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Coated-wire organic ion-selective electrodes in titrations based on ion-pair formation
Analytica
Chimica
Acta,
141 (1982)
377-382
Elsevier Scientific Publishing Company, Amsterdam -
Printed in The Netherlands
Short Communication COATED-WIRE ORGANIC ION-SELECTIVE ELECTRODES TITRATIONS BASED ON ION-PAIR FORMATION Part 3. Determination of Cationic Triarylmethane Dyestuffs=
K. VYTRAS*
IN
and WI. DAJKOVAb
Department of Analytical (Czechoslouakia)
Chemistry,
College of Chemical
Technology,
53-3 10 Pardubice
(Received 2nd November 1981) Aqueous solutions of seven cationic triarylmethane dyestuffs can be titrated with sodium tetraphenyiborate using as indicator electrode an aluminium wire coated
Summary.
2-ethylhexyl ether or with a PVC membrane plasticized with Z-nitrophenyl phate. Titrations are possible in unbuffered and buffered media within 4-10. Characteristic titration data are given for determinations of malachite glaucin @, brilliant green, fuchsine, methyl violet, crystal violet, and Victoria
tricresylphosthe pH range green B. seto-
blue B.
triarylmethane dyestuffs can be precipitated from aqueous with an anionic reagent such as sodium tetraphenylborate. Although such ion-pairs are often employed as electroactive components in liquid membranes of various ion-selective electrodes (especially for anions such as nitrate, perchlorate, alkylsulphates and arenesulfonates), few papers have dealt with the determination of these dyes by potentiometric titration. Simple titrations based on ion-pair formation with visual end-points are difficult or impossible because the dye solutions are usually deeply coloured. The titration of crystal violet with sodium tetraphenylborate can be followed [2] by using a liquid-state electrode with a natural rubber membrane saturated with solutions of crystal violet- 12-tungosilicate or tetraphenylborate in 1,2-dichlorobenzene. This titration can also be followed with the commercial Crytur 19-15 potassium ion-selective electrode based on a PVC membrane containing valinomycin and dipentylphthalate [33, with the Orion tetrafluoroborate or cyanide electrode 143, or even with simple coated-wire electrodes [ 53 _ Acidic dyestuffs can be titrated with crystal violet solutions [ 63 . A picrate-selective, liquid-membrane electrode based on crystal violet picrate in nitrobenzene is suitable for following two-phase titrations of crystal violet and other basic dyes with picric or 3,5-dinitrosalicylic acid [ 7, S] . The
cationic
solutions
aFor Part 2, see [I] _ bPresent address: Regional Agricultural Frjrdek-Mistek, Czechoslovakia.
0003-2670/82/0000-0000/$02.75
Laboratory,
Agrochemical
Corporation,
0 1982 Elsevier Scientific Publishing Company
738
01
378
It has been shown [l, 5, 9] that, for potentiometric titration purposes, the electrodes can be covered with blank membranes containing only a plastic matrix and a plasticizer. In such titrations, as is well known, the potential change at the end-point must be well defined, but the slope of the electrode response need be neither reproducible nor Nemstian and the actual potential at the end-point is of secondary interest. In the present communication, results are reported for a wide selection of the triarylmethane dyestuffs (Table l), to assess further the possibilities of such electrodes. Experimental Solutions_ Sodium tetraphenylborate stock solution (2.5%) was prepared, stored and standardized as described previously [I, 9]_ Titrant solutions were prepared by suitabie dilution_ Stock solutions of the triarylmethane dyestuffs (Table 1) were prepared from commercial specimens at the approximate concentrations given in parentheses: crystal violet (0.02 M), tichsine (0.01 M), malachite green B (0.02 M), methyl violet (0.02 M), brilliant green (0.02 M) [all from Lachema, TABLE
1
List of titrated dyestuffs
L
Common name
Malachite green B Setoglaucin 0 Brilliant green Fuchsine Methyl viol&’ Crystal violet Victoria blue B
Colour Indes No. and C-1. Name [lo]
Substituents [lo] R Ar
42000 Basic 42025 Basic 42040 Basic 42510 Basic 42535 Basic 42555 Basic 44045 Basic
CH,
Green 1 Blue 1
X-
Molecular mass [lo]
phenyl
&Cl;
1403.35h
CH,
2-chlorophenyl
cl-
399.37
C1 H,
phenyl
&Cl;
593.34c
H
3-methyl-&aminophenyl 4-(A’-methyl)aminophenyl 4-(l\‘,iV-dimethyl)aminophenyl 4
Cl-
409.92d
cl-
393.96
cl-
407.99
cl-
506.09
Green 1 Violet 14 Violet 1 Violet 3 Blue 26
CH, CH, CH,
“A mixture of the hydrochlorides of the more highly methylated pararosanilines, mainly the N-tetra-, penta-, and hexamethyl derivatives. The manufacturer’s data are for the pentamethyl derivative. bFor 3C,,H&IN, + 2ZnC1, + 2H,O, similarly for Cdihydrate and dtetrahydrate.
379
Czechoslovakia] ~ Victoria blue B (0.01 M), and setoglaucin 0 (0.02 M) [both from Geigy, Switzerland]. Equipment_ A universal pH meter OP-204/l (Radelkis, Budapest) was used. The potential changes were monitored by an appropriate ion-selective electrode and a double-junction saturated calomel reference electrode (0.01 M &NO, salt bridge). Coated-wire ion-selective electrodes were prepared by using an aluminium conductor [ 591 from tetrahydrofuran solutions of PVC and plasticizer (diamylphthalate, dioctylphthalate, didecylphthalate, dioctylsebacate, tricresylphosphate, or 2-nitrophenyl 2-ethylhesyl ether)_ A poly(vinylbutyral) (PVB) membrane plasticized with 2-nitrophenyl 2-ethylhexyl ether was also tested. Ion-eschangers were not added; the electrodes were pre-conditioned by the appropriate ion-pair titration_ Procedure. Titrations were made at room temperature_ About 50 ml of the solution was titrated in a lOO-ml beaker; the titrant was added from a IO-ml automatic burette and stirring was done magnetically_ If necessary, the pH value of the solution titrated was adjusted by addition of Britton-Robinson buffer and checked with glass and saturated calomel electrodes. When strongly acidic media were needed, hydrochloric acid was added. Resu Its and discussion Titrations of unbuffered
solutions. As was shown for titrations of organic cations with tetraphenylborate [3], the shape of the titration curve is governed primarily by the solubility product of the precipitate fo-rmed. With increasing mass of the cation, the solubility of the corresponding tetraphenylbomte usually decreases, thus both the steepness and the overall potential break increase. As the triarylmethane dyestuffs tested are ionic compounds of high molecular mass and almost symmetrical charge distribution, most of the dyes were titrated without difficulties. In titrations of crystal violet, methyl violet, fuchsine, and setoglaucin 0, the equilibrium potential of the pre-conditioned electrodes was established very quickly after each addition of titrant (this contrasts with the titrations when the Crytur 19-15 electrode was used, which took up to 45 min [3]). Figure 1 shows that the electrode membranes plasticized with 2-nitrophenyl 2-ethylhesyl ether or tricresylphospham gave the most useful titration curves. These electrodes (working code numbers 878C and 878M) were therefore preferred in further studies. Titrations of malachite green B and Victoria blue B were somewhat slower but the titration curves could be reproducibly recorded after small delays in the vicinity of the end-point_ Such delays are usually observed when cationic impurities are present in the sample, but no other coloured species were observed when the purity of these samples was tested chromatographically. In contrast, methyl violet, which is a mixture of methylated pararosaniline derivatives, gave a clean potential break without delays, all the species being titrated together. In titrations of brilliant green, the electrode potential was established slowly, which made the titrations longer and the curves less reproducible. The titration curves of these dyestuffs are shown in Fig. 2.
