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Pyrocatecholsulphonphthalein complexan as an analytical reagent
Talmo, Vol. 31, No. 12, pp. 1121-1124, Printed in Great Britain
1984
0039-9140/84 $3.00 + 0.00 Pergamon Press Ltd
ANALYTICAL
DATA
PYROCATECHOLSULPHONPHTHALEIN COMPLEXAN AS AN ANALYTICAL REAGENT* Ru-QIN
Department
of Chemistry
Yu,
ZHENG-QI
and Chemical
(Received
ZHANG
Kiirbl et al.’ reported that a number of chelating agents can be obtained from Pyrocatechol Violet (PV), Pyrogallol Red and rosolic acid by condensation with iminodiacetic acid and formaldehyde according to the procedure described by Schwarzenbath et al.,’ but the synthesis and analytical application of these products have not been described. The acid-base indicator Cresol Red, from which Xylenol Orange (X0) is derived, is structurally related to PV. The structure of the product derived from condensation of PV and iminodiacetic acid therefore seems likely to be
so,
0
This compound, pyrocatecholsulphonphthalein complexan (PSC), is in essence X0 with its methyl groups replaced by phenolic groups, so PSC has the functional groups of both X0 and PV. The question then is whether PSC possesses the characteristic properties of both X0 and PV. For complexometric titration of bismuth, a mixture of PV and X0 has been recommended as the indicator.3 PSC may therefore give even better results for such titrations. In this paper the synthesis of PSC and its characteristic properties as an analytical reagent are reported. EXPERIMENTAL
of PSC
Finely powdered PV (4.0 g) and iminodiacetic acid (5.4 g) were dissolved in 15 ml of water containing 3.2 g of sodium hydroxide, and 80 ml of glacial acetic acid were added, with *Research supported by Chem. Grant No. 566 of Science Foundation, Academia Sinica, PRC. 1121
ZHANG
Changsha,
People’s Republic
of
22 June 1984)
(PSC) has been synthesized from Pyrocatechol by Mannich condensation. Its acid dissociation characteristic properties of Xylenol Orange (X0) X0 and PV in sensitivity as a chromogenic reagent of some metal ion-PSC complexes are reported. stirring. The mixture was transferred into a 250-ml threenecked, round-bottomed flask fitted with a reflux condenser, stirrer and dropping funnel containing 40% aqueous formaldehyde. After addition of 2.4 ml of formaldehyde the mixture was heated at 55-60” for 12 hr, with stirring. A further 2.4 ml of formaldehyde were added and the mixture was stirred for a total of 7 days at 55-60”. The formation of PSC was checked before proceeding further. To do this, a few drops of reaction mixture were added to 5 ml of acetone; the dark-green precipitate produced was separated and dissolved in a small amount of water. The solution gave a purple-red colour with La(II1) in acetate buffer at pH 5.0. Completion of the Mannich reaction was identified by paper chromatography with butan-1-ol-glacial acetic acid-water (3: 1: 1, v/v). The reaction was considered complete if the chromatogram showed only a green spot at R, = 0. The hot reaction mixture was then filtered by suction, and the filtrate was poured into 1 litre of acetone, with stirring. After standing for I hr the dark-green precipitate was filtered off under suction and washed with a small amount of acetone. The product was dried at 40” under vacuum. The yield was about 5.6 g of crude product (56%). Purification
Scheme 1.
