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Coated-wire organic ion-selective electrodes in titrations based on ion-pair formation
Analytica Chimica Acta, 124 (1981) 91-98 o EXsevier Scientific Publishing Company, Amsterdam -
Printed in The Netherlands
COATED-WIRE ORGANIC IONSELECTIVE ELECTRODES IN TITRATIONS BASED ON ION-PAIR FORMATION Determination of Arenediazoniuxn Salts with Sodium Tetraphenylborate
K. VY’I%.AB*,
M. REMESa and H. KUB&OVkSVOBODOVAb
Department of Analytical Chemistry, College of Chemical Technology, (Czechoslovakia)
532 10 Pardubice
(Received 29th July 1980)
SUMMARY
Titrimetric determinations of arenediazonium salts can be based on ion-pair formation between the diazonium cation and tetraphenylborate. Titrations are done under cooling with ice and are followed potentiometrically with organic ion-selective electrodes comprising PVC membranes plasticized with polar solvents and coated on aluminium wires. The method was tested in determinations of arenediazonium salts derived from 20 aromatic amines, including aniline, toluidines, naphthylamines and their derivatives. Except for compounds containing hydrophylic groups such as -COOH and -OH, the potentiometric titration curves have well defined end-points. The results are reproducible, with relative standard deviations in the range 0.4-1.4% at the millimolar level. The method can be used for reliable determinations of arenediazonium salts in analytical control of azo dyestuff production_
Most methods of the classical type for the determination of arenediazonium salts are based either on their oxidation properties or on azo-coupling reactions; all these methods include some tedious procedures. For example, titrimetric azocoupling with 0.1 M solutions of I-phenyl-3-methyl&pyrazolone, m-phenylenediamine [ 13, or secondary aliphatic amines [2] is time-consuming because of the external end-point detection by spot testing. Titrations with titanium(II1) solutions [ 31 must be done under an inert atmosphere (carbon dioxide or nitrogen). Decomposition of diazonium salts catalyzed by copper(I) chloride in hydrochloric acid and accompanied by evolution of nitrogen [I] is characterized by all the troubles of gasometric methods. Titrations with perchloric acid in a dioxane medium [4] are very unselective In the work reported here, conditions for the determination of arenediazonium salts were studied by means of organic ion-selective electrodes, =Present address: Department of Research Management and Technical information, DNICHEM, General Directorate of Chemical Industry, 532 06 Parduhice, Czechoslovakia. bPr&ent address:. Analytical Department of Plant Research, East-Bohemian Chemical Plants Synthesia, 532 17 PardubiceSemtin, Czechoslovakia.
92
which can be applied generally for precipitation titrations of univalent organic cations with sodium tetraphenylborate (see [ 51). EXPEFUMENTAL Solutions
and methods
of measurements
Sodium tefraphenylborate solution (Na13Ph4, 2.5%) was prepared by dissolving 25 g of the substance (pa., Lachema) in 500 ml of water; about 5 g of alumina was added, and the bottle was shaken occasionally for about 16 h. Then the solution was filtered, adjusted with sodium hydroxide to pH 9, and diluted to 11 with water. The solution was standardized potentiometrically against recrystallized thallium(I) nitrate [ 5,6]. Aromatic atnines were converted to hydrochlorides by additions of hydrochloric acid, and solutions were prepared containing 0.2-0.01 mol of the substance per liter. Solutions of arenediazonium salts were prepared by titration of precisely measured volumes of the respective amine solutions with sodium nitrite under cooling to 0°C with ice; the end-point of the diazotization was checked both potentiometrically (with a Pt. electrode vs. SCE) and with iodide-starch papers. Some of amines were diazotized in suspension. Specific conditions required for the diazotization of each of the amines [7] are summarized in Table 1. The solutions of the arenediazonium salts were kept in an icebox, for not longer than 8 h. For the potentiometric measurements, OP-204/l, OP-205, and OP-208 pH meters (Radelkis, Budapest) were used. The electrochemical cell consisted of the appropriate ion-selective electrode (see below) and a saturated calomel electrode (SCE) with a 0.01 M NaN03 salt bridge to avoid contamination of the titrated solution with potassium ions. Titrations were made in the usual way: the solution of the diazonium salt was transferred to a beaker (100 or 150 ml), the volume was adjusted to 50-75 ml (to obtain a ca 0.005 mol 1-l solution of the titrated substance), and the NaBPh, t&ant (2.5%) was added from an automatic burette (10 ml). The titration vessel was cooled with ice to 0°C externally, or an ice cube was dropped directly into the solution. If necessary, the pH value of the titrated solution was adjusted by addition of concentrated B&ton-Robinson buffer (a 0.4 M HJP0,-0.4 M CH,COOH0.4 M H,BO, stock solution mixed with desired amount of 2 M NaOH), and was checked potentiometrically with calibrated glass and saturated calomel electrodes. Ion-selective
electrodes
Coated-wire ion-selective electrodes were prepared by using an isolated aluminium conductor. The bare wire was dipped into a solution of poly(vinyl chloride) (PVC, 0.09 g) and plasticizer (0.2 ml) in tetrahydrofuran (3 ml) and the solvent was allowed to evaporate. Ion-exchangers were not added to the membranes. Electrodes were preconditioned by using them in titrations of thallium(I) nitrate or the particular organic cation with
93 1 Lit of titrated solutions and characterization of titration curves Arenedhzonium
cation titrated
Titration conditions
Titzationcurve
Cont.
pH
Electrode used
Steepness (mV/O.lml)
Overall Potential change
878C 878D 8781 878C
lo-14 5-8 5-7 18-24 2O-34 48-50 4447 38 5 6 6 24-26 30-32 34-38 33-36 30-34 18--20 2O-30
2oO-210 14O-160 120-130 200-205 220 230 240 230 115-135 155-170 130-135 230 26O-280 270-290 260 260 190 180-215
8-12 14-15 30 8-10 8-10 7-8 5-8 15-17 7-8 7-10 12-16 17 14-17 15-18 18-22 15-18 2O-25 23-25 26 28 4-6
180-190 250 240 245 210-220 130-160 160 230 llO-115 125-145 19O-205 195 200 220 215 175-200 210 235 235 235 llO-120
(mall-')
Benzene
3Chlorobenzene-
4-ChIorcbenzene4Bromobenzene-
P.BDichIoroberueneP-Nitrobenzene-
0.2 M aniline (50 ml) ~MNsxNO,~
0.006
HCI
0.2 M 3chloroaniline 1MNaNOP
0.006
HCI
0.004
: 8 10 HCI
0.2 M4-chIoroaniIine 1 M NcxNO,~ 0.2 M4-bromoaniIine 1 M NzxNO,~
0.004
0.04 M 2.5-dichIoroaniIine 0.006 0.5 M NaNo,” 0.2 M P-nitroaniline 0.006 lMNaNOza
HCl HCl 4 6 8 10 HCl HCl=
878~ 878C 878~ 878C
2O-15 878C
3-Nitrobenrene-
0.2 M I-nitmaniline lMNaNOza
0.006
HCI 4 6 8= HCI
I-Nitrobenzene
0.2 M 4nitroanUine lMNaNOza
0.006
HCI
3-Methoxybenzene-
O.lM3-methoryaailine 0.5 M NaNo2d
0.005
4Methoxybeozene-
O.lM4methoxynniIine 0.5Na~o~d
0.005
3-Hwkorybenzene-
0.2 M 3-amhopbenoI lMNaNO,a O.lM 4-amInobenzoic acid 1 M N&ozd 0.2M 3-tohidine 1MNaNOP
0.006
HCl 4 6 8 10 HCI 4 6 8 10 HCl
878C
0.006
HCI
878C
0.006
HCl 4
878C
4-Carboxybenzenea-Methylbenrenc
4Methylbenzene-
0.2 M4toluIdine 1 M NaNOza
0.006
3-N&o-4-methylbenzene-
0.2M 3-nitro+methylaniIine.0.5 M NaNOza
0.005
N-AcetyM-aminobenzene-
0.1 M N-acetyl-3-~henylene- 0.01 diamhe. 1 M NaNOp
: HCI
HCI 4 6 HCl 3
878C
878C 878D 8781 878C 878D 8781 878C
878C
878C 878D 8781 878C
878C
1 lO-12 19-20 20-25 25-35 23-26 12-17 14-16 5-6 12-14 14-16 6-8 8-11
50-70 205 205 225 240 205-220 160 170-180 140-155 185 196 180-205 176
94 TABLE 1 (continued) Arenediazouium cation titrated
Preparation
TitrationcondiLions
Titrationcurve
Cont. pH
Steepness
OVeldl
(mV/O.lml)
potential
Electrode used
change
I-Naphthakne-
0_025Ml-naphthylamine 0.25 M NaNOZe
0.003
5 7= HCI 4 6 8= HCl
4Bromo-lraphthalene
4hromo-l-naphthylaminef 0.25MNaNOze 4nitro-1-naphthylamineg 0.25 M NaNOze 0.025 M 2-naphthylamine 0.25M NaNOze
0.005
HCl
878A 878B 878C 878D 8781 878J 878M 878N 8781 20-15 878C
0.003
HCI
878C
0.003
HCI 4 6 8
878C
0.1 M N-acetyEB-phenylene- 0.005 diaminehydrosulphate. 0.5M NaNOzd
4-Nitrc-1-naphthalene2-Naphthalene-
878C
lo-11 10-17 20-30 20-25 30-39 17-30 14-17 17-20 22-25 17-20 15-20 9-10 15-17 17-20 8-12 20-26 30-37
175 180 220-245 220 195 190 170-180 170-190 200-210 160 190 10&200 190-210 170-200 135-145 215-220 215-220
15-18
230
30-35
260-265
46-50 58-62 63-67
290-305 295-300 305-315
_tThe stock solution was 0.1 M_ b0_02 M stock solution. ‘Xower results at given pH. d0.05 M stock solution_ eO.O1 M stock solution_ ‘0.55 g diazotized in suspension in 40 ml of 11 IV!HCl and 20 ml of water. go.47 g diazotized in suspension, as in footnote 1.