380
V
Fig. 1. Influence of plasticizer on the shape of titration curves of crystal violet (ca. 5 X 10e3 M, 50 ml) with sodium tetraphenylborate (ca. 7 x lo-’ M) for coated-wire electrodes plasticized with (1) 2-nitrophenyl 2-ethylhexyl ether; (2) diamylphthalate; (3) dioctylphthalate; (4) didecylphthalate; (5) tricresylphosphate; and (6) dioctylsebacate. Full lines are for PVC membranes, and the dashed line for a PVB membrane.
Titrations in buffered media. All the dyes were also tested in titrations over the pH range 3-10. The presence of the buffers did not significantly influence the shape of the particular titration curve. The potentials were usually established more slowly, but the curves were reproducible and the end-point readings remained the same. Another situation can occur if the medium is such that the dye can be involved in protolytic equilibria. For example, crystal violet can form yellowish green or orange cations with different positive charges [ 111. This protonation also influenced the shapes of the titration curves of crystal violet in such media; for 5 X 10m3M crystal violet, the potential jump was sharp in 0.01 M HCl, but showed two inflections in 0.1 M and 1 M HCl; the latter curve was particularly diffuse. Influence of dilution. The possibilities of determining triarylmethane dyestuffs in more diIute solutions were tested for crystal violet. Stock solutions of the dye and tetraphenylborate were diluted lo-, loo-, and lOOO-fold compared to their initial concentrations, and the titration curves were
Fig. 2. Titration curves of ca. 5 X lo-’ M solutions of (1) crystal violet; (2) methyl violet; (3) malachite green B; (4) brilliant green; (5) Victoria blue B; (6) setoglaucin 0; and (7) fuchsine with ca. 6 x lo-’ M sodium tetraphenylborate using an 878C electrode.
381 TABLE
2
Analytical
data for titrations
Sample
Crystal
violet
Fuchsine
hlalacbite green B Methyl violet Setoglaucin 0 Brilliant green Victoria blue B “Mean
of 3-4
of triarylmethane
dyestuffs
Given
Assay”
Buffer
(mg)
(“-1
(PHI
Electrode used
122.4 111.0 111.0 111.0 111.0
86.6 i 1.2 87.2 f 1.3 86.6 f 1.9 86.5 f 1.9 85.4 f 1.3
4.0 5.5 7.2
878C
83.6 56.3 56.3 44.9 44.9 150.3 118.2 119.8 167-l 151.8
87.0 87.4 86.3 86.9 89.6 75.1 86.2 27.4 33.6 46.1
5.6 9.5 -
878C 878M
titrations
with
+ + f i + f f I f ?
relative
2.1 1.3 0.9 1.5 2.3 3.s 1.0 2.2 78 1.3
standard
878C STSC 878C S78C 87SC
Titration
curve
Overall
Steepness
potential break (mV)
near end-point (mV/O.l ml)
600-700
90-120
540-560 510-530 420-460
75-80 75-80 iO-80
430-460 420-460 420-440 420-460 420-460 500-520 560-620 550-560 330-340 540-560
SO-100 90-100 90-100 80-100 90-100 56-80 75-90 80-90 80-100 60-SO
deviation.
recorded. The dilution caused both the steepness and the overall potentia! jump of the titration curves to decrease, of course, but the potential jumps were surprisingly well pronounced for titrations of 10 mg and 1 mg of crystal violet and were usable even for 0.1 mg. Unfortunately, the end-points did not remain constant, being shifted to higher values by the dilution. Yet the titration curves were well reproducible so that the problem can be solved by standardization against a verified dye sample at about the same concentration. Precision and accuracy. Finding standard samples is probably the most difficult problem because commercial specimens of triarylmethane dyestuffs usually contain many impurities. As a sufficiently reliable and independent reference method, e.g. photometry, is always influenced by the choice of standard substance, it is practically impossible to assess the accuracy of the proposed titrations. As indicated by the assay values given in Table 2, not a dyestuff tested was of 100% purity. The only comparable values available are for crystal violet: assays of other commercial samples of it have given results of 89.3% [ 31 and 87_6% [4] _ Selig [ 41 also showed, by elemental analysis, that the precipitate formed with crystal violet has a I:1 stoichiometric ratio of cation to anion; this is expected also for other samples_ This kind of ionpair was observed also in other studies of triphenylmethane dyes [12] _ Despite these problems, the titrations of triarylmethane dyestuffs with tetra-
3S2
phenylborate are reproducibIe as far as relative deviations can be recommended for analytical production control.
