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Spectrophotometric study of the reaction of thallium with brompyrogallol red: The effect of the cationogenic tenside Septonex
MICROCHEMICAL
JOURNAL
36, 301-305 (1987)
Spectrophotometric Brompyrogallol
Study of the Reaction of Thallium with Red: The Effect of the Cationogenic Tenside Septonex
I. NGMcovA,~ E. HAVELKOVA, V. SUK, AND B. EVTIMOVA* Department of Analytical Chemistry, Faculty of Natural Sciences, Charles University, Albertov 2030, 128 40 Prague 2, Czechoslovakia, and *Department of Analytical Chemistry, Faculty of Chemistry, The K. Ochridsky University, Sofia, Bulgaria
Received April 14, 1987; accepted June 30, 1987 The reaction of TI(III) with brompyrogallol red was studied and the effect of the cationogenie tenside Septonex on this reaction was considered. Optimal conditions were found for the formation of the binary and ternary complexes and regions of validity of the Beero 1987 Academic PXSS, IIIC. Lambert law were determined.
INTRODUCTION Brompyrogallol red (5,5’-dibrompyrogallolsulfophthalein, DG) forms colored complexes with trivalent and quadrivalent metal cations in acid medium and with divalent cations in neutral or alkaline medium. These complexes can be successfully used for the spectrophotometric determination of metals. The complexes are usually red, purple, or blue, except for the complexes of Ag(1) (I) and Au(II1) (2), which form yellow complexes with DG, with maximum absorbance at about 400 nm. The color of the complex with Ag(1) changes to blue in the presence of l,lOphenanthroline (A = 635 nm), with considerably increased sensitivity of the determination (3). The addition of the cationogenic tenside Septonex (2) leads to a slight increase in the sensitivity of the determination of the Au(II1) complex and shifts complex formation to lower pH values. We have found that Tl(II1) also yields a yellow complex with brompyrogallol red, with maximum absorbance at 398 nm. The optimum conditions for complex formation were studied and the effect of Septonex on this reaction was also followed. The results obtained were employed to develop a new spectrophotometric method for determining thallium, with sensitivity comparable to that of the most sensitive determinations of this metal. These methods are based mostly on the formation of the ion associate of the complex halide anion of Tl(II1) and a basic, mostly triphenylmethane or xanthene dye (e.g., rhodamine B (4)). The water-insoluble associates formed must be extracted into an organic solvent, while the complex formed between Tl(II1) and DG can be measured spectrophotometrically directly in the aqueous solution. Instruments.
The absorption
EXPERIMENTAL spectra were measured
on a recording
spectro-
r To whom correspondence should be addressed. 301 0026-265X/87 $1 SO Copyright 0 1987 by Academic Press, Inc. All tights of reproduction in any form reserved.
302
NtiMCOVli
ET AL.
photometer from Pye Unicam Instruments Ltd. (Cambridge, Great Britain). The other spectrophotometric measurements were carried out on a single-beam SPEKOL deflection spectrophotometer from the Carl Zeiss Co. (Jena, GDR). Measurements were carried out using quartz cuvettes with an internal diameter of 1.00 cm. The concentration of hydrogen ions was measured using a digital PHM 64 pH meter from Radiometer Research (Copenhagen, Denmark), with a combined GK 2401 electrode. The instrument was calibrated using standard solutions from the same company. Solutions. The solution of 5 x lop4 M brompyrogallol red (Lachema, Brno, Czechoslovakia) was prepared by grinding 0.0288 g of the purified indicator (2) with 20 ml of ethanol in a mortar. After complete dissolution, the solution was transferred to a loo-ml volumetric flask and diluted with distilled water to the mark. This solution was used within 48 h. The stock solution of 5 x 10m3 M TlNO, was prepared by dissolving 0.2664 g of the substance (BDH Chemicals Ltd., Poole, England) in 200 ml of distilled water. The stock solution of 5 x 10e4 M Tl(II1) was prepared (4) by oxidation of the lo-ml solution of Tl(I) using 3 ml of acidified bromine water (300 ml bromine water + 15 ml 0.1 M HCl), in the presence of 20 ml 2 M KCI. Excess bromine was removed by heating the solution on a water bath to 80°C. The colorless solution was cooled and diluted with distilled water to 100 ml. This solution has a pH of 2.8 and should be used within 14 days. The solution of 1 x 10m2 M Septonex (carbethoxypentadecyltrimethylammonium bromide, Spofa, Czechoslovakia) was prepared by dissolving 0.8448 g of the substance in 200 ml distilled water. The pH of the solution was adjusted using buffers (Walpole, L. Michaelis) prepared according to published procedures (5). The ionic strength was adjusted with 2 M KCl. RESULTS Absorption
AND DISCUSSION
Spectra
Figure 1 depicts the absorption spectra giving the curve for DG (curve l), the curve recorded after addition of 2 x 10m5 M Tl(II1) (curve 2), and the same curves in the presence of Septonex (curves 3 and 4), which exhibit slight hyperchromic and bathochromic shifts compared to the original curves. The measurements were carried out at the optimal pH for complex formation, It is apparent that, in both cases, the reaction of DG with Tl(II1) leads to a decrease in the absorption maximum of brompyrogallol red (A,, = 560 nm without Septonex and 582 nm with Septonex) and to the formation of a new maximum at 398 or 405 nm, corresponding to complex formation. Thus Tl(II1) can be determined using brompyrogallol red on the basis of either the decrease in the absorption maximum of DG or the increase in the absorption maximum of the complex. The former approach is more sensitive.
