THE AUTOXIDATION O F STANNOUS CHLORIDE. 111 THEPHOTOCHEMICAL REACTION' ROBERT C . HARING
AND
JAMES H. WALTON
Department of Chemistry, University of Wisconsin, Madison, Wisconsin Received January
4, 1033
In the preceding paper (1) the thermal reaction of stannous chloride with atmospheric oxygen was discussed, and evidence given to show that it is probably a thermal chain reaction similar to the autoxidation of sodium sulfite and of benzaldehyde, investigated by Backstrom (2) and Alyea and Backstrom (3). In order to secure further proof of the chain mechanism of the autoxidation of stannous chloride, the effect of visible and ultra violet radiation on the reaction was investigated. APPARATUS
The apparatus used was similar to that used in the studies of the thermal reaction, A reaction flask was made from a quartz tube of :1 inches inside diameter, and 7 inches long. Indentations were made in the bottom, as was done with the glass flasks, to insure a better agitation. It was found impracticable to use a flask exactly like those of glass (1) with a bulb in the lower part. The quartz flask did not give a high enough area of gasliquid interface to keep the solution saturated with oxygen and thus give the same reaction velocity as determined in the glass flasks, but it gave reproducible results, and thus allows a comparison of the results with positive catalysts and inhibitors. The light source used was a capillary mercury vapor lamp of the type described by Daniels and Heidt (4). This lamp gives a high intensity in the ultra-violet region, and allows more accurate measurements than a low intensity lamp. Some absorption measurements were made on the quartz monochromator described by Heidt and Daniels ( 5 ) . Spectrograph work was done by means of a Kriiss quartz spectrograph equipped with a single 60' prism. ABSORPTION SPECTRUM O F STANNOUS CHLORIDE
Preliminary experiments having shown that ultra-violet radiation was effectivein speeding up the autoxidation of stannous chloride, it wasneces1 This research was financed by a grant from the Research Committee of the University of Wisconsin, Dean C. S. Slichter, Chairman, 375
376
ROBERT C. HARING AND JAMES H. WALTON
sary to determine what wave lengths were active in causing this result. The absorption spectrum for the stock solutions of stannous chloride (32 grams per liter of SnClz; 0.8 N in HCI) was determined on the spectrograph, and it was found that the region of complete absorption extends up to 3000 8. Decreasing the concentration of stannous chloride shifts the limit of complete absorption toward the lower wave lengths. CORRECTION FACTOR FOR POLYCHROMATIC LIGHT
The monochromator was not suitable for the full set of experiments on quantum yield, using a shaken cell, and no filter could be !rranged which would give a high transmission of t,he mercury line at 3020 A. and still shut out al1,lines at 3130 .& and higher, so that it was necessary to use polychromatic light in these experiments. The light intensity in the quantum yield experiments was determined by the use of the uranyl oxalate acTABLE 1 Effect of light intensity on quantum yield la
I
4
quanta per minute
7.7 5.2 3.2 2.0
(10117
33. 29.
14. 12.
tinometer described by Leighton and Forbes (6). A weighted average quantum yield of 0.553 was calculated for the range of wave lengths used, from the data given in their article on quantum yields for monochromatic light. In the determination, the uranyl oxalate actinometer was shaken at the same speed in the same flask as was used for stannous chloride in order to give comparable results. Since the range of absorption of uranyl oxalate and stannous chloride is different it was necessary to determine a correction factor to be applied to the number of quanta obtained by the actinometer. This factor was obtained by the use of two similar quartz cells in series. The actinometer solution was placed in the second cell. The first cell was filled successively with distilled water and stannous chloride stock solution, and the decomposition of the actinometer solution measured in each case. The ratio of the number of quanta absorbed by the stannous chloride to the number absorbed by uranyl oxalate was found to be 0.62 f O . O 1 . This factor was applied to all the runs with different lamps, since Daniels and Heidt have shown (4) that the distribution of energy in the ultra-violet region varies only slightly with the voltage at a constant wattage.
AUTOXIDATION O F STANNOUS CHLORIDE. I11
377
VARIATION O F QUANTUM YIELD WITH THE LIGHT INTENSITY
It is known that for many photochemical reactions the quantum yield C$ varies with the intensity of the light.