Synthesis
ZHI-HUA
University,
10 May 1983. Revised 7 May 1984. Accepted
Summary-Pyrocatecholsulphonphthalein complexan Violet (PV), iminodiacetic acid and formaldehyde equilibria have been studied potentiometrically. Some and PV are found in PSC, which is slightly superior to for bismuth. The spectrophotometric characteristics
0
and
Engineering, Hunan China
and analysis of PSC
A 2.0-cm bore chromatographic column 80 cm long was prepared with Polyamide 6 (Beijing). About 5 g of the crude product was dissolved in the minimum amount of water, applied to the top of the column, and eluted with water at a flow-rate of 0.5 ml/min. The green fraction of the eluate (ca. 50 ml) was evaporated to dryness at 40” under vacuum. The product was evaluated by paper chromatography as described above, and also by thin-layer chromatography. A 9: 1 w/w mixture of Polyamide 6 and cellulose powder slurried with methanol was spread on a 6.5 x 18 cm glass plate to give a layer 0.3-0.5 mm thick. The sample, dissolved in water, was applied to the air-dried plate and the chromatogram developed with 0.05M sodium hydroxide (R,= 0.83). The product was a dark green hvgroscooic solid with metallic lustre. Analysis: C,H,,N,O,,S requires 4.08%N, 4.66x& found, 4.2%N, 4.5x.S. The principal infrared bands were at 525, 700, 900, 1380, 1590, 2980 and 3350 cm-‘.
All solutions were prepared with demineralized water and analytical-reagent grade chemicals unless otherwise stated. The stock solution of lO_*A4 PSC was prepared from the purified reagent and diluted as required. Cetyltrimethylammonium bromide (CTMAB, Fluka) was used
1122
ANALYTICAL
without further purification. Standard solutions of rareearth ions were prepared from the corresponding spectrally pure oxides (except La,O,, 99.99% and Y,O,, 99.999% pure). Solutions of other metal ions were standardized, when necessary, by EDTA titration. The carbonate-free hydroxide solutions used in the potentiometric titrations n = KJH+]
Potentiometric studies of dissociation equilibria PSC solutions were potentiometrically titrated with potassium hydroxide under an atmosphere of nitrogen.“ The flow of nitrogen was stopped during readings. The degree of neutralization a and average number of bound protons R were calculated and the A-pH curves constructed. The ionization constants were calculated by the Bjerrum method.5 AND DISCUSSlON
and purijication
of PSC
Preliminary experiments showed that in aqueous alcohol medium the Mannich reaction was difficult to control, owing to the tendency to polymerization. Of the tested reaction media, glacial acetic acid gave the best yield. The paper chromatography described showed that the spot of crude product remained at the starting point (R,= 0) and did not contain unreacted PV (R, = 0.45). For the separation on the Polyamide 6 column, water was used as the eluent to avoid introducing alkali-metal ions. A blue fraction of impurities appeared before the green fraction for some batches of crude product. The blue fraction was discarded and only the green fraction collected. The main part of the impurities (brown in colour) remained on the column. Dissociation
tertiary
amino
groups
(&-K,,,)
takes
place in acidic medium. The constants J&K~,, , are related to the dissociation of the carboxyl and phenol groups. The Bjerrum formation function fi for the protonation of L9- may be written as
+ K,.,K.,,*[H+12 + .
were prepared by passing 0.2M potassium chloride through a column (8 mm bore, 10 mm long) of Zerolit FF (OHform, 200 mesh). The potassium hydroxide solution obtained was collected in a thisk protected with a soda-lime tube, and standardized by acid-base titration.
Synthesis
protonated
+ 2Ka,,K,,z[H+]2 + . . . + 11K,zK~~2. . Ka,,,[H+]”
1 + KJH+]
RESULTS
DATA
+ K,,,K.,,2.
(1)
. K =.,,[H+]”
and can be calculated
from the equation?
ii = (9 - a)T,,
+ PHml T P=
- W+l
(2)
where Tpscis the total PSC concentration and a is the degree of neutralization, defined as ratio of moles of sodium hydroxide added to moles of PSC taken. The formation curves for protonation of L9- in the absence and presence of CTMAB (1.5 x lo-‘M) are shown in Fig. 1. The PK.,,, values calculated are presented in Table 1. The presence of CTMAB inhibits the dissociation of the first carboxylic acid group (KJ, but promotes the remaining steps. Figure 2 shows the absorption spectra of PSC at various pH values. The equilibrium corresponding to constant Ka,, is mainly responsible for the colour change of the indicator from yellow to blue. On addition of CTMAB to the yellow PSC solution at pH 5.0, the Ka,, value increases remarkably and the solution turns blue. The values of Ka,, and K& can easily be determined spectrophotometrically. From the pH-absorbance curves at 620 nm (for Ka,,) or 610 nm (for K&), pK,,, and pK& have been found to be 5.36 and 4.49, respectively. These values are in good agreement with those obtained from potentiometric measurements.
