338
EDWARD S. AMIS AND JOSEPH B. PRICE
EFFECT OF DIELECTRICS A S D SOLS'EST UPON THE REGESERL4TIOS I S ACID SOLUTIOX OF ALKALI-FADED BROXIOPHEXOL BLUE EDWtiRD S. AMIS
AND
JOSEPH B. PRICE
Charles Edward Coates Chemzcal Laboratory, Louzsiana State Gnztersity, Baton Rouge,
Louzszana Receiaed J a n u a r y 2 , IS@
Amis and La Mer (3), studying the kinetics of the fading of bromophenol blue, observed that the dye could be regenerated at a measurable rate if the alkaline solutions were neutralized and a slight ewess of acid added. The mechanism of the fading reaction was given by these authors to be the formation of the colorless carbinol from the negative univalent hydroxide ion and the negative bivalent bromophenol blue ion. Amis and La Mer further showed that electrostatic effects accounted mainly for the data obtained on the reaction in isocomposition and in isodielectric water-ethyl alcohol and in mater-methyl alcohol media. Hochberg and La Mer (7) showed that the fading of several sulfonphthalein dyes conformed to the mechanism of carbinol formation, and that the regeneration reaction rate for bromocresol purple in alkaline solution was directly proportional to the activity of xater in water-methyl alcohol solvent. In this investigation a study mas made of the regeneration of alkali-faded bromophenol blue by hydrochloric acid of various normalities in water, and by 0.09 N hydrochloric acid in water-ethyl alcohol and in vater-methyl alcohol media of various compositions and of various constant dielectric constants. PREPARATION OF MATERIALS
The bromophenol blue used in these studies mas Eastman S o . 752 tetrabromophenolsulfonphthalein of the same quality as that used by Anis and La RIer (3). Stock solutions (2.4 X molar) of this dye were made up as described by these authors; the sodium hydroxide solutions were made up and standardized by the same methods they used. Hydrochloric acid stock solutions (about 1.5000 N ) were prepared by diluting Baker's C. P. Analyzed hydrochloric acid and standardized and checked to one part per thousand. Absolute ethyl alcohol and C.P. methyl alcohol, acetone-free, were further purified by the method described by Lund and Bjerrum (12). All volumetric apparatus and all weights were calibrated; the temperatures were determined with U. S. Bureau of Standards calibrated thermometers, and were constant to llt0.01"C. EXPERIMENTAL
molar stock solution of bromophenol blue Ten milliliters of the 2.4 X were made up to 100 ml. with water and a volume of the standard alkali sufficient
REGENERATION OF FADED BROhIOPHEHOL BLUE
339
to make the resulting mixture 0.1 normal in sodium hydroxide and 2.4 X molar in dye. This solution was allowed to fade to the colorless condition, the time required being from 4 to 15 hr. depending upon the temperature. The faded solution and mater were placed in a thermostat and allowed to come to the temperature a t which the run was to be made. Ten milliliters of the faded dye solution was diluted with water and sufficient stock solution of hydrochloric acid to give a mixture 2.4 X molar in faded dye and having the acid normality desired. A standard neutral solution of unfaded dye of the same molarity as the faded dye in the run was also placed in the thermostat. To make a reading, 5 ml. of regenerating solution was pipetted into a clean dry test tube, and then 5 ml. of the standard was pipetted into another clean dry test tube. One milliliter of sodium hydroxide solution of the correct strength to give a solution 0.1 N in alkali was added to the tube containing the standard. The regenerating solution was also brought to 0.1 N in sodium hydroxide by the addition of 1 ml. of alkali of a different normality. The time was noted, the solutions shaken vigorously and poured into the colorimeter