THALLIUM
A 1.6
-
1.2
-
REACTION
WITH
400
500
BROMPYROGALLOL
RED
303
0.6 -
FIG. 1. Absorption 4.0); and (4) TI(II1) x 10-4&f.
Optimal
Reaction
600
A In4
curves (1) DG (pH 5.7); (2) TI(II1) + DG (pH = 5.7); (3) DG + Septonex (pH + DG + Septonex (pH 4.0). coo = 3 x 10m5 M; cm(IIIj = 2 x 1O-5 M; cscpt, = 8
Conditions
It was found that the absorbance of the binary Tl(III)-DG system attains a maximum constant value in the range pH 5.6-6.2; in the presence of Septonex, this range is shifted to pH 3.9-4.9. Thus the addition of cationogenic tenside increases the selectivity of this spectrophotometric determination of Tl(II1). The reaction is quantitative at a metal:DG ratio of 1:l. The Septonex concentration must be at least 6 x 10e4 M, corresponding to the critical micelle concentration of Septonex. At laboratory temperature, maximum complex coloration is achieved after 5 min from the time of component mixing; the solution absorbance then remains practically constant for a period of 40 min. Heating in a boiling water bath did not yield better absorbance values and, in fact, heating for more than 6 min led to a decrease in the absorbance. In the presence of Septonex, the maximum absorbance is attained when the buffer is first mixed with the solution of Tl(II1) and DG and the solution is left to stand for 5 min. Then the Septonex solution is added, the volumetric flask is tilled to the mark with distilled water, and the absorbance is measured. The absorbance of solutions prepared in this way is constant for 20 h. Heating the solution led to a decrease in the absorbance value. The Effect of Ionic Strength The ionic strength of the binary Tl(III)-DG system has no effect on the position and height of the absorption maximum, as in the case of DG alone. In the presence of Septonex, the addition of a strong electrolyte affects the absorbance
NfiMCOVP;
304
0.2
0.4
ET AL.
0.6
0.0
I
2. The effect of the ionic strength on the absorbance of solutions containing Septonex. (1) DG + Septonex; (2) Tl(II1) + DG + Septonex (cDG = 3 X 10m5 M; cnclm = 2 x 10e5 M; cseP,, = 8 x 10-4 M; pH 4.0; A = 582 nm); (3) DG + Septonex; (4) Tl(II1) + DG + Septonex (cDG = 4 x 10-5 M; C,(,,I) = 3 x 10-S M; Csq. = 8 x 1O-4 M; pH 4.0; A = 405 nm). FIG.
