56
Anal. Chem. 1981, 53, 56-60
for the /3 term and a resulting increase in scattering of data points. Since the ratio of the coefficients in eq 3 is greater than unity (s/a = 1-73),it is clear that phenol blue is more sensitive to solvent polarity than to influences from hydrogen bonding by the solvent. Thus, its greater significance as a solvatochromic indicator lies in the determination of more precise ?r* values for both aprotic and HBD solvents. For purposes of verification, the bathcchromic shifts for phenol blue in HBD media were used with the regression in eq 3; and the uncertainty in the computed a values (AO.06 cm-’ X lo3 standard deviation) is equivalent to recently published results (4). Likewise, the ratio ( s / a ) conforms to the more restricted requirements proposed by Taft and Kamlet for a-scale indicators (12).
LITERATURE CITED (1) Kamlet, M.; Tan, R. J. Am. Chem. Soc. 1976, 98, 377. (2) Kamlet, M.; Abboud, J.; Tan, R. J. Am. Chem. Soc. 1977, 99, 6027. (3) Abboud, J.; Taft, R. J. phys. Chem. 1070, 83, 412. (4) Kamlet, M.; Jones, M.; Taft, R.; Abboud, J. J . Chem. Soc., P&ln Trans. 2 1979, 324. (5) Kolllng, 0.:Goodnlght, J. Anal. Chem. 1073, 45, 160. (6) McRae, E. Q. J . mys. Chem. 1957. 87, 562. (7) Figueras, J. J. Am. Chem. Soc. 1971, 93. 3255. (8) Figueras, J.; Scuhrd, P.; Ma&, A. J . Org. Chem. 1971, 36, 3497. (9) Kdling, 0.; ooodnight, J. Anal. Chem. 1974, 46, 482. (IO) R M k , J.; Bwger, W. “Organic Solvents”, 3rd ed: Wiley-Intersclence: New York, 1970;Chapter 5. (11) Tan, R.; Kamlet. M. J. Am. Chem. Soc. 1076, 98, 2886. (12) Tan, R.; Kamlet, M. J . Chem. Soc., Perkln Trans. 2 1070, 1723.
RECEIVED for review August 26,1980. Accepted October 20, 1980.
Spectrophotometric Determination of Aniline by the Diazotization-Coupling Method with 8-Amino- 1-hydroxynaphthalene-3,6-disulfonic Acid as the Coupling Agent Leorge Norwitr and Peter N. Keliher’ Chemistry Department, Villanova University, Villanova, Pennsylvania 19085
A comprehenslve study was made of the spectrophotometrlc detennlnatlon of anlllne by the dlazotlzatbn-coupllngmethod, using H-acld (8-amlno-l-hydroxynaphthalene-3,6-dlsulfonlc acid) as the coupllng agent. Among the most Important factors Investigated were necesslty for purifylng the H-acid, effect of acid concentration on the dlazotlzatkn, effect of nttrlte concentration on the dlazotlzatlon, effect of temperature on the dlazotlzatlon, necesslty for destroylng the excess nitrite, Interference of alcohol, effect of pH on the coupllng reaction, and effect of light on the color. The recommended method Is far more accurate and precise than prevlously published dlazotlzatlon-coupllng procedures and Is adaptable to the macro, semlmlcro, and micro scale.
The determination of aniline is very important from the point of view of chemical processes and toxicology. A frequently used spectrophotometric method for the determination involves diazotization and reaction with a suitable coupling agent to produce an intensely colored azo dye. The method has been used primarily for the determination of smaller percentages of aniline and leaves much to be desired in the way of accuracy and reproducibility. A complete literature survey or comprehensive study of the method has not been made. Previously recommended conditions for the method (1-17), which are summarized in Table I, vary considerably and are often contradictory, even with the same coupling agent. It is the purpose of the present investigation to study the many factors involved in the method. The coupling agent chosen for most of the work was H-acid (80003-2700/81/0353-0056$01.00/0
amino-l-hydroxynaphthalene-3,6-disulfonic acid), which was found in this laboratory to be the most useful coupling agent.
