- Email: [email protected]
Fluorimetric determination of iron(III) by a kinetic method
MICROCHEMICAL
JOURNAL
Fluorimetric
31, 50-55 (1985)
Determination
of Iron(lll)
by a Kinetic
Method
A. NAVAS AND F. SANCHEZ ROJAS Department
of Analytical
Chemistry,
Faculty
of Sciences, The University,
Mdlaga-4,
Spain
Received January 19, 1983 A sensitive and relatively interference-free method for the kinetic determination of iron(III) is described. The method is based on the catalytic action of this ion on the autoxidation process of 4,8-diamino-l,5-dihydroxyanthraquinone-2,6-disuifonate (disodium salt), to produce a strong pink fluorescence that increases with time. The reaction is monitored at 585 nm, when excited at 525 nm, and the initial rate method is applied to perform the analytical procedure. The influence of reaction variables and the effect of foreign ions are discussed. Iron(II1) contents between 0.1 and 1 pg ml-’ can be determined with R.S.D. of *3.7%. 8 1985 Academic Press, Inc.
INTRODUCTION The use of molecular fluorescence as an analytical technique for the determination of transition or paramagnetic metal ions has been limited traditionally by the structural complexity of these ions that facilitates spin-orbit coupling and energy dissipation of the excited states by nonradiative processes. In practice, the use of this technique is limited to those species that participate in redox processes acting upon a fluorogenic substrate whose concentration in the medium is proportional to those species of analytical interest which brought about the transformation (6, 13). Few methods for the fluorimetric determination of iron have been described, and most of these are based on quenching phenomena (Z-4). One of these is based on kinetic measurements that involve following the rate of decrease of the fluorescence intensity which accompanies the ferric ion-catalyzed reaction of a carboxymethyl derivative of a stilbenedisulfonic acid with hydrogen peroxide (2). To date only two fluorimetric methods for the determination of iron(W), based on the appearance of fluorescence, have been reported (I 1, 12). Both involved the oxidation of 1,4-diamino-2,3-dihydroanthraquinone which results in a fluorescent product. One of these employed kinetic measurements (12), and the other when the equilibrium has been reached (II). Iron(III), a paramagnetic ion, can be determined by fluorimetric technique, based on its catalytic action in the autoxidation process of 4,8-diamino-l,Sdihydroxyanthraquinone-2,6-disulfonate (disodium salt). The quenching effect of iron(III) on the fluorescence of the solution is not critical because of the relative nature of the initial rate measurements. This paper discusses the more interesting aspects of the development of a kinetic-fluorimetric method for the determination of traces of iron(II1). 50 0026-265X/85 $1.50 Copyright 0 1985 by Academic Press. Inc. All rights of reproduction in any form reserved.
Fe DETERMINATION
MATERIALS
BY KINETIC
METHOD
51
AND METHODS
Apparatus
Fluorescence was monitored with a Perkin-Elmer Model MPF-43A grating spectrofluorimeter with facilities for maintaining the cell holders at constant temperature. A Perkin-Elmer Model 023 recorder was used to register the kinetic curves from which the analytical information was obtained. The fluorescence data are reported without spectral correction. Reagents and Solutions
Sodium 4,8-diamino-I ,5-dihydroxyanthraquinone-2,6-disulfonate (DADHADS) (7) stock solution. Prepare a 0.01% m/v solution in deionized water. This solution was stable for at least 1 month. Standard iron(II1) solution: 1.79 x lo-* M. This solution was prepared by dissolving (NO&Fe * 9H20 in lo-* M hydrochloric acid and standardizing it complexometrically. All chemicals used were analytical-grade reagents, and deionized distilled water was used throughout. The dilute solutions were prepared immediately before use, the reagents and analyte solution bottles were inmersed in the same temperaturecontrolled cell holder. Procedure
Introduce 1 ml of 2.109 x 10e4 M DADHADS solution and 2 ml of 0.15 M hydrochloric acid in a 25-ml volumetric flask. Add enough deionized water to make up to the final volume of 25 ml when all the reagents are present, and an aliquot of sample containing 2.5-25 pg of iron(II1). Monitor the fluorescence intensity (A,, = 525 nm; A,, = 585 nm) starting 60 set after the addition of sample solution. RESULTS AND DISCUSSION
It has been found that DADHADS undergoes a very slow transformation in strongly acidic media by the action of the oxygen dissolved in water, catalyzed 3 t
I 0.5
1
1.5
2
lg ([HCII lo3 M) FIG.
