F. C. Thyrionl Ecole Nationale Polytechnique Alger 10, Algeria
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I
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Photo-Oxidation of Leuro Methyl Crystal Violet A physical chemistry experiment
Photolytic reactions generally require expensive and improved apparatus though an inexpensive and easily constructed photochemical reactor has been described pre~iously.~ The experiment of carbon tetrachloride-sensitized oxidation of leuco methyl crystal violet (LMCV) described in this paper can be easily achieved in 3 hr and does not require special procedures that are often necessary in this field. Furthermore, this reaction is an illustration of experimental zero-order reaction which is often badly understood in kinetics. Instrument necessary to perform the experiment is a double-beam spectrophotometer (we used a Beckman, Model DB) equipped with uv lamp power supply. The hydrogen lamp is supported by a wood plate and isolated from it by an asbestos sheet as shown in Figure Epoxy cement is used to fasten the lamp holder over a 2-mm thick Pyrex plate. The dimensions of this plate are calculated to cover perfectly the cell holder top of the spectrophotometer. The position of the lamp holder must be adjusted so that the output window of the uv lamp is just in line with the axis of the 1-em path length cell containing the solution to be photolyzed. The Pyrex plate is darkened allowing an undarkened square area which will transmit wavelengths down to 290 nm. When the apparatus is in operation, a light shield is placed near the lamp to protect the eyes of anyone in the vicinity from stray radiation. A capillary tube or syringe needle is placed along the reaction cell wall for flushing the system with nitrogen during irradiation. Procedure
A standard solution of 2 X
M LMCV ethanol
' On leave from Louvain University, Belgium.
STARR, J. E., AND EABTMAN, R. H., J. CHEM.EDUC., 41. 394 (1964).
766
/ Journal of Chemical Educalion
Figure 1.
Devise for photolyring reodion cell in ritu.
solution (Reagent Grade) is prepared and maintained in the dark. Two milliliters aliquot of this solution is then transferred to a clean, dry reaction cell. The reference cell is filled with the same solution. During irradiation, no change in absorbance will be observed a t 590 nm (A,., colored cationic dye = 590 nm, c., 88,000 l/mole. cm). The second experiment is conducted using 2 X 10W2 M CCL as sensitizer. The absorbance variations are followed for 30 min. A third experiment can be made using a lower concentration in LMCV, for instance 4 X M . In both cases, the plot of the absorbance in function of time is linear. This means that the experimental order of the reaction with respect to LMCV is zero, inasmuch as the LMCV concentration is sufficient to absorb the radiation completely. Two other experiments can be performed in presence of 2 X 10-2 M CC14 solutions to establish the mechanism of photo-oxidation. The first one will use 2X M LMCV and lo-' M HC1; the second will he performed on 4 ml of 10-4 M LMCV solution. Figure 2 shows results obtained a t 20°C.
Figure 2. Rate of dye formation under roriour consentrotions of leuco methyl sryrtol violet and carbon tetrachloride a t 20°C.
with an exponent n. The size of n defines the molecularity of the elementary reaction involved in the activated complex formation. It is very important to point out, at this stage, the difference between the rate expression and the reaction mechanism of the reaction. Some reactions do not depend on the concentration of a reactant (zero-order with respect to it) because the reactant concentration either is in large excess or is maintained constant by continuous feeding. I n our particular case, this feeding is performed by the light intensity inasmuch as the reactant concentration is sufficient to absorb completely the light passing through the solution. By changing the incident intensity 10 or the path length L, we might be able to determine n. We found that the reaction is first-order with respect to the leuco compound, in agreement with MacLaclan's flash photolysis result^.^ The following mechanism can be postulated to explain the results. On photolysis, the absorbing species D H is excited.
Mechanism and Discussion
DH
Under homogeneous conditions and nearly complete absorption, the absorbed intensity Iab, per unit time and volume is
where I. is the light intensity per unit of time a t the front of the solution; A, the irradiated area of cell; V , the volume of the solution; L, the thickness of the solution or path length of light. The expression of a light-initiated reaction in presence of a sensitizer S can be written S
+ DH-Ph"
and the rate is V
=
k[S]'[DH]"
where [DH] is the concentration of light-excited molecules. This last quantity can be replaced by the value I.,,. The specificrate becomes then
At CCla concentration near 2 X loF2iM,the specific rate does not depend on the sensitizer concentration, hence
Experimentally, we observed the following relation, where o.d. is the optical density
which can be related to the preceding expression as
+ hv-DH*
(1)
The question arises now as to whether the excited species is a singlet or triplet state. The oxygen effect postulates the existence of a triplet state. In presence of CCL, the following reactions would occur in a scheme similar to that postulated by MacLachlan DH*
+ CCL DH+'
-=
DH+' D.
+ CCL-
+ H+
(2(
(3)
DH+'+D.+D++DH
(4 )
Reaction (3) expresses the hydrochloric effect. It inhibits the D + formation by displacing the equilibrium backward. This reaction would be the rate-determining step and would explain the first-order kinetics with respect to DH. On decreasing CCla concentration, reaction (2) becomes the rate-determining step and the total order is 2. Suggestions for Further Work
The student may study the oxygen effect upon the reaction. Oxygen is known to be a very effective triplet quencher. The decrease in rate constants on photolyzing oxygenated solutions lets us assume the existence of the triplet state DHT*. Experiments at decreasing CCla concentrations would also be valuable to verify kinetics with respect to it. The quantum yield can also be determined by taking advantage of the known quantum yield of crystal violet leuco nitrile (a = 1 in e t h a n ~ l ) . ~As the quantum yield is defined by the relation
it will be independant from incident intensity and path length if n = 1.
We see that the rate is zero-order with respect to the 7
1
absorbing species and is proportional to the quantity 2 L
a
MACLACHLAN, A,, J. Phgs. Chem., 71, 718 (1966).
'SPORER, A.H., Trans.FamdaySoc., 57,983
Volume 48, Number
(1961).
I I, November 1971 / 767