cups, and the average of five settings of the run against the standard taken as the reading. The total time consumed in making a reading averaged about 3 min. Any fading of the run during the time of reading was offset by a similar fading of the standard. For 'the runs in water-alcohol solvent mixtures, the dilutions of the stock dye solutions were made with the correct proportion, determined from Akeriof's data (l), of the two solvents to give the percentage composition or dielectric constant of solvent desired. Standards were made to conform to the runs in solvent composition. The runs in water-alcohol were all made to an acid strength of 0.09 N . The instrument used for analysis was a Klett colorimeter. A Mazda lamp with a blue filter furnished illumination. D.4TA
Typical runs are recorded in tables 1 and 2. The kN values in all cases were calculated as pseudo-unimolecular rate constants. These values were then converted to the bimolecular constants, k, in moles per liter per day by dividing by the concentration of acid and adjusting the time factor. Duplicate runs gave average k~ values which did not vary more than fl per cent from the mean. Table 3 contains k values measured in water a t 25O, 35O, and 45"C.,and a t various normalities of hydrochloric acid and various ionic strengths. This table also contains energies of activation and frequency factor values for the temperature intervals 25' to 35OC. and 35" to 45OC. A plot of the log k versus 4;is given for the three temperatures 25O, 35",and 45OC.in figure 1 . While the curves have the direction of slope required by the reaction between the negative carbinol ion and the oxonium ion, it is observed that there is a marked decrease of the bimolecular rate constant with increasing ionic strength or with increasing acidity, since p was increased by adding hydrochloric acid in greater proportion. The rate of decrease of the constant is greatest in the region of low ionic strength, gradually becomes less marked, and approaches zero a t about
340
EDWARD S. AMIS AND JOSEPH B. PRICE
di = 0.25. This change of the reaction velocity constant with acidity has the appearance of a rate composed of an acid-uncatalyzed portion plus a portion TABLE 1 Kinetics of the regeneration 'bromophenol blue i n water TEMPEPAILXE, 3S"c.; HCI
X concentration of bromophenol blue regenerated
Time
minutes
0.375 0.442 0.538 0.720 0,912 1.10 1.17 1.44 1.82 1.92
565 855 965 1225 1385 2105 2555 3655 4385 4955
Time
tion of bromophenol blue regenerated
Time
minuler
M E T E Y L ALCOCEOL,
0.08.V
105 X concentra-
IO5
2.40 1.93 2.15 2.34 2.74 2.30 2.02 1.92 2.29 2.20 2.23(av.
22.0 PER CEVT; D = 65; TEUPEPATURE, 35°C.;HCI, 0.09 'A 106 X concentration of bromophenol blue regenerated
23 48 75 93 121
7.73 1440 - = 139 0.08 103
k(days-1) = - x
D = 63;TEMPERATUBE, 35'C.; HCI, 0.09 iV
METEYL ALCOHOL, 25.9 PER C E N T ;
X concentration of bramophenol blue regenerated
106
Time
1Oak.v ( m k - 1 )
I _ _
k(days-1) =
IVkN (mk-1)
_____
minnler
mitrzrler
210 230 250 265 280 435
8.20 7.62 7.60 7.66 7.56 7.73(av.)
0.499 0.833 1.25 1.47 1.73
1.20 1.25 1.34 1.44 1.57 1.97
,
2.58 1440
2.57 2.46 2.51 2.52 2.79 2.64 2.58 (av.)
-X = 41 3 0.09 102
250 325 355 390 478 -
1.25 1.39 1.48 1.54 1.76 1
2.05 1440 09 103
2.27 2.02 2.03 1.95 1.96 2.05(av.)
k(days-1) = -- X - = 32 8
o
RECIENERATION OF FADED BROhfOPHENOL BLUE
341
to zero acid concentration, obtaining the values of k x = 0 indicated in that figure. These extrapolations show that there is a regeneration reaction due to the water molecules reacting with the dye ions. There is also an acid-catalyzed reaction, so that the over-all rate may be expressed by the equation k,v = ko 23
14
I
+ k'C
I
I
1
I
LOG 135' LOG k25+ 0.500 LOG h45 5 LOG h25 + 0900 2'-\
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