1O-~A4.
1. Effect
of acidity
on reaction
rate. (DADHADSI
= 1.68 x 1O-5 M; /Fe3+1 = 1.78 x
52
NAVAS AND SANCHEZ ROJAS
2.5 -
&-%
2-
E c" 1.5-
I 0.7
0.9
1.1
1.3
1.5
lg ([RI 106Mnl FIG. 2. Effect of DADHADS concentration on the reaction rate. IHCll = 1.2 x IO-* M; IFe3+l = 1.78 x 1O-5 M.
by certain cations (5, 10). This oxidation process leads to the transformation of DADHADS to a diquinone-type compound (5). The oxidized product presents a strong pink fluorescence (A,, = 525 nm; A,, = 585 nm) that may be used for monitoring the reaction by fluorescence technique (9). Effect of the Reaction Variables The optimum concentration of a species was chosen by changing its initial concentration, the remaining variables being fixed. The optimum concentrations are those at which the relative standard deviation in the initial rate measurements is minimal. This is found when the reaction order of a species is zero or as close
1
2
3 t(minl
FIG. 3. Influence of the temperature on the reaction rate. lHCl[ = 1.2 x lo-* M; [DADHADSI = 8.44 x 10e6 M; IFe3+1 = 1.78 x 10m5M. Curves: (1) 4”C, (2) lO”C, (3) 2O”C, (4) 25°C. (5) 3O”C, (6) 4O"C, (7) 60°C (8) 70°C.
Fe DETERMINATION
BY KINETIC
TABLE 1 SUMMARY OF KINETIC DATA FOR THE REACTION BETWEEN DADHADS v = IH+I
53
METHOD
AND Fe(II1)
IH+J 5 7.94 x 1O-3 M
v = lH+l”
7.94 x 1O-3 M sIH+J C 0.016 M
v = IH+I-’
0.016 M ZIH+I
v = IR/’
5 0.08 M
IRI 5 6.75 x 1O-6 M
v x lRlo
6.75 x 1O-6 M SIRI Z 1.27 x lO-5 M
v = IRI-3’4
1.27 x 1O-6 M SIRI 5 2.53 x 1O-5 M
v x IFe3+/
1.79 x 1O-6 M sIFe3+/ 5 1.79 x 1O-5 M
LIDependence
of the reaction
rate (v) on experimental
-
variables.
to it as possible, since at this point small fluctuations in concentration will not affect the initial reaction rate on a zero-order species. The results for IHCI( optimization are summarized in Fig. 1. For [HCll values between 8 x lop3 and 1.6 x 1O-2 M, the initial rate is independent of this variable, and a 1.2 x 10P2M concentration of hydrochloric acid was chosen to establish the proposed method. The effect of IDADHADS( is shown in Fig. 2, from which it may be deduced that the initial rate is independent of DADHADS for reagent concentrations between 6.75 x 10m6and 1.27 x lop5 M. Thus, a 8.44 x 1O-6 M concentration was chosen. The temperature effect on the kinetic curves (Fig. 3) is critical and shows a dramatic enhancement on raising the temperature from 4 to 6O”C, and therefore must be precisely controlled. The work reported here was done at 25 + O.YC, since working close to room temperature permits an easier control. Kinetic Data
By plotting the logarithm of the initial rate against the logarithm of the species concentration, a curve is obtained whose slope at a given concentration is equal to the reaction order of the species. These data were calculated for hydrochloric acid, DADHADS, and iron( They are summarized in Table 1.
FIG. 4. Calibration
graph for iron(II1).
wm Fe (III) [HCll = 1.2 x 10m2 M; IDADHADSJ = 8.44 x 1O-6 M,
54
NAVAS AND SANCHEZ ROJAS TABLE 2 EFFECT OF DIVERSE
IONS ON THE DETERMINATION
Ion IO,Ca(I1) Co(H), Ni(II), Cu(II), Y(III), Au(III), Th(IV)
OF 0.5 pg
ml-’ Fe(III) Cion/CFeY
Mn(II), Mg(II), AC-, ClO,UO,(II), Al(II1) Zn(II), Cd(II), Hg(II), Cr(III), citrate Mo(VI), Tl(III), BrO,-, EDTA Ce(IV), V(V)
1000 400 160 80 40 4 0.2 co.2
’ Tolerable concentration ratio (4% relative error).