of brompyrogallol red, especially at 582 nm (see Fig.2, curves 1 and 3). The absorbance of the ternary Tl(III)-DG-Septonex system at 582 nm attains a constant value in the ionic strength interval 0.5-0.8 (curve 2), while the presence of a strong electrolyte has little effect on the absorbance at 405 nm (curve 4). Up to an ionic strength of 0.2, the increase in the absorbance equals 5%; the absorbance then remains constant up to an ionic strength of 0.8. Thus it is preferable to carry out the determination of Tl(II1) with brompyrogallol red in the presence of Septonex at a wavelength of 405 nm. Composition of the Complex
It was found using the Job method of continuous variations that Tl(II1) and DG react to form a 1: 1 complex. The presence of Septonex has no effect on this ratio. Calibration Curve
It was found that, for the determined optimal conditions (pH 5.7; cno = 4 X 10e5 M) and at a wavelength of 560 nm, the binary Tl(III)-DG complex obeys the Beer-Lambert law in the concentration range 0.4-5.7 pg Tl(II1) * ml-l. The Sandell sensitivity is S = 4.2 x 1O-3 pg * cm-2; E = 4.7 x lo4 liters * mol-’ * cm- l. The relative standard deviation equals 0.7- 8.31%. At a wavelength of X = 398 nm, the Beer-Lambert law is obeyed in the range 0.7-6.9 pg Tl(II1) * ml-‘; S = 14.3 x lop3 kg * cm-2; E = 1.4 x lo4 liters * mole1 * cm-‘. In the presence of 8 x 10e4 M Septonex, cDo = 4 x 10e5 M, and pH 4.0 at A = 582 nm, the Beer-Lambert law is obeyed in the range 0.4-5.6 p,g Tl(II1) * ml-‘; S = 3.81 x 10e3 pg * cm-2; E = 5.3 x 104 liters * mol-’ * cm-‘. The relative standard deviation is 0.71-10.97%. At A = 405 nm, the Beer-Lambert law is obeyed in the concentration range 0.7-6.9 Fg Tl(II1) . ml-‘; S = 12.6 X 1O-3 pg . 10d3 pg. cm-2; E = 1.6 x lo4 liters * mol-1 * cm-l.
THALLIUM
REACTION
WITH BROMPYROGALLOL
RED
305
It follows from the values obtained that, in the presence of Septonex, the sensitivity and selectivity of the reaction are increased somewhat and the reproducibility is slightly decreased. The sensitivity of the determination is higher for measurement at the wavelength of the maximum of DG (or DC + Septonex); however, in the presence of Septonex, the measurement at this wavelength is affected by the ionic strength. Interferences
Amounts of foreign ions leading to an absorbance change of greater than 5% were considered to interfere. The measurements were carried out in the binary system at 560 nm and in the ternary system at 405 nm. In the binary system the determination was not disturbed by the ions Ca*+ , Ba2+, Mg*+ , Cl-, Br- , SOi-, PO:-, CO:-, F-, and NO; up to a ratio of 1:2000; Co*+ ions to a ratio of 1:lOOO; Mn*+ and Cd2+ ions to a ratio of 1:300; Hg*+ and Ni2+ ions to a ratio of 1:30; Zn2+, Cr3+, and Ag+ to a ratio of l:lO, Fe3+ to a ratio of 1:5; and AP+ and Pb*+ to a ratio of 1:0.5. The interference from Co2+ and Hg*+ increases in the ternary system (1:200 and 1:5, respectively), while that from Zn*+ (1:300) and Pb2+ (1:5) decreases. Procedure for the Determination of Tl(ZZZ)Ions The binary Tl(lZI)-DG system. An amount of thallium solution is pipetted into a 25-ml volumetric flask so that the final concentration is less than 5 kg/ml. Two drops of a 0.1% pentamethoxyl red solution are added and the acidity of the solution is adjusted with sodium acetate to yield the colorless form of the indicator. Then 8 ml of acetate buffer, pH 5.7, and 2 ml of 5 x 1O-4 M brompyrogall01 red are added. The flask is filled to the mark and the absorbance is measured after 5 min at 560 or 398 nm. The measurement must be carried out within 40 min after the reagents are mixed. The ternary Tl(ZIZ)-DG-Septonex system. As previously described, a Tl(II1) solution is pipetted into a 25ml-volumetric flask and the pH is adjusted, followed by addition of buffer with pH 4.0 and a brompyrogallol red solution. After the mixture stands for 5 min, 2 ml of 1 x lo-* M Septonex solution is added and the flask is tilled with distilled water to the mark. The absorbance is then measured at 405 or 582 nm. REFERENCES 1. 2. 3. 4. 5.
Dagnall, R. M.; West, T. S. Talanla, 1961, 8, 711-719. MatouSkovB, E.; NtmcovB, I.; Suk, V. Microchem. 1980, 25, 403-409. Dagnall, R. M.; West, T.S. Tdanta, 1964, 11, 1533-1541. Onishi R. Bull. Chem. Sot. Japan, 1956, 29, 945. Cihalik, J.; DvoTBk, J.; Suk, V. pH Measurement Handbook. SNTL, Prague, 1975.
