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Isodifferential derivative approach to the spectrophotometric determination of nickel and cobalt mixtures
Analytica Chimica Acta, 197 (1987) 275-280 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Short Communication
ISODIFFERENTIAL DERIVATIVE APPROACH TO THE SPECTROPHOTOMETRIC DETERMINATION OF NICKEL AND COBALT MIXTURES
F. GARCIA SANCHEZ*,
M. HERNANDEZ
Department (Spain)
Chemistry,
of Analytical
Faculty
LOPEZ and J. C. MARQUEZ GOMEZ of Sciences,
The University,
Malaga-29071
(Received 12th May 1986)
Summary. A graphical method for measuring derivative amplitudes in binary mixtures with overlapping spectra is described. The method is based on the interference-free character of the isodifferential points m the derivative calibration graphs. Cobalt and nickel mixturesareanalyzed in the range 0.01-2.5 Mgml-’ by the formation of coloured chelates with benzyl-2-pyridylketone 2-pyridylhydrazone, with relative standard deviations d 1.5%.
Several very sensitive reagents have been proposed for the individual spectrophotometric determination of nickel and cobalt. However, determination of both in a mixture is often troublesome because of the considerable overlap between their broad absorption spectra [l] . Generally, the resolution of such binary mixtures requires the measurement of total absorbance at two wavelengths and solving a set of simultaneous equations. Derivative spectra, however, provide additional possibilities in this respect. Derivative spectrophotometry enhances the detectability of minor spectral features; its consequent capability to discriminate between closely related spectral features make it useful in several areas of analytical spectrometry [Z, 31. Recently spectrophotometers [ 4, 51 giving the use of rapid “scanning” diode-array digital data output, offers new possibilities for the resolution of mixtures [ 61, particularly when combined with modern liquid chromatography [ 7,8] . Several approaches have been used in the above reports for the quantitative evaluation of the derivative amplitudes of analyte solution mixtures with closely related spectral shapes, generally based on trial and error. The model used in this paper is based on the fact that Beer’s Law is obeyed for the nickel and cobalt complexes of a particular ligand in the concentration range studied, and therefore is also obeyed for the total absorbance. The derivative absorbances (first and second) of a compound band are also the sum of its component derivative absorbances.
0003-2670/87/$03.50
o 1987 Elsevier Science Publishers B.V.
216
Experimental Apparatus and reagents. Spectral measurements were made with a Shimadzu UV-240 Graphicord recording spectrophotometer in l-cm quartz cells. The spectra were obtained with a spectral bandwidth of 0.5 nm, a scanning speed of 3 nm s-’ and recording chart speed of 10 nm cm-‘. First and second derivative ultraviolet spectra were obtained with a Shimadzu derivative spectrum attachment with optional program/interface (model OPI-2) giving first to fourth derivatives, Ah 1, 2 and 4 nm. The synthesis of benzyl-2-pyridylketone 2-pyridylhydrazone (BPKPH) was as reported previously [9]. Solutions (1 X 1O-3 M) were prepared weekly by dissolving 0.0288 g of BPKPH in 100 ml of absolute ethanol. A 0.1 M nickel stock solution was prepared from nickel nitrate hexahydrate and standardized gravimetrically with dimethylglyoxime. Cobalt stock solution (0.1 M) was prepared from cobalt nitrate hexahydrate and standardized by EDTA titration (xylenol orange indicator). Working solutions were prepared by appropriate dilutions with water. A pH 9.0 buffer solution was prepared from 0.1 M boric acid and 0.1 M sodium hydroxide. Unless otherwise stated, the reagents used were of