are concerned,
and
REFERENCES 1 K. Vyti%s, M. Dnjkovj: and V. Mach, Anal_ Chim. Acta, 127 (1981) 166. 2 A. G. Fogg and K. S. Yoo, in E. Pungor and I. Buz&r (Eds.), Ion Selective Electrodes Amsterdam (and Akadiimiai Kiad6, (Conf., Budapest, September 1977), Eisevier, Budapest), 1978, p. 369. 3 K. VytFas, Collect. Czech. Chem. Commun., 42 (1977) 3168. 4 W. Selig. Mikrochim. Acta, (lSSO/II) 133. 5 K. Vyti%s, M. Dajkovi and M. RemeS. Cesk. Farm., 30 (1981) 61. 6 A. G. Fogg, A. A. Al-Sibaai and K. S. Yoo, Anal. Lett., 10 (1977) 173. 7 I. A\. Gurev and T. S. Vyatchanina, Zh. Anal. Khim., 34 (1979) 976. S A. A. Kalugin and I_ A. Gurev, Zh. Anal. Khim., 35 (1980) 2424. 9 K. Vyt%s, ill. RemeS and H. KubeSovi-Svobodova, Anal. Chim. Acta, 124 (1981) 91. 10 Colour Index. Vol. 4, 3rd edn., The Society of Dyers and Colourists, Bradford, 1971. 11 E. Bdnyai, in E. Bishop (Ed.), Indicators, Pergamon, Oxford. 1972. 12 S. Motomizu, S. Fujiwara and K. Toei, Anal. Chim. Acta, 12s (1981) 185.
Chimica
Acta,
141 (1982)
377-382
Elsevier Scientific Publishing Company, Amsterdam -
Printed in The Netherlands
Short Communication COATED-WIRE ORGANIC ION-SELECTIVE ELECTRODES TITRATIONS BASED ON ION-PAIR FORMATION Part 3. Determination of Cationic Triarylmethane Dyestuffs=
K. VYTRAS*
IN
and WI. DAJKOVAb
Department of Analytical (Czechoslouakia)
Chemistry,
College of Chemical
Technology,
53-3 10 Pardubice
(Received 2nd November 1981) Aqueous solutions of seven cationic triarylmethane dyestuffs can be titrated with sodium tetraphenyiborate using as indicator electrode an aluminium wire coated
Summary.
2-ethylhexyl ether or with a PVC membrane plasticized with Z-nitrophenyl phate. Titrations are possible in unbuffered and buffered media within 4-10. Characteristic titration data are given for determinations of malachite glaucin @, brilliant green, fuchsine, methyl violet, crystal violet, and Victoria
tricresylphosthe pH range green B. seto-
blue B.
triarylmethane dyestuffs can be precipitated from aqueous with an anionic reagent such as sodium tetraphenylborate. Although such ion-pairs are often employed as electroactive components in liquid membranes of various ion-selective electrodes (especially for anions such as nitrate, perchlorate, alkylsulphates and arenesulfonates), few papers have dealt with the determination of these dyes by potentiometric titration. Simple titrations based on ion-pair formation with visual end-points are difficult or impossible because the dye solutions are usually deeply coloured. The titration of crystal violet with sodium tetraphenylborate can be followed [2] by using a liquid-state electrode with a natural rubber membrane saturated with solutions of crystal violet- 12-tungosilicate or tetraphenylborate in 1,2-dichlorobenzene. This titration can also be followed with the commercial Crytur 19-15 potassium ion-selective electrode based on a PVC membrane containing valinomycin and dipentylphthalate [33, with the Orion tetrafluoroborate or cyanide electrode 143, or even with simple coated-wire electrodes [ 53 _ Acidic dyestuffs can be titrated with crystal violet solutions [ 63 . A picrate-selective, liquid-membrane electrode based on crystal violet picrate in nitrobenzene is suitable for following two-phase titrations of crystal violet and other basic dyes with picric or 3,5-dinitrosalicylic acid [ 7, S] . The
cationic
solutions
aFor Part 2, see [I] _ bPresent address: Regional Agricultural Frjrdek-Mistek, Czechoslovakia.
0003-2670/82/0000-0000/$02.75
Laboratory,
Agrochemical
Corporation,
0 1982 Elsevier Scientific Publishing Company
738
01
378
It has been shown [l, 5, 9] that, for potentiometric titration purposes, the electrodes can be covered with blank membranes containing only a plastic matrix and a plasticizer. In such titrations, as is well known, the potential change at the end-point must be well defined, but the slope of the electrode response need be neither reproducible nor Nemstian and the actual potential at the end-point is of secondary interest. In the present communication, results are reported for a wide selection of the triarylmethane dyestuffs (Table l), to assess further the possibilities of such electrodes. Experimental Solutions_ Sodium tetraphenylborate stock solution (2.5%) was prepared, stored and standardized as described previously [I, 9]_ Titrant solutions were prepared by suitabie dilution_ Stock solutions of the triarylmethane dyestuffs (Table 1) were prepared from commercial specimens at the approximate concentrations given in parentheses: crystal violet (0.02 M), tichsine (0.01 M), malachite green B (0.02 M), methyl violet (0.02 M), brilliant green (0.02 M) [all from Lachema, TABLE
1
List of titrated dyestuffs
L
Common name
Malachite green B Setoglaucin 0 Brilliant green Fuchsine Methyl viol&’ Crystal violet Victoria blue B
Colour Indes No. and C-1. Name [lo]
Substituents [lo] R Ar
42000 Basic 42025 Basic 42040 Basic 42510 Basic 42535 Basic 42555 Basic 44045 Basic
CH,
Green 1 Blue 1
X-
Molecular mass [lo]
phenyl
&Cl;
1403.35h
CH,
2-chlorophenyl
cl-
399.37
C1 H,
phenyl
&Cl;
593.34c
H
3-methyl-&aminophenyl 4-(A’-methyl)aminophenyl 4-(l\‘,iV-dimethyl)aminophenyl 4
Cl-
409.92d
cl-
393.96
cl-
407.99
cl-
506.09
Green 1 Violet 14 Violet 1 Violet 3 Blue 26
CH, CH, CH,
“A mixture of the hydrochlorides of the more highly methylated pararosanilines, mainly the N-tetra-, penta-, and hexamethyl derivatives. The manufacturer’s data are for the pentamethyl derivative. bFor 3C,,H&IN, + 2ZnC1, + 2H,O, similarly for Cdihydrate and dtetrahydrate.