Calibration, Accuracy, and Precision In the context of the preceding discussion, a linear calibration graph for iron(III), covering a range of 0.1-l pg ml-‘, was found (Fig. 4). For a series of nine measurements on 0.5 kg ml-’ iron(II1) the relative error was 2.8%, and relative standard deviation 3.7%. Effect of Foreign Ions The effect of various ions on the determination of iron(II1) at the level 0.5 pg ml-’ was investigated by first testing a lOOO-foldm/m ratio of interferer to iron, and (if interference occurred) the ratio was reduced progressively until interference ceased. Higher ratios were not tested. The criterion for interference was a variation on the initial rate of more than +4% from the value expected for iron alone. The results are given in Table 2. Interferences in this method arise from four main chemical sources: (1) species causing a decrease in the actual concentration of DADHADS via complexation reactions (Th(IV)) (8); (2) complexing agents for iron(II1) (EDTA, F-); (3) oxidizing substances acting against DADHADS (Ce(IV), BrO,-); and (4) catalysts of the autoxidation process (V(V), Au(II1)). Spectral interferences should be minimum due to inherent selectivity of fluorimetry, together with the relative character of kinetic measurements. In general, the tolerance of foreign ions is sufticiently high, except for Th(IV), V(V), Ce(IV), Au(III), and F-. REFERENCES 1. Block, J., and Morgan, E., Determination of parts-per-billion iron by fluorescence extinction. Anal. Chem. 34, 1647-1649 (1962). 2. Bozhevol’nov, E. A., Kreingold, S. U., Lastovskii, R. P., and Sidorenko, V. V., Use of luminescence reagents in the kinetic methods of analysis. Dokl. Akud. Nauk SSSR 153, 97(1963). 3. Dancknortt, P., and Eisenbrand, J., “Lumineszenzanalyze in filtrierten Ultravioletten Licht,” Leipzig, 1956. 4. Fink, D., Pivnichny, J., and Ohnesorge, W., Determination of iron at parts-per-billion levels by quenching of 2,2’,2”-terpyridine luminescence. Anal. Gem. 41, 833-834 (1969). 5. Garcia Sanchez, F., Navas, A., Santiago, M., and Grases, F., Kinetic fluorimetric determination
Fe DETERMINATION
6. 7. 8. 9. 10. 11. 12. 13.
BY KINETIC
METHOD
55
of traces of vanadium(V) based on a catalysed autoxidation process. Tulanra 28, 833-837 (1981). Garcia Sanchez, F., Navas, A., and Lasema, J. J., Kinetic fluorimetric determination of inorganic species by bromate oxidation of chelating agent and complexation with metal ions. Anal. Chem. 55, 253-256 (1983). Navas, A., and Garcia Sanchez, F., Determination espectrofotometrica de Cu(I1) mediante 4,8diamino-l,S-dihidroxiantraquinona-2,6-disulfonato scklico. An. Quim. 75, 506-510 (1979). Navas, A., and Garcia Sanchez, F., Determination espectrofotomttrica de Th(IV) con 4,8-diamino-1,5-dihidroxiantraquinona-2,6-disulfonato sodico. An. Quim. 511-513 (1979). Navas, A., Sanchez Rojas, F., and Garcia Sanchez, F., Kinetic fluorimetric determination of microamounts of cerium(IV) by means of sodium 4,8-diamino- I ,5-dihydroxyanthraquinone-2,6disulphonate. Mikrochim. Acta 1(3-4), 175-181 (1982). Navas, A., and Sanchez Rojas, F., Determination cinetico fluorimetrica de trazas de oro(II1) mediante un proceso de autoxidacion catalizado. Quim. Anal. 11(2), 112- 122 (1983). Salinas, F., Garcia Sanchez, F., and Genestar, C., Fluorimetric determination of iron and thallium with 1,4-diamino-2,3-dihydroanthraquinone. Anal. Lett. lS(A9), 747-755 (1982). Salinas, F., Genestar, C., and Grases, F., Kinetic fluorimetric determination of iron and thallium based on oxidation transformation of 1,4-diamino-2,3-dihydroanthraquinone. Anal. Chim. Acta 130, 337-344 (1981). Schulman, S. G., “Fluorescence and Phosphorescence Spectroscopy: Physicochemical Principles and Practice.” Pergamon, Oxford, 1977.