of PSC
Under strongly acidic conditions PSC can take up two protons to give a form that will be represented by H,,L2+. The acid-base equilibria of PSC are: 80 Ia
H,,L:‘+H,,L+~H,L+==+
H&
*
H,L’-
PK,.? G+
7.0 -
P&J
H,L3- * .I
60
-
50
-
Ph.6
40
H 5L4- ++
H,L’-
pKa.9
d
20
H2L7m +
HL8- &=!Z% d
The dissociation
-
30-
a pK,8 H3L6- 2
d
-
,
,
I
I
n
~9
P&II
of the sulphonate
I
12345678
group and the two
Fig. 1. The formation curves for protonation of PSC (L9.-): I, in the absence of CTMAB; II, in the presence of CTMAB (1.5 x 10~‘M).
ANALYTICAL
1123
DATA
Table 1. Acid dissociation constants of PSC pi,,
2.03 4
3.08 5
pKI,
2.60 2.95
,JL,LlW+l. ‘.I F-L;+,Ll
K
’
9.36 8 10.20 9 11.40 10 12.15 11 3.92 6 5.40 7 3.30 4.50
8.25
0.5
9.65 10.60 11.70
lomc strength = 0.1 (KCI); the K:,,
values are the apparent acid dissociation constants determined in the presence of CTMAB.
0.4 z 5 a k 2 a
PSC forms intensely coloured water-soluble complexes with many metal ions. The characteristics of these reactions are summarized in Table 2. The complexation reaction of Bi(II1) with PSC has been taken as an example for more detailed investigation. In 0.10M nitric acid medium, at least two Bi(III)-PSC complex species are formed. In the presence of excess of Bi(III), a blue 1: 1 complex is formed, with absorption maximum at 620 nm. When PSC is in excess, a red complex with metal:ligand ratio of I:2 is formed, with an absorption maximum at 505 nm. The stepwise stability constants of the 1: 1 and 1:2 complexes, determined by the method of corresponding solutions modified by Guan et al.’ are log K, = 5.45 and log K2 = 3.89 (in O.lM nitric acid/O. 1M potassium nitrate medium). The apparent molar absorptivity of the 1:2 complex is 2.15 x lo4
Metal ion AI(II1) Be(I1) Bi(II1) Cd(I1) Ce(II1) Co(II1) Cr(II1) Cu(II1) Er(II1) Fe(U) Fe(II1) Ga(II1) Gd(II1) Hg(II) La(II1) Mg(II) Mn(I1) Nb(V) Nd(II1) Ni(I1) Pb(I1) Pr(II1) Sm(II1) Sn(I1) Sn(IV) V(V) Y(II1) Zn(I1) Zr(IV) *HOAc-NaOAc tHOAc-NaOAc
0.2
0.1
Colour reactions of PSC with metal ions
Table 2. Spectrophotometric
0.3
0.0 420
600
500 X
(nm)
Fig. 2. Absorption spectra of PSC at various pH values: 1, 2.41; 2, 3.30; 3, 4.00; 4, 6.10; 5, 9.46; 6, 10.50; 7, 11.80; 8, 13.00.