analytical-reagent grade. Distilled, demineralized water was used throughout. Determination of binary mixtures. Place an aliquot of a sample containing 0.25-1.001 pg ml-’ cobalt and 0.5-2.00 pg ml-’ nickel into a lo-ml volumetric flask. Add 5 ml of 1 X 10e3 M BPKPH ethanolic solution and 2 ml of pH 9.0 borate buffer and dilute to the mark with deionized water. Record the firstderivative spectrum between 400 and 550 nm against a reagent blank, with Ah = 4 nm, at a scanning speed of 3 nm cm-‘. Measure the firstderivative analytical value as the vertical difference in the absorbance change (dA) scale from the corresponding isodifferential point (h 1 = 483 nm for nickel, h2 = 453 nm for cobalt) to the intersection with the firstderivative curve. The concentration of nickel and cobalt in the sample is found from calibration graphs previously run under the same conditions as those for the mixture. Results and discussion BPKPH behaves as a sensitive chromogenic ligand for several metal ions [lo]. Under alkaline conditions, the ligand is tridentate and the complexes, formed with the anionic form of the ligand, are only sparingly soluble in water. The chelates are soluble, however, if the medium contains sufficient ethanol. BPKPH reacts with cobalt(I1) to form a yellow-orange complex with absorption maximum at 483 nm, over the pH range 2-l 3.5 [lo]. The complexation of BPKPH with nickel occurs in a narrower pH range; maximal and constant absorbance is obtained in the pH range lo--13 at the absorption maximum of 453 nm. In 50% ethanol, chelate formation is complete with both metals after addition of borate buffer of pH 9.0, with a five-fold molar excess of BPKPH. Colour development is instant and the colour is stable for at least 4 h. Slight variations in absorbance can be obtained by modifying the
concentration of ethanol on the medium. No changes in absorbance were observed when the order of addition of the reagents was changed. A mole ratio plot shows that the mole ratio of cobalt or nickel to BPKPH at pH 10.40 is 1:2. Figure 1A shows the spectra obtained for the nickel and cobalt complexes. The absorption spectrum of the 1:l metal ion mixture consists of a broad band which is the sum of the individual components, and which does not allow discrimination between the components. The first derivatives of these spectra are shown in Fig. 1B. Again, the severe overlap of the derivative spectra makes it difficult to determine each metal ion. Suitable selection of the wavelength from which the derivative amplitudes are measured, however, allows accurate determination of nickel and cobalt in mixtures from the firstderivative spectra, as will be demonstrated below. Figure 2A shows the absorption spectra of two series of solutions containing increasing concentrations of the BPKPH chelates of nickel and cobalt. Again they emphasize that the closely related spectral behaviour of both chelates provides superimposed spectra at practically all wavelengths and concentration ratios, and individual quantitation is impeded. The wavelengths of maximum absorption of the chelates are 453 (nickel) and 483 nm (cobalt) at all concentrations. At each wavelength, the change in absorbance dA with changing wavelength is zero for one of the chelates, so that the other component can be determined without interference at that wavelength. This can be seen in Fig. 2B. From Fig. 2A and B it may be inferred that greatest sensitivity would be obtained when the firstderivative
I,(
1
A 0,825
0.275
420
520 X (nm)
6
)
0,ll'
425
525
f
x (ml)
Fig. 1. (A) Absorption spectra of reagent (R), cobalt complex (Co), nickel complex (Ni) and mixtures of cobalt + nickel (1 :l) ( w:w) complexes (1 wg ml-’ Co’+ or Ni”, 5 X 10T4 M BPKPH; pH 10.4). (B) First derivatives of the spectra in (A).