JOURNAL
36, 301-305 (1987)
Spectrophotometric Brompyrogallol
Study of the Reaction of Thallium with Red: The Effect of the Cationogenic Tenside Septonex
I. NGMcovA,~ E. HAVELKOVA, V. SUK, AND B. EVTIMOVA* Department of Analytical Chemistry, Faculty of Natural Sciences, Charles University, Albertov 2030, 128 40 Prague 2, Czechoslovakia, and *Department of Analytical Chemistry, Faculty of Chemistry, The K. Ochridsky University, Sofia, Bulgaria
Received April 14, 1987; accepted June 30, 1987 The reaction of TI(III) with brompyrogallol red was studied and the effect of the cationogenie tenside Septonex on this reaction was considered. Optimal conditions were found for the formation of the binary and ternary complexes and regions of validity of the Beero 1987 Academic PXSS, IIIC. Lambert law were determined.
INTRODUCTION Brompyrogallol red (5,5’-dibrompyrogallolsulfophthalein, DG) forms colored complexes with trivalent and quadrivalent metal cations in acid medium and with divalent cations in neutral or alkaline medium. These complexes can be successfully used for the spectrophotometric determination of metals. The complexes are usually red, purple, or blue, except for the complexes of Ag(1) (I) and Au(II1) (2), which form yellow complexes with DG, with maximum absorbance at about 400 nm. The color of the complex with Ag(1) changes to blue in the presence of l,lOphenanthroline (A = 635 nm), with considerably increased sensitivity of the determination (3). The addition of the cationogenic tenside Septonex (2) leads to a slight increase in the sensitivity of the determination of the Au(II1) complex and shifts complex formation to lower pH values. We have found that Tl(II1) also yields a yellow complex with brompyrogallol red, with maximum absorbance at 398 nm. The optimum conditions for complex formation were studied and the effect of Septonex on this reaction was also followed. The results obtained were employed to develop a new spectrophotometric method for determining thallium, with sensitivity comparable to that of the most sensitive determinations of this metal. These methods are based mostly on the formation of the ion associate of the complex halide anion of Tl(II1) and a basic, mostly triphenylmethane or xanthene dye (e.g., rhodamine B (4)). The water-insoluble associates formed must be extracted into an organic solvent, while the complex formed between Tl(II1) and DG can be measured spectrophotometrically directly in the aqueous solution. Instruments.
The absorption
EXPERIMENTAL spectra were measured
on a recording
spectro-
r To whom correspondence should be addressed. 301 0026-265X/87 $1 SO Copyright 0 1987 by Academic Press, Inc. All tights of reproduction in any form reserved.
302
NtiMCOVli
ET AL.
photometer from Pye Unicam Instruments Ltd. (Cambridge, Great Britain). The other spectrophotometric measurements were carried out on a single-beam SPEKOL deflection spectrophotometer from the Carl Zeiss Co. (Jena, GDR). Measurements were carried out using quartz cuvettes with an internal diameter of 1.00 cm. The concentration of hydrogen ions was measured using a digital PHM 64 pH meter from Radiometer Research (Copenhagen, Denmark), with a combined GK 2401 electrode. The instrument was calibrated using standard solutions from the same company. Solutions. The solution of 5 x lop4 M brompyrogallol red (Lachema, Brno, Czechoslovakia) was prepared by grinding 0.0288 g of the purified indicator (2) with 20 ml of ethanol in a mortar. After complete dissolution, the solution was transferred to a loo-ml volumetric flask and diluted with distilled water to the mark. This solution was used within 48 h. The stock solution of 5 x 10m3 M TlNO, was prepared by dissolving 0.2664 g of the substance (BDH Chemicals Ltd., Poole, England) in 200 ml of distilled water. The stock solution of 5 x 10e4 M Tl(II1) was prepared (4) by oxidation of the lo-ml solution of Tl(I) using 3 ml of acidified bromine water (300 ml bromine water + 15 ml 0.1 M HCl), in the presence of 20 ml 2 M KCI. Excess bromine was removed by heating the solution on a water bath to 80°C. The colorless solution was cooled and diluted with distilled water to 100 ml. This solution has a pH of 2.8 and should be used within 14 days. The solution of 1 x 10m2 M Septonex (carbethoxypentadecyltrimethylammonium bromide, Spofa, Czechoslovakia) was prepared by dissolving 0.8448 g of the substance in 200 ml distilled water. The pH of the solution was adjusted using buffers (Walpole, L. Michaelis) prepared according to published procedures (5). The ionic strength was adjusted with 2 M KCl. RESULTS Absorption
AND DISCUSSION
Spectra
Figure 1 depicts the absorption spectra giving the curve for DG (curve l), the curve recorded after addition of 2 x 10m5 M Tl(II1) (curve 2), and the same curves in the presence of Septonex (curves 3 and 4), which exhibit slight hyperchromic and bathochromic shifts compared to the original curves. The measurements were carried out at the optimal pH for complex formation, It is apparent that, in both cases, the reaction of DG with Tl(II1) leads to a decrease in the absorption maximum of brompyrogallol red (A,, = 560 nm without Septonex and 582 nm with Septonex) and to the formation of a new maximum at 398 or 405 nm, corresponding to complex formation. Thus Tl(II1) can be determined using brompyrogallol red on the basis of either the decrease in the absorption maximum of DG or the increase in the absorption maximum of the complex. The former approach is more sensitive.