JOURNAL
Fluorimetric
31, 50-55 (1985)
Determination
of Iron(lll)
by a Kinetic
Method
A. NAVAS AND F. SANCHEZ ROJAS Department
of Analytical
Chemistry,
Faculty
of Sciences, The University,
Mdlaga-4,
Spain
Received January 19, 1983 A sensitive and relatively interference-free method for the kinetic determination of iron(III) is described. The method is based on the catalytic action of this ion on the autoxidation process of 4,8-diamino-l,5-dihydroxyanthraquinone-2,6-disuifonate (disodium salt), to produce a strong pink fluorescence that increases with time. The reaction is monitored at 585 nm, when excited at 525 nm, and the initial rate method is applied to perform the analytical procedure. The influence of reaction variables and the effect of foreign ions are discussed. Iron(II1) contents between 0.1 and 1 pg ml-’ can be determined with R.S.D. of *3.7%. 8 1985 Academic Press, Inc.
INTRODUCTION The use of molecular fluorescence as an analytical technique for the determination of transition or paramagnetic metal ions has been limited traditionally by the structural complexity of these ions that facilitates spin-orbit coupling and energy dissipation of the excited states by nonradiative processes. In practice, the use of this technique is limited to those species that participate in redox processes acting upon a fluorogenic substrate whose concentration in the medium is proportional to those species of analytical interest which brought about the transformation (6, 13). Few methods for the fluorimetric determination of iron have been described, and most of these are based on quenching phenomena (Z-4). One of these is based on kinetic measurements that involve following the rate of decrease of the fluorescence intensity which accompanies the ferric ion-catalyzed reaction of a carboxymethyl derivative of a stilbenedisulfonic acid with hydrogen peroxide (2). To date only two fluorimetric methods for the determination of iron(W), based on the appearance of fluorescence, have been reported (I 1, 12). Both involved the oxidation of 1,4-diamino-2,3-dihydroanthraquinone which results in a fluorescent product. One of these employed kinetic measurements (12), and the other when the equilibrium has been reached (II). Iron(III), a paramagnetic ion, can be determined by fluorimetric technique, based on its catalytic action in the autoxidation process of 4,8-diamino-l,Sdihydroxyanthraquinone-2,6-disulfonate (disodium salt). The quenching effect of iron(III) on the fluorescence of the solution is not critical because of the relative nature of the initial rate measurements. This paper discusses the more interesting aspects of the development of a kinetic-fluorimetric method for the determination of traces of iron(II1). 50 0026-265X/85 $1.50 Copyright 0 1985 by Academic Press. Inc. All rights of reproduction in any form reserved.
Fe DETERMINATION
MATERIALS
BY KINETIC
METHOD
51
AND METHODS
Apparatus
Fluorescence was monitored with a Perkin-Elmer Model MPF-43A grating spectrofluorimeter with facilities for maintaining the cell holders at constant temperature. A Perkin-Elmer Model 023 recorder was used to register the kinetic curves from which the analytical information was obtained. The fluorescence data are reported without spectral correction. Reagents and Solutions
Sodium 4,8-diamino-I ,5-dihydroxyanthraquinone-2,6-disulfonate (DADHADS) (7) stock solution. Prepare a 0.01% m/v solution in deionized water. This solution was stable for at least 1 month. Standard iron(II1) solution: 1.79 x lo-* M. This solution was prepared by dissolving (NO&Fe * 9H20 in lo-* M hydrochloric acid and standardizing it complexometrically. All chemicals used were analytical-grade reagents, and deionized distilled water was used throughout. The dilute solutions were prepared immediately before use, the reagents and analyte solution bottles were inmersed in the same temperaturecontrolled cell holder. Procedure
Introduce 1 ml of 2.109 x 10e4 M DADHADS solution and 2 ml of 0.15 M hydrochloric acid in a 25-ml volumetric flask. Add enough deionized water to make up to the final volume of 25 ml when all the reagents are present, and an aliquot of sample containing 2.5-25 pg of iron(II1). Monitor the fluorescence intensity (A,, = 525 nm; A,, = 585 nm) starting 60 set after the addition of sample solution. RESULTS AND DISCUSSION
It has been found that DADHADS undergoes a very slow transformation in strongly acidic media by the action of the oxygen dissolved in water, catalyzed 3 t
I 0.5
1
1.5
2
lg ([HCII lo3 M) FIG.
