The anions that did not interfere were: OAc-, NOz-, SOa-2, S03-2, S20s-2, .BrOs-, Io3-, HzPOd-, C03-2, and C4H406-2. IO4- formed a browri precipitate of AgIOd. The presence of Br03- did not interfere immediately but after standing for 1 hour it formed a white precipitate of AgBrOs which destroyed the chelate. Extraction Studies. Thirteen organic solvents were purified (6), generally by distillation to remove peroxides, and
employed as extracting agents. They were benzene, hexane, cyclohexane, n-heptane, decalin, benzyl alcohol, isoamyl alcohol, n-hexyl alcohol, xylene, diethyl ether, methyl ethyl ketone, chloroform, and Skelly C. Not one extracted the chelate nor did mixtures of 2 or 3 of the solvents have any effect. RECEIVED August 24, 1966. Accepted January 3, 1967. (6) C. E. Meloan, Ph.D. Thesis, Purdue University, 1959.
2,4,6-Trichlorophenolin a Constant Ionic Strength Buffer System for pH Range 5.5-6.5 Albert L. Caskeyl Department of Chemistr y , Iowa State University, Ames, Iowa
BUFFER SOLUTIONSof varying pH but fixed ionic strength, prepared from monoba dc acid and monoacidic base systems, have been described by Bates ( I ) . While determining the naphtholic acid dissociation constant of 2-nitroso-1-naphthol4-sulfonic acid (2), the need arose for such a buffer usable at pH 6. Hydroxy1amin:-hydroxylammonium chloride, the only available one, wa!; eliminated for use because it might react with the quinont:-oxime tautomer. Work delimiting 2,4,6-trichlorophenol for use as such a buffer is reported here; it is appreciably water soluble, it has been studied more extensively than others, and the acid ionization constant is of the correct order. Reported pK, values ranged from 7.59 (3) to 5.50 ( 4 ) . Many values fell in between (5-9); a recently reported one is 15.2(IO). A simple acid base indicator was not available for titration of 2,4,6-trichlorophenol with standard base as a simple, rapid method of determination. A suitable indicator, briefly described by Csanyi, (;], I2), 2,2‘,3,3 ’-tetramethylphenolphthalein [3,3-bis(4-hydroxy-2,3-xylyl)phthalide], was prepared. E YPERIMENTAL
Reagents. Reagent ,gade chemicals meeting ACS specifications were used whenever commercially available. The 2,4,6-trichlorophenol (Eastman Kodak 1469) was sublimed, dried, and stored over anhydrous Mg(C10&: neutralization 1 Present address, Department of Chemistry, Southern Illinois University, Carbondale, Ill. 62901
(1) Roger G. Bates, “Elcctrometric pH Determinations,” Wiley, New York, 1954. (2) A. L. Caskey, Dept. of Chem., Iowa State University, Ames, Iowa, unpublished work, 1966. (3) N. T. Crabb and F. E Critchfield, Talanta, 10,271 (1963). (4) A. G. Ogston, J . Chen.8. SOC.,134, 1713 (1936). (5) L. F. Fieser and M. Fieser, “Organic Chemistry,” 3rd ed., D. C. Heath and Co., Eloston, 1956. (6) A. Hantzsch, Ber. Deht. Chem. Ges., 32, 3066 (1899). (7) J. A. A. Ketelaar, H. R. Gersmann, and M. Beck, Rec. Trau. Chim.,71,497 (1952). (8) G. J. Tiessens. Rec. Trau. Chim.. 48. 1066 (1929). (9) G. J . Tiessens, Zbid.,50, 112 (1931). (10) E. C. Steiner and J. M. Gilbert, J. Am. Chem. Soc., 85, 3054 (1963). (11) W. Csanyi, Austrian Datent80633 (15 October 1919). (12) W. Csanyi, Z. Elektrochem., 27, 64 (1921).