NaBPh, solution; within a few conditioning (plasticizer)
in the membrane
became
titrations,
gradually
saturated
the organic solvent with
precipitated
tekaphenylborate [ 81, and the titration curves were usually reproducible after the second titration. Different plasticizers were used (electrode working code numbers are given in parentheses): dioctylphthalate (878A), diamylphthalate (878B), 2-nitrophenyl 2-ethylhexyl ether (878C), diamyloxalate (878D), didecylphthalate (878I), dioctylsebacate (878J), tricresylphosphate (technical, 878M, or with gas chromatographic purity, 878N), and dimethoxybenzene (878P). A commercial Ca*’ -selective electrode (Crytur 20-l 5; PVC membrane containing a Ca*+-selective ligand and a trace of sodium tetraphenylborate, plasticized with 2-nitrophenyl n-octyl ether) with internal eIectroIyte (CaC12) and internal reference (Ag/AgCl) electrode was used for comparison.
95 RESULTS AND DISCUSSION
Titration curves The
titrimetric
detexxnination
of
arenediazonium
salts
is based
on ion-
pair formation of the cation with tetraphenylborate Ar-N+=N
+ BPh; = I&N;,
BP&]
The ion-pair compound is practically insoluble in water. Both the steepness of the break in the potentiometric titration curve and the overall size of the potential break are governed by the solubility product of the precipitate formed_ However, the shape of the titration curve can also be significantly influenced by the plasticizer in the electrode membrane. This influence was studied in titrations of benzene-, 4chlorobenzene-, 4-bromobenzene-, 3- and 4nitrobenzene-, 4toluene-, and I-naphthalenediazonium salts. In all cases, the most advantageous shapes of the potentiometric titration curves were observed when 2nitrophenyl 2ethylhexyl ether was used to plasticize the electrode membrane. This electrode (code number 878C) was then preferred in most further studies. The role of the membrane plasticizer can be seen from Fig. 1, where 10 titration curves recorded for the determination of 1-naphthalenediazonium
chloride are shown. The probability of extraction of the precipitated l-naph-
thalenediazonium tetraphenylborate into a membrane plasticizer increases with its increasing polarity and, consequently, both the steepness and size of the potential break increase. It should be mentioned that electrode selectivity towards organic ions is generally directly proportional to the distribution ratio of the ion-pairs formed, as shown by Scholer and Simon [9] and Freiser et al. [lo].
Fig. 1. Influence of the electrode membrane plasticizer on metric titration curves of 1-naphthalenediazonium chloride (0.063 M). The indicating electrodes used were: (1) 8785; (2) (6) 878P; (6) 878A; (7) 878B; (8) S781; (9) 878C; (10) Crytur
the shape of the potentio(0.0025 M) with NaBPh, 878D; (3) 878N; (4) 878M; 20-15.
96
Accuracy, reproducibility, and interferences When the results of the determinations with tetraphenylborak
solutions which are summarized in Table 1 were compared with the amounts of the amines weighed originally, statistically insignificant differences were obtained only in titrations of the Pmethoxybenzene- and 2naphthalenediazonium salts_ In other determinations, somewhat lower results were obtained. it should be mentioned that most of the amines used here were not completely pure and that reliable check dete rminations depend largely on the choice and availability of a suitable standard substance. When the results were compared with the consumptions of sodium nitrite solution obtained when the diazonium salts were prepared from the corresponding amines, differences were usually not observed. When interferences in the titrated solutions were avoided, the results were readily reproducible, with relative standard deviations lying in the range 0.4-1.4%. The first two titration curves for each salt usually had an atypical shape but subsequent curves were practically identical. Both the accuracy and reproducibility of the dete nninations can be influenced by interferences as outlined below. Since the stabilities of the arenediazonium salts are different with regard to temperature and light, particular conditions are required for the determination of each compound The main decomposition reaction in acidic aqueous medium can be described by the scheme ~r-h” ._,++..;k++{;: k,
ka
.{z+H+
From the kinetic point of view, the decomposition is limited by the rate of the first reaction; the carbonium cation formed then reacts with a nucleophilic agent (e.g., water or halide) to form the respective derivative. In nearly neutral medium, the decomposition of arenediazonium salts is complicated by other reactions, e.g., azo coupling of the salt with the phenol formed, Bamberger’s hydrolytic reaction, etc. In alkaline solutions, the arenediazonium ion decomposes according to the scheme _&-N;
+ H,O = Ar-N,-OH
+ H+ = AI-N2-O-
+ 2 H’.
