- Email: [email protected]
Spectrophotometric determination of gallium(III) with phenylfluorone in the presence of hexadecylpyridinium bromide and pyridine
Ta/mra, Vol. 37, No. 6, pp. 637440, 1990 Printed in Great Britain. All rights reserved
0039-914OpO$3.00 + 0.00
Copyright 0 1990Pergamon Press plc
SPECTROPHOTOMETRIC DETERMINATION OF GALLIUM(II1) WITH PHENYLFLUORONE IN THE PRESENCE OF HEXADECYLPYRIDINIUM BROMIDE AND PYRIDINE SATORUSAKURABA Industrial Research Institute of Aomori prefecture, Fukuro-machi, Hirosaki-shi, 036 Japan (Received 26 August 1988. Revised 23 August 1989. Accepted 9 December 1989) Summary-Phenylfhtorone reacts with gallium in the presence of hexadecylpyridinium bromide and pyridine to form a water-soluble chelate with an absorption maximum at 570 nm and constant absorbance in the pH range 4.0-5.5. At this wavelength, Beer’s law is obeyed up to 4.3 x 10T6M gallium. The sensitivity is very high and the molar absorptivity is 1.48 x lo5 l.mole-‘.cm-I. The chelate has been utilized in the determination of gallium at the pg level. The ratio of gallium to phenylfluorone in the complex is 1:2.
Of the xanthene dyes, 2,3,7-trihydroxy-9phenyl-6-fluorone ( phenylfluorone) has been used for spectrophotometric determination of several metal ions.lm3 Recently it was used in conjunction with cationic surfactants for the spectrophotometric determination of germanium4 and titanium.5 Phenylfluorone and its metal chelates are soluble in ethanol but the solutions are not stable and turbidity develops. However, when a cationic surfactant and a unidentate ligand such as pyridine or nitrite are also present the turbidity does not appear. We have applied this type of reaction in the determination of cobalt,6 nickel,’ copper’ and zinc.9 Gallium reacts with phenylfluorone in the presence of hexadecylpyridinium bromide and pyridine or nitrate to form a water-soluble red chelate. Pyridine is preferred because it gives a molar absorptivity that is nearly 1.5 times that obtained with nitrite. The spectrophotometric determination of gallium by means of this reaction is described in this communication. For the spectrophotometric determination of gallium Semi-Methylxylenol Blue,” Chromazurol S” and 4-(2-pyridylazo)resorcinol’2 have been used as the chromogenic reagent in aqueous solution, and 2 - (2 - pyridylazo) - 5 - monoethylamino -p - cresol (PAEAC),13 Rhodamine B”18 etc.19 have been used in conjunction with extraction into organic solvents, but the present method is more sensitive than any of those methods. Moreover, it is much simpler than the PAEAC and Rhodamine B methods since no extraction is required. TAL 37,6-G
EXPERIMENTAL
Apparatus
A Hitachi 320 A double-beam recording spectrophotometer and a Hitachi 139 spectrophotometer were used for measuring absorbances, with fused-silica cells of 10 mm path-length. Measurements were made at 25.0 + 0.1”. A Beckman Expandomatic SS-2 pH meter was used for pH measurement. Reagents
A standard stock gallium solution was prepared by dissolving 1.00 g of gallium metal (99.999% pure) in 10 ml of concentrated nitric acid. The solution was boiled to expel nitrogen oxides and diluted accurately to 1 litre with water. Working standards were prepared by appropriate dilution. A standard solution of phenylfluorone (PF) was prepared by dissolving 0.0322 g of the reagent (Merck) in about 50 ml of ethanol containing a few drops of concentrated hydrochloric acid. Then the solution was transferred to a loo-ml standard flask and diluted to volume with ethanol. Working solutions were prepared by appropriate dilution. Pyridine (Py) solution (2.48M) and 2.0 x IO-‘M hexadecylpyridiniumbromide (HPB) solution were prepared by dissolving a suitable amount of the reagent in water. The pH was adjusted with a buffer solution made by mixing 1M sodium acetate and 1M hydrochloric acid.
637
SATORU SAKURABA
638
All reagents were of guaranteed-reagent grade. Demineralized water was used throughout.
0.5 0.4
Procedure
r r
l,...-.-C.-.-*_
/
l-•-.
/
Two ml of the buffer solution, 2.5 ml of the HPB solution, 2.5 ml of the pyridine solution and 2.0 ml of the phenylfluorone solution were added to 1 ml of sample solution, in that order. The absorbance was measured at 570 nm against a reagent blank after the solution had stood for 30 min at 25”. RESULTS AND DISCUSSION
Absorption spectra
Gallium reacts with phenylfluorone to form a red complex in the presence of HPB and pyridine. The absorption spectra of the gallium complex in the four-component system and of the reagent blank at pH 4.8 are shown in Fig. 1. For comparison, the spectra for other combinations of gallium with the reagents at the same pH are also given. The four-component system shows a pronounced absorption maximum at 570 nm, whereas a less well-defined absorption with a peak at 500 nm appears in the absence of HPB and pyridine. The deep red complex is formed only if both HPB and pyridine are present, and in the absence of either, the absorbance is much smaller than that of the red complex (Fig. 1, curves 4 and 5). Eflect of pH
The effect of pH on the colour development of the complex is shown in Fig. 2. Maximum
g 0.3 x b z 0.2 u
l
0.1
t OW
10
PH
Fig. 2. Effect of pH on absorbance. Ga(II1) 2.86 x W6M, PF 8.0 x 10-6M, HPB 2.0 x 10-‘&f, Py 0.248M.
and practically constant absorbance is obtained over the pH range 4.0-5.5. E#ect of reagent concentrations
The concentrations of the reagents were optimized one at a time, with other conditions constant. Maximum colour formation was obtained with slightly greater than 2-fold molar ratio of phenylfluorone, 300-fold molar ratio of HPB and 3 x 104-fold molar ratio of pyridine relative to gallium. Stability of the colour
Full colour development was reached in about 20 min after the reagents were added. The colour, once developed, was very stable and the absorbance remained constant for at least several hours. Calibration graph
A calibration graph made under the optimum conditions was linear up to 4.3 x 10e6M gallium. The molar absorptivity was 1.48 x lo5 l.mole-‘.cm-‘. The reproducibility of the method, expressed as the relative standard deviation of the absorbance, was 0.5% for gallium (9 replicates). Effect of diverse ions
Wavelength,
nrn
Fig. 1. Absorption spectra of the gallium complex and reagent combinations. (1) HPB-Py-PF, (2) Ga-HPBPy-PF, (3) Ga-PF, (4) Ga-Py-PF, (5) Ga-HPB-PF. References are water for (1) and reagent blank for (2)-(S). Concentrations of Ga(III), HPB, Py and PF 4 x IOe6M, 2.0 x lo-)M, 2.48 x IO-‘M and 8 x 10-6M, respectively.
The effect of diverse ions on the determination of 5 pg of gallium is summarized in Table 1. Aluminium, iron, tin, titanium, cadmium, manganese and vanadium caused some error when present in 1: 1 weight ratio to the gallium. Use of nitrite instead of pyridine completely eliminates the interference of aluminium if fluoride is also present but the sensitivity is reduced: the absorption maximum of the nitrite
Determination
of gallium(II1) with phenylfluorone
Table 1. Influence of various foreign ions on determination (measured at 570 mm) Foreign ion 2:: cu2+ Zn2+ Pd2+ In’+ Fe’+ Fe’+ Fe’+* Al’+ Al’+? Al’+* Sn4+
Amount added, pg
Ga found, !Qr
Error, Irg
50.0 25.0 50.0 25.0 25.0 10.0 10.0 5.0 100.0 5.0 25.0 100.0 5.0
5.0 5.0 5.0 5.0 5.0 5.0 1.6 4.8 3.6 5.2 5.2 7.1
88 0:o 0.0 0.0 0.0 0.0 -3.4 0.2 -1.4 0.2 0.2 2.1
639
of 5 pg of gallium
Amount Foreign ion
Ga added, I&?
found, /Jg
Error, &3
Ti4+ Cd2+ Mn2+ V(V) v(V)* Mo(V1) NO, ClFF-t so:PO:-
5.0 5.0 5.0 50.0 5.0 50.0 50.0 50.0 100.0 50.0 50.0
2.3 4.3 3.2 5.8 5.1 5.0 5.0 3.6 1.9 4.8 5.1 1.6
-2.7 -0.7 -1.8 0.8 0.1 0.0 0.0 -1.6 -3.1 -0.2 0.1 -3.4
*Gallium was separated by ether extraction. tin the presence of nitrite instead of pyridine, with 0.5 ml of 0.5% sodium fluoride solution added, measurement at 580 nm.
complex is at 580 nm and the molar absorptivity is 1.05 x lo5 l.molee’.cm-‘. Iron(II1) can be masked by reduction with 10% hydroxylamine hydrochloride or 5% ascorbic acid before colour development. If large quantities of iron(II1) and aluminium are present, gallium can be separated from them by extraction into diethyl ether from 6-844 hydrochloric acid medium after prior reduction of the iron(III).20 A similar separation can be used when a large quantity of vanadium(V) is present. In both cases the organic phase is washed twice with 6M hydrochloric acid, and gallium is stripped by two extractions with water. This aqueous extract is evaporated to
,,I-
4
I 0
,
lo-s/b
1
, 2
I 3
lo-?b2
Fig. 3. Relationship between a/(A - A’) and I/b”. Concentrations of Ga(III), HPB, Py and PF: 2.0 x 10m6M, 2.0 x IO-‘M, 0.248M and 6.0 x 10-6-1.2 x 10-s,44, respectively. O-0 n = I; 0-O n = 2; measured at 570 nm.
dryness after addition of a little nitric acid. The residue is dissolved in a small quantity of dilute nitric acid and diluted with water, then the gallium determined as already described. Composition of the complex The Benesi and Hildebrand method22 was used to find the composition of the complex. For a GaL, complex (L = ligand) the following relationship may be derived: a/@ -A’)
= {l/(6, - nc2)} + (K’/(c, - nc,)b”}
where A is the absorbance of the solution measured against the reagent blank, a and b are the initial concentrations of gallium and the reagent respectively, cl and c2 (c2b = A’) are the molar absorptivities of the complex and the reagent blank, respectively, at a constant pH value. The plots of a/(A -A’) against l/b” at 570 nm are shown in Fig. 3. A straight line is obtained for n = 2. When similar plots were made for varied concentrations of pyridine (0.054 IOM), a straight line was obtained for n = 2. Therefore it is concluded that a 1: 2: 2 complex is formed between gallium, phenylfluorone and pyridine. It may be considered that this complex is formed in a micelle since the presence of a surfactant such as HPB is important. Acknowledgements-The author is sincerely grateful to Professor Masao Maruyama of Chuoh University for his kind encouragement, and Professor Kengo Uchida of Hirosaki University for his kind advice and helpful discussions throughout this study.
640
kOllU SAKUMJ3.4 REFERENCES
1. J. Gillis, J. Hoste and A. Claeys, Anal. Chim. Acta, 1947, 1, 302. 2. E. B. Sandell, Calorimetric Determination of Traces of Metals, 3rd Ed., p. 862. Interscience, New York, 1959. 3. C. L. Luke and M. E. Campbell, Anal. Chem., 1956,28, 1273. 4. E. M. Donaldson, Takmta, 1984, 31, 997. 5. Wu Quianfeng, ibid., 1985, 32, 507. 6. S. Sakuraba and M. Kojima, Nippon Kagaku Kaishi, 1976, 345. 7. Ia’em, ibid., 1978, 208. 8. Idem, ibid., 1980, 209. 9. S. Sakuraba, Talanta, 1984, 31, 840. 10. J. Ueda and T. Kitadani, Nippon Kagaku Kaishi, 1982, 1914. 11. L. I. Ganago and N. N. Ishchenko, Zh. Analit. Khim., 1980, 35, 1718; Anal. Abstr., 1981, 48, 5B68. 12. K. Hagiwara, M. Nakane, Y. Osumi, E. I&ii and Y. Miyake, Bunseki Kagaku, 1961, 10, 1379.
13. S. Miwa and S. Shibata, Nagoya Kogyo Gijutsu Shikensho Hokoku, 1981, 30, 350; Chem. Abstr., 1982, 96, 17354Om. 14. H. Onishi and E. B. Sandell, Anal. Chim. Acta, 1955,13, 159. 15. K. Nishimura and T. Imai, Bunseki Kagaku, 1964, 13, 518. 16. F. Culkin and J. P. Riley, Analyst, 1958, 83, 208. 17. Y. Hasagawa, T. Inagake, Y. Karasawa and A. Fujita, Talanta, 1983, 30, 721. 18. G. N. Lypka and A. Chow, Anal. Chim. Acta, 1972.60, 65. 19. F. J. Barragirn de la Rosa, R. Escobar Godoy and J. L. Gbmez Ariza, Talanta, 1988, 35, 343. 20. E. B. Sandell, Calorimetric Determination of Traces of Metals, 3rd Ed., p. 471. Interscience, New York, 1959. 21. G. H. Morrison and H. Freiser, Solvent Extraction in Analytical Chemistry, p, 208. Wiley, Interscience, New York, 1957. 22. H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Sot., 1949, 71, 2703.
0039-914OpO$3.00 + 0.00
Copyright 0 1990Pergamon Press plc
SPECTROPHOTOMETRIC DETERMINATION OF GALLIUM(II1) WITH PHENYLFLUORONE IN THE PRESENCE OF HEXADECYLPYRIDINIUM BROMIDE AND PYRIDINE SATORUSAKURABA Industrial Research Institute of Aomori prefecture, Fukuro-machi, Hirosaki-shi, 036 Japan (Received 26 August 1988. Revised 23 August 1989. Accepted 9 December 1989) Summary-Phenylfhtorone reacts with gallium in the presence of hexadecylpyridinium bromide and pyridine to form a water-soluble chelate with an absorption maximum at 570 nm and constant absorbance in the pH range 4.0-5.5. At this wavelength, Beer’s law is obeyed up to 4.3 x 10T6M gallium. The sensitivity is very high and the molar absorptivity is 1.48 x lo5 l.mole-‘.cm-I. The chelate has been utilized in the determination of gallium at the pg level. The ratio of gallium to phenylfluorone in the complex is 1:2.
Of the xanthene dyes, 2,3,7-trihydroxy-9phenyl-6-fluorone ( phenylfluorone) has been used for spectrophotometric determination of several metal ions.lm3 Recently it was used in conjunction with cationic surfactants for the spectrophotometric determination of germanium4 and titanium.5 Phenylfluorone and its metal chelates are soluble in ethanol but the solutions are not stable and turbidity develops. However, when a cationic surfactant and a unidentate ligand such as pyridine or nitrite are also present the turbidity does not appear. We have applied this type of reaction in the determination of cobalt,6 nickel,’ copper’ and zinc.9 Gallium reacts with phenylfluorone in the presence of hexadecylpyridinium bromide and pyridine or nitrate to form a water-soluble red chelate. Pyridine is preferred because it gives a molar absorptivity that is nearly 1.5 times that obtained with nitrite. The spectrophotometric determination of gallium by means of this reaction is described in this communication. For the spectrophotometric determination of gallium Semi-Methylxylenol Blue,” Chromazurol S” and 4-(2-pyridylazo)resorcinol’2 have been used as the chromogenic reagent in aqueous solution, and 2 - (2 - pyridylazo) - 5 - monoethylamino -p - cresol (PAEAC),13 Rhodamine B”18 etc.19 have been used in conjunction with extraction into organic solvents, but the present method is more sensitive than any of those methods. Moreover, it is much simpler than the PAEAC and Rhodamine B methods since no extraction is required. TAL 37,6-G
EXPERIMENTAL
Apparatus
A Hitachi 320 A double-beam recording spectrophotometer and a Hitachi 139 spectrophotometer were used for measuring absorbances, with fused-silica cells of 10 mm path-length. Measurements were made at 25.0 + 0.1”. A Beckman Expandomatic SS-2 pH meter was used for pH measurement. Reagents
A standard stock gallium solution was prepared by dissolving 1.00 g of gallium metal (99.999% pure) in 10 ml of concentrated nitric acid. The solution was boiled to expel nitrogen oxides and diluted accurately to 1 litre with water. Working standards were prepared by appropriate dilution. A standard solution of phenylfluorone (PF) was prepared by dissolving 0.0322 g of the reagent (Merck) in about 50 ml of ethanol containing a few drops of concentrated hydrochloric acid. Then the solution was transferred to a loo-ml standard flask and diluted to volume with ethanol. Working solutions were prepared by appropriate dilution. Pyridine (Py) solution (2.48M) and 2.0 x IO-‘M hexadecylpyridiniumbromide (HPB) solution were prepared by dissolving a suitable amount of the reagent in water. The pH was adjusted with a buffer solution made by mixing 1M sodium acetate and 1M hydrochloric acid.
637
SATORU SAKURABA
638
All reagents were of guaranteed-reagent grade. Demineralized water was used throughout.
0.5 0.4
Procedure
r r
l,...-.-C.-.-*_
/
l-•-.