278
L 0 420
6
520
1
“I5
1-45
x (nm)
X (nm)
Fig. 2. (A) Absorption spectra of two series of cobalt and nickel complexes at the concentrations (rg ml-l) indicated on the curves (5 x 10“ BPKPH; pH 10.4). (B) Superimposed first derivatives of the spectra in (A).
amplitude of component 1 is zero at the same wavelength at which the firstderivative amplitude of component 2 is maximal. This condition is satisfied at the wavelength where the first derivative of component 1 is zero and the second derivative of component 2 is zero. In turn, this is satisfied when the wavelength difference between absorption maxima corresponding to both absorbing species is equal to the half band-width of each spectral band. These considerations are of analytical interest because it becomes possible to predict if two overlapping bands can be measured satisfactorily by using the isodifferential derivative approach. Only a few data about the fundamental spectra of the overlapping compounds are needed, i.e., maximum spectral intensity (A), half band-width at half maximum intensity (l/2 B) and separation between wavelength maxima of both compounds (C). Although the spectral profiles of two compounds can vary in a broad manner, the considerations can be simplified if the overlapping bands are confined to a di;creased spectral zone in which they have a Gaussian profile. On this assumption, the absorbances of bands 1 and 2 are given as a function of h (wavelength) by I,(h) = A1 exp [(--h - C,)2/2B:]
(1)
I,(h) = A2 exp [(-X - C2)2/2B:]
(2)
The first derivative of band 1 is expressed as dl,(h)/dh
= [-A,(h
-
C,)/Bf]
exp [-(X - C,)2/2B:]
(3)
and the second derivative of band 2 as d212(h)/dh2 = (A,/@){
[(h - C2)2/Bi] - 1) exp [-(h - C2)2/2B2,1
(4)
279
The maximum sensitivity and precision are obtained in the isodifferential derivative method when the first derivative amplitude of a component is zero. The solutions for d212(h)/dX2 = 0 and for dli(h)/dh = 0 are satisfied when C1 = h and (X - C,)/B, = 1, i.e., when C1 - C2 = B2. These conclusions are illustrated in Fig. 3, in which two overlapping Gaussian bands are examined for three distinct values of ratios B1/B2 and AI/A*. From Fig. 2A it may be seen that the band maxima separation for the nickel and cobalt chelates is 30 nm. The half band-width corresponding to the nickel chelate is 35 nm and that for the cobalt chelate is 40 nm. As C1 - C2 = B2 the half band-width must optimally be 30 nm so there is only a small difference from this optimal value in first-derivative amplitude (Fig. 2B). The dependence of the first- and second-derivative spectra on the instrumental parameters follows the general patterns of derivative spectroscopy [ 111. Thus Ah = 4 nm, a medium scanning speed (3 nm cm-‘) and the response time associated with the derivative circuit give satisfactory results. The calibration graphs prepared by plotting absorbance and dA against nickel and cobalt concentrations (in r.cgml-‘) were linear for O-l.25 pg ml-’ and O-2.25 pg ml-‘, respectively. The equations obtained by the least-squares treatment of data for mixtures were dA = O.l04[Ni] - 0.007, r = 0.9994 (n = 5), and dA = O.O38[Co] - 0.001, r = 0.9954 (n = 5). The analytical characteristics are summarized in Table 1. The results obtained for the determination of each ion in various binary mixtures are shown in Table 2. The method gives satisfactory results. We thank The Comision Asesora de Investigation Cientifica y Tecnica for supporting this study (Project No. 3007/83 CO2-02).
C=
8,
C -
B,
C=
B,-
B,
Fig. 3. Effect of band-width and separation between wavelength maxima on the usefulness of the isodifferential derivative approach.