The variation in intensity was produced by different lamps and by varying the wattage of the lamps. Table 1 shows the effect on the quantum yield of the photochemical autoxidation of stannous chloride. These intensities are given as the actual intensities causing reaction in the stannous chloride, that is, they are the actinometer intensities multiplied by the correction factor. The quantum yields are not accurate in the last figure, but two figures are given for comparison purposes. The following sample of the calculations is given for reference. Rim No. 617 “Light” reaction = Total = 7.86 =
7.00 .-.-.. 716 273 22,400 760 298
-
“dark” reaction
- 0.86
7.00 cc. of 02 used in 22 minutes
2(6*06) (10)*3 = 1.48(10)19molecules of stannous chloride reacting 22 per minute
The actinometer solution decreased 2.80 cc. of 0.0328 N potassium permanganate in titer, after one hour exposure t o the light.
‘
(1.48) (10119
= (5.18) (10117
=
28.6 molecules per quanta
EFFECT OF CATALYSTS ON QUANTUM YIELD
If the thermal and photochemical reactions are completely analogous, accelerators and inhibitors should have the same effect on the photochemical reaction as on the thermal reaction. This is true for the catalysts used in the experiments. A few of the substances found to have pronounced effects on the thermal reaction were used, and gave the expected results as shown in table 2. It is to be noticed that picric acid, a strong inhibitor for the thermal reaction, reduces the quantum yield to almost exactly 1. The accelerators for the thermal reaction also accelerate the photochemical reaction as shown by the results. The last column of table 2 shows the relative effect of the catalysts used. In determining these values, the values for the quantum yield of the uncatalyzed reaction were interpolated from the results of table 1, expressed as a straight line proportionality between C$ and Io.
378
ROBERT C. HARING AND JAMES H. WALTON
TABLE 2 E$ect of catalysts o n quantum yield
I CATALYST
I
0.00002 M Picric acid.. . . . . . . . . . . . . . . 0.1 M Amyl alcohol.. . . . . . . . . . . . . . . . . 0.1 M Allyl alcohol. . . . . . . . . . . . . . . . . . 0.1 M Propionic acid.. . . . . . . . . . . . . . . . 0.002 M Thiourea.. . . . . . . . . . . . . . . . . . . 0.065 M Hydroquinone.. . . . . . . . . . . . . . 0.065 M Catechol.. . . . . . . . . . . . . . . . . . . 0.065 M Resorcinol.. . . . . . . . . . . . . . . . . . 1.75 M Isopropyl alcohol . . . . . . . . . . . . 0.1 ill tertiary Butyl alcohol. . . . . . . . . .
Io
MOLECULES REACI'INQ PER MINUTE
1
1
PER CENT OF UNCAT, ALYZED 6
quanta per minute
7.2 (lo)*? 6.8 6.1 4.1 3.2 3.2 3.2 3.2 3.2 2.0
6.2 (10)' 262. 266. 187. 83. 115. 86. 125, 140. 42.
1. 39. 44. 46. 26. 36. 27. 39. 45. 21.
3. 120. 150. 230. 160. 220. 170. 240. 280. 210.