THALLIUM
A 1.6
-
1.2
-
REACTION
WITH
400
500
BROMPYROGALLOL
RED
303
0.6 -
FIG. 1. Absorption 4.0); and (4) TI(II1) x 10-4&f.
Optimal
Reaction
600
A In4
curves (1) DG (pH 5.7); (2) TI(II1) + DG (pH = 5.7); (3) DG + Septonex (pH + DG + Septonex (pH 4.0). coo = 3 x 10m5 M; cm(IIIj = 2 x 1O-5 M; cscpt, = 8
Conditions
It was found that the absorbance of the binary Tl(III)-DG system attains a maximum constant value in the range pH 5.6-6.2; in the presence of Septonex, this range is shifted to pH 3.9-4.9. Thus the addition of cationogenic tenside increases the selectivity of this spectrophotometric determination of Tl(II1). The reaction is quantitative at a metal:DG ratio of 1:l. The Septonex concentration must be at least 6 x 10e4 M, corresponding to the critical micelle concentration of Septonex. At laboratory temperature, maximum complex coloration is achieved after 5 min from the time of component mixing; the solution absorbance then remains practically constant for a period of 40 min. Heating in a boiling water bath did not yield better absorbance values and, in fact, heating for more than 6 min led to a decrease in the absorbance. In the presence of Septonex, the maximum absorbance is attained when the buffer is first mixed with the solution of Tl(II1) and DG and the solution is left to stand for 5 min. Then the Septonex solution is added, the volumetric flask is tilled to the mark with distilled water, and the absorbance is measured. The absorbance of solutions prepared in this way is constant for 20 h. Heating the solution led to a decrease in the absorbance value. The Effect of Ionic Strength The ionic strength of the binary Tl(III)-DG system has no effect on the position and height of the absorption maximum, as in the case of DG alone. In the presence of Septonex, the addition of a strong electrolyte affects the absorbance
NfiMCOVP;
304
0.2
0.4
ET AL.
0.6
0.0
I
2. The effect of the ionic strength on the absorbance of solutions containing Septonex. (1) DG + Septonex; (2) Tl(II1) + DG + Septonex (cDG = 3 X 10m5 M; cnclm = 2 x 10e5 M; cseP,, = 8 x 10-4 M; pH 4.0; A = 582 nm); (3) DG + Septonex; (4) Tl(II1) + DG + Septonex (cDG = 4 x 10-5 M; C,(,,I) = 3 x 10-S M; Csq. = 8 x 1O-4 M; pH 4.0; A = 405 nm). FIG.