280 TABLE 1 Characteristics of the first-derivative isodifferential procedure Metal
Sensitivity*
Detection limitb (rg ml-‘)
Linear range h3 ml-‘)
RSD (a)
Cobalt Nickel
1.05 x lo-” 8.00 x lo-’
0.0138 0.0065
0.045-2.25 0.022-1.25
1.34 (n = 5) 0.75 (n = 5)
aFrom slope of calibration graph. b 30 value. TABLE 2 Analysis of binary mixtures of nickel and cobalt Ratio Ni :Co (w/w)
Taken
Founda
1:20 1:4 1:2 1:l 1:l 2:l 7.5:1 1O:l
0.100 0.500 0.500 1.000 0.500 1 .ooo 1.500 1.000
0.110 0.494 0.501 0.923 0.520 1.005 1.485 0.998
Error (%)
Nickel (rg ml-‘)
r f * * f. f + *
0.008 0.001 0.021 0.015 0.010 0.002 0.005 0.012
+10.0 -1.2 +o.o -7.7 + 4.0 +0.5 -1.5 -0.2
Cobalt (pg ml-‘) Taken
Found”
2.000 2.000 1 .ooo 1.000 0.500 0.500 0.200
1.995 2.040 0.963 0.963 0.523 0.530 0.184
fr 0.018 c 0.034 f 0.017 2 0.015 * 0.013 Yt0.010 f 0.015
Error (%) -0.2 +2.0 -3.7 -3.7 + 4.6 + 6.0 -8.0
aMean * SD of three separate determinations.
REFERENCES 1 Z. Marczenko, Spectrophotometric Determination of Elements, Horwood, Chichester, 1976. 2 G. Talsky, L. Mayring and H. Kreuzer, Angew. Chem., Int. Ed. Engl., 17 (1978) 785. 3T. C. O’Haver, Anal. Proc., 19 (1982) 22. 4 D. G. Jones, Anal. Chem., 57 (1985) 1057A. 5 D. G. Jones, Anal. Chem., 57 (1985) 1207A. 6 D. T. Rossi and H. L. Pardue, Anal. Chim. Acta, 175 (1985) 153. 7 M. J. Milan0 and E. Grushka, J. Chromatogr., 133 (1977) 352. 8 B. J. Clark, A. F. Fell, H. P. Scott and D. Westerlund, J. Chromatogr., 286 (1984) 261. 9 J. J. Laserna, A. Navas and F. Garcia Sanchez, Anal. Chim. Acta, 121 (1980) 295. 10 F. Garcia Sanchez, A. Navas, J. J. Laserna and A. Arbaizar, Analyst, 107 (1982) 35. 11 J. Medinilla, F. Ales and F. Garcia Sanchez, Talanta, 33 (1986) 329.
Short Communication
ISODIFFERENTIAL DERIVATIVE APPROACH TO THE SPECTROPHOTOMETRIC DETERMINATION OF NICKEL AND COBALT MIXTURES
F. GARCIA SANCHEZ*,
M. HERNANDEZ
Department (Spain)
Chemistry,
of Analytical
Faculty
LOPEZ and J. C. MARQUEZ GOMEZ of Sciences,
The University,
Malaga-29071
(Received 12th May 1986)
Summary. A graphical method for measuring derivative amplitudes in binary mixtures with overlapping spectra is described. The method is based on the interference-free character of the isodifferential points m the derivative calibration graphs. Cobalt and nickel mixturesareanalyzed in the range 0.01-2.5 Mgml-’ by the formation of coloured chelates with benzyl-2-pyridylketone 2-pyridylhydrazone, with relative standard deviations d 1.5%.
Several very sensitive reagents have been proposed for the individual spectrophotometric determination of nickel and cobalt. However, determination of both in a mixture is often troublesome because of the considerable overlap between their broad absorption spectra [l] . Generally, the resolution of such binary mixtures requires the measurement of total absorbance at two wavelengths and solving a set of simultaneous equations. Derivative spectra, however, provide additional possibilities in this respect. Derivative spectrophotometry enhances the detectability of minor spectral features; its consequent capability to discriminate between closely related spectral features make it useful in several areas of analytical spectrometry [Z, 31. Recently spectrophotometers [ 4, 51 giving the use of rapid “scanning” diode-array digital data output, offers new possibilities for the resolution of mixtures [ 61, particularly when combined with modern liquid chromatography [ 7,8] . Several approaches have been used in the above reports for the quantitative evaluation of the derivative amplitudes of analyte solution mixtures with closely related spectral shapes, generally based on trial and error. The model used in this paper is based on the fact that Beer’s Law is obeyed for the nickel and cobalt complexes of a particular ligand in the concentration range studied, and therefore is also obeyed for the total absorbance. The derivative absorbances (first and second) of a compound band are also the sum of its component derivative absorbances.