Test of Beer's law jor stannous chloride solutions SOLUTION
CONCENTRATION OF STANNOUS CHLORIDE
CONCENTRATION OB' HYDROCELORIC ACID
grams per Lifer
grams per liter
3.0 8.0 13.0 21 .o 31.0
6.0 16.0 26.0 42.0 62.0
PER CENT LIQHT TRANSMITTED
87.6 66.7 37.9 8.4 1.o
6.5 7.4 10.9 17.2 21.8
BEER'S LAW STUDIES FOR STANNOUS CHLORIDE
In general, light-absorbing media follow Beer's law, which gives the variation in absorption of a given wave length with varying concentration and thickness of the absorbing layer. Z
z=
e-acd
Where I = the intensity of the transmitted light, lo = the intensity of the incident light, Q! = the molecular absorption coefficient, c = the concentration (in moles per liter), and d = the thickness of the layer. ( d = 1.3 cm. for the cell used). Table 3 shows the results of these experiments on stannous chloride solutions. Hydrochloric acid was used to keep the stannous chloride from hydrolyzing, a constant ratio of hydrochloric acid to stannous chloride
379
AUTOXIDATION O F STANNOUS CHLORIDE. I11
being used in all solutions. The wave length used was the 3130 8. line, isolated by the use of the quartz monochromator. It can be seen that the value of (Y changes about threefold for a tenfold change in concentration of stannous chloride, showing that Beer’s law does not hold. It was thus assumed that stannous chloride is not the absorbing medium. To test this hypothesis further, runs were made using a constant amount of stannous chloride with varying amounts of hydrochloric acid. The results are shown in table 4. In this case the value of a changes twelvefold for a thirtysixfold change in the hydrochloric acid concentration. Hydrochloric acid alone shows no appreciable absorption‘at this wave length, so that the TABLE 4 Effect of varying amounts of hvdrochloric acid on the Beer’s law constant SOLUTION
CONCENTRATION OF STANNOUS CHLORIDE
1
CONCENTRATION O F HYDROCHLORIC ACID
grams per liter
grams per liter
13.0 13.0 13.0
4.4 26.0 156.0
g C
f
WAVE LENQTH
2850 d. 3020 3130 3340 3650
PER CENT LIGHT TRANSMITTED
69.7 37.9 1.1
PER CENT LIGHT TRANSMITTED
0.0 4.9 28.4 97.9 100.0
4.1 10.9 50.3
U
to
13.4 5.6 0.095 0.0
great change in a! must be due to the influence of the hydrochloric acid on the stannous chloride. Thus the medium which absorbs the light quantum and becomes activated is not stannous chloride, nor the Sn+f ion, but must be some substance whose concentration increases with addition of hydrochloric acid to the system. The absorbing medium is thus either the complex chloro acid HSnCL or HZSnC14, or the ions SnCb- and SnCla of those acids. That these complex ions are present in an acid solution was shown by Prytz (7). MOLECULAR ABSORPTION COEFFICIENT
The molecular absorption coefficient was determined for a stannous chloride solution containing 32.7 grams of stannous chloride per liter and
380
ROBERT C. HARING AND JAMES H. WALTON
0.811 N in hydrochloric acid, by use of the monochromator. Table 5 gives the results. These values of LY are not applicable to other concentrations, because of the non-validity of Beer’s law, but are included to show the range of partial and complete absorption. SUMMARY
1. The autoxidation of stannous chloride is shown to be a photochemical chain reaction by the quantum yields of more than unity. 2. The photochemical and thermal reactions are affected similarly by accelerators and inhibitors; thus the hypothesis of a chain mechanism is supported for the thermal reaction. 3. Stannous chloride fhows complete absorption below a certain limit; the limit is about 3000 A. for the stock solutions used in the research, containing about 32 grams of stannous chloride per liter and being 0.8 N in hydrochloric acid, and is lower for more dilute solutions. 4. Stannous chloride was shown not to obey Beer’s law. This was explained by the assumption that the complexes HSnCL and H2SnC14 are the active agents in absorbing the light, since the concentrations of these substances are increased by addition of hydrochloric acid to the system. 5. The molecular absorption coefficients for one stock solution are given, showing complete absorpotion of light at 2850 A. and below, partial absorption from 3020A. to 3340 A., and completetransmission at 3650 A. and above. REFERENCES (1) (2) (3) (4) (5) (6) (7)
HARING, ROBERT C., AND WALTON, JAMESH . : J. Phys. Chem. 37, 133 (1933). BXCKSTROM, H. L. J.: J. Am. Chem. SOC.49, 1460 (1927). ALYEA,H. N.,AND BSCKSTROM, H. L. J.: J. Am. Chem. SOC.61, 90 (1929). DANIELS, F., AND HEIDT,L. J . : J. Am. Chem. SOC.64, 2381 (1932). HEIDT,L. J., AND DANIELS, F.: J. Am. Chem. SOC.64,2384 (1932). LEIGHTON, W. G., AND FORBES,G. S.: J. Am. Chem. SOC.62, 3139 (1930). PRYTZ, M.: Z. anorg. Chem. 172, 147 (1928).