EXPERIMENTAL SECTION Apparatus and Reagents. Bausch and Lomb Model 70 spectrophotometer (1-cm cell). All chemicals used were reagent grade except the H-acid. Standard Aniline Solution No. I (1 mL = 10.00 rng of Aniline). Dissolve 1.OOO g of aniline in ethanol and dilute to 100 mL with ethanol in a volumetric flask. Prepare fresh weekly. Standard Aniline Solution No. 2 (1 mL = 0.050 mg of Aniline). Dilute standard aniline solution No. 1very accurately 200-fold with water in a volumetric flask. Prepare fresh every 3 days. Sodium Nitrite Solution ( 2 % ) and Sulfamic Acid Solution (3%). Prepare fresh every 3 weeks. Purification of H-acid. Transfer 20 g of technical grade H-acid (8amino-l-hydroxynaphthalene3,6-disdfonic acid, monosodium (Eastman Kodak or salt, NH2Cl$-14(OH)(S03H)S03Na.H20) Fisher Scientific Co.) to a 600-mL beaker containing about 300 mL of boiling water and boil for several minutes with stirring to dissolve. Cool in ice. Filter the precipitated salt through a small Buchner funnel containingWhatman No. 41 filter paper and wash four times with cold water and three times with acetone. Spread the salt onto a large filter paper and allow to stand in the air for 1-2 h to volatilize the residual acetone. Do not use a vacuum desiccator. H-acid Reagent (0.75%). Prepare fresh every 3 days from purified H-acid and store in a brown bottle. Preparation of Calibration Curve. Transfer 0.00,1.00,2.00, 3.00, and 4.00 mL of standard aniline solution no. 2 (1mL = 0.050 mg of aniline) to 100-mL volumetric flasks and dilute to about 75 mL with water. Add 2.0 mL of hydrochloricacid (2.5 to 100) and 1.0 mL of sodium nitrite solution (2%) and swirl. Allow to stand 5 min. Add 2.0 mL of sulfamic acid solution (3%), swirl, wash down the neck of the flask to remove residual nitrite, and 0 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
swirl again. Allow to stand 10 min. Add 10.0 mL of sodium bicarbonate solution (6%), swirl, add 2.0 mL of H-acid reagent (0.75%),and swirl again. Dilute to the mark, mix well, and store in the dark. Measure the absorbance in the interval 15-45 min against water at 526 nm, deduct the blank, and plot absorbance against milligrams of aniline per 100 mL. Procedure. Transfer an aliquot of the sample containing preferably 0.075-0.175 mg of aniline to a 1WmL volumetric flask, dilute to about 75 mL, and proceed as described for the preparation of the calibration curve. RESULTS AND DISCUSSION Preparation of H-Acid Reagent. Technical grade H-acid is dark grayish brown and dissolves in water to give a dark brown but clear solution. Two milliliters of a 0.75% solution prepared from the technical grade material gave a very high blank (absorbance of about 0.1) when carried through the procedure. It is essential, therefore, to purify the H-acid. At first, the purification was performed by dissolution in dilute hydrochloric acid and treatment with activated carbon as recommended by English (8)in the purification of l-amino&hydroxynaphthalene-2,4-disulfonicacid. This technique was found to be unsatisfactory and troublesome when applied to H-acid, so the simple recrystallization procedure from boiling water was used. The yield in the recrystallization process without any attempt to rework the filtrates was about 50%; costwise, this is satisfactory since technical grade H-acid is cheap. The recrystallized material is white with a faint tint of gray. It is very stable and is not affected by light. A solution of H-acid, however, darkens rapidly when exposed to light and subsequently gives a high blank. The solution must therefore be stored in a brown bottle; when so stored it remains pract i d y colorless for 3 days. The absorbances of blanks obtained by using freshly prepared and 3 day old H-acid solution (0.75%), both stored in brown bottles, were 0.005 and 0.010, respectively. Some experiments were conducted on the stabilization of H-acid solution by the addition of sodium sulfite, as has been recommended by Willstaedt (I). This treatment improved the stability but was less effective than storage in a brown bottle. The mechanism of the deterioration of H-acid solution to light is not known. It does not involve air oxidation, since a solution of H-acid in a brown bottle does not darken when subjected to a current of air for 10 min. Effect of Acidity on the Diazotization. When no acid is present, diazotization does not take place even with a large excess of nitrite. Full color development was achieved after diazotization in the presence of 0.1-5.0 mL of hydrochloric acid (10 to 100) or 0.1-5.0 mL