Anal. Chem. 1983, 55, 1226-1229
1226
Spectrophotometric Determination of Aminophenols, Phenylenecliamines, Dinitroanilines, Trichloroanilines, and Tetrachloroaniline by the Diazotization-Coupling Technique with N-( I-Naphthy1)ethylenediamine as the Coupling Agent George Norwitz and Peter N. Keliher" Chemistry Department, Villanova Universl& Villanova, Pennsylvania 19085
Methods are described for the determination of 2-, 3-, and 4-aminophenol, m - and p -phenylenediamine, 2,4- and 2,6dinltroaniilne, 2,4,5- and 2,4,6-trichloroanillne, and 2,4,5,6tetrachloroaniiineby the diazotization coupling spectrophotometric technique, using N-( I-naphthyi)ethyienediamlne (also called N-( l-naphthalenyi)-1,2-ethanediamine or N-na) as the coupling agent. The optimum acidity for the diazotizationcoupling of these aromatic amines varies considerably. The weakly basic aromatic amines require a very high acidity. The method is not applicable to o-phenylenediamine, since that compound undeirgoes intramolecular diazotization-coupiing with the production of 1,2,3-benzotriazole.
Recently, this laboratory investigated the spectrophotometric determination of aromatic amines by the diazotization-coupling technique, using H-acid (8-amino-1-hydroxynaphthalene-3,6-disulfonicacid) and N-(1-naphthyl)ethylenediamine (also called N-(1-naphthaleny1)-1,2-ethanediamine or N-na) as the coupling agents (I). For various reasons, the method clould not be applied to the determination of aminophenols, phenylenediamines, dinitroanilines, trichloroanilines, or tetrachloroanilines (I). These aromatic amines, although they are for the most part of considerable commercial importance, have not been successfully determined by the diazotization-coupling technique, even by using other coupling agents besides H-acid or N-na; many have not been determined spectrophotometrically by any method. It is the purpose of the present paper to show how the aromatic amines in question can be determined by the N-na method by the use of special techniques. The H-acid method cannot be used for these aromatic amines, since little or no color is produced.
EXPERIMENTAL SECTION Apparatus and Reagents. Standard 2-, 3-, and 4-Aminophenol Solution A (1 mL = 2.50 mg).Prepare fresh every 3 days by dissolving 0.2500 g in a mixture of 40 mL of water, 40 mL of ethanol, and 5 mL of aiulfuric acid (1to 1) (room temperature) and diluting to 100 mlL in a volumetric flask with water. Standard 2-, 3-,and 4-Aminophenol Solution B (1mL = 0.025 mg). Prepare fresh daily by diluting a 5-mL aliquot of standard 2-, 3-, or 4-aminophenol solution A to 500 mL in a volumetric flask with water. Standard m- and p-Phenylenediamine Solution A (1mL = 2.50 mg). Prepare fresh every 3 days by dissolving 0.2500 g of the crushed sample in (EL mixture of 75 mL of water and 5 mL of concentrated hydrochloric acid (room temperature) and diluting to 100 mL in a volumetric flask with water. Standard m- and pPhenylenediamine Solution B (1 mL = 0.025 mg). Prepare fresh daily by diluting a 5-mL aliquot of standard m- or p-phenylenediamine solution A to 500 mL in a volumetric flask with water. Standard 2,4- and 2,6-Dinitroaniline Solution A ( 1 mL = 2.00 mg). Dissolve 0.1000 g in concentrated sulfuric acid and dilute to 50 mL in a volumetric flask with concentrated sulfuric acid. 0003-2700/83/0355-1226$0 I .S.O./ OJ _.