0003-2670/87/$03.50
o 1987 Elsevier Science Publishers B.V.
216
Experimental Apparatus and reagents. Spectral measurements were made with a Shimadzu UV-240 Graphicord recording spectrophotometer in l-cm quartz cells. The spectra were obtained with a spectral bandwidth of 0.5 nm, a scanning speed of 3 nm s-’ and recording chart speed of 10 nm cm-‘. First and second derivative ultraviolet spectra were obtained with a Shimadzu derivative spectrum attachment with optional program/interface (model OPI-2) giving first to fourth derivatives, Ah 1, 2 and 4 nm. The synthesis of benzyl-2-pyridylketone 2-pyridylhydrazone (BPKPH) was as reported previously [9]. Solutions (1 X 1O-3 M) were prepared weekly by dissolving 0.0288 g of BPKPH in 100 ml of absolute ethanol. A 0.1 M nickel stock solution was prepared from nickel nitrate hexahydrate and standardized gravimetrically with dimethylglyoxime. Cobalt stock solution (0.1 M) was prepared from cobalt nitrate hexahydrate and standardized by EDTA titration (xylenol orange indicator). Working solutions were prepared by appropriate dilutions with water. A pH 9.0 buffer solution was prepared from 0.1 M boric acid and 0.1 M sodium hydroxide. Unless otherwise stated, the reagents used were of analytical-reagent grade. Distilled, demineralized water was used throughout. Determination of binary mixtures. Place an aliquot of a sample containing 0.25-1.001 pg ml-’ cobalt and 0.5-2.00 pg ml-’ nickel into a lo-ml volumetric flask. Add 5 ml of 1 X 10e3 M BPKPH ethanolic solution and 2 ml of pH 9.0 borate buffer and dilute to the mark with deionized water. Record the firstderivative spectrum between 400 and 550 nm against a reagent blank, with Ah = 4 nm, at a scanning speed of 3 nm cm-‘. Measure the firstderivative analytical value as the vertical difference in the absorbance change (dA) scale from the corresponding isodifferential point (h 1 = 483 nm for nickel, h2 = 453 nm for cobalt) to the intersection with the firstderivative curve. The concentration of nickel and cobalt in the sample is found from calibration graphs previously run under the same conditions as those for the mixture. Results and discussion BPKPH behaves as a sensitive chromogenic ligand for several metal ions [lo]. Under alkaline conditions, the ligand is tridentate and the complexes, formed with the anionic form of the ligand, are only sparingly soluble in water. The chelates are soluble, however, if the medium contains sufficient ethanol. BPKPH reacts with cobalt(I1) to form a yellow-orange complex with absorption maximum at 483 nm, over the pH range 2-l 3.5 [lo]. The complexation of BPKPH with nickel occurs in a narrower pH range; maximal and constant absorbance is obtained in the pH range lo--13 at the absorption maximum of 453 nm. In 50% ethanol, chelate formation is complete with both metals after addition of borate buffer of pH 9.0, with a five-fold molar excess of BPKPH. Colour development is instant and the colour is stable for at least 4 h. Slight variations in absorbance can be obtained by modifying the
concentration of ethanol on the medium. No changes in absorbance were observed when the order of addition of the reagents was changed. A mole ratio plot shows that the mole ratio of cobalt or nickel to BPKPH at pH 10.40 is 1:2. Figure 1A shows the spectra obtained for the nickel and cobalt complexes. The absorption spectrum of the 1:l metal ion mixture consists of a broad band which is the sum of the individual components, and which does not allow discrimination between the components. The first derivatives of these spectra are shown in Fig. 1B. Again, the severe overlap of the derivative spectra makes it difficult to determine each metal ion. Suitable selection of the wavelength from which the derivative amplitudes are measured, however, allows accurate determination of nickel and cobalt in mixtures from the firstderivative spectra, as will be demonstrated below. Figure 2A shows the absorption spectra of two series of solutions containing increasing concentrations of the BPKPH chelates of nickel and cobalt. Again they emphasize that the closely related spectral behaviour of both chelates provides superimposed spectra at practically all wavelengths and concentration ratios, and individual quantitation is impeded. The wavelengths of maximum absorption of the chelates are 453 (nickel) and 483 nm (cobalt) at all concentrations. At each wavelength, the change in absorbance dA with changing wavelength is zero for one of the chelates, so that the other component can be determined without interference at that wavelength. This can be seen in Fig. 2B. From Fig. 2A and B it may be inferred that greatest sensitivity would be obtained when the firstderivative