of brompyrogallol red, especially at 582 nm (see Fig.2, curves 1 and 3). The absorbance of the ternary Tl(III)-DG-Septonex system at 582 nm attains a constant value in the ionic strength interval 0.5-0.8 (curve 2), while the presence of a strong electrolyte has little effect on the absorbance at 405 nm (curve 4). Up to an ionic strength of 0.2, the increase in the absorbance equals 5%; the absorbance then remains constant up to an ionic strength of 0.8. Thus it is preferable to carry out the determination of Tl(II1) with brompyrogallol red in the presence of Septonex at a wavelength of 405 nm. Composition of the Complex
It was found using the Job method of continuous variations that Tl(II1) and DG react to form a 1: 1 complex. The presence of Septonex has no effect on this ratio. Calibration Curve
It was found that, for the determined optimal conditions (pH 5.7; cno = 4 X 10e5 M) and at a wavelength of 560 nm, the binary Tl(III)-DG complex obeys the Beer-Lambert law in the concentration range 0.4-5.7 pg Tl(II1) * ml-l. The Sandell sensitivity is S = 4.2 x 1O-3 pg * cm-2; E = 4.7 x lo4 liters * mol-’ * cm- l. The relative standard deviation equals 0.7- 8.31%. At a wavelength of X = 398 nm, the Beer-Lambert law is obeyed in the range 0.7-6.9 pg Tl(II1) * ml-‘; S = 14.3 x lop3 kg * cm-2; E = 1.4 x lo4 liters * mole1 * cm-‘. In the presence of 8 x 10e4 M Septonex, cDo = 4 x 10e5 M, and pH 4.0 at A = 582 nm, the Beer-Lambert law is obeyed in the range 0.4-5.6 p,g Tl(II1) * ml-‘; S = 3.81 x 10e3 pg * cm-2; E = 5.3 x 104 liters * mol-’ * cm-‘. The relative standard deviation is 0.71-10.97%. At A = 405 nm, the Beer-Lambert law is obeyed in the concentration range 0.7-6.9 Fg Tl(II1) . ml-‘; S = 12.6 X 1O-3 pg . 10d3 pg. cm-2; E = 1.6 x lo4 liters * mol-1 * cm-l.
THALLIUM
REACTION
WITH BROMPYROGALLOL
RED
305
It follows from the values obtained that, in the presence of Septonex, the sensitivity and selectivity of the reaction are increased somewhat and the reproducibility is slightly decreased. The sensitivity of the determination is higher for measurement at the wavelength of the maximum of DG (or DC + Septonex); however, in the presence of Septonex, the measurement at this wavelength is affected by the ionic strength. Interferences
Amounts of foreign ions leading to an absorbance change of greater than 5% were considered to interfere. The measurements were carried out in the binary system at 560 nm and in the ternary system at 405 nm. In the binary system the determination was not disturbed by the ions Ca*+ , Ba2+, Mg*+ , Cl-, Br- , SOi-, PO:-, CO:-, F-, and NO; up to a ratio of 1:2000; Co*+ ions to a ratio of 1:lOOO; Mn*+ and Cd2+ ions to a ratio of 1:300; Hg*+ and Ni2+ ions to a ratio of 1:30; Zn2+, Cr3+, and Ag+ to a ratio of l:lO, Fe3+ to a ratio of 1:5; and AP+ and Pb*+ to a ratio of 1:0.5. The interference from Co2+ and Hg*+ increases in the ternary system (1:200 and 1:5, respectively), while that from Zn*+ (1:300) and Pb2+ (1:5) decreases. Procedure for the Determination of Tl(ZZZ)Ions The binary Tl(lZI)-DG system. An amount of thallium solution is pipetted into a 25-ml volumetric flask so that the final concentration is less than 5 kg/ml. Two drops of a 0.1% pentamethoxyl red solution are added and the acidity of the solution is adjusted with sodium acetate to yield the colorless form of the indicator. Then 8 ml of acetate buffer, pH 5.7, and 2 ml of 5 x 1O-4 M brompyrogall01 red are added. The flask is filled to the mark and the absorbance is measured after 5 min at 560 or 398 nm. The measurement must be carried out within 40 min after the reagents are mixed. The ternary Tl(ZIZ)-DG-Septonex system. As previously described, a Tl(II1) solution is pipetted into a 25ml-volumetric flask and the pH is adjusted, followed by addition of buffer with pH 4.0 and a brompyrogallol red solution. After the mixture stands for 5 min, 2 ml of 1 x lo-* M Septonex solution is added and the flask is tilled with distilled water to the mark. The absorbance is then measured at 405 or 582 nm. REFERENCES 1. 2. 3. 4. 5.
Dagnall, R. M.; West, T. S. Talanla, 1961, 8, 711-719. MatouSkovB, E.; NtmcovB, I.; Suk, V. Microchem. 1980, 25, 403-409. Dagnall, R. M.; West, T.S. Tdanta, 1964, 11, 1533-1541. Onishi R. Bull. Chem. Sot. Japan, 1956, 29, 945. Cihalik, J.; DvoTBk, J.; Suk, V. pH Measurement Handbook. SNTL, Prague, 1975.