Standard 2,4- and 2,6-Dinitroaniline Solution B (1 mL = 0.020 mg). Pipet a 5-mL aliquot of standard 2,4- or 2,6-dinitroaniline solution A into a 500-mL volumetric flask containing 10 mL of sulfuric acid (1to l),dilute to about 450 mL with water, cool to room temperature, and dilute to the mark with water. Standard 2,4,5- and 2,4,6-Trichloroaniline Solution A (1 mL = 2.50 mg). Dissolve 0.2500 g in ethanol and dilute to 100 mL in a volumetric flask with ethanol. Standard 2,4,5- and 2,4,6-Trichloroaniline Solution B (1mL = 0.025 mg). Pipet a 5-mL aliquot of standard 2,4,5- or 2,4,6trichloroaniline solution A into a 500-mL volumetric flask containing 10 mL of sulfuric acid (1 to l),dilute to about 450 mL with water, cool to room temperature, and dilute to the mark with water. Standard 2,4,5,6-Tetrachloroaniline Solution A ( 1 mL = 2.00 mg). Dissolve 0.1000 g in concentrated sulfuric acid and dilute to 50 mL in a volumetric flask with concentrated sulfuric acid. Standard 2,4,5,6-Tetrachloroaniline Solution B (1 mL = 0.020 mg). Pipet a 5-mL aliquot of standard 2,4,5,6-tetrachloroaniline solution A into a 500-mL volumetric flask containing 25 mL of sulfuric acid (1to l),dilute to about 450 mL with water, cool to room temperature, and dilute to the mark with water. Preparation of Calibration Curves. 2-, 3-, and 4-Aminophenol and p-Phenylenediamine. Transfer aliquots of standard 2-, 3-, or 4-aminophenolsolution B or p-phenylenediamine solution B to 50-mL volumetric flasks. The aliquots for all the aromatic amines in this paper should be chosen in accordance with the sensitivities indicated in Table I t o cover the range of about 0 to 0.6 absorbance unit. Add a few milliliters of water and the amount of acid indicated in Table I. Dilute to about 30-35 mL , with water, add 1.0 mL of sodium nitrite solution ( l % )swirl, and allow to stand 5 min. Add 1.0 mL of sulfamic acid solution (3%), swirl, wash down the neck of the flask, and allow to stand 10 min. Add the amount of N-na reagent (0.75%) indicated in Table I, dilute to the mark, and mix. Measure the absorbance against water at the wavelength and time indicated in Table I. Deduct the blank and plot absorbance against milligrams of the aromatic amine per 50 mL. m-Phenylenediamine,2,4- and 2,6-Dinitroaniline, 2,4,5- and Proceed 2,4,6-Trichloroaniline, and 2,4,5,6-Tetrachloroaniline. as with the aminophenols and p-phenylenediamine till after the addition of the acid indicated in Table I. Dilute to about 40-42 mL with water (this high dilution is especially important for 2,4,5,6-tetrachloroaniline, since the excess nitrite is not destroyed by the sulfamic acid when a large amount of sulfuric acid is added and the dilution kept low). Cool in an ice bath to about 10 "C. Remove the flasks from the ice bath, add the sodium nitrite, and proceed as described for the aminophenols and p-phenylenediamine, but insert stoppers and mix after the addition of the sulfamic acid (instead of washing down the neck of the flask). Note. In the determination of m- and p-phenylenediamine, it is essential that the operation consisting of the addition of the N-na, dilution to the mark, and measurement of the absorbance be performed individually on each solution. Method. Transfer an appropriate aliquot to a 50-mL volumetric flask and proceed as described in the preparation of the calibration curves. RESULTS AND DISCUSSION Aminophenols. When the N-na method was applied to 2- and 4-aminophenol, it was found that the use of standard 0 lQ83 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
1227
Table I. Spectrophotometric Determination of Aminophenols, Phenylenediamines, Dinitroanilines, Trichloroanilines, and Tetrachloroaniline by the N-na Method
aromatic amine
mL of reagent
mL of acid
2-aminophenol 3-aminophenol 4-aminophenol p-phenylenediamine m-phenylenediamine 2,4-dinitroaniline 2,6-dinitroaniline 2,4,5-trichloroaniline 2,4,6-trichloroaniline 2,4,5,6-tetrachloroaniline
6.0 of 1 N HC1 2.0 of 1 N HC1 3.0 of 1 N HCl 4.0 of H,SO, (1to 1) 25.0 of H,SO, (1to 1) 25.0 of H,SO, (1to 1) 30.0 of H,SO, (1to 1) 25.0 of H,SO, (1to 1) 25.0 of H,SO, (1to 1) 30.0 of concn H,SO,
10.0
2.5 10.0 2.5 2.5 2.5 2.5 2.5 2.5 2.5
color
nm
time for development
violet purplishred violet cherry red cherry red cherry red cherry red purplish red purplish red purplish red
575 556 582 527 523 527 521 536 535 532
16-24h 90 min 16-24h 0.25-2 rnin 0.25-2 rnin 30min 30 rnin 30 min 30 min 1.25-2 h
A,,,
A (0.05 mg/50 mL)
0.35 0.33 0.31 0.46 0.32 0.32 0.23 0.28 0.25 0.13
