ROBERTLIVINGSTON AND RUDOLPH PARISER
1510
cal. deg.-l mole-' was shown to be due to internal rotation, corresponding to a potential barrier of 4700 cal. mole-l.
[CONTRIBUTION FROM
THE SCHOOL OF
Vol. 70
The entropy a t 298.1' K. and 1 atmosphere was found to be 65.91 cal. deg. mole-'. BERKELEY,CALIFORNIA
CHEMISTRY, INSTITUTE
OF
RECEIVEDN O V E ~ E6,R 1947
TECHNOLOGY, UNIVERSITY O F MINNESOTA]
The Chlorophyll-sensitized Photooxidation of Phenylhydrazine by Methyl Red. Reactivity of the Several Forms of Methyl Red'
11.
BY ROBERTLIVINGSTON AND RUDOLPH PARISER The several solutions were made from stock solutions In a previous study2 of this reaction a mechanism was suggested which led to the prediction of methyl red and of either sodium methylate or alcoholic acid. All measurements were made with that the quantum yield should increase with in- hydrochloric 2 X 10-6 M methyl red. As only ordinary precautions creasing concentration of phenylhydrazine. Di- were taken t o keep the solutions out of contact with laborarect measurements, in the range from 0.01 to 0.20 tory air, it is probable that the sodium methylate solutions some carbonate and hydroxide. M phenylhydrazine, showed a definite although contained Methods.-The photometric measurements were made small decrease in the yield with increasing concen- with a Beckmann spectrophotometer a t room temperature trations. Since it was noticed that the color of (24 to 27'). Measurements were made a t 100 A. intervals methyl red was also affected by this change in con- in the range from y 3600 t o 6000 A. Duplicate preparacentration, a series of measurements using varying tions and measurements were made for each solution mixtures of phenylhydrazine and phenylhydra- studied. Experimental Results zine hydrochloride were made. Both the concentration of a "red" form of the methyl red and the The solutions studied were made up to contain, yield of the reaction increased with increasing in addition to 2 X 10-6 M methyl red, the followacidity of the solution, the yield approaching a ing added substances: (1) 0.40 M HCl, (2) loF3 limiting value of about 0.5. Since methyl red has M HC1, (3) M HC1, (4) M HC1, ( 5 ) lo4 three colored forms,3it was necessary to determine M NaOCHa, (6) 10-6 M NaOCHa, (7) 2 X M the absorption spectra of each of these forms, be- NaOCH8, (8) 5 X 10-5 M NaOCH3, and (9) fore it was'possible to analyze the solutions spec- M NaOCH3. trophotometrically. Using this method of analFurther increase of the hydrochloric acid concenysis, it was found that only the intermediate form tration above 0.40 M did not affect the extinction of methyl red reacted in the solutions used. curve. At the other end of the range, use of conMaking allowance for the variation of quantum centrated sodium methylate results in a fading of yield with dye concentration, it can be shown that the yellow color. This fading is reversible. It is the yield is also a (symbatic) function of the phen- not complete even in very basic solutions. The exylhydrazine concentration. The yield appears to tinction coefficients a t wave lengths near the be independent of intensity. A relatively simple maximum decrease about 30% as the concentrareaction mechanism is consistent with these ob- tion of methylate is increased from to 1 M. servations. The absorption spectrum of the dye is practically Part I. Spectrophotometric Analysis of Methyl unchanged in the range from 2 X lov4to 5 X M NaOCH3. Red The extinction curves for solutions 1,3,6,7, and Experimental Methods and Materials Materials.-The methanol was purified by treating 9 are plotted in Fig. 1. The curves corresponding synthetic methanol with an amount of sodium estimated to solutions 4, 5 and 8 have been omitted from t o be three times as much as was required t o react with the plot to simplify it. They belong to the same the water present, refluxing with an excess of methyl ~ h t h a l a t e and , ~ then distilling through an efficient packed family of curves as those plotted. It should be column. The purification of the methyl red has been de- noted that the curves intersect a t one of two scribed.2 Alcoholic hydrochloric acid was prepared by points, corresponding to either X 4360 or 4800 k. bubbling dry hydrogen chloride into methanol. Sodium One curve, number 11, passes through both points methylate solutions were prepared by allowing weighed quantities of clean sodium t o react completely with meth- of intersection. It is apparent that the dye can exist in three anol. The concentrations of the acid and base solutions were checked by titration with aqueous standard solutions. different colored forms. From the variation of the absorption curves with the acidity of the solutions, (1) This work was supported jointly by the Graduate School of it may be safely assumed that curves 9 and 7 corthe University of Minnesota and by the Office of Naval Research (Contract N6ori-212, T. 0. I) to whom the authors are indebted. respond, respectively, to the pure forms I and III.6 (2) R. Livingston, n. Sickle and A. Uchiyama, J. Phys. Colloid It is impossible to calculate exactly the extincChcm., 61, 775 (1947). (8) A. Thiel, A. Dassler and P. Wiilfkin, Forlsch. Chem. Phyrik. p h y t i k . Chcm., 18, no. 3 (1924). (4) We are indebted to Dr. R. Arnold of the Organic Division of this department for suggesting thin method.
( 5 ) It is possible that solution 9 contains a trace of the colorless form. However, the practical independence of the curve from the methylate concentrations over a wide range is evidence that the percentage of the dye present in the colorless form is small.
April, 1948
PHOTO~XIDATION REACTIVITY OF METHYL RED
1511
6
5
-
4
I
23 X U
2
1
36
38
48 60 52 54 56 58 10-1. coefficients of methyl red dissolved in methanol containing acid or base.