I,(
1
A 0,825
0.275
420
520 X (nm)
6
)
0,ll'
425
525
f
x (ml)
Fig. 1. (A) Absorption spectra of reagent (R), cobalt complex (Co), nickel complex (Ni) and mixtures of cobalt + nickel (1 :l) ( w:w) complexes (1 wg ml-’ Co’+ or Ni”, 5 X 10T4 M BPKPH; pH 10.4). (B) First derivatives of the spectra in (A).
278
L 0 420
6
520
1
“I5
1-45
x (nm)
X (nm)
Fig. 2. (A) Absorption spectra of two series of cobalt and nickel complexes at the concentrations (rg ml-l) indicated on the curves (5 x 10“ BPKPH; pH 10.4). (B) Superimposed first derivatives of the spectra in (A).
amplitude of component 1 is zero at the same wavelength at which the firstderivative amplitude of component 2 is maximal. This condition is satisfied at the wavelength where the first derivative of component 1 is zero and the second derivative of component 2 is zero. In turn, this is satisfied when the wavelength difference between absorption maxima corresponding to both absorbing species is equal to the half band-width of each spectral band. These considerations are of analytical interest because it becomes possible to predict if two overlapping bands can be measured satisfactorily by using the isodifferential derivative approach. Only a few data about the fundamental spectra of the overlapping compounds are needed, i.e., maximum spectral intensity (A), half band-width at half maximum intensity (l/2 B) and separation between wavelength maxima of both compounds (C). Although the spectral profiles of two compounds can vary in a broad manner, the considerations can be simplified if the overlapping bands are confined to a di;creased spectral zone in which they have a Gaussian profile. On this assumption, the absorbances of bands 1 and 2 are given as a function of h (wavelength) by I,(h) = A1 exp [(--h - C,)2/2B:]
(1)
I,(h) = A2 exp [(-X - C2)2/2B:]
(2)
The first derivative of band 1 is expressed as dl,(h)/dh
= [-A,(h
-
C,)/Bf]
exp [-(X - C,)2/2B:]
(3)
and the second derivative of band 2 as d212(h)/dh2 = (A,/@){
[(h - C2)2/Bi] - 1) exp [-(h - C2)2/2B2,1
(4)
279
The maximum sensitivity and precision are obtained in the isodifferential derivative method when the first derivative amplitude of a component is zero. The solutions for d212(h)/dX2 = 0 and for dli(h)/dh = 0 are satisfied when C1 = h and (X - C,)/B, = 1, i.e., when C1 - C2 = B2. These conclusions are illustrated in Fig. 3, in which two overlapping Gaussian bands are examined for three distinct values of ratios B1/B2 and AI/A*. From Fig. 2A it may be seen that the band maxima separation for the nickel and cobalt chelates is 30 nm. The half band-width corresponding to the nickel chelate is 35 nm and that for the cobalt chelate is 40 nm. As C1 - C2 = B2 the half band-width must optimally be 30 nm so there is only a small difference from this optimal value in first-derivative amplitude (Fig. 2B). The dependence of the first- and second-derivative spectra on the instrumental parameters follows the general patterns of derivative spectroscopy [ 111. Thus Ah = 4 nm, a medium scanning speed (3 nm cm-‘) and the response time associated with the derivative circuit give satisfactory results. The calibration graphs prepared by plotting absorbance and dA against nickel and cobalt concentrations (in r.cgml-‘) were linear for O-l.25 pg ml-’ and O-2.25 pg ml-‘, respectively. The equations obtained by the least-squares treatment of data for mixtures were dA = O.l04[Ni] - 0.007, r = 0.9994 (n = 5), and dA = O.O38[Co] - 0.001, r = 0.9954 (n = 5). The analytical characteristics are summarized in Table 1. The results obtained for the determination of each ion in various binary mixtures are shown in Table 2. The method gives satisfactory results. We thank The Comision Asesora de Investigation Cientifica y Tecnica for supporting this study (Project No. 3007/83 CO2-02).