l.mole-‘.cm-‘. The sensitivity of PSC as a colorimetric reagent for Bi(II1) is better than that of X0 (&530 = 1.6 x lo4 l.mole-‘.cm-‘)’ or PV(&,,, = 1.37 x lo4 1. mole-‘. cmm’).9 PSC is a useful indicator for the EDTA titration of bismuth, giving a colour change from blue to yellow in 0.5M nitric acid medium. A transient purple-red colour develops just before the equivalence point, and is a useful signal of the approaching end-point. The relative standard devi-
characteristics of metal-PSC complexes formed with excess of PSC PH*
4.0 5.0 O.lM HNO, 5.0 4.2 5.0 4.0t 5.0 4.20 5.0 3.6 1.5 (HNO,) 4.20 5.0 4.20 10.0 5.0 2.4 (ClCH,COOH-HCI) 4.20 5.0 5.0 4.20 4.20 4.0 O.lM HC1 3.615 4.20 5.0 O.lM H2S0,
E.__, nm 530 500 505 580 550 560 530 560 550 570 530 520 550 510 580 550 530 530 550 620 540 550 550 530 530 540 560 550 530
6, lo4
I.mole-‘.cm-’
0.26 0.32 2.15 0.70 1.50 0.94 1.31 1.06 1.09 1.20 1.84 0.55 1.28 0.54 1.81 1.25 0.51 1.50 0.93 1.23 1.20 1.oo 0.95 1.00 1.87 0.86 1.85 1.70 1.80
buffer unless otherwise stated. buffer containing ascorbic acid. The colour develops after heating in
a boiling water-bath for 10 min. §Heating in a boiling water-bath for 5 min.
700
1124
ANALYTICAL
ation is found to be 0.06%. Details will be published elsewhere. In acetate solution buffered at pH 4.2, lanthanum(II1) forms a 1:2 red complex with PSC, apparent stability constant 1.23 x lo9 (PH 4.2, ionic strength O.lM). Similarly, yttrium give a 1:2 complex with an apparent stability constant of 6.17 x 10’ (pH 4.2, ionic strength O.lM). In the presence of CTMAB, lanthanum and yttrium form blue ternary complexes with PSC, the molar absorptivities of the La(III) and Y(III) complexes being 6.5 x lo4 (650 nm) and 7.31 x lo4 l.mole-’ .cm-’ (640 nm), respectively (for [metal] = 1 x lo-‘M, [PSC] = 3.5 x 10m4M). A preliminary investigation shows that the increase in sensitivity seems to be the formation of complexes with unusual metal-to-ligand molar ratios. It is interesting that addition of an organic solvent such as ethylene glycol depresses the formation of the lanthanum ternary complex but remarkably enhances the absorptivity of the Y(IIItPSC-CTMAB species. This effect can be utilized for the determination of yttrium in the presence of lanthanum. When PSC solution is mixed with an aged zirconium(IV) solution, a slow complexation reaction takes place. Trace amounts of fluoride catalyse this reaction, even at a fluoride concentration as low as 5.0 x lo-“M. This provides a promising means of kinetic assay of traces of fluoride. It is difficult to detect fluoride in water at levels below lo-‘M with a
DATA
lanthanum fluoride electrode, because traces of fluoride are released from the membrane surface itself into the test solution, and the PSC-Zr(IV) reaction will easily distinguish between two samples of demineralized water, one of which has been in brief contact with a lanthanum fluoride electrode. This is a very sensitive qualitative test, but its analytical characteristics depend on the history of the zirconium solution, and quantitative application will require further investigation. Acknowledgement-The authors are grateful to Professor Shu-Chuan Liang, Institute of Chemistry, Academia Sinica, Beijing, for valuable comments and encouragement. REFERENCES 1. J. Kiirbl, V. Svoboda and D. Terziska, Chem. Ind. London, 1958, 1232. 2. G. Anderegg, H. Flaschka, R. Sallmann and G. Schwarzenbach, Helv. Chim. Acta, 1954, 37, 113. 3. N. Zhou, Huaxue Shiji 1979, 273. 4. A. Albert and E. P. Serjeant, Ionization Constants of Acids and Bases, pp. 16-68. Methuen, London, 1962. 5. J. Bjerrum, Metal Amine Formation in Aqueous Solution, Haase, Copenhagen, 1941. 6. J. Inc&dy, Analytical Applications of Complex Equilibria, D. 98. Horwood. Chichester. 1976. 7. Y. W. &an and G. X. Xu, Acta Chim. Sinica, 1963.29, 37. 8. B. Bud%insk$, in Chelates in Analytical Chemistry, H. A. Flaschka and A. J. Barnard (eds.), Vol. 1, p. 21. Dekker, New York, 1967. 9. M. Malit, Z. Anal. Chem., 1962, 186, 418.