Table 11. Effect of Acid Concentration on the Colors absorbance for different amounts of 1 N hydrochloric acid aromatic amine 2-aminophenola, 3-aminophenolaac 4-aminophenola , aromaticamine m-phenylenediamine e p-phenylenediaminee 2,4-dinitroanilineg 2,6-dinitroaniline g 2,4,5-trichloroanilineg 2,4,6-trichloroanilineg 2,4,5,6-tetrachloroaniline
0.5
0.5
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
10.0
25.0
0.47
0.38 0.50 0.38
0.43 0.50 0.47
0.48 0.50 0.48
0.53 0.49 0.46
0.53 0.49 0.44
0.53 0.49 0.43
0.53 0.47 0.41
0.53 0.47 0.39
0.49 0.40 0.35
0.32 0.32 0.28
1.5
absorbance for different amounts of concentrated sulfuric acidd 2.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 0.31
0.31
0.31
0.15
0.16
0.25
0.32
0.64
0.64
0.60
0.52
0.41
0.44
0.34
0.50
0.56
0.57
0.42
0.50
0.50
0.50
0.27
0.28
0.46
0.44
0.06
0.54
0.58
0.03
0.18
0.23
0.43
0.46
0.46
0.16 0.15
0.27
0.08
0.12
0.32
0.35
0.13
30.0
0.32
>2.0f
0.54
0.70
>2.0f
0.36
0.17
0.31
0.45
0.13
25.0
0.39
0.45
0.37
0.30
0.39
10.0 mL of N-na reagent was used and the absorbance measured after the solution stood overnight. a 0.075 mg/50 mL. 2.5 mL of N-na reagent was used and the absorbance measured after 90 min. For sulfuric acid of 17.5 mL or less, the equivalent amount of 1 to 1 acid was used. e 0.050 mg/50 mL. f An intense purple color developed due to undestroyed nitrite. 0.10 mg/50 mL. 0.15 mg/50 mL.
solutions of these compounds in water or ethanol gave erratic results and high blanks. It is believed that this is due to partial oxidation of these compounds to 2- and 4-quinonimine and subsequent hydrolysis of the latter compounds to 2- and 4quinone (2). It was found that the tendency of 2- and 4aminophenol to give erratic results and high blanks could be eliminated by use of stock solutions containing about 2.5% sulfuric acid so as to form the stable amine salt. The solution should also contain a high concentration of ethanol (about 40%). The use of the water-ethanol-sulfuric acid solution is also recommended for 3-aminophenol, although a water solution of that compound is quite stable. A study of the effect of acidity on the diazotization-coupling of 2-, 3-, and 4aminophenol (Table 11) showed that the acidity is moderately critical for 2- and 3-aminophenol and quite critical for 4aminophenol. The recommended optimum acidities for 2-, 3-, and 4-aminophenol are 6.0, 2.0, and 3.0 mL of 1 N hydrochloric acid per 50 mL, respectively. The diazotization of all the aminophenols is complete in 5 min a t room temperature. The diazotized salt is then stable for about 20-30 min and then deteriorates slowly. Cooling in ice increased the time needed for diazotization and did not eliminate the deterioration. The color obtained for 3-aminophenol developed fully in 90 min with 2.5 mL of the N-na reagent, but the colors
obtained for 2- and 4-aminophenol required overnight standing and the use of 10 mL of the N-na reagent. When only 2.5 mL of the N-na reagent was used for the 2- and 4-aminophenol and the solution allowed to stand overnight, the recoveries obtained were less than 50% of the recoveries obtained by using 10 mL of the reagent and the results were erratic. The reason why 2- and 4-aminophenol do not couple readily may be related to the peculiar resonance structure of phenols. It is known that phenols, in addition to the Kekul6 forms, have charged forms with a negative charge a t the ortho and para positions of the molecule (3). It is plausible to expect that these negative charges would decrease the polarization and coupling capabilities of the diazonium ion obtained from 2and 4-aminophenol. The amount of acid for the diazotization-coupling, the amount of N-na reagent, the type of color obtained, the wavelength of maximum absorption (Am), the time for color development, and the absorbance for 0.05 mg/50 mL for all the aromatic amines tested in this paper are shown in Table
I. Phenylenediamines. The N-na method was found to be applicable to m- and p-phenylenediamine but not to ophenylenediamine. The reason the method is not applicable to o-phenylenediamine is that this compound undergoes in-
1228
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
tramolecular diazotization-coupling to produce the heterocyclic
,
I
compound, 1,2,3-benzotriazole (C,H,NHN=N) ( 4 ) . For mand p-phenylenediamine (especially the latter), the use of standard solutions in water or ethanol gave erratic resulls and high blanks. In the case of p-phenylenediamine, this may be due to partial oxidation to 4-quinone (2). It was found that the tendency of m- and p-phenylenediamine to give erratic results and high blanks could be eliminated by the use of stock solutions containing