C=
8,
C -
B,
C=
B,-
B,
Fig. 3. Effect of band-width and separation between wavelength maxima on the usefulness of the isodifferential derivative approach.
280 TABLE 1 Characteristics of the first-derivative isodifferential procedure Metal
Sensitivity*
Detection limitb (rg ml-‘)
Linear range h3 ml-‘)
RSD (a)
Cobalt Nickel
1.05 x lo-” 8.00 x lo-’
0.0138 0.0065
0.045-2.25 0.022-1.25
1.34 (n = 5) 0.75 (n = 5)
aFrom slope of calibration graph. b 30 value. TABLE 2 Analysis of binary mixtures of nickel and cobalt Ratio Ni :Co (w/w)
Taken
Founda
1:20 1:4 1:2 1:l 1:l 2:l 7.5:1 1O:l
0.100 0.500 0.500 1.000 0.500 1 .ooo 1.500 1.000
0.110 0.494 0.501 0.923 0.520 1.005 1.485 0.998
Error (%)
Nickel (rg ml-‘)
r f * * f. f + *
0.008 0.001 0.021 0.015 0.010 0.002 0.005 0.012
+10.0 -1.2 +o.o -7.7 + 4.0 +0.5 -1.5 -0.2
Cobalt (pg ml-‘) Taken
Found”
2.000 2.000 1 .ooo 1.000 0.500 0.500 0.200
1.995 2.040 0.963 0.963 0.523 0.530 0.184
fr 0.018 c 0.034 f 0.017 2 0.015 * 0.013 Yt0.010 f 0.015
Error (%) -0.2 +2.0 -3.7 -3.7 + 4.6 + 6.0 -8.0
aMean * SD of three separate determinations.
REFERENCES 1 Z. Marczenko, Spectrophotometric Determination of Elements, Horwood, Chichester, 1976. 2 G. Talsky, L. Mayring and H. Kreuzer, Angew. Chem., Int. Ed. Engl., 17 (1978) 785. 3T. C. O’Haver, Anal. Proc., 19 (1982) 22. 4 D. G. Jones, Anal. Chem., 57 (1985) 1057A. 5 D. G. Jones, Anal. Chem., 57 (1985) 1207A. 6 D. T. Rossi and H. L. Pardue, Anal. Chim. Acta, 175 (1985) 153. 7 M. J. Milan0 and E. Grushka, J. Chromatogr., 133 (1977) 352. 8 B. J. Clark, A. F. Fell, H. P. Scott and D. Westerlund, J. Chromatogr., 286 (1984) 261. 9 J. J. Laserna, A. Navas and F. Garcia Sanchez, Anal. Chim. Acta, 121 (1980) 295. 10 F. Garcia Sanchez, A. Navas, J. J. Laserna and A. Arbaizar, Analyst, 107 (1982) 35. 11 J. Medinilla, F. Ales and F. Garcia Sanchez, Talanta, 33 (1986) 329.