1O-~A4.
1. Effect
of acidity
on reaction
rate. (DADHADSI
= 1.68 x 1O-5 M; /Fe3+1 = 1.78 x
52
NAVAS AND SANCHEZ ROJAS
2.5 -
&-%
2-
E c" 1.5-
I 0.7
0.9
1.1
1.3
1.5
lg ([RI 106Mnl FIG. 2. Effect of DADHADS concentration on the reaction rate. IHCll = 1.2 x IO-* M; IFe3+l = 1.78 x 1O-5 M.
by certain cations (5, 10). This oxidation process leads to the transformation of DADHADS to a diquinone-type compound (5). The oxidized product presents a strong pink fluorescence (A,, = 525 nm; A,, = 585 nm) that may be used for monitoring the reaction by fluorescence technique (9). Effect of the Reaction Variables The optimum concentration of a species was chosen by changing its initial concentration, the remaining variables being fixed. The optimum concentrations are those at which the relative standard deviation in the initial rate measurements is minimal. This is found when the reaction order of a species is zero or as close
1
2
3 t(minl
FIG. 3. Influence of the temperature on the reaction rate. lHCl[ = 1.2 x lo-* M; [DADHADSI = 8.44 x 10e6 M; IFe3+1 = 1.78 x 10m5M. Curves: (1) 4”C, (2) lO”C, (3) 2O”C, (4) 25°C. (5) 3O”C, (6) 4O"C, (7) 60°C (8) 70°C.
Fe DETERMINATION
BY KINETIC
TABLE 1 SUMMARY OF KINETIC DATA FOR THE REACTION BETWEEN DADHADS v = IH+I
53
METHOD
AND Fe(II1)
IH+J 5 7.94 x 1O-3 M
v = lH+l”
7.94 x 1O-3 M sIH+J C 0.016 M
v = IH+I-’
0.016 M ZIH+I
v = IR/’
5 0.08 M
IRI 5 6.75 x 1O-6 M
v x lRlo
6.75 x 1O-6 M SIRI Z 1.27 x lO-5 M
v = IRI-3’4
1.27 x 1O-6 M SIRI 5 2.53 x 1O-5 M
v x IFe3+/
1.79 x 1O-6 M sIFe3+/ 5 1.79 x 1O-5 M
LIDependence
of the reaction
rate (v) on experimental
-
variables.
to it as possible, since at this point small fluctuations in concentration will not affect the initial reaction rate on a zero-order species. The results for IHCI( optimization are summarized in Fig. 1. For [HCll values between 8 x lop3 and 1.6 x 1O-2 M, the initial rate is independent of this variable, and a 1.2 x 10P2M concentration of hydrochloric acid was chosen to establish the proposed method. The effect of IDADHADS( is shown in Fig. 2, from which it may be deduced that the initial rate is independent of DADHADS for reagent concentrations between 6.75 x 10m6and 1.27 x lop5 M. Thus, a 8.44 x 1O-6 M concentration was chosen. The temperature effect on the kinetic curves (Fig. 3) is critical and shows a dramatic enhancement on raising the temperature from 4 to 6O”C, and therefore must be precisely controlled. The work reported here was done at 25 + O.YC, since working close to room temperature permits an easier control. Kinetic Data
By plotting the logarithm of the initial rate against the logarithm of the species concentration, a curve is obtained whose slope at a given concentration is equal to the reaction order of the species. These data were calculated for hydrochloric acid, DADHADS, and iron( They are summarized in Table 1.
FIG. 4. Calibration
graph for iron(II1).
wm Fe (III) [HCll = 1.2 x 10m2 M; IDADHADSJ = 8.44 x 1O-6 M,
54
NAVAS AND SANCHEZ ROJAS TABLE 2 EFFECT OF DIVERSE
IONS ON THE DETERMINATION
Ion IO,Ca(I1) Co(H), Ni(II), Cu(II), Y(III), Au(III), Th(IV)
OF 0.5 pg
ml-’ Fe(III) Cion/CFeY
Mn(II), Mg(II), AC-, ClO,UO,(II), Al(II1) Zn(II), Cd(II), Hg(II), Cr(III), citrate Mo(VI), Tl(III), BrO,-, EDTA Ce(IV), V(V)
1000 400 160 80 40 4 0.2 co.2
’ Tolerable concentration ratio (4% relative error).