1984
0039-9140/84 $3.00 + 0.00 Pergamon Press Ltd
ANALYTICAL
DATA
PYROCATECHOLSULPHONPHTHALEIN COMPLEXAN AS AN ANALYTICAL REAGENT* Ru-QIN
Department
of Chemistry
Yu,
ZHENG-QI
and Chemical
(Received
ZHANG
Kiirbl et al.’ reported that a number of chelating agents can be obtained from Pyrocatechol Violet (PV), Pyrogallol Red and rosolic acid by condensation with iminodiacetic acid and formaldehyde according to the procedure described by Schwarzenbath et al.,’ but the synthesis and analytical application of these products have not been described. The acid-base indicator Cresol Red, from which Xylenol Orange (X0) is derived, is structurally related to PV. The structure of the product derived from condensation of PV and iminodiacetic acid therefore seems likely to be
so,
0
This compound, pyrocatecholsulphonphthalein complexan (PSC), is in essence X0 with its methyl groups replaced by phenolic groups, so PSC has the functional groups of both X0 and PV. The question then is whether PSC possesses the characteristic properties of both X0 and PV. For complexometric titration of bismuth, a mixture of PV and X0 has been recommended as the indicator.3 PSC may therefore give even better results for such titrations. In this paper the synthesis of PSC and its characteristic properties as an analytical reagent are reported. EXPERIMENTAL
of PSC
Finely powdered PV (4.0 g) and iminodiacetic acid (5.4 g) were dissolved in 15 ml of water containing 3.2 g of sodium hydroxide, and 80 ml of glacial acetic acid were added, with *Research supported by Chem. Grant No. 566 of Science Foundation, Academia Sinica, PRC. 1121
ZHANG
Changsha,
People’s Republic
of
22 June 1984)
(PSC) has been synthesized from Pyrocatechol by Mannich condensation. Its acid dissociation characteristic properties of Xylenol Orange (X0) X0 and PV in sensitivity as a chromogenic reagent of some metal ion-PSC complexes are reported. stirring. The mixture was transferred into a 250-ml threenecked, round-bottomed flask fitted with a reflux condenser, stirrer and dropping funnel containing 40% aqueous formaldehyde. After addition of 2.4 ml of formaldehyde the mixture was heated at 55-60” for 12 hr, with stirring. A further 2.4 ml of formaldehyde were added and the mixture was stirred for a total of 7 days at 55-60”. The formation of PSC was checked before proceeding further. To do this, a few drops of reaction mixture were added to 5 ml of acetone; the dark-green precipitate produced was separated and dissolved in a small amount of water. The solution gave a purple-red colour with La(II1) in acetate buffer at pH 5.0. Completion of the Mannich reaction was identified by paper chromatography with butan-1-ol-glacial acetic acid-water (3: 1: 1, v/v). The reaction was considered complete if the chromatogram showed only a green spot at R, = 0. The hot reaction mixture was then filtered by suction, and the filtrate was poured into 1 litre of acetone, with stirring. After standing for I hr the dark-green precipitate was filtered off under suction and washed with a small amount of acetone. The product was dried at 40” under vacuum. The yield was about 5.6 g of crude product (56%). Purification
Scheme 1.
Synthesis
ZHI-HUA
University,
10 May 1983. Revised 7 May 1984. Accepted
Summary-Pyrocatecholsulphonphthalein complexan Violet (PV), iminodiacetic acid and formaldehyde equilibria have been studied potentiometrically. Some and PV are found in PSC, which is slightly superior to for bismuth. The spectrophotometric characteristics
0
and
Engineering, Hunan China
and analysis of PSC
A 2.0-cm bore chromatographic column 80 cm long was prepared with Polyamide 6 (Beijing). About 5 g of the crude product was dissolved in the minimum amount of water, applied to the top of the column, and eluted with water at a flow-rate of 0.5 ml/min. The green fraction of the eluate (ca. 50 ml) was evaporated to dryness at 40” under vacuum. The product was evaluated by paper chromatography as described above, and also by thin-layer chromatography. A 9: 1 w/w mixture of Polyamide 6 and cellulose powder slurried with methanol was spread on a 6.5 x 18 cm glass plate to give a layer 0.3-0.5 mm thick. The sample, dissolved in water, was applied to the air-dried plate and the chromatogram developed with 0.05M sodium hydroxide (R,= 0.83). The product was a dark green hvgroscooic solid with metallic lustre. Analysis: C,H,,N,O,,S requires 4.08%N, 4.66x& found, 4.2%N, 4.5x.S. The principal infrared bands were at 525, 700, 900, 1380, 1590, 2980 and 3350 cm-‘.