of sulfuric acid (3to loo), added to a volume of about 75 mL of solution (providing that sufficient sodium bicarbonate was added to obtain the proper pH prior to the addition of the H-acid). No experiments were conducted with amounts of acid greater than 5 mL of hydrochloric acid (10 to 100) or 5 mL of sulfuric acid (3 to 100). The full color development with 0.1 mL of acid (pH about 3) was unexpected; however, this amount of acid still fulfills the requirement that 2 equiv of acid are needed for every equiv + NaN02+ 2HX C6HSN2+X-+ 2H20 of aniline: + NaX, where X = C1, HS04,etc. (This equation represents the overall reaction; in actuality, diazotization is a complex stepwise process (18,19).) The use of 0.5 mL of hydrochloric acid (10 to 100) or 0.5 mL of sulfuric acid (3to 100) (equivalent to 2.0 mL of hydrochloric acid (2.5 to 100) and 2.0 mL of sulfuric acid (0.75 to 100), respectively) is recommended for the method. Effect of Amount of Nitrite and Alcohol. The amount of nitrite is critical in a peculiar way. When amounts of sodium nitrite solution (2%) from 0-5.0 mL (together with 0.5 mL of hydrochloric acid (10 to 100)) were added to solutions containing aniline and the color was developed by adding the sodium bicarbonate and H-acid, the absorbances increased
-
Table 11. Interference of Ethanol, Methanol, and Acetone with the Recommended Method (0.100 mg of Aniline Present per 100 mL)
recovery of aniline (mg/100 mL) in the presence of various amounts of ethanol, methanol, and acetone
3.0
solvent ethanol methanol acetone
mL
5.0 mL
7.5 mL
10
mL
20 mL
30
mL
50 mL
0.101 0.096 0.088 0.078 0.070 0.033 0.020 0.098 0.088 0.075 0.067 0.041 0.101 0.096 0.093 0.086 0.072 0.046 0.026
steadily over the range 0-0.9 mL of sodium nitrite solution and then leveled of somewhat over the range 0.9-1.2 mL. Over the range 1.2-5.0 mL, the samples assumed a reddish purplish tint and gave high and erratic results. With less than 1.2 mL of sodium nitrite solution, the blanks were satisfactory; with more than 1.2 mL, the blanks were purplish and gave high absorbance readings (0.1-0.3). Ethanol and methanol (but not acetone) in amounts as little as 0.2 mL had the effect of markedly increasing the formation of the interfering purplish color in samples containing aniline and in the blanks. The formation of the interfering color involves a reaction between the excess nitrite and the H-acid, possibly nitrosation. The effect of ethanol and methanol in enhancing the formation of the color may involve the production of ethyl and methyl nitrite and subsequent hydrolysis of these compounds. In view of the above results, consideration was given to using 0.9 mL of sodium nitrite solution (2%) without destroying the excess nitrite; however, the accuracy and precision obtained by this method of approach were only fair. Because of this limited accuracy and precision and the interfering effect of alcohol, the decision was made to destroy the excess nitrite. When the excess nitrite was destroyed (by use of a sufficient amount of sulfamic acid), a plateau of absorbance for samples containing 0.10 mg of aniline was obtained over the range 0.6-5.0 mL of sodium nitrite solution (2%). The use of 1.0 mL of the sodium nitrite solution is recommended. Ethanol and methanol have another affect on the color that is not readily apparent in the presence of excess nitrite, namely, the characteristic of repressing the formation of the color. This phenomenon, which is also shown by acetone, has nothing to do with excess nitrite. The results obtained in the finally recommended method (described in the Procedure) in the presence of varying amounts of ethanol, methanol, and acetone after destroying the excess nitrite are shown in Table 11. It is seen that up to 5 mL of ethanol or acetone and up to 3 mL of methanol can be present in the recommended method. Time for t h e Diazotization. The same result was obtained by using 0.5 mL of hydrochloric acid (10 to 100) (together with 1.0 mL of sodium nitrite solution (2%)) at room temperature over the time interval 2-30 min. For 30-120 min, the results were erratic. On standing overnight no color developed, indicating that the benzene diazonium chloride had undergone some kind of transformation. Use of a n Accelerator for t h e Diazotization. The use of sodium or potassium bromide as an accelerator for the diazotization, as recommended by Ponomerenko (3),Kuritayn and Kuritsyna (5), English (a), and Belyakov and Gorbgleva (II), is not necessary. Effect of Temperature on Diazotization. The same result was obtained over the temperature range 10-30 "C, so it is not necessary to cool the solution in ice. Low results were obtained at temperatures over 35 "C. Investigators who recommended cooling the solution in ice prior to the diazotization (see Table I) were apparently influenced by the methods used in preparative chemistry. In such methods, the