Calibration, Accuracy, and Precision In the context of the preceding discussion, a linear calibration graph for iron(III), covering a range of 0.1-l pg ml-‘, was found (Fig. 4). For a series of nine measurements on 0.5 kg ml-’ iron(II1) the relative error was 2.8%, and relative standard deviation 3.7%. Effect of Foreign Ions The effect of various ions on the determination of iron(II1) at the level 0.5 pg ml-’ was investigated by first testing a lOOO-foldm/m ratio of interferer to iron, and (if interference occurred) the ratio was reduced progressively until interference ceased. Higher ratios were not tested. The criterion for interference was a variation on the initial rate of more than +4% from the value expected for iron alone. The results are given in Table 2. Interferences in this method arise from four main chemical sources: (1) species causing a decrease in the actual concentration of DADHADS via complexation reactions (Th(IV)) (8); (2) complexing agents for iron(II1) (EDTA, F-); (3) oxidizing substances acting against DADHADS (Ce(IV), BrO,-); and (4) catalysts of the autoxidation process (V(V), Au(II1)). Spectral interferences should be minimum due to inherent selectivity of fluorimetry, together with the relative character of kinetic measurements. In general, the tolerance of foreign ions is sufticiently high, except for Th(IV), V(V), Ce(IV), Au(III), and F-. REFERENCES 1. Block, J., and Morgan, E., Determination of parts-per-billion iron by fluorescence extinction. Anal. Chem. 34, 1647-1649 (1962). 2. Bozhevol’nov, E. A., Kreingold, S. U., Lastovskii, R. P., and Sidorenko, V. V., Use of luminescence reagents in the kinetic methods of analysis. Dokl. Akud. Nauk SSSR 153, 97(1963). 3. Dancknortt, P., and Eisenbrand, J., “Lumineszenzanalyze in filtrierten Ultravioletten Licht,” Leipzig, 1956. 4. Fink, D., Pivnichny, J., and Ohnesorge, W., Determination of iron at parts-per-billion levels by quenching of 2,2’,2”-terpyridine luminescence. Anal. Gem. 41, 833-834 (1969). 5. Garcia Sanchez, F., Navas, A., Santiago, M., and Grases, F., Kinetic fluorimetric determination
Fe DETERMINATION
6. 7. 8. 9. 10. 11. 12. 13.
BY KINETIC
METHOD
55
of traces of vanadium(V) based on a catalysed autoxidation process. Tulanra 28, 833-837 (1981). Garcia Sanchez, F., Navas, A., and Lasema, J. J., Kinetic fluorimetric determination of inorganic species by bromate oxidation of chelating agent and complexation with metal ions. Anal. Chem. 55, 253-256 (1983). Navas, A., and Garcia Sanchez, F., Determination espectrofotometrica de Cu(I1) mediante 4,8diamino-l,S-dihidroxiantraquinona-2,6-disulfonato scklico. An. Quim. 75, 506-510 (1979). Navas, A., and Garcia Sanchez, F., Determination espectrofotomttrica de Th(IV) con 4,8-diamino-1,5-dihidroxiantraquinona-2,6-disulfonato sodico. An. Quim. 511-513 (1979). Navas, A., Sanchez Rojas, F., and Garcia Sanchez, F., Kinetic fluorimetric determination of microamounts of cerium(IV) by means of sodium 4,8-diamino- I ,5-dihydroxyanthraquinone-2,6disulphonate. Mikrochim. Acta 1(3-4), 175-181 (1982). Navas, A., and Sanchez Rojas, F., Determination cinetico fluorimetrica de trazas de oro(II1) mediante un proceso de autoxidacion catalizado. Quim. Anal. 11(2), 112- 122 (1983). Salinas, F., Garcia Sanchez, F., and Genestar, C., Fluorimetric determination of iron and thallium with 1,4-diamino-2,3-dihydroanthraquinone. Anal. Lett. lS(A9), 747-755 (1982). Salinas, F., Genestar, C., and Grases, F., Kinetic fluorimetric determination of iron and thallium based on oxidation transformation of 1,4-diamino-2,3-dihydroanthraquinone. Anal. Chim. Acta 130, 337-344 (1981). Schulman, S. G., “Fluorescence and Phosphorescence Spectroscopy: Physicochemical Principles and Practice.” Pergamon, Oxford, 1977.