379
Czechoslovakia] ~ Victoria blue B (0.01 M), and setoglaucin 0 (0.02 M) [both from Geigy, Switzerland]. Equipment_ A universal pH meter OP-204/l (Radelkis, Budapest) was used. The potential changes were monitored by an appropriate ion-selective electrode and a double-junction saturated calomel reference electrode (0.01 M &NO, salt bridge). Coated-wire ion-selective electrodes were prepared by using an aluminium conductor [ 591 from tetrahydrofuran solutions of PVC and plasticizer (diamylphthalate, dioctylphthalate, didecylphthalate, dioctylsebacate, tricresylphosphate, or 2-nitrophenyl 2-ethylhesyl ether)_ A poly(vinylbutyral) (PVB) membrane plasticized with 2-nitrophenyl 2-ethylhexyl ether was also tested. Ion-eschangers were not added; the electrodes were pre-conditioned by the appropriate ion-pair titration_ Procedure. Titrations were made at room temperature_ About 50 ml of the solution was titrated in a lOO-ml beaker; the titrant was added from a IO-ml automatic burette and stirring was done magnetically_ If necessary, the pH value of the solution titrated was adjusted by addition of Britton-Robinson buffer and checked with glass and saturated calomel electrodes. When strongly acidic media were needed, hydrochloric acid was added. Resu Its and discussion Titrations of unbuffered
solutions. As was shown for titrations of organic cations with tetraphenylborate [3], the shape of the titration curve is governed primarily by the solubility product of the precipitate fo-rmed. With increasing mass of the cation, the solubility of the corresponding tetraphenylbomte usually decreases, thus both the steepness and the overall potential break increase. As the triarylmethane dyestuffs tested are ionic compounds of high molecular mass and almost symmetrical charge distribution, most of the dyes were titrated without difficulties. In titrations of crystal violet, methyl violet, fuchsine, and setoglaucin 0, the equilibrium potential of the pre-conditioned electrodes was established very quickly after each addition of titrant (this contrasts with the titrations when the Crytur 19-15 electrode was used, which took up to 45 min [3]). Figure 1 shows that the electrode membranes plasticized with 2-nitrophenyl 2-ethylhesyl ether or tricresylphospham gave the most useful titration curves. These electrodes (working code numbers 878C and 878M) were therefore preferred in further studies. Titrations of malachite green B and Victoria blue B were somewhat slower but the titration curves could be reproducibly recorded after small delays in the vicinity of the end-point_ Such delays are usually observed when cationic impurities are present in the sample, but no other coloured species were observed when the purity of these samples was tested chromatographically. In contrast, methyl violet, which is a mixture of methylated pararosaniline derivatives, gave a clean potential break without delays, all the species being titrated together. In titrations of brilliant green, the electrode potential was established slowly, which made the titrations longer and the curves less reproducible. The titration curves of these dyestuffs are shown in Fig. 2.
380
V
Fig. 1. Influence of plasticizer on the shape of titration curves of crystal violet (ca. 5 X 10e3 M, 50 ml) with sodium tetraphenylborate (ca. 7 x lo-’ M) for coated-wire electrodes plasticized with (1) 2-nitrophenyl 2-ethylhexyl ether; (2) diamylphthalate; (3) dioctylphthalate; (4) didecylphthalate; (5) tricresylphosphate; and (6) dioctylsebacate. Full lines are for PVC membranes, and the dashed line for a PVB membrane.
Titrations in buffered media. All the dyes were also tested in titrations over the pH range 3-10. The presence of the buffers did not significantly influence the shape of the particular titration curve. The potentials were usually established more slowly, but the curves were reproducible and the end-point readings remained the same. Another situation can occur if the medium is such that the dye can be involved in protolytic equilibria. For example, crystal violet can form yellowish green or orange cations with different positive charges [ 111. This protonation also influenced the shapes of the titration curves of crystal violet in such media; for 5 X 10m3M crystal violet, the potential jump was sharp in 0.01 M HCl, but showed two inflections in 0.1 M and 1 M HCl; the latter curve was particularly diffuse. Influence of dilution. The possibilities of determining triarylmethane dyestuffs in more diIute solutions were tested for crystal violet. Stock solutions of the dye and tetraphenylborate were diluted lo-, loo-, and lOOO-fold compared to their initial concentrations, and the titration curves were
Fig. 2. Titration curves of ca. 5 X lo-’ M solutions of (1) crystal violet; (2) methyl violet; (3) malachite green B; (4) brilliant green; (5) Victoria blue B; (6) setoglaucin 0; and (7) fuchsine with ca. 6 x lo-’ M sodium tetraphenylborate using an 878C electrode.