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
Table 111. Effect of the Amount of Sodium Bicarbonate on the Development of the Color (0.100mg of Aniline Present per 100 mL) mL of 6% equivalence NaHCO, in g of pH after sol NaHCO, H-acid 1.0 2.0 3.0 5.0 7.5 10.0 12.5 15.0 20.0
0.06 0.12 0.18 0.30 0.45 0.60 0.75 0.90 1.2 1.5" 2.0" 6.0a
2.5 6.0 6.6 6.8 6.9 7.2 7.3 7.4 7.5 7.6 7.6 7.8
abs 0.00 0.27 0.35 0.37 0.38 0.39 0.39 0.395 0.40 0.405 0.43 0.44
Added as the solid.
solution is cooled with ice because large amounts of diazonium salts are unstable, especially since diazotization is an exothermic process. Use of Sulfamic Acid to Destroy the Excess Nitrite. As stated earlier, it was found necessary to destroy the excess nitrite. Sulfamic acid was found to be preferable to urea for this purpose. No experiments were conducted with sodium or ammonium sulfamate. We found ethanol to be ineffective for destroying nitrite. Two milliliters of sulfamic acid (3%) was found to be satisfactory for destroying the residual nitrite after initially adding 1.0 mL of sodium nitrite solution (2%). A larger amount of sulfamic acid would necessitate the use of more sodium bicarbonate for the neutralization. The same result was obtained on allowing the solution to stand for 5-30 min after adding the sulfamic acid. A time period of 10 min is recommended. Effect of pH on the Coupling Reaction. Sodium bicarbonate is recommended for adjusting the pH for the coupling reaction. To test the effect of different amounts of sodium bicarbonate, we added various quantities of this reagent to solutions containing 0.10 mg of aniline, and the samples were carried through the procedure. The results (Table 111) show that the absorbance of the cherry red dye tends to increase with increasing amounts of sodium bicarbonate, but the increase is slight over the range 7.5-15 mL of sodium bicarbonate solution (6%). The pH of the solutions measured after adding the H-acid also tends to increase with increasing amounts of sodium bicarbonate (Table 111). The use of 10.0 mL of sodium bicarbonate (6%) (which gives a pH of about 7.2) is recommended. Some work was done on developing the color by the addition of 6 g of solid sodium bicarbonate (which gives a pH of about 7.8); however, the results were not as satisfactory as with 10.0 mL of sodium bicarbonate solution (6%). Experiments showed that the color could also be developed by adding 4 g or more sodium acetate (which gives a pH of about 6); however, sodium acetate seemed to offer no advantage over sodium bicarbonate. Some work was done on the use of sodium hydroxide solution in the development of the color. It was found that the absorbance
decreased as the amount of sodium hydroxide increased, although neither the hue of the color nor its stability changed. When 5 mL of sodium hydroxide solution (10%)was added, the decrease in absorbance was about 50%. The probable explanation of the decrease in absorbance is that at high pH the diazonium ion is converted to diazotate ion, which does not couple (19,201. Coupling reactions are complicated processes that involve the orientation effects of the various substituent groups and rearrangements (19, 20). When a diazotized compound is coupled to H-acid in an acid solution, the amino group of the H-acid is the directing influence and the coupling takes place in the 7 position; when the coupling takes place in alkaline solution the hydroxyl group of the H-acid is the directing influence and the coupling takes place in the 2 position (20). There is no information on the coupling in a sodium bicarbonate medium; however, since the hue of the color remains the same whether the color is developed in sodium bicarbonate or sodium hydroxide medium, it can be deduced that the coupling in the sodium bicarbonate medium takes place in the 2 position. The formula of the cherry red dye would, therefore, be as follows:
