-1QLASTITA1 TIT’E STUDY O F SOME PHOTOCHEJIICXL EFFECTS PRODUCED BJ7 ULTRA4-T710LETI,IGHT1 BY J . HOIV.IKD hl.lTHEIVS
.IND L E O S H . DEU-EI’
The object of this in\-estigation rvas to follon-, quantitatively, the decomposition and oxidation of various solutions under the action of ultra--violet light. For this purpose solutions of sodium sulphite, potassium permanganate, potassiiim dichromate, and oxalic acid were used. -1Cooper-Hen-itt quartz mercury i-apor lamp \!-as used as the source of ultra-violet rays. The lamp was run on a direct current and used three amperes with a potential drop across the terminals of ;o volts. Care \vas exercised to keep the current strength constant, as the 1-elocity of the reactions studied is so highly dependent upon the strength of the illumina ti on, The flasks containing the solutions to he il-orked with \\-ere made of transparent quartz, a material lvhich alloir-s the ultraviolet rays to pass freely. They were supported in direct light of the lamp a t a distance of I O cm. Due to the considerable amount of heat generated by the lamp in action, arid the heat effect produced hy the absorption of the rays, the temperature had to be controlled. This was acconiplished by supporting a 17-ater-bath immediately under the lamp. connecting it with a Ivater tap and so regulating the flon. of water as to keep a constant temperature in the bath. The The temperature of the ii.ater iras kept a t about 20’. quartz flasks n-ere supported on floats in this bath in such a manner that one-half of their surface was exposed to the light, the other being exposed to the cooling action of the water. *I thermometer kept in the solution indicