T H E MECHANISM OF THE PHOTOCHEMICAL REACTION BETWEEN BROMINE AND WATER H. ARMIN PAGEL AND WARNER W. CARLSON Chemical Laboratory of the University of Nebraska, Lincoln, Nebraska Received December 18, 1936
The mechanism of the photochemical reaction between bromine and water, which yields hydrobromic acid and oxygen, apparently has never been studied. Several qualitative experiments, however, are given in the literature. Balard (1) states that bromine water exposed to sunlight slowly decolorizes with the formation of hydrobromic and bromic acids. Lowig (4), however, found that the reaction products are hydrobromic acid and oxygen. Pebal (5) found that saturated bromine water in a sealed container does not appreciably decolorize after three months exposure to sunlight. He was unable to detect any increase of gaseous pressure upon opening the container; however the solution was found to be slightly acid and gave a positive test for bromide. Joseph (2) showed that only 2 per cent of the theoretical amount of hydrobromic acid is formed if bromine water is exposed to sunlight for over one month in colorless glass bottles. The results of the quantitative investigation carried out in this laboratory are described below. MATERIALS AND APPARATUS
Water was prepared by distilling tap water from alkaline permanganate, followed by redistillation. To oxidize possible volatile’ organic matter, the water was further treated with a small portion of pure bromine and allowed to stand in sunlight for several hours. The bromine was then completely boiled out and the residue distilled, and the distillate finally redistilled. A 12-liter, all Pyrex glass still was used. Grasselli’s C.P. hydrochloric acid was likewise treated with bromine, followed by gentle boiling and aeration to remove the bromine. Mallinckrodt’s C.P. bromine was purified by agitating with potassium bromide solution for several hours, followed by five washings with conductivity water in a separatory funnel. To remove small amounts of insoluble impurity the bromine was then dissolved in a large volume of conductivity water and reclaimed by distillation, using the still previously mentioned. The purification was completed by distilling once from concentrated sulfuric acid and finally redistilling. Upon evaporation, the bromine left no residue and gave 613
614
H. ARMIN PAGEL AND WARNER W. CARLSON
negative tests for sulfuric acid. Sodium thiosulfate solutions, 0.05 N and 0.12 N , were prepared from Baker’s C.P. analyzed chemical. Mallinckrodt’s “Reagent quality” potassium iodide was used. This showed no trace of iodate. I n order to duplicate titration conditions, the temperature of the iodine solutions to be titrated was controlled to 20 & 2”C., and a mechanical stirrer operating at constant speed was used. The photochemical reactions were carried o u t in Pyrex reaction chambers resembling a volumetric pipet. These had a capacity of about 55 cc., and were 15 cm. in length by 25 mm. in diameter. A 4 cm. x 5 mm. tube was sealed on one end, and a 6 em. x 11 mm. tube was sealed on the other end. The reaction chambers were cleaned with sulfuric acid-dichromate cleaning solution, thoroughly washed, and finally steamed. Accurately weighed amounts of bromine were dispensed in small sealed glass bulbs. A 200-watt, frosted, Mazda electric light bulb was used as the light source. This was mounted vertically and rotated mechanically a t about 20 R.P.M. to provide average uniform illumination in a horizontal plane. The photochemical reactions were carried out in a photographic dark room, with temperatures thermostatically controlled to 25.0 h0.5OC. PROCEDURE
The smaller tube on the reaction chamber was first sealed. The bromine bulb was then introduced into the chamber through the larger tube, followed by a solid glass plunger 8 cm. long by 6 mm. in diameter. A measured amount of water (45 cc.) was then added. The larger tube was then heated well away from the open end and drawn to a slender curved constriction. This was then connected to an aspirator pump for several minutes and sealed, while evacuated, a t the constricted point. By quickly inverting the reaction chamber the glass plunger was allowed to strike and break the bulb, thus liberating the bromine into the water. The reaction tubes were then mounted vertically, equidistant from the light source. The amount of reaction which had taken place during the various time intervals was then found by iodometrically determining the amount of bromine remaining. The following technique was used to avoid loss of bromine vapor: A file mark was cut about 1 cm. from the sealed end of the smaller tube on the reaction chamber, and a calcium chloride tube was connected by means of a rubber coupling. One gram of potassium iodide in 10 cc. of 0.5 N hydrochloric acid was then added into the calcium chloride tube. By applying transverse pressure the tube was broken at the file mark, thereby forming a valve within the rubber coupling. After the iodide solution had been introduced into the reaction chamber, the latter was vigorously shaken to extract all of the bromine vapor in the free space. The reaction chamber was then lowered into a 500-cc. Erlenmeyer flask
PHOTOCHEMICAL REACTION BETWEEN BROMINE AND WATER
615
containing 200 cc. of 0.1 N hydrochloric acid and 1 g. of potassium iodide. By applying firm downward pressure the curved constricted end of the larger tube was then broken open against the bottom of the flask. This now permitted free drainage and thorough rinsing of the reaction chamber. The liberated iodine was then immediately titrated with thiosulfate. The latter was always standardized immediately after completing the analyses as follows: Weighed amounts of bromine in the sealed glass bulbs were introduced into a 500-cc. glass-stoppered Erlenmeyer flask containing the same volume of acid solution and potassium iodide as used in the analysis. The tightly stoppered flask was then vigorously shaken to break the bromine bulb and also insure complete reaction of the bromine vapor before titration. TABLE 1 Reaction between bromine and water SERIES B
SERIES A
Time in hours
Bromine reacted'
12 24 60 108 180
1.51 2.28 3.49 4.58 4.82
K X
10-8
0.89 0.91 0.91 0.96 0.87
Time in hours
Bromine reacted'
K X 10-
Time in hours
1.26 1.85 2.72 3.72 4.51
1.42 1.34 1.36 1.38 1.34
12 24 48 96 144
12 24
48 96 180
* Calculated t o moles per liter
SERIES C
Bromine K X 10-8 reacted'
1.75 2.97 4.05 5.05 5.79
1.30 1.48 1.48 1.41 1.44
X lo-'. EXPERIMENTAL
Three series of experiments were carried out: series A, using 0.0221 M bromine at 50 cm. from the light source; series B, using 0.0221 M bromine at 71 cm.; and series C, using 0.0469 M bromine at 50 cm. The data and reaction velocity constants found are tabulated in table 1. DISCUSSION
The reaction appears to take place as follows: Brz + HtO e H + + BrHBrO + hv -+H+ + Br-
+ HBrO + 40,
The reaction velocity constants given above were determined as follows: The initial concentrations of hypobromous and hydrobromic acids were calculated from the hydrolysis equilibrium equation (3) [H+][Br-][HBrO] = K ~ =~5.8. IBrzl
x
10-9
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H. ARMIN PAGEL AND WARNER W. CARLSON
The number of moles of hydrobromic acid formed at any stage of the reaction always equals twice the moles of bromine reacted, as determined iodometrically. Since the hydrolysis reaction rapidly attains equilibrium (3), the concentrations of both hypobromous and hydrobromic acids can be calculated by inserting { [HBr]initial + [HBr] formed } into the hydrolysis equilibrium equation. By integrating the curves [HBrO]/[H+][Br-1, it was found that the reaction velocities can be expressed:
- d[Brzl/dt
=
IK[HBrO]/[H+][Br-]
The following mechanism appears logical: HBrO
+ hv
--f
Br+’ + OH-
therefore, -d[Br+’]/dt = Ik[HBrO] The Br+’ reacts simultaneously, Br+’ + OH-
--+
H + + Br-
+
$03
(a)
and Br+’ + Br-
-+
Brz
+E
From reaction a, -d[Br+’]/dt
=
k’[Br+’][OH-]
Within the range studied, the bromine concentrations remain practically constant, owing to the reversal of the hydrolysis reaction with increasing hydrobromic acid concentrations, therefore we may assume -d[Br+’]/dt = k”[Br+’l[Br-I for reaction b. The mole fractions of Br+’ taking part in the competing reactions a and b are thus directly proportional to k’[OH-] and k”[Br-1, respectively. The rate equation may then be written - d[Brz]/dt = IK[HBrOl[OH-I/[Br-I
to explain the observed decrease in reaction rate with respect to the increasing concentrations of hydrobromic acid. SUMMARY
1. The photochemical reaction between bromine and water has been studied. 2. The reaction velocity equation was found, and a reaction mechanism shown to be in logical agreement with it has been discussed.
PHOTOCHEMICAL RHACTION BETWEEN BROMINE AND WATER
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REFERENCES (1) BALARD, A. J.: Ann. chim. phys. 32, 364 (1826). (2) JOSEPH,A. F.: J. SOC. Chem. Ind. 29, 1190 (1910). H.A.: J. Am. Chem. SOC.66, 1504 (1934). (3) LIEBHAFSKY, , Das Brom und seine chemischen Verhaltnisse, p. 22. Heidelberg, (4) L ~ W I GC.: 1829. (5) PEBAL,L.: Ann. 231, 148 (1885).