40
42
44
46 A,
Fig. 1.-Extinction
A. x
60
tion curve of the third form from the curves for the TABLE I two pure forms and from any number of curves for EXTINCTIONCOEFFICIENTSOF THE COLOREDFORMSOF mixtures.6 However, a reasonably precise nuMETHYLRED merical approximation, to the curve of form 11, ai X 10-4 as X 10-4 a1 X 10-4 x, A. (lit./mole)a (lit./mole)a (lit./mole)* may be readily ~ b t a i n e d . ~The , ~ curves which 3600 0.99 0.243 0.187 pass through the point a t 4360 A. correspond to 3700 1.44 .40 ,117 solutions which are freg from form 111; and those 3800 1.91 .55 ,075 which cross a t 4800 A. represent solutions free 3900 2.32 .79 .057 from form I. The analysis of the data was sim4000 2.64 1.00 ,065 plified by the fact that one curve (for solution num4060 2.71b ber 4) which is not shown on Fig. 1 p,asses very 4100 2.67 1.26 ,112 close to both points of intersection. Accordingly 4200 2.45 1.50 ,212 this solution must contain chiefly form I1 of the 4300 2.07 1.75 .39 dye. For those solutions whose curves cross a t 4400 1.72 1.97 .66 4360 A. the observed extinction coefficient pi 4500 1.37 2.22 1.05 equals
+
4600 1.01 2.53 1.61 4700 0.70 2.87 2.32 when C1 and Cz are the concentrations of form I 4800 .44 3.18 3.13 and I1 in the ith solution and al,x and aZ,xare the 4900 ,262 3.33 3.94 extinction coefficients of the pure form I and I1 a t 4910 3,34b the wave length X. Similarly for curves crossing 5000 ,138 3.27 4.78 a t 4800 5100 .085 2.99 5.42 &,A = QZ,XGaa,xCa 5200 .038 2.44 5.70 5210 5.71b Approximately the values of &A are equal to those 5300 .020 1.72 5.60 of ( Y Z , ~ . Curve I1 has been obtained from the val5400 .OlO 1.04 5.35 ues of solution No. 4 by a process of successive 5500 .005 0.57 4.79 approximations, as follows. Minor adjustments .005 .270 3.56 5600 were made in these approximate values of aZx .005 ,130 2.02 5700 until all of the mixture curves (2, 3, 5, 6, 7 and 8) 5800 .000 .075 0.89 could be fitted in terms of the preceding two 5900 .om .025 .295 equations and a series of values of al,x, aZ,xand .000 ,000 .082 6000 q3A. It is assumed that these values are close apa The values of the extinction coefficients are in terms of proximations to the true values for the extinction logarithms. b The maximum value of the excoefficients of the three pure forms of the dye. common tinction coefficient. Bi,x =
A.
al,xC~ LUZ,XCZ
+
The three solid curves of Fig. 1 are plots of these values which are listed in Table I.
(6) Compare E. Q. A d a m and L. Rosenstein, THISJOURNAL, 16, 1462 (1914).
It is interesting to compare these results with those obtained by Thiel, Dassler and Wfdfkin (ref. 3, Fig. 13) for aqueous solutions of methyl
ROBERTLIVINGSTON AND RIJDOLPH PARISER
1512
I T O l . 70
tion, before and after illumination, with the Beckmann spectrophotometer. Two or more wave lengths were used and the measured values were corrected for the (small) absorption due to chloroTABLEI1 phyll. Control experiments showed that this method of analysis did not produce any change COMPARISON OF MAXIMUM ABSORPTION IN AQUEOUS AXD METHANOL SOLUTIONS in the concentration and that it gave results conAqueous solutions Methanol solutions sistent with the analytical method used in the Solution x,A. a x 10-4 x,b. a x 10-4 earlier measurements. 2.71 2.08 4060 Basic 4470 The results of a number of determinations of 5.33 4910 3.34 Intermediate 5300 the quantum yield, in terms of the disappearance 5.25 5210 5.71 Acidic 3170 of methyl red, are presented in Table 111. Chlorophyll A a t a concentration of 5 X l o + M was Part 11. Photochemical Measurements used in all experiments. The solvent was methExperimental Methods and Materials anol. The experiments were performed a t room The reagents used were similar to those de- temperature, which varied between 25 and 28'. scribed by Livingston, Sickle and Uchiyama.2 The actinic light was a red band, having a maxiThe method of purifying methanol is described in mum a t 6200 A., cutting off sharply a t 6000 A., the first part of the paper. The apparatus, de- and tailing off gradually to about 7300 A. The scribed by them,2 was slightly improved mechan- special symbols used in the table have the followically and by the use of a voltage stabilizer with ing significance: I, number of quanta absorbed the light source. Cylindrical reaction vessels, 12 per second in the reaction cell, (Ph), molarity of mm. in length, were used in all of the present ex- phenylhydrazine, (PhHCl), molarity of phenylperimen ts. hydrazine hydrochloride, (D)o,initial molarity of Except for the analytical method, the experi- the total methyl red, and (D")o, initial molarity mental procedure and the routine computations of the intermediate form of methyl red. The quanwere similar to those previously described.2 All tum yield, Cp, is the average value and is defined by solutions were analyzed for methyl red concentra- the equation
red. The data of Table 117suggest that, while two of the colored forms are chemically identical in methanol and in water, the remaining forms differ.
(7) As was pointed out by Thiel in 1924,athe probable structures of methyl red in aqueous solutions are as follows.
0
//
-
8 =
[(DO) - (D)ri..~I(moles/liter) I (quanta/sec.) [TO- Tri,,,~] (sec.) V(1) x N (molecules/mole)
c-oI
The significance of the values tabulated in the last three columns is discussed in a later section of the paper. Basic :