IE all cases, when old solutions of arenediazonium salts were titrated with
sodium telraphenylborate, low results were obtained. Decomposition of the salt in acidic medium is retarded by electronegative substituents, but the same substituents accelerate the diazotate formation in alkaline medium. Diazonium salts containing hydrophilic groups such as hydroxyl and carbovyl tend to form zwitterions. The potentiometrlc titration curves of these salts showed only slight curvature and the end-point readings were ambiguous.- Salts containing a sulphonic acid group could not be determined titrimetrically. Decomposition of the precipitated arenediazonium tetraphenylborate
97
was observed in some cases; this was accompanied by evolution of gas bubbles. The reaction was not studied in detail but it probably involves formation of nitrogen, triphenylboron and an ArPh compound. The decomposition does not influence the arenediazonium ion concentration in solution, and so titration curves can be recorded as usual. Erroneous results were obtained when the diazonium salts prepared contain an excess of either amine or sodium nitrite. Aromatic amines, converted to substituted ammonium salts in mineral acid medium, are also precipitated and titrated with tetraphenylborate [ 111. Excess of nitrite decomposes tetraphenylborate by oxidation. Small excesses of both amine and nitrite cause high end-point readings. If the excess is large, two potential breaks appear in the titration curve; the first is due to precipitation of the arenediazonium ion but the end-point is again erroneous. Interference of nitrite ion can be suppressed if the titrated solution is buffered to pH > 4, the titration error then being less than 13% (see Table 2). This titrimetric method for the determination of arenediazonium salts TABLE
2
Influence of the excess of amine or sodium nitrite on the end-point reading in titrations of 75-mi volumes Diazonium salt
Amount taken
Interference
Amount added (ml)
pH
4-Bromobenzene
3 ml 6.1 M
NaNO, (0.1 hl)
1 3 2
4-Bromoaniiine (0.1 M)
1 3 2
NaNO, (0.1 M)
1.5
2-Naphthylamine (0.25 M)
5
HCi HCI HCl 4 6 8 10 HCi HCI HCI 4 6 8 HCI 4 6 8 HCi 3 5
2-Naphthalene
25 mi O-01 M
=Partiai aso coupling. bQuantitative azo-coupling with an amine added.
Error in end-point (a) +28 -23 +9.5 +7.0 +0.6 + 0.6 +1.9 i-11 +39 +37 +13 -21= -23= i- 9.6 -2-7 -2.5 -2.7 +19.3 +3.7 -51.7b -51.7u -51.7b
98
with sodium tetzaphenylborate solution has been tested on some technical samples [ 121. It proved to be quick, simple, and sufficiently reliable. The main applications of the method will probably arise in the analytical control of azo dyestuffs production. REFERENCES 1 2 3 4 5 6 7
8 9 10 11 12
M. Jurecek, Organicka Anaiysa, Vol. II, p. 405, Nakladatelstvi CSAV, Prague, 1957. A_ P. Terentev and I. S. Tubina, Zh_ Anal. Khim., 18 (1963) 113. E. Knecht and L. Thompson, J. Sot. Dyers Colour., 36 (1920) 215; 41 (1925) 94. C. W. Pifer and E. G. Woihsh, Anal. Chem., 24 (1952) 300. K. Vyh%s, Am. Lab., 11(1979) No. 2,93;Int. Lab., 9 (1979) No. 2,35. K. Vytias, V. Rrfha and S_ Kotriy, Sb. Ved. Pr., Vys. Sk. Chemickotechnol., Pardubice. 35 (1976) 41. R. P. Lastovskii and Yu_ I. Vainshtein, Tekhnicheskii anaiiz v proizvodstve prome zhutochnykh produktov i krasitelei, 3rd. edn; pp_ 142-165, Goskhimizdat, Moscow, 1958. K. Vyti%s, M. Dajkoti and M. Rem&, Cesk. Farm., in press. R. Scholcr and W. Simon, Helv. Chim. Acta, 55 (1972) 1801. H. J. James, G. D. Carrnack and H. Freiser, Anal. Chen, 44 (1972) 856. K. Vytias, Collect. Czech. Chem. Commun., 42 (1977) 3168. K. Vytias, T. Capoun, H. Svobodovg and J. Latitik, unpublished results.