381 TABLE
2
Analytical
data for titrations
Sample
Crystal
violet
Fuchsine
hlalacbite green B Methyl violet Setoglaucin 0 Brilliant green Victoria blue B “Mean
of 3-4
of triarylmethane
dyestuffs
Given
Assay”
Buffer
(mg)
(“-1
(PHI
Electrode used
122.4 111.0 111.0 111.0 111.0
86.6 i 1.2 87.2 f 1.3 86.6 f 1.9 86.5 f 1.9 85.4 f 1.3
4.0 5.5 7.2
878C
83.6 56.3 56.3 44.9 44.9 150.3 118.2 119.8 167-l 151.8
87.0 87.4 86.3 86.9 89.6 75.1 86.2 27.4 33.6 46.1
5.6 9.5 -
878C 878M
titrations
with
+ + f i + f f I f ?
relative
2.1 1.3 0.9 1.5 2.3 3.s 1.0 2.2 78 1.3
standard
878C STSC 878C S78C 87SC
Titration
curve
Overall
Steepness
potential break (mV)
near end-point (mV/O.l ml)
600-700
90-120
540-560 510-530 420-460
75-80 75-80 iO-80
430-460 420-460 420-440 420-460 420-460 500-520 560-620 550-560 330-340 540-560
SO-100 90-100 90-100 80-100 90-100 56-80 75-90 80-90 80-100 60-SO
deviation.
recorded. The dilution caused both the steepness and the overall potentia! jump of the titration curves to decrease, of course, but the potential jumps were surprisingly well pronounced for titrations of 10 mg and 1 mg of crystal violet and were usable even for 0.1 mg. Unfortunately, the end-points did not remain constant, being shifted to higher values by the dilution. Yet the titration curves were well reproducible so that the problem can be solved by standardization against a verified dye sample at about the same concentration. Precision and accuracy. Finding standard samples is probably the most difficult problem because commercial specimens of triarylmethane dyestuffs usually contain many impurities. As a sufficiently reliable and independent reference method, e.g. photometry, is always influenced by the choice of standard substance, it is practically impossible to assess the accuracy of the proposed titrations. As indicated by the assay values given in Table 2, not a dyestuff tested was of 100% purity. The only comparable values available are for crystal violet: assays of other commercial samples of it have given results of 89.3% [ 31 and 87_6% [4] _ Selig [ 41 also showed, by elemental analysis, that the precipitate formed with crystal violet has a I:1 stoichiometric ratio of cation to anion; this is expected also for other samples_ This kind of ionpair was observed also in other studies of triphenylmethane dyes [12] _ Despite these problems, the titrations of triarylmethane dyestuffs with tetra-
3S2
phenylborate are reproducibIe as far as relative deviations can be recommended for analytical production control.
are concerned,
and
REFERENCES 1 K. Vyti%s, M. Dnjkovj: and V. Mach, Anal_ Chim. Acta, 127 (1981) 166. 2 A. G. Fogg and K. S. Yoo, in E. Pungor and I. Buz&r (Eds.), Ion Selective Electrodes Amsterdam (and Akadiimiai Kiad6, (Conf., Budapest, September 1977), Eisevier, Budapest), 1978, p. 369. 3 K. VytFas, Collect. Czech. Chem. Commun., 42 (1977) 3168. 4 W. Selig. Mikrochim. Acta, (lSSO/II) 133. 5 K. Vyti%s, M. Dajkovi and M. RemeS. Cesk. Farm., 30 (1981) 61. 6 A. G. Fogg, A. A. Al-Sibaai and K. S. Yoo, Anal. Lett., 10 (1977) 173. 7 I. A\. Gurev and T. S. Vyatchanina, Zh. Anal. Khim., 34 (1979) 976. S A. A. Kalugin and I_ A. Gurev, Zh. Anal. Khim., 35 (1980) 2424. 9 K. Vyt%s, ill. RemeS and H. KubeSovi-Svobodova, Anal. Chim. Acta, 124 (1981) 91. 10 Colour Index. Vol. 4, 3rd edn., The Society of Dyers and Colourists, Bradford, 1971. 11 E. Bdnyai, in E. Bishop (Ed.), Indicators, Pergamon, Oxford. 1972. 12 S. Motomizu, S. Fujiwara and K. Toei, Anal. Chim. Acta, 12s (1981) 185.