410 nm. In strongly acidic media, maximum emission occurs at 435 nm with excitation at 383 nm because the HzL+ species is formed.
265
Quenching of the fluorescence intensity with time was observed in all media, the decrease being 11.6% after 60 min of irradiation. The temperature has a great influence; above 25°C the intensity decreases, below 15°C it increases. The intensity remains constant in the range 15-25%. Ionic strength did not affect the fluorescence intensity up to 0.5 M in sodium perchlorate or nitrate. Determination of the apparent pK, of 3-hydroxyflavone. The related acidity constants were calculated from plots of the variation of fluorescence intensity vs. the concentration of proton in the medium [9], The fluorescence intensities were measured at A,, = 354, 410 and 383 nm (characteristic of HL, L- and HzL+ species, respectively) and A,, = 520 nm for pH l-13 and 435 nm for the Hammett acidity zone (Fig. 1). For the equilibria HzL+ * HL + H+ (K&) and HL + L- + H+ (K$), the values obtained were pK$ = -3.45 k 0.05 and pKzl = 8.85 k 0.05 at 20°C and ionic strength 0.1 M. These pK,* values in the excited state are different from those found for the ground state, which were pKaz = -2.40 f 0.10 and pKal = 9.00 * 0.10 because the acid-base properties of HL are different in the excited and ground states, particularly in strongly acidic media. Spectrofluorimetric study of the chromium(IV) system in benzene medium. In a 24% (v/v) ethanol/water medium, chromium(V1) did not affect the fluorescence of the neutral species of 3-hydroxyflavone. Less polar media were therefore tested and benzene proved to be the most suitable. The efficiency of extraction of 3-hydroxyflavone and its Cr(V1) complex was studied by determining the ligand and Cr(V1) concentrations remaining in the aqueous layer. The extraction efficiency was found to be almost 100% at
lr
0 -Ho
I
’
0
4
6
l2
pH
Fig. 1. Variation of the fluorescence intensity of 3-hydroxyflavone (HL) as a function of [H+]. [HL] = 1.0 x 10” M. Medium: 24% ethanol/water (v/v). Curves: (1) Aex = 383 nm, hem= 435 nm; (2) A, = 354 nm, Aem = 520 nm; (3) h, = 410 nm, A, = 520 nm.
266
pH <9 for the ligand; at pH >9 the extraction decreased because of ionization of the ligand. The extraction efficiency for the chromium(V1) complex was found to be near 100% when H,, = -0.6. A shaking time of 1 min sufficed to reach equilibrium between the aqueous and organic layers. The fluorescence emission of the complex in the benzene layer was measured at 530 nm with excitation at 358 nm (4-nm slit-widths) after extraction from solution at H, = -0.6 (in perchloric acid). The fluorescence intensities after 60 min of irradiation were stable for <5 min. The fluorescence quenching observed was 5% after 5 and 12% after 30 min. The recorded spectra of the complex at different acidities were similar to those for the ligand itself in the same media; they differed only in the intensities measured, the fluorescence being quenched when the complex was formed. The most suitable acidity was H, = -0.6 in perchloric acid. At lower acidities, the complex seemed not to be formed. Stoichiometry and stability constant. The stoichiometry of the Cr(V1) complex was established by the mole ratio and continuous variations methods, under the experimental conditions giving maximum emission. The composition of the complex was 1:l. The apparent dissociation constant in the excited state ~was calculated as pK * = 7.8 f 0.1, at 20°C and p = 0.1. In other work, it was found that the complex in the ground state had a stoichiometry (1:2 Cr(VI)/L) and infrared spectroscopy indicated that the uncharged ligand present in the ground state becomes a zwitterion in the excited state. This is in agreement with reported data on 4-methyl-7-hydroxycoumarin [lo]. It seems likely that the zwitterionic species reacts with HCrOi to form a sevenmembered ring complex. Calibration graph. A linear relationship between the fluorescence intensity and the Cr(V1) concentration was found for the range 10-650 nm ml-‘; the correlation coefficient was 0.991. The detection limit for a signal/noise ratio of 3 [ll] was 10 ng ml-’ Cr(V1). To examine the statistics of the method, a 325 ng ml-’ Cr(V1) solution was taken through the procedure 10 times; the mean result was 332 + 45.6 ng ml-’ at the 95% probability level, the relative error being 4.4%, and the relative standard deviation 6.1%. Interferences. The interferences of foreign substances were studied by mixing 335 ng 1-l Cr(VI), the foreign substance and enough perchloric acid Ions or sodium hydroxide to give H,, = -0.6 before the benzene extraction. which did not interfere at loo-fold molar concentrations compared to Cr(V1) were BO;, SO;-, SO’,-, SCN-, PO:-, F-, ClO;, ClO;, Cl-, NO;, tartrate, Li, Na, K, NH:, Al, As(V), Pb(II), Cd, Zn, W(VI), Th(IV), Os(VIII), Se(V1) or Te(V1). There was no interference at lo-fold molar levels from Cu(II), Be, Sr, Ni(II), Co(II), Ga, Mn(II), Ce(III), U(VI), Tl(1) or oxalate. Equimolar concentrations of Zr, Mo(VI), La, Bi, Fe(II1) and Pd(I1) were tolerated. Excess of fluoride masked the interference of lOO-fold molar amounts of Fe(III), Sn(IV), Zr, U(V1) and Ga. Iron(II1) could also be masked by 15% phosphoric acid. Interference was defined as any variation in the fluorescence
267 TABLE 1 Determination of chromium in steels Sample
lb 2c 3e
Error
Chromium content (%) Certified
Founda
2.72 0.70 1.18
2.60 * 0.3 0.62 * 0.2 1.15 + 0.1
-0.12 -0.08 -0.03
aAverage of five determinations with standard deviation. bChromium-nickel steel (Hoepfner Gebr., Hamburg) containing 0.29% MO and 0.66% Ni. CSteel F-154 (CENIM, Madrid) containing 0.11% C, 0.415% Mn, 0.108% Si, 0.0141% P, 0.0129% S and 2.889% Ni. dSteel F-122 (CENIM, Madrid) containing 0.33% C, 0.57% Mn, 0.26% Si, 0.015% P, 0.012% S and 4.18% Ni.
intensity measured exceeding 2.5 times the standard deviation obtained for the measurement of Cr(V1) alone. Application to the determination of chromium in steels. The proposed method was applied to the determination of chromium in various steels (Table 1). The samples (0.5-1.0 g) were dissolved in 50 ml of acid mixture (3:l HCL/HNOB for sample 1 and 4:3:1 HN03/HC104/HC1 for samples 2 and 3). The solutions obtained were diluted to exactly 250 ml and aliquots were treated with 0.1 N permanganate in 50% phosphoric acid in order to oxidize chromium to Cr(V1) and to mask iron. The excess of permanganate was then reduced by addition of 1% sodium azide. These solutions were suitable for application of the method proposed above. The results for chromium are shown in Table 1. REFERENCES 1 M. KatyaI, TaIanta, 15 (1968) 95. 2 T. Kanno, Japan Analyst (London), 10 (1961) 8. 3 M. Katyai and S. Prakash, TaIanta, 24 (1977) 367. 4 E. B. Sandell and H. Onishi, Photometric Determination of Traces of Metals, Part 1, 4th edn., InterscienceWiIey, New York, 1978, p. 325. 5 B. S. Garg and R. P. Singh, TaIanta, 18 (1971) 761. 6 T. Hayashi, K. Hara, S. Kawai and T. Ohno, Chem. Pharm. Bull., 18 (1970) 1112. 7 A. Mukara, E. Omae and T. Suzuki, Bunseki Kagaku, 29 (1980) 780; Chem. Abs., 94 (1981) 95139t. 8 W. Tkacz and L. Pszonicki, Chem. Anal., 22 (1977) 1013. 9 T. Forster, Z. Elektrochem., 54 (1950) 42. 10 G. Takatan, R. J. Juneau and S. G. Schuhnan, Anal. Chim. Acta, 64 (1972) 1044. 11 J. D. Winefordner, W. J. McCarthy and P. A. St. John, J. Chem. Educ., 44 (1967) 80.