All solutions were prepared with demineralized water and analytical-reagent grade chemicals unless otherwise stated. The stock solution of lO_*A4 PSC was prepared from the purified reagent and diluted as required. Cetyltrimethylammonium bromide (CTMAB, Fluka) was used
1122
ANALYTICAL
without further purification. Standard solutions of rareearth ions were prepared from the corresponding spectrally pure oxides (except La,O,, 99.99% and Y,O,, 99.999% pure). Solutions of other metal ions were standardized, when necessary, by EDTA titration. The carbonate-free hydroxide solutions used in the potentiometric titrations n = KJH+]
Potentiometric studies of dissociation equilibria PSC solutions were potentiometrically titrated with potassium hydroxide under an atmosphere of nitrogen.“ The flow of nitrogen was stopped during readings. The degree of neutralization a and average number of bound protons R were calculated and the A-pH curves constructed. The ionization constants were calculated by the Bjerrum method.5 AND DISCUSSlON
and purijication
of PSC
Preliminary experiments showed that in aqueous alcohol medium the Mannich reaction was difficult to control, owing to the tendency to polymerization. Of the tested reaction media, glacial acetic acid gave the best yield. The paper chromatography described showed that the spot of crude product remained at the starting point (R,= 0) and did not contain unreacted PV (R, = 0.45). For the separation on the Polyamide 6 column, water was used as the eluent to avoid introducing alkali-metal ions. A blue fraction of impurities appeared before the green fraction for some batches of crude product. The blue fraction was discarded and only the green fraction collected. The main part of the impurities (brown in colour) remained on the column. Dissociation
tertiary
amino
groups
(&-K,,,)
takes
place in acidic medium. The constants J&K~,, , are related to the dissociation of the carboxyl and phenol groups. The Bjerrum formation function fi for the protonation of L9- may be written as
+ K,.,K.,,*[H+12 + .
were prepared by passing 0.2M potassium chloride through a column (8 mm bore, 10 mm long) of Zerolit FF (OHform, 200 mesh). The potassium hydroxide solution obtained was collected in a thisk protected with a soda-lime tube, and standardized by acid-base titration.
Synthesis
protonated
+ 2Ka,,K,,z[H+]2 + . . . + 11K,zK~~2. . Ka,,,[H+]”
1 + KJH+]
RESULTS
DATA
+ K,,,K.,,2.
(1)
. K =.,,[H+]”
and can be calculated
from the equation?
ii = (9 - a)T,,
+ PHml T P=
- W+l
(2)
where Tpscis the total PSC concentration and a is the degree of neutralization, defined as ratio of moles of sodium hydroxide added to moles of PSC taken. The formation curves for protonation of L9- in the absence and presence of CTMAB (1.5 x lo-‘M) are shown in Fig. 1. The PK.,,, values calculated are presented in Table 1. The presence of CTMAB inhibits the dissociation of the first carboxylic acid group (KJ, but promotes the remaining steps. Figure 2 shows the absorption spectra of PSC at various pH values. The equilibrium corresponding to constant Ka,, is mainly responsible for the colour change of the indicator from yellow to blue. On addition of CTMAB to the yellow PSC solution at pH 5.0, the Ka,, value increases remarkably and the solution turns blue. The values of Ka,, and K& can easily be determined spectrophotometrically. From the pH-absorbance curves at 620 nm (for Ka,,) or 610 nm (for K&), pK,,, and pK& have been found to be 5.36 and 4.49, respectively. These values are in good agreement with those obtained from potentiometric measurements.
of PSC
Under strongly acidic conditions PSC can take up two protons to give a form that will be represented by H,,L2+. The acid-base equilibria of PSC are: 80 Ia
H,,L:‘+H,,L+~H,L+==+
H&
*
H,L’-
PK,.? G+
7.0 -
P&J
H,L3- * .I
60
-
50
-
Ph.6
40
H 5L4- ++
H,L’-
pKa.9
d
20
H2L7m +
HL8- &=!Z% d
The dissociation
-
30-
a pK,8 H3L6- 2
d
-
,
,
I
I
n
~9
P&II
of the sulphonate
I
12345678
group and the two
Fig. 1. The formation curves for protonation of PSC (L9.-): I, in the absence of CTMAB; II, in the presence of CTMAB (1.5 x 10~‘M).
ANALYTICAL
1123
DATA
Table 1. Acid dissociation constants of PSC pi,,
2.03 4
3.08 5
pKI,
2.60 2.95
,JL,LlW+l. ‘.I F-L;+,Ll
K
’
9.36 8 10.20 9 11.40 10 12.15 11 3.92 6 5.40 7 3.30 4.50
8.25
0.5
9.65 10.60 11.70
lomc strength = 0.1 (KCI); the K:,,
values are the apparent acid dissociation constants determined in the presence of CTMAB.
0.4 z 5 a k 2 a
PSC forms intensely coloured water-soluble complexes with many metal ions. The characteristics of these reactions are summarized in Table 2. The complexation reaction of Bi(II1) with PSC has been taken as an example for more detailed investigation. In 0.10M nitric acid medium, at least two Bi(III)-PSC complex species are formed. In the presence of excess of Bi(III), a blue 1: 1 complex is formed, with absorption maximum at 620 nm. When PSC is in excess, a red complex with metal:ligand ratio of I:2 is formed, with an absorption maximum at 505 nm. The stepwise stability constants of the 1: 1 and 1:2 complexes, determined by the method of corresponding solutions modified by Guan et al.’ are log K, = 5.45 and log K2 = 3.89 (in O.lM nitric acid/O. 1M potassium nitrate medium). The apparent molar absorptivity of the 1:2 complex is 2.15 x lo4
Metal ion AI(II1) Be(I1) Bi(II1) Cd(I1) Ce(II1) Co(II1) Cr(II1) Cu(II1) Er(II1) Fe(U) Fe(II1) Ga(II1) Gd(II1) Hg(II) La(II1) Mg(II) Mn(I1) Nb(V) Nd(II1) Ni(I1) Pb(I1) Pr(II1) Sm(II1) Sn(I1) Sn(IV) V(V) Y(II1) Zn(I1) Zr(IV) *HOAc-NaOAc tHOAc-NaOAc
0.2
0.1
Colour reactions of PSC with metal ions
Table 2. Spectrophotometric
0.3
0.0 420
600
500 X
(nm)
Fig. 2. Absorption spectra of PSC at various pH values: 1, 2.41; 2, 3.30; 3, 4.00; 4, 6.10; 5, 9.46; 6, 10.50; 7, 11.80; 8, 13.00.
l.mole-‘.cm-‘. The sensitivity of PSC as a colorimetric reagent for Bi(II1) is better than that of X0 (&530 = 1.6 x lo4 l.mole-‘.cm-‘)’ or PV(&,,, = 1.37 x lo4 1. mole-‘. cmm’).9 PSC is a useful indicator for the EDTA titration of bismuth, giving a colour change from blue to yellow in 0.5M nitric acid medium. A transient purple-red colour develops just before the equivalence point, and is a useful signal of the approaching end-point. The relative standard devi-
characteristics of metal-PSC complexes formed with excess of PSC PH*
4.0 5.0 O.lM HNO, 5.0 4.2 5.0 4.0t 5.0 4.20 5.0 3.6 1.5 (HNO,) 4.20 5.0 4.20 10.0 5.0 2.4 (ClCH,COOH-HCI) 4.20 5.0 5.0 4.20 4.20 4.0 O.lM HC1 3.615 4.20 5.0 O.lM H2S0,
E.__, nm 530 500 505 580 550 560 530 560 550 570 530 520 550 510 580 550 530 530 550 620 540 550 550 530 530 540 560 550 530
6, lo4
I.mole-‘.cm-’
0.26 0.32 2.15 0.70 1.50 0.94 1.31 1.06 1.09 1.20 1.84 0.55 1.28 0.54 1.81 1.25 0.51 1.50 0.93 1.23 1.20 1.oo 0.95 1.00 1.87 0.86 1.85 1.70 1.80
buffer unless otherwise stated. buffer containing ascorbic acid. The colour develops after heating in
a boiling water-bath for 10 min. §Heating in a boiling water-bath for 5 min.
700
1124
ANALYTICAL
ation is found to be 0.06%. Details will be published elsewhere. In acetate solution buffered at pH 4.2, lanthanum(II1) forms a 1:2 red complex with PSC, apparent stability constant 1.23 x lo9 (PH 4.2, ionic strength O.lM). Similarly, yttrium give a 1:2 complex with an apparent stability constant of 6.17 x 10’ (pH 4.2, ionic strength O.lM). In the presence of CTMAB, lanthanum and yttrium form blue ternary complexes with PSC, the molar absorptivities of the La(III) and Y(III) complexes being 6.5 x lo4 (650 nm) and 7.31 x lo4 l.mole-’ .cm-’ (640 nm), respectively (for [metal] = 1 x lo-‘M, [PSC] = 3.5 x 10m4M). A preliminary investigation shows that the increase in sensitivity seems to be the formation of complexes with unusual metal-to-ligand molar ratios. It is interesting that addition of an organic solvent such as ethylene glycol depresses the formation of the lanthanum ternary complex but remarkably enhances the absorptivity of the Y(IIItPSC-CTMAB species. This effect can be utilized for the determination of yttrium in the presence of lanthanum. When PSC solution is mixed with an aged zirconium(IV) solution, a slow complexation reaction takes place. Trace amounts of fluoride catalyse this reaction, even at a fluoride concentration as low as 5.0 x lo-“M. This provides a promising means of kinetic assay of traces of fluoride. It is difficult to detect fluoride in water at levels below lo-‘M with a
DATA
lanthanum fluoride electrode, because traces of fluoride are released from the membrane surface itself into the test solution, and the PSC-Zr(IV) reaction will easily distinguish between two samples of demineralized water, one of which has been in brief contact with a lanthanum fluoride electrode. This is a very sensitive qualitative test, but its analytical characteristics depend on the history of the zirconium solution, and quantitative application will require further investigation. Acknowledgement-The authors are grateful to Professor Shu-Chuan Liang, Institute of Chemistry, Academia Sinica, Beijing, for valuable comments and encouragement. REFERENCES 1. J. Kiirbl, V. Svoboda and D. Terziska, Chem. Ind. London, 1958, 1232. 2. G. Anderegg, H. Flaschka, R. Sallmann and G. Schwarzenbach, Helv. Chim. Acta, 1954, 37, 113. 3. N. Zhou, Huaxue Shiji 1979, 273. 4. A. Albert and E. P. Serjeant, Ionization Constants of Acids and Bases, pp. 16-68. Methuen, London, 1962. 5. J. Bjerrum, Metal Amine Formation in Aqueous Solution, Haase, Copenhagen, 1941. 6. J. Inc&dy, Analytical Applications of Complex Equilibria, D. 98. Horwood. Chichester. 1976. 7. Y. W. &an and G. X. Xu, Acta Chim. Sinica, 1963.29, 37. 8. B. Bud%insk$, in Chelates in Analytical Chemistry, H. A. Flaschka and A. J. Barnard (eds.), Vol. 1, p. 21. Dekker, New York, 1967. 9. M. Malit, Z. Anal. Chem., 1962, 186, 418.