Wavelength for Measuring the Color. The absorbance c w e obtained on a Hewlett-Packard Model 8450A recording spectrophotometer showed a broad peak with a maximum at 526 nm. However, the wavelength setting in the method is not critical. Practically the same absorbance readings were obtained by using the Bausch and Lomb Model 70 spectrophotometer over the range 524-530 nm. Effect of Amount of H-acid. A plateau for absorbance was obtained over the range 1.5-5.0 mL of H-acid reagent (0.75%)when 0.10 mg of aniline was present. The use of 2.0 mL of the reagent is recommended. Effect of Light on the Color. Light did not seem to have an appreciable effect on the color; however, it produced a significant error in the method by causing the absorbance of the blank to increase markedly after a few minutes, due to the deterioration of the H-acid, especially in strong sunlight. Apparently, the deterioration does not take place when enough aniline is present to produce the red dye, which acts like an internal filter. To overcome the effect of light, it is recommended that all the solutions be stored in the dark in the interval between the final mixing and the color measurement. A satisfactory alternative to storing the solutions in the dark is to use volumetric flasks made from low actinic glass. Bahensky and gulc (4) recommended the addition of 1 mL of sodium bisulfite solution (10%)after the addition of H-acid in order to stabilize the color and blank. We found that this treatment was less effective than storing the samples in the dark.
Table IV. Effect of Time on the Development of the Color mg of aniline (per100mL)
8min
15min
blank 0.050 0.100 0.150 0.200
0.005 0.19 0.385 0.58 0.77
0.005 0.195 0.395 0.59 0.78
absorbance aft;er different time intervals 30min 45min l h 2h 0.005 0.195 0.395 0.59 0.78
0.005 0.20 0.40 0.59 0.78
59
0.005 0.20 0.405 0.59 0.78
0.01 0.205 0.41 0.60 0.78
3h
4h
24 h
0.01 0.21 0.41 0.60 0.79
0.02 0.21 0.41 0.61 0.80
0.02 0.22 0.43 0.63 0.83
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
Effect of Temperature on t h e Color. Temperature had no significant effect on the color; the same absorbance was obtained whether the color was developed at room temperature or by placing the volumetric flask in boiling water for a few minutes. The color is not susceptible to air oxidation, as judged by the fact that the absorbance did not change when a stream of air was passed through the solution for a few minutes. Effect of Time on t h e Development of t h e Color. A study of the effect of time on the development of the color (Table IV) showed that the color developed practically completely in 15 min and then increased at a very slight rate for a period of up to 24 h. Measurement of the color in the interval 15-45 min is recommended. Effect of Dissolved Gases. Nitrogen is produced by the reaction of nitrite with sulfamic acid, while carbon dioxide is produced by the reaction of hydrochloric and sulfamic acids with sodium bicarbonate. Most of gases are driven off in the swirling and mixing and the residual amounts do not interfere. The use of a stream of air to drive out the carbon dioxide, as recommended by Behenskg and h l c (4), does no harm but is not necessary. Application to t h e Semimicro and Micro Scale. Experiments indicated that the method could be adapted to the semimicro and micro scale, as well as the macro scale. In connection with work on a semimicro scale, a calibration curve was prepared by transferring 1.00, 2.00,3.00, and 4.00 mL of standard aniline solution no. 3 (1mL = 0.0050mg of aniline) (prepared by diluting standard aniline solution no. 2 tenfold) to 10-mL volumetric flasks, diluting to about 7 mL, and proceeding as in the macro method but using 0.20 mL of hydrochloric acid (2.5 to loo),0.10 mL of sodium nitrite solution (2%),1.0 mL of sodium bicarbonate solution (6%), and 0.20 mL of H-acid reagent (0.75%). The calibration curve so obtained was practically identical with that obtained for the macro method. In connection with work on the micro scale, 0.100,0.200,0.300,and 0.400 mL of standard aniline solution no. 3 were transferred to 1-mL volumetric flasks and the volumes brought up to about 0.7 mL. The color was developed as in the macro method but using 0.020 mL of hydrochloric acid (2.5 to NO), 0.010 mL of sodium nitrite solution (2%), 0.10 mL of sodium bicarbonate solution (6%),and 0.020 mL of H-acid reagent (0.75%). Syringes were used in the micro method for measuring the standard aniline solution and reagents. After the addition of each reagent, the stoppers were inserted into the 1-mL volumetric flasks and the solutions mixed. The colors obtained in the micro method followed a logical progression but we did not have a spectrophotometer that could measure the absorbance of the small volumes. Certainly, it would seem that the colors could be compared visually with standards prepared in the same way.
Accuracy and Precision. To check the accuracy and precision of the method we analyzed the four concentrations of aniline used for the preparation of calibration curve on four successive days. For all concentrations, the maximum spread for the four absorbance readings was 0.015. The calibration curve followed Beer’s law. Application of t h e Findings. It is believed that the procedure described in this paper will have extensive application in the determination of large and small amounts of aniline in process chemistry and toxicology. Also, the procedure should be applicable to the determination of other aromatic amines, including pharmaceuticals, but this was not investigated. The findings concerning conditions for diazotization would seem applicable to methods using other coupling agents besides H-acid, since the diazotization is a separate entity. The authors are currently investigating the application of the findings to the N-l-naphthylethylenediamine method. ACKNOWLEDGMENT We thank the Hewlett-Packard Co. for the loan of the Model 8450A spectrophotometer and Jay Levine of the company for useful discussion. LITERATURE CITED (1) W i l l ~ t a d t H. , Blochem. 2. 1034, 269, 182-188. (2) Korenman, I. M.; Ganichev, P. A. Uch. Zap. Gor’k. Unlv. 1053, 24, 123-125. ‘ (3) Ponomerenko, B. V. Tr. Kom. Ami. Khim. Akad. Nauk SSSR Inst. W h l m . Anal. Khlm. 1056, 7, 289-294. (4) Bahenskg, V.; h l C , J. KOf028 Ochr. Mater. 1058, 2 , 85-68. (5) Kwitsyn, L. V.; Kwitsyna, V. M. Izv. Vyssh. Uchebn. Zaved., Khlm. Khlm. Tekhnol. 1072, 15, 461-462. (6) Chrastll, J. Analyst(London) 1976. 101, 522-527. (7) Eikins. H. B. “The Chemistry of Industrial Toxicology”, 2nd 4.;Wiley: New York, 1959; pp 293-294. (8) English, F. L. AM/. Chem. 1047, 19, 457-459. (9) Daniel, J. W. Ana&st(London) 1081, 86, 840-843. (10) Clipson, J. L.; Thomas, L. C. Analyst (London) 1063, 88, 971-972. (1 1) Betyakov, A. A.; Oorb9ieva, N. V. Tr. Kom. Anal. Khim., Akad. Nauk SSSR 1060, 1 1 , 436-446. (12) Elssner, W. Arch. Pharm. (Athens) 1030, 268, 322-323. (13) Bandelin, F. J.; Kemp, C. R. Anal. Chem. 1046, 18. 470-471. (14) Montgomery, M.; Freed, V. H. J . Agrk. Food ch8ffl. 1050, 7, 817-61 8. (15) Hanson, N. W.; Reiiiy, D. A.; Stagg. H. E. “The Determination of Toxic Substances in Air, A Manual of IC1 Practice”, Heffer: CambrMge, England, 1965; pp 58-60. (16) Stewart, C. P.;Stolman, A. “Toxicology, Mechanisms and Analytical Methods”; Academic Press: New York, 1961; Vol. 11, pp 81-82. (17) Kratochvii, V. Z . AM/. hem. 1061, 183, 267-272. (18) Ridd. J. H. Q . Rev., Chem. Soc. 1061, 15, 418-441. (19) Zdiinger. H. “Diazo and Azo Chemistry”; Interscience: New York, 1981. (20) “Kirk-Othmer Encyclopedia of Chemical Technology”, 3rd 4.:Wiiey: New York, 1978; Voi. 3, pp 387-433.
RECEIVED for review July 11,1980. Accepted September 25, 1980.