/
Two ml of the buffer solution, 2.5 ml of the HPB solution, 2.5 ml of the pyridine solution and 2.0 ml of the phenylfluorone solution were added to 1 ml of sample solution, in that order. The absorbance was measured at 570 nm against a reagent blank after the solution had stood for 30 min at 25”. RESULTS AND DISCUSSION
Absorption spectra
Gallium reacts with phenylfluorone to form a red complex in the presence of HPB and pyridine. The absorption spectra of the gallium complex in the four-component system and of the reagent blank at pH 4.8 are shown in Fig. 1. For comparison, the spectra for other combinations of gallium with the reagents at the same pH are also given. The four-component system shows a pronounced absorption maximum at 570 nm, whereas a less well-defined absorption with a peak at 500 nm appears in the absence of HPB and pyridine. The deep red complex is formed only if both HPB and pyridine are present, and in the absence of either, the absorbance is much smaller than that of the red complex (Fig. 1, curves 4 and 5). Eflect of pH
The effect of pH on the colour development of the complex is shown in Fig. 2. Maximum
g 0.3 x b z 0.2 u
l
0.1
t OW
10
PH
Fig. 2. Effect of pH on absorbance. Ga(II1) 2.86 x W6M, PF 8.0 x 10-6M, HPB 2.0 x 10-‘&f, Py 0.248M.
and practically constant absorbance is obtained over the pH range 4.0-5.5. E#ect of reagent concentrations
The concentrations of the reagents were optimized one at a time, with other conditions constant. Maximum colour formation was obtained with slightly greater than 2-fold molar ratio of phenylfluorone, 300-fold molar ratio of HPB and 3 x 104-fold molar ratio of pyridine relative to gallium. Stability of the colour
Full colour development was reached in about 20 min after the reagents were added. The colour, once developed, was very stable and the absorbance remained constant for at least several hours. Calibration graph
A calibration graph made under the optimum conditions was linear up to 4.3 x 10e6M gallium. The molar absorptivity was 1.48 x lo5 l.mole-‘.cm-‘. The reproducibility of the method, expressed as the relative standard deviation of the absorbance, was 0.5% for gallium (9 replicates). Effect of diverse ions
Wavelength,
nrn
Fig. 1. Absorption spectra of the gallium complex and reagent combinations. (1) HPB-Py-PF, (2) Ga-HPBPy-PF, (3) Ga-PF, (4) Ga-Py-PF, (5) Ga-HPB-PF. References are water for (1) and reagent blank for (2)-(S). Concentrations of Ga(III), HPB, Py and PF 4 x IOe6M, 2.0 x lo-)M, 2.48 x IO-‘M and 8 x 10-6M, respectively.
The effect of diverse ions on the determination of 5 pg of gallium is summarized in Table 1. Aluminium, iron, tin, titanium, cadmium, manganese and vanadium caused some error when present in 1: 1 weight ratio to the gallium. Use of nitrite instead of pyridine completely eliminates the interference of aluminium if fluoride is also present but the sensitivity is reduced: the absorption maximum of the nitrite
Determination
of gallium(II1) with phenylfluorone
Table 1. Influence of various foreign ions on determination (measured at 570 mm) Foreign ion 2:: cu2+ Zn2+ Pd2+ In’+ Fe’+ Fe’+ Fe’+* Al’+ Al’+? Al’+* Sn4+
Amount added, pg
Ga found, !Qr
Error, Irg
50.0 25.0 50.0 25.0 25.0 10.0 10.0 5.0 100.0 5.0 25.0 100.0 5.0
5.0 5.0 5.0 5.0 5.0 5.0 1.6 4.8 3.6 5.2 5.2 7.1
88 0:o 0.0 0.0 0.0 0.0 -3.4 0.2 -1.4 0.2 0.2 2.1
639
of 5 pg of gallium
Amount Foreign ion
Ga added, I&?
found, /Jg
Error, &3
Ti4+ Cd2+ Mn2+ V(V) v(V)* Mo(V1) NO, ClFF-t so:PO:-
5.0 5.0 5.0 50.0 5.0 50.0 50.0 50.0 100.0 50.0 50.0
2.3 4.3 3.2 5.8 5.1 5.0 5.0 3.6 1.9 4.8 5.1 1.6
-2.7 -0.7 -1.8 0.8 0.1 0.0 0.0 -1.6 -3.1 -0.2 0.1 -3.4
*Gallium was separated by ether extraction. tin the presence of nitrite instead of pyridine, with 0.5 ml of 0.5% sodium fluoride solution added, measurement at 580 nm.