Printed in The Netherlands
COATED-WIRE ORGANIC IONSELECTIVE ELECTRODES IN TITRATIONS BASED ON ION-PAIR FORMATION Determination of Arenediazoniuxn Salts with Sodium Tetraphenylborate
K. VY’I%.AB*,
M. REMESa and H. KUB&OVkSVOBODOVAb
Department of Analytical Chemistry, College of Chemical Technology, (Czechoslovakia)
532 10 Pardubice
(Received 29th July 1980)
SUMMARY
Titrimetric determinations of arenediazonium salts can be based on ion-pair formation between the diazonium cation and tetraphenylborate. Titrations are done under cooling with ice and are followed potentiometrically with organic ion-selective electrodes comprising PVC membranes plasticized with polar solvents and coated on aluminium wires. The method was tested in determinations of arenediazonium salts derived from 20 aromatic amines, including aniline, toluidines, naphthylamines and their derivatives. Except for compounds containing hydrophylic groups such as -COOH and -OH, the potentiometric titration curves have well defined end-points. The results are reproducible, with relative standard deviations in the range 0.4-1.4% at the millimolar level. The method can be used for reliable determinations of arenediazonium salts in analytical control of azo dyestuff production_
Most methods of the classical type for the determination of arenediazonium salts are based either on their oxidation properties or on azo-coupling reactions; all these methods include some tedious procedures. For example, titrimetric azocoupling with 0.1 M solutions of I-phenyl-3-methyl&pyrazolone, m-phenylenediamine [ 13, or secondary aliphatic amines [2] is time-consuming because of the external end-point detection by spot testing. Titrations with titanium(II1) solutions [ 31 must be done under an inert atmosphere (carbon dioxide or nitrogen). Decomposition of diazonium salts catalyzed by copper(I) chloride in hydrochloric acid and accompanied by evolution of nitrogen [I] is characterized by all the troubles of gasometric methods. Titrations with perchloric acid in a dioxane medium [4] are very unselective In the work reported here, conditions for the determination of arenediazonium salts were studied by means of organic ion-selective electrodes, =Present address: Department of Research Management and Technical information, DNICHEM, General Directorate of Chemical Industry, 532 06 Parduhice, Czechoslovakia. bPr&ent address:. Analytical Department of Plant Research, East-Bohemian Chemical Plants Synthesia, 532 17 PardubiceSemtin, Czechoslovakia.
92
which can be applied generally for precipitation titrations of univalent organic cations with sodium tetraphenylborate (see [ 51). EXPEFUMENTAL Solutions
and methods
of measurements
Sodium tefraphenylborate solution (Na13Ph4, 2.5%) was prepared by dissolving 25 g of the substance (pa., Lachema) in 500 ml of water; about 5 g of alumina was added, and the bottle was shaken occasionally for about 16 h. Then the solution was filtered, adjusted with sodium hydroxide to pH 9, and diluted to 11 with water. The solution was standardized potentiometrically against recrystallized thallium(I) nitrate [ 5,6]. Aromatic atnines were converted to hydrochlorides by additions of hydrochloric acid, and solutions were prepared containing 0.2-0.01 mol of the substance per liter. Solutions of arenediazonium salts were prepared by titration of precisely measured volumes of the respective amine solutions with sodium nitrite under cooling to 0°C with ice; the end-point of the diazotization was checked both potentiometrically (with a Pt. electrode vs. SCE) and with iodide-starch papers. Some of amines were diazotized in suspension. Specific conditions required for the diazotization of each of the amines [7] are summarized in Table 1. The solutions of the arenediazonium salts were kept in an icebox, for not longer than 8 h. For the potentiometric measurements, OP-204/l, OP-205, and OP-208 pH meters (Radelkis, Budapest) were used. The electrochemical cell consisted of the appropriate ion-selective electrode (see below) and a saturated calomel electrode (SCE) with a 0.01 M NaN03 salt bridge to avoid contamination of the titrated solution with potassium ions. Titrations were made in the usual way: the solution of the diazonium salt was transferred to a beaker (100 or 150 ml), the volume was adjusted to 50-75 ml (to obtain a ca 0.005 mol 1-l solution of the titrated substance), and the NaBPh, t&ant (2.5%) was added from an automatic burette (10 ml). The titration vessel was cooled with ice to 0°C externally, or an ice cube was dropped directly into the solution. If necessary, the pH value of the titrated solution was adjusted by addition of concentrated B&ton-Robinson buffer (a 0.4 M HJP0,-0.4 M CH,COOH0.4 M H,BO, stock solution mixed with desired amount of 2 M NaOH), and was checked potentiometrically with calibrated glass and saturated calomel electrodes. Ion-selective
electrodes
Coated-wire ion-selective electrodes were prepared by using an isolated aluminium conductor. The bare wire was dipped into a solution of poly(vinyl chloride) (PVC, 0.09 g) and plasticizer (0.2 ml) in tetrahydrofuran (3 ml) and the solvent was allowed to evaporate. Ion-exchangers were not added to the membranes. Electrodes were preconditioned by using them in titrations of thallium(I) nitrate or the particular organic cation with
93 1 Lit of titrated solutions and characterization of titration curves Arenedhzonium
cation titrated
Titration conditions
Titzationcurve
Cont.
