JUNE, 1966 THE ANALYST Vol. 91, No. 1803 The Determination of Phenol, o-Cresol and p-Cresol in Aqueous Solution by a Kinetic Method BY A. E. BURGESS AND J. L. LATHAM (Department of Chemistrjl and Biology, Harris College, Corporation Street, Preston, Lancashire) Certain phenols can be determined in aqueous solution by a kinetically controlled bromination in which the only measurement is the time taken for the bleaching of an indicator. This time is proportional to the concentration of the indicator. Since no specialised apparatus is needed, the method is not restricted to the laboratory. PHENOLIC substances are often determined by bromination in aqueous so1ution.l y 2 9, One method is to add an excess of a standard solution of bromate and bromide ions to an acidified solution of the phenol.After a suitable interval of time, during which the bromination reaction proceeds to completion, an excess of iodide ions is added, and the iodine liberated is back-titrated with standard thiosulphate solution. Phenol has also been determined in micro amounts by using the electrochemical generation of b r ~ m i n e . ~ This paper shows that phenol, o-cresol and 9-cresol can be determined bromometrically in dilute aqueous solution, without the necessity of using a titration or an electrochemical technique. Analytical use is made of the kinetic characteristics of the following two reac- tions- (a) the production of bromine in an aqueous solution containing bromide, bromate and hydrogen ions ; ( b ) the nuclear mono-bromination of the phenol. The observed variable is the time of bleaching of an indicator, and the conditions are chosen so that the observed time is directly proportional to the concentration of phenol used.The theoretical basis of the method is that when a mixture of bromide and bromate ions to which phenol has been added is acidified, a stationary-state concentration of bromine is set up in accordance with the following equations- * . (1) . . * * (2) HfBr0,- -+ H+Br- -% products (ultimately bromine) . . ArH, + Br, -% ArH,Br + H+Br- * . . . . . where Ar represents an aromatic nucleus. The rate constant for reaction (2) is large, as would be expected from the fact that the addition of bromine water to aqueous phenol produces an immediate precipitate of tri- bromophenol. Bell and Rawlinson5 found the value of k , for phenol, at 25" C, to be 1.8 x lo5 litre per mole per second.Because of the large rate constant for nuclear bromination, the concentration of bromine is so low that the stationary-state hypothesis may be applied to the system until the point is reached at which the phenol is completely converted into tribromophenol. Consequently, at all stages of the reaction, both before and after the mono-bromination stage, the rate of nuclear bromination equals the rate of generation of bromine from bromide and bromate ions. Under these stationary-state conditions, the rate of nuclear bromination is independent of the chemical nature and the concentration of the phenol, for it is determined only by the rate of reaction (1). A further consequence of the stationary state is that, during mono-bromination, the concentration of bromine is so low that an azo dye, such as methyl orange, is not bleached significantly in the course of a few minutes.The essential feature of the analytical method proposed is that the de-activating effect of the bromine atom on the aromatic nucleus causes the rate constant for di-bromination to 343344 [Analyst, Vol. 91 be much less than that for mono-bromination. As a result, the stationary-state concentration of the bromine rises rapidly as the mono-bromination stage is approached, when it reaches a level at which methyl orange is rapidly bleached. This is confirmed potentiometrically for, at the time when the methyl orange bleaches, there is a sharp increase in the slope of the curve of redox potential against time.The time of bleaching for given initial concentrations of bromate, bromide and hydrogen ions a t a given temperature depends only on the concen- tration of the phenol. The over-all stoicheiometry for the mono-bromination of a phenolic molecule (ArH) by an acidified aqueous solution of bromate and bromide ions, is given by the equation- As has been shown above, the rate of nuclear bromination is independent of the concentration of phenol, and so the reaction shown in equation (3) obeys the kinetic rate law6 for the bro- mide - bromate reaction, namely- SELECTION OF OPTIMUM INITIAL COKCENTRATIOE- Let a Ee the initial concentration of bromate in the reaction mixture, b, the initial concentration of the phenol, R,, the initial rate of the bromate - bromide reaction, A,, the rate of the bromate - bromide reaction when the indicator is bleached, k , the fourth order rate constant of the bromate - bromide reaction (equation 4), and t,, the time of bleaching of the indicator.To obtain a linear calibration graph of phenol concentration against time of bleaching, the rate of the bromate - bromide reaction (given by equation 4) must be effectively constant. Mathematical analysis of equations (3) and (4) shows that deviations from linearity are minimised if the initial concentrations of bromate, bromide and hydrogen ions are in the ratio in which they are consumed in the bromination reaction, namely 1 : 2 : 3. If this is so, then- As 3 molecules of phenol are consumed for each bromate ion consumed, then- BURGESS AND LATHAM : DETERMINATION OF PHENOL, 0-CRESOL BrO,- + 2Rr- + 3H+ + 3ArH -+ 3ArBr + 3H,O .. . . * - (3) - d[BrO,-]/dt = R = K[BrO,-] [Br-j [H+I2 . . . . - * (4) . . * * (5) * * (6) . . - - (7) Hence R,/R, = (1 - b / 3 ~ ) ~ . . . . a.e. R,/R,= 1 - 4b/3a, if b < 3a . . .. - - ( 8 ) R, = 18ka4 . . . . . . R, = 18k(a - b/3)4 where b < 3a . . Equation (8) shows that the rate at the time of bleaching is always less than the initial rate, but, providing that b < a , the rate is effectively constant and the approximation Rl = R, is nearly exact. If this is so, then, since b mole per litre of the phenol are mono- brominated in time t,- R, = 18ka4 c= b/3t, . . . . * . * * (9) i.e. Substitution of a known value of k in equation (10) enables the initial value of a to be calculated for any desired value of t,.Examination of equation (8) shows that the condition for the rate to be constant up to the time of bleaching is that b / a must be so small that K J R , fi 1. However, for a pre-selected time of bleaching, a and b are related by equation (10). This equation shows that for a given time (t,) and concentration of phenol (b),"the value of b/a decreases as the rate constant ( k ) decreases. Lowering the temperature reduces k and, consequently, also reduces b/a. In practice, a value of t, of up to 200 seconds at 0" C proves to be convenient, as this enables an ice-bath to be used for temperature control. METHOD REAGENTS- potassium bromate, and 0.2 M with respect to potassium bromide. respect to sulphuric acid, containing 10 mg of methyl orange per litre of solution.Bromate - bromide stock solzttion-Prepare an aqueous solution, 0.1 M with respect to Sulfihziric acid - methyl orange solzctioqi-Prepare an aqueous solution, 0-15 M (0.3 x) withJune, 19661 AND @-CRESOL IX AQUEOUS SOLUTIONS BY A KINETIC METHOD 345 PROCEDURE- Place 25 ml of the sulphuric acid - methyl orange solution in a 50-ml calibrated flask and add an aliquot of the unknown phenol solution. Dilute the contents of the flask to the mark with water. The resulting sample solution should be not more than 0.004 M with respect to the phenol. Place 10 ml of the bromate - bromide stock solution and 20 ml of the sample solution prepared above, in separate clean, dry boiling-tubes and allow the solutions to cool to 0" C in an ice-bath.Start a stop-clock, and after noting the time, pour the contents of one tube quickly into the other tube. Uniformity o f composition is ensured by transferring the result- ing solution quickly from one tube to the other twice more. Then place the reaction mixture back in the ice-bath over a white tile, and look vertically down through the solution. Record the time when the last tinge of red colour of the indicator disappears. Calibrate the method by using a solution of phenol of known concentration (0.01 M is suitable), and by varying the amount added in making the sample solution. To standardise the timing procedure, a blank experiment is made with the mixed solutions of sulphuric acid - methyl orange and bromate - bromide, but without phenol.This gives a bleaching time of about 3 seconds. The value for the blank is subtracted from the values obtained in the determination, and corrected times are used. The bleaching of the indicator occurs over the course of a few seconds. RESULTS AND DISCUSSION The results of 18 measurements on phenol, o-cresol and 9-cresol are shown in Fig. 1. The standard deviation The continuous line was calculated by the "least squares" method. from this line is 3.3 seconds. 0 Fig. 1 . Time of bleaching for various concentratioris: 0, phenol; A, o-crcsol; and ;< , p-cresol These results suggest that the method is capable of giving rapid analyses to an accuracy of 3 per cent. Equation (10) requires the slope of the calibration graph, shown in Fig. 1, to be 54ka4.The observed slope is 2-0 x mole per litre per second, whereas the calculated value under the quoted experimental conditions is 2.8 x mole per litre per second, assuming k to be 0-42 litre3 per mole3 per second. This agreement is acceptable, bearing in mind, (a), that this value of k has been extrapolated from a value7 at 25" C by using the Arrhenius equation, and, ( b ) , that the rate constant for this reaction varies considerably with ionic strength.8 In the examples quoted in Fig. 1, the time for the first precipitation of di-brominated or tri-brominated products was greater than that for the bleaching of the indicator. However, with m-cresol, precipitation occurred before the indicator was bleached. Examination of the structures of o-cresol, m-cresol and 9-cresol shows that it is only in the meta isomer that the346 BURGESS AND LATHAM [Analyst, Vol.91 methyl group is ortho or para to the site of substitution. As the methyl group activates strongly at the ortho and para positions and has little effect at the meta position, the rate constant for the bromination of m-cresol is much higher than that of o-cresol and 9-cresol. Hence, the mono-brominated m-cresol can be di-brominated at a rate comparable to that for mono-bromination of o-cresol and 9-cresol, and consequently the indicator method fails for m-cresol. REFERENCES 1. 2. 3. \.’ogel, A. I . , ‘ I A 4 Text-Book of Quantitative Inorganic Analysis,” Third Edition, Longmans, 4. 5. 6. 7 . 8. Koppeschaar, W. F., 2. analyt. Chem., 1876, 15, 233. Riemschneider, R., Chim. Ind., 1951, 66, 806. Green & Co. Ltd., London, 1961, p. 388. Kozak, G. S., and Fernando, Q., Analytica Chim. Acta, 1962, 26, 541. Bell, R. P., and Rawlinson, D. J.. J . Chern. SOC., 1961, 54, 63. Skrabal, A,, and Weberitsch, S. R., Mh. Chem., 1915, 36, 211. “Tables of Chemical Kinetics, Homogeneous Reactions,” National Bureau of Standards ( U . S . ) , Bray, W. C., and Liebhafsky, H. A4., J . Amer. Chem. SOC., 1935, 57, 51. Circular 510, 1951, 669. Received June 28th, 1966