complex is at 580 nm and the molar absorptivity is 1.05 x lo5 l.molee’.cm-‘. Iron(II1) can be masked by reduction with 10% hydroxylamine hydrochloride or 5% ascorbic acid before colour development. If large quantities of iron(II1) and aluminium are present, gallium can be separated from them by extraction into diethyl ether from 6-844 hydrochloric acid medium after prior reduction of the iron(III).20 A similar separation can be used when a large quantity of vanadium(V) is present. In both cases the organic phase is washed twice with 6M hydrochloric acid, and gallium is stripped by two extractions with water. This aqueous extract is evaporated to
,,I-
4
I 0
,
lo-s/b
1
, 2
I 3
lo-?b2
Fig. 3. Relationship between a/(A - A’) and I/b”. Concentrations of Ga(III), HPB, Py and PF: 2.0 x 10m6M, 2.0 x IO-‘M, 0.248M and 6.0 x 10-6-1.2 x 10-s,44, respectively. O-0 n = I; 0-O n = 2; measured at 570 nm.
dryness after addition of a little nitric acid. The residue is dissolved in a small quantity of dilute nitric acid and diluted with water, then the gallium determined as already described. Composition of the complex The Benesi and Hildebrand method22 was used to find the composition of the complex. For a GaL, complex (L = ligand) the following relationship may be derived: a/@ -A’)
= {l/(6, - nc2)} + (K’/(c, - nc,)b”}
where A is the absorbance of the solution measured against the reagent blank, a and b are the initial concentrations of gallium and the reagent respectively, cl and c2 (c2b = A’) are the molar absorptivities of the complex and the reagent blank, respectively, at a constant pH value. The plots of a/(A -A’) against l/b” at 570 nm are shown in Fig. 3. A straight line is obtained for n = 2. When similar plots were made for varied concentrations of pyridine (0.054 IOM), a straight line was obtained for n = 2. Therefore it is concluded that a 1: 2: 2 complex is formed between gallium, phenylfluorone and pyridine. It may be considered that this complex is formed in a micelle since the presence of a surfactant such as HPB is important. Acknowledgements-The author is sincerely grateful to Professor Masao Maruyama of Chuoh University for his kind encouragement, and Professor Kengo Uchida of Hirosaki University for his kind advice and helpful discussions throughout this study.
640
kOllU SAKUMJ3.4 REFERENCES
1. J. Gillis, J. Hoste and A. Claeys, Anal. Chim. Acta, 1947, 1, 302. 2. E. B. Sandell, Calorimetric Determination of Traces of Metals, 3rd Ed., p. 862. Interscience, New York, 1959. 3. C. L. Luke and M. E. Campbell, Anal. Chem., 1956,28, 1273. 4. E. M. Donaldson, Takmta, 1984, 31, 997. 5. Wu Quianfeng, ibid., 1985, 32, 507. 6. S. Sakuraba and M. Kojima, Nippon Kagaku Kaishi, 1976, 345. 7. Ia’em, ibid., 1978, 208. 8. Idem, ibid., 1980, 209. 9. S. Sakuraba, Talanta, 1984, 31, 840. 10. J. Ueda and T. Kitadani, Nippon Kagaku Kaishi, 1982, 1914. 11. L. I. Ganago and N. N. Ishchenko, Zh. Analit. Khim., 1980, 35, 1718; Anal. Abstr., 1981, 48, 5B68. 12. K. Hagiwara, M. Nakane, Y. Osumi, E. I&ii and Y. Miyake, Bunseki Kagaku, 1961, 10, 1379.
13. S. Miwa and S. Shibata, Nagoya Kogyo Gijutsu Shikensho Hokoku, 1981, 30, 350; Chem. Abstr., 1982, 96, 17354Om. 14. H. Onishi and E. B. Sandell, Anal. Chim. Acta, 1955,13, 159. 15. K. Nishimura and T. Imai, Bunseki Kagaku, 1964, 13, 518. 16. F. Culkin and J. P. Riley, Analyst, 1958, 83, 208. 17. Y. Hasagawa, T. Inagake, Y. Karasawa and A. Fujita, Talanta, 1983, 30, 721. 18. G. N. Lypka and A. Chow, Anal. Chim. Acta, 1972.60, 65. 19. F. J. Barragirn de la Rosa, R. Escobar Godoy and J. L. Gbmez Ariza, Talanta, 1988, 35, 343. 20. E. B. Sandell, Calorimetric Determination of Traces of Metals, 3rd Ed., p. 471. Interscience, New York, 1959. 21. G. H. Morrison and H. Freiser, Solvent Extraction in Analytical Chemistry, p, 208. Wiley, Interscience, New York, 1957. 22. H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Sot., 1949, 71, 2703.