pH
Electrode used
Steepness (mV/O.lml)
Overall Potential change
878C 878D 8781 878C
lo-14 5-8 5-7 18-24 2O-34 48-50 4447 38 5 6 6 24-26 30-32 34-38 33-36 30-34 18--20 2O-30
2oO-210 14O-160 120-130 200-205 220 230 240 230 115-135 155-170 130-135 230 26O-280 270-290 260 260 190 180-215
8-12 14-15 30 8-10 8-10 7-8 5-8 15-17 7-8 7-10 12-16 17 14-17 15-18 18-22 15-18 2O-25 23-25 26 28 4-6
180-190 250 240 245 210-220 130-160 160 230 llO-115 125-145 19O-205 195 200 220 215 175-200 210 235 235 235 llO-120
(mall-')
Benzene
3Chlorobenzene-
4-ChIorcbenzene4Bromobenzene-
P.BDichIoroberueneP-Nitrobenzene-
0.2 M aniline (50 ml) ~MNsxNO,~
0.006
HCI
0.2 M 3chloroaniline 1MNaNOP
0.006
HCI
0.004
: 8 10 HCI
0.2 M4-chIoroaniIine 1 M NcxNO,~ 0.2 M4-bromoaniIine 1 M NzxNO,~
0.004
0.04 M 2.5-dichIoroaniIine 0.006 0.5 M NaNo,” 0.2 M P-nitroaniline 0.006 lMNaNOza
HCl HCl 4 6 8 10 HCl HCl=
878~ 878C 878~ 878C
2O-15 878C
3-Nitrobenrene-
0.2 M I-nitmaniline lMNaNOza
0.006
HCI 4 6 8= HCI
I-Nitrobenzene
0.2 M 4nitroanUine lMNaNOza
0.006
HCI
3-Methoxybenzene-
O.lM3-methoryaailine 0.5 M NaNo2d
0.005
4Methoxybeozene-
O.lM4methoxynniIine 0.5Na~o~d
0.005
3-Hwkorybenzene-
0.2 M 3-amhopbenoI lMNaNO,a O.lM 4-amInobenzoic acid 1 M N&ozd 0.2M 3-tohidine 1MNaNOP
0.006
HCl 4 6 8 10 HCI 4 6 8 10 HCl
878C
0.006
HCI
878C
0.006
HCl 4
878C
4-Carboxybenzenea-Methylbenrenc
4Methylbenzene-
0.2 M4toluIdine 1 M NaNOza
0.006
3-N&o-4-methylbenzene-
0.2M 3-nitro+methylaniIine.0.5 M NaNOza
0.005
N-AcetyM-aminobenzene-
0.1 M N-acetyl-3-~henylene- 0.01 diamhe. 1 M NaNOp
: HCI
HCI 4 6 HCl 3
878C
878C 878D 8781 878C 878D 8781 878C
878C
878C 878D 8781 878C
878C
1 lO-12 19-20 20-25 25-35 23-26 12-17 14-16 5-6 12-14 14-16 6-8 8-11
50-70 205 205 225 240 205-220 160 170-180 140-155 185 196 180-205 176
94 TABLE 1 (continued) Arenediazouium cation titrated
Preparation
TitrationcondiLions
Titrationcurve
Cont. pH
Steepness
OVeldl
(mV/O.lml)
potential
Electrode used
change
I-Naphthakne-
0_025Ml-naphthylamine 0.25 M NaNOZe
0.003
5 7= HCI 4 6 8= HCl
4Bromo-lraphthalene
4hromo-l-naphthylaminef 0.25MNaNOze 4nitro-1-naphthylamineg 0.25 M NaNOze 0.025 M 2-naphthylamine 0.25M NaNOze
0.005
HCl
878A 878B 878C 878D 8781 878J 878M 878N 8781 20-15 878C
0.003
HCI
878C
0.003
HCI 4 6 8
878C
0.1 M N-acetyEB-phenylene- 0.005 diaminehydrosulphate. 0.5M NaNOzd
4-Nitrc-1-naphthalene2-Naphthalene-
878C
lo-11 10-17 20-30 20-25 30-39 17-30 14-17 17-20 22-25 17-20 15-20 9-10 15-17 17-20 8-12 20-26 30-37
175 180 220-245 220 195 190 170-180 170-190 200-210 160 190 10&200 190-210 170-200 135-145 215-220 215-220
15-18
230
30-35
260-265
46-50 58-62 63-67
290-305 295-300 305-315
_tThe stock solution was 0.1 M_ b0_02 M stock solution. ‘Xower results at given pH. d0.05 M stock solution_ eO.O1 M stock solution_ ‘0.55 g diazotized in suspension in 40 ml of 11 IV!HCl and 20 ml of water. go.47 g diazotized in suspension, as in footnote 1.
NaBPh, solution; within a few conditioning (plasticizer)
in the membrane
became
titrations,
gradually
saturated
the organic solvent with
precipitated
tekaphenylborate [ 81, and the titration curves were usually reproducible after the second titration. Different plasticizers were used (electrode working code numbers are given in parentheses): dioctylphthalate (878A), diamylphthalate (878B), 2-nitrophenyl 2-ethylhexyl ether (878C), diamyloxalate (878D), didecylphthalate (878I), dioctylsebacate (878J), tricresylphosphate (technical, 878M, or with gas chromatographic purity, 878N), and dimethoxybenzene (878P). A commercial Ca*’ -selective electrode (Crytur 20-l 5; PVC membrane containing a Ca*+-selective ligand and a trace of sodium tetraphenylborate, plasticized with 2-nitrophenyl n-octyl ether) with internal eIectroIyte (CaC12) and internal reference (Ag/AgCl) electrode was used for comparison.
95 RESULTS AND DISCUSSION
Titration curves The
titrimetric
detexxnination
of
arenediazonium
salts
is based
on ion-
pair formation of the cation with tetraphenylborate Ar-N+=N
+ BPh; = I&N;,
BP&]
The ion-pair compound is practically insoluble in water. Both the steepness of the break in the potentiometric titration curve and the overall size of the potential break are governed by the solubility product of the precipitate formed_ However, the shape of the titration curve can also be significantly influenced by the plasticizer in the electrode membrane. This influence was studied in titrations of benzene-, 4chlorobenzene-, 4-bromobenzene-, 3- and 4nitrobenzene-, 4toluene-, and I-naphthalenediazonium salts. In all cases, the most advantageous shapes of the potentiometric titration curves were observed when 2nitrophenyl 2ethylhexyl ether was used to plasticize the electrode membrane. This electrode (code number 878C) was then preferred in most further studies. The role of the membrane plasticizer can be seen from Fig. 1, where 10 titration curves recorded for the determination of 1-naphthalenediazonium
chloride are shown. The probability of extraction of the precipitated l-naph-
thalenediazonium tetraphenylborate into a membrane plasticizer increases with its increasing polarity and, consequently, both the steepness and size of the potential break increase. It should be mentioned that electrode selectivity towards organic ions is generally directly proportional to the distribution ratio of the ion-pairs formed, as shown by Scholer and Simon [9] and Freiser et al. [lo].
Fig. 1. Influence of the electrode membrane plasticizer on metric titration curves of 1-naphthalenediazonium chloride (0.063 M). The indicating electrodes used were: (1) 8785; (2) (6) 878P; (6) 878A; (7) 878B; (8) S781; (9) 878C; (10) Crytur
the shape of the potentio(0.0025 M) with NaBPh, 878D; (3) 878N; (4) 878M; 20-15.
96
Accuracy, reproducibility, and interferences When the results of the determinations with tetraphenylborak
solutions which are summarized in Table 1 were compared with the amounts of the amines weighed originally, statistically insignificant differences were obtained only in titrations of the Pmethoxybenzene- and 2naphthalenediazonium salts_ In other determinations, somewhat lower results were obtained. it should be mentioned that most of the amines used here were not completely pure and that reliable check dete rminations depend largely on the choice and availability of a suitable standard substance. When the results were compared with the consumptions of sodium nitrite solution obtained when the diazonium salts were prepared from the corresponding amines, differences were usually not observed. When interferences in the titrated solutions were avoided, the results were readily reproducible, with relative standard deviations lying in the range 0.4-1.4%. The first two titration curves for each salt usually had an atypical shape but subsequent curves were practically identical. Both the accuracy and reproducibility of the dete nninations can be influenced by interferences as outlined below. Since the stabilities of the arenediazonium salts are different with regard to temperature and light, particular conditions are required for the determination of each compound The main decomposition reaction in acidic aqueous medium can be described by the scheme ~r-h” ._,++..;k++{;: k,
ka
.{z+H+
From the kinetic point of view, the decomposition is limited by the rate of the first reaction; the carbonium cation formed then reacts with a nucleophilic agent (e.g., water or halide) to form the respective derivative. In nearly neutral medium, the decomposition of arenediazonium salts is complicated by other reactions, e.g., azo coupling of the salt with the phenol formed, Bamberger’s hydrolytic reaction, etc. In alkaline solutions, the arenediazonium ion decomposes according to the scheme _&-N;
+ H,O = Ar-N,-OH
+ H+ = AI-N2-O-
+ 2 H’.
IE all cases, when old solutions of arenediazonium salts were titrated with
sodium telraphenylborate, low results were obtained. Decomposition of the salt in acidic medium is retarded by electronegative substituents, but the same substituents accelerate the diazotate formation in alkaline medium. Diazonium salts containing hydrophilic groups such as hydroxyl and carbovyl tend to form zwitterions. The potentiometrlc titration curves of these salts showed only slight curvature and the end-point readings were ambiguous.- Salts containing a sulphonic acid group could not be determined titrimetrically. Decomposition of the precipitated arenediazonium tetraphenylborate
97
was observed in some cases; this was accompanied by evolution of gas bubbles. The reaction was not studied in detail but it probably involves formation of nitrogen, triphenylboron and an ArPh compound. The decomposition does not influence the arenediazonium ion concentration in solution, and so titration curves can be recorded as usual. Erroneous results were obtained when the diazonium salts prepared contain an excess of either amine or sodium nitrite. Aromatic amines, converted to substituted ammonium salts in mineral acid medium, are also precipitated and titrated with tetraphenylborate [ 111. Excess of nitrite decomposes tetraphenylborate by oxidation. Small excesses of both amine and nitrite cause high end-point readings. If the excess is large, two potential breaks appear in the titration curve; the first is due to precipitation of the arenediazonium ion but the end-point is again erroneous. Interference of nitrite ion can be suppressed if the titrated solution is buffered to pH > 4, the titration error then being less than 13% (see Table 2). This titrimetric method for the determination of arenediazonium salts TABLE
2
Influence of the excess of amine or sodium nitrite on the end-point reading in titrations of 75-mi volumes Diazonium salt
Amount taken
Interference
Amount added (ml)
pH
4-Bromobenzene
3 ml 6.1 M
NaNO, (0.1 hl)
1 3 2
4-Bromoaniiine (0.1 M)
1 3 2
NaNO, (0.1 M)
1.5
2-Naphthylamine (0.25 M)
5
HCi HCI HCl 4 6 8 10 HCi HCI HCI 4 6 8 HCI 4 6 8 HCi 3 5
2-Naphthalene
25 mi O-01 M
=Partiai aso coupling. bQuantitative azo-coupling with an amine added.
Error in end-point (a) +28 -23 +9.5 +7.0 +0.6 + 0.6 +1.9 i-11 +39 +37 +13 -21= -23= i- 9.6 -2-7 -2.5 -2.7 +19.3 +3.7 -51.7b -51.7u -51.7b
98
with sodium tetzaphenylborate solution has been tested on some technical samples [ 121. It proved to be quick, simple, and sufficiently reliable. The main applications of the method will probably arise in the analytical control of azo dyestuffs production. REFERENCES 1 2 3 4 5 6 7
8 9 10 11 12
M. Jurecek, Organicka Anaiysa, Vol. II, p. 405, Nakladatelstvi CSAV, Prague, 1957. A_ P. Terentev and I. S. Tubina, Zh_ Anal. Khim., 18 (1963) 113. E. Knecht and L. Thompson, J. Sot. Dyers Colour., 36 (1920) 215; 41 (1925) 94. C. W. Pifer and E. G. Woihsh, Anal. Chem., 24 (1952) 300. K. Vyh%s, Am. Lab., 11(1979) No. 2,93;Int. Lab., 9 (1979) No. 2,35. K. Vytias, V. Rrfha and S_ Kotriy, Sb. Ved. Pr., Vys. Sk. Chemickotechnol., Pardubice. 35 (1976) 41. R. P. Lastovskii and Yu_ I. Vainshtein, Tekhnicheskii anaiiz v proizvodstve prome zhutochnykh produktov i krasitelei, 3rd. edn; pp_ 142-165, Goskhimizdat, Moscow, 1958. K. Vyti%s, M. Dajkoti and M. Rem&, Cesk. Farm., in press. R. Scholcr and W. Simon, Helv. Chim. Acta, 55 (1972) 1801. H. J. James, G. D. Carrnack and H. Freiser, Anal. Chen, 44 (1972) 856. K. Vytias, Collect. Czech. Chem. Commun., 42 (1977) 3168. K. Vytias, T. Capoun, H. Svobodovg and J. Latitik, unpublished results.