equivalent, before sublimation, 196.5, after sublimation 197.7 + 0.2 (potentiometric titration in 50z ethanol with standard NaOH); theor., 197.46. The 2,3-dimethylphenol (K and K Laboratories, Inc.) was sublimed: m.p. 74” C, b.p. 213” C ; lit. m.p. 74” C, b.p. 213°C (13). The 2,2’,5,5’tetramethylphenolphthalein was prepared by a reported method (14) and recrystallized from tetraline and ethanolwater: m.p. 285.5-7” C ; lit. m.p. 276” C (14, 286-7” C (15). The hydroxylammonium chloride, NHzOH-HCI, was recrystallized [from dilute, aqueous HCl, washed with ethanol, dried 30 minutes at 110” C, and stored over anhydrous Mg(C104)2]: assay, 100.15z (rel. std. dev. 0 . 3 0 z , 4 trials) by titration with 0.1009NAgN03 (rel. std. dev. 0.23 6 trials) using dichlorofluorescein adsorption indicator. The 0.100M potassium hydroxide in 0.1000M potassium chloride was prepared by dissolving dried KC1, 7.4555 grams, in deionized water, converting it into KOH using an anion exchange column containing Amberlite IRA-410 in the OHform, adding KC1, 7.4568 grams, and diluting to 1 liter. The solution was 0.09942N in KOH; 43.82 ml neutralized 0.8897 gram potassium acid phthalate. The buffer solution of NH,OH.HCl was prepared 0.05000M in both KCl and NH20H.HC1, with an ionic strength of 0.1000, following the practice of Bates ( I ) . The stock solutions of 2,4,6-trichlorophenol were prepared 0.1000M in KC1. The weight, grams of 2,4,6-trichlorophenol, the volume, ml, of 0.100M KOH (in 0.1000M KCl), and the weight, grams, of KC1, respectively, in 1 liter of each of the solutions were: for the study at 312 mp, 0.3871, 20.00, and 7.3069; for the study at 286.5 mp, 0.8294, 42.00, and 7.1425; for the study at 246 mp, 0.2106, 11.00, and 7.3733. Solutions used in the measurement of the pK, were prepared by pipetting into each of several 100-ml volumetric flasks, 25.00 ml of stock solution of potassium 2,4,6-trichlorophenolate, 10.00 ml of NH20H.HC1-KC1 buffer mixture and either 0.0971MHCl or 0.100MKOH (in 0.1000M KCl). Deionized water was added to each strongly basic solution and each solution was diluted with 0.1000M KCl. Equipment. The pH meter used was a Beckman Model G equipped with a general use glass electrode and ground sleeve
z,
(13) “Handbook of Chemistry and Physics,” 40th Ed., D. Hodgman, ed., Chemical Rubber Publishing Co., Cleveland, Ohio, 1958. (14) A. Thiel and L. Junger, 2. Atiorg. Allgem. Chem., 178, 49 (1929). (15) M. Dominikiewiez, Roczniki Chem., 11, 113 (1931). VOL. 39, NO. 3, MARCH 1967
385
RESULTS AND DISCUSSION
Table 1. Titrations of 2,4,6-TrichlorophenoI with Standard Base; 2,2',3,3'-Tetramethyfphenolphthalein as Indicator 2,4,6-Tri0.12002N 2,4,6-Trichlorophenol, base chlorophenol Av. value grams added, ml found, and std. dev.
type, saturated calomel electrode system calibrated using NBS standard buffers ( I ) . Measured p H values were corrected for calibration drift. A Cary Recording Spectrophotometer, Model 12, equipped with matched, silica, 1.000-cm cells, and located in a room thermostated to 25" C was used to determine spectra. Potentiometric measurements at constant temperature were done on solutions in a constant temperature bath maintained at 25.0" i. 0.01 C. Preparation of 2,3',3,3'-Tetramethylphenolphthalein. A mixture of 2,3-dimethylphenol, 6.1 1 grams (0.050 mole), o-phthalic anhydride, 3.7 grams (0.025 mole), and excess anhydrous stannic chloride was refluxed for 1-5 hours. The red colored solution and dark solid resulting were cooled to near 0" C and extracted with small portions of ice cold 1 :1 HC1-diethyl ether. A polymeric by-product was filtered from the liquids before they were separated, the product remaining in the ether, which was then washed three times with dilute HCl, water, and dilute N a H C 0 3 and finally dilute NaHC03-Na2C03 at p H 9.5-10 until the aqueous layer was nearly colorless. The indicator was extracted into dilute aqueous NaOH, the extract acidified with HC1, and the indicator extracted back into diethyl ether, which was evaporated to dryness. Unreacted 2,3-dimethylphenol was sublimed from the brown solid (or extracted with 60-90" C. petroleum ether) and identified: m.p. 74" C. The product was recrystallized from carbon tetrachloride and ethanolwater yielding a tan solid: weight 1 gram, m.p. 275" C ; probably slightly impure. Procedures. Ethanol-water mixtures, varying from 100 ethanol to 100% water, were used as solvents in titrations of 2,4,6-trichlorophenol with standard base. The 2,2',3,3'tetramethylphenolphthalein was used like phenolphthalein is used. Stenstrom and Goldsmith in 1929 (16) briefly described the method used for the determination of a pK,, noting that it requires the species obey Beer's law. Diehl and Lindstrom (17), as well as others, described in detail the method used in this work. The determination of the pK, was done with NHzOH.HC1 buffers using conditions under which 2,4,6trichlorophenol and its potassium salt obeyed Beer's law. Three independent measurements were made at the available wavelengths, 312, 286.5, and 246 mp, at an ionic strength of 0.1000, and at 25.0" C. The instability of hydroxylamine in a basic solution, due to its disproportionation into nitrogen and ammonia (I8), necessitated use of the solutions prepared for the measurement of the pK, as soon as they reached constant temperature in the bath. Less than 10 minutes elapsed between the measurement of the pH and the determination of the spectrum of each solution.
Phenolphthalein, thymolphthalein, and 2,2 ',5,5 '-tetramethylphenolphthalein were unsuitable for titration of 2,4,6trichlorophenol with standard base. Results, as purity, were low for the former indicator and high for the latter indicator, although not as high as for thymolphthalein. Changes in solvent varying from water to a high concentration of ethanol did not change the results. The solvent effects were less than the experimental error. The ionizations of the halogenated phenol and the indicators were effected in the same direction and in the same order by the changes in solvent. 2,2 ',3,3 '-Tetramethylphenolphthalein was obtained as a tan material in a low yield (about 15z). Reflux times of 1.5-2 hours give the highest yields before isolation, The indicator dissolves readily in ethanol to give a pale yellow solution, which upon the addition of excess base becomes pure, intense blue. Under titration conditions, in aqueous solution the color change is colorless to blue, first appearing at about pH 8.85. The respective, apparent pK, is 9.48 =t 0.06 (at 25" 3 1°C and 0.100 ionic strength as KC1). The wavelength maximum in the visible region occurred at 585 mp with a molar absorptivity of 2.79 X lo4liter-mole-km-'. The phthalein is stable in 0.01N K O H for a t least 3 weeks. The suitability of 2,2 ',3,3 '-tetramethylphenolphthalein as an indicator for titration of 2,4,6-trichlorophenol with base was studied by classical titration of samples dissolved to give 25 ethanol solutions at the end point. The approach of the end point was very evident and similar to that with phenolphthalein. The first 6 titrations run, which were done in about 30 minutes, are summarized in Table I. The NaOH used was 0.12002N (rel. std. dev. 0.045 8 trials). An identical titration was followed potentiometrically. The titration curve was that of a typical weak acid titrated with a strong base. The end point break upon addition of 0.10 ml of base was slightly greater than 0.4 pH. The color change occurred during addition of the first portion of this aliquot. The potentiometrically determined equivalence point gave a purity of l00.08%, in good agreement with the value found by titration with the visual indicator. The observed ultraviolet-visible adsorption spectra of 2,4,6-trichlorophenol and its sodium salt have characteristics similar to those reported for dichlorophenols (19) and for trichlorophenols (20-22). Spectral measurements made at 0.100 ionic strength showed that 2,4,6-trichlorophenol and its anion obeyed Beer's law throughout the UV-visible region. Molar absorptivities were determined from the slope of Beer's law plots. Solutions saturated with 2,4,6trichlorophenol showed no adsorption at wavelengths longer than 340 mp in an acidic medium and longer than 358 mp in a basic medium, but showed a weakly absorbing shoulder at about 320 mp in an acidic medium. When ethanol was used as the solvent instead of water, all of the maxima were shifted slightly to longer wavelengths and were more intense, but the spectra were similar otherwise. Spectra characteristics: found; solvent, wavelength, mp [molar absorptivity]; in acidic aqueous solution, 246 [360], 286.5 maximum [2193], 293 maximum [2170], 312 [very small]; in basic aqueous solution, 246 maximum [8704], 286.5 [1236], 293 [2038], 312
(16) W. Stenstrom and N. Goldsmith, J . Phys. Chem., 30, 1683 (1926). (17) H. Diehl and F. Lindstrom, A~SAL. CHEU.,31, 414 (1959). (18) T. Moeller, "Inorganic Chemistry," Wiley, New York, 1952.
(19) R . A. Robinson, J . Res. Natl. Bur. Std., A68, 159 (1964). (20) C. L. Hilton, Textile Res. J . , 28, 263 (1958). (21) D. Logie, A m l y s f , 82, 563 (1957). (22) J. E. Purvis, J . Chem. SOC.,1913, p. 1638.
1,1587 1.0013 0.7975 1.1059 1,1336 0.9171
48.93 42,29 33,74 46.76 47.84 38.75
100.07 100.09
100. 12
100.26 100.29 100.01 100.13
u =
0.09 or
0.09%
z
386
ANALYTICAL CHEMISTRY
z,
Table 11. Data for Determination of pK, of 2,4,6-Trichlorophenol 0.0971N HC1, ml 5.00 2.00
0.100N KOH, ml 1 .oo 1 ,511 2.00 2.o o c 2.2.5 2.50 2.70 2.7.5 3.00 3.05 3.25 3.50 3.75 4.00 4.2:s 4.50 4.75 6.00 8.00
HzO, ml
2865 m p
246 mp5 Ab 0,010 0 025
PH 2.37 2.73
9.458 0.790 2.160 0.895 0.972 1.012
5.45 5.80 11.10 5.88 5.95 5.98
6.03 6.14 6.28 6.41
1.225 1.335 1,455 1.575 1,695
6.17 6.24 6.37 6.48 6.61
1,430
6.83
1.940
6.89
1.315 1.320
10.65 11.23
2,265d 2,320d
10.68d 11.27d
312 mpa Ab 0.005 0.005 0.008 0.011 0.038
PH 2.38 2.65 3.30 3.54 4.18
2.297 2.295 2.285 2.265 2.175
0.262
5.09
2.080 2.025
5.50 5.63
0 :580
5.54
1.950 1.892
5.73 5.87
0.837 0.985 1,235 1.423 1.655 1.865 2.075 2.350d 2.345
5.74 5.91 6.07 6.22 6.42 6.63 6.94 7.57d 10.84
1.800 1.730 1.655 1,582
Ab
PH 2.43 2.73 3.59 4.30 5.15
2.00
1 .oo 3.00
A different concentrai ion of 2,4,6-trichlorophenolwas used at each wavelength. The blanks contained buffer solution diluted with 0.1000MKCI. c This solution did not contain buffer. d There was evidence OF decomposition of NHzOH; data were not used. 0
b
maximum [4793]; in aqueous solution, 273.8 isosbestic point [965 =t 31, 293.5 isostiestic point [2165]; in acidic ethanol solution 290 [2.7 x 1031,297 [2.7 X 1031; and in basic ethanol solution 222 [>lo4], 249 [>lo4], 319 [5.4 X lo3]; literature, in methanol, 289 maximum, 296 maximum ( 2 1 ) ; in 2,2,4trimethylpentane, 295.7'5 maximum [2.8 X lo3] (20); in absolute ethanol, 294.1 maximum [2900] (23); and in aqueous solution, 273 isosbesti: point, 295.5 isosbestic point (24). The presence of N H 2 0 H 'HCl as a 1 solution has no effect on the absorption spectra of 2,4,6-trichlorophenol and its sodium salt, except where slight absorption by the amine occurred below 240 mp which restricted "*OH . HCl buffers to use at longer wavelengths. 0.1000M KCl had no effect on any of the spectra. Halogenated phenols can be oxidized to quinones (25, 26). Partial oxidation results in polymer formation. Triiodophenol undergoes this type of decomposition forming a red, polymeric, oxidized prc'duct in a basic solution (27). A red, water insoluble impurity was found in the commercial 2,4,6trichlorophenol used. 13asic solutions of 2,4,6-trichloropheno1 are stable as the spectxum of an aqueous, basic solution did not change during a 7-week period, the duration of the study.
Data describing the solutions on which measurements were made to determine the pK, are shown in Table 11. When absorbance was plotted as a function of pH, curves were obtained similar to those reported by Diehl and Lindstrom (19,but showing only one break. Well defined linear regions existed. At 312, 286.5, and 246 mp, values of 6.019, 6.016, and 6.044, respectively, were obtained for the pK, of 2,4,6-trichlorophenol (av. 6.026, std. dev. 0.015). A value of 6.02 seems to be most reasonable because the value of 6.044 obtained at 246 mp is more uncertain than the other two values inasmuch as Beer's law plots at 246 mp showed more scattering of the data. 2,4,6-TrichlorophenoI can be used in buffer systems of varying pH but fixed ionic strength when hydroxylamine is not satisfactory. In spectrophotometric work, it can be used at wavelengths above 360 mp. The concentration of the buffer on the low pH side is limited by the solubility of the free phenol in water. Solutions that are 0.004M functioned satisfactorily (2), and may be used at pH values down to 5.85.9. Unlike hydroxylamine, 2,4,6-trichlorophenol is stable in basic solution. ACKNOWLEDGMENT
(23) A . Burawoy and J. 1'. Chamberlain, Ibid., 1952,p. 2310. (24) T. Kashima and T. Konda, Shokuhin Eiseigku Zasshi, 5, 35 (1964); C.A., 61, 12638d(1964). (25) E.Leger, Bull. Soc. Chim. France, 3, 578 (1908). (26) E.Leger, Compt. Rend., 146,694 (1908). (27) G.H. Woollett, J . Ain. Chem. Soc., 38, 2472 (1916).
The assistance and guidance of Harvey Diehl during the course of this work are gratefully acknowledged. RECEIVED for review January 7,1966. Resubmitted December 14, 1966. Accepted January 3, 1967.
VOL. 39, NO. 3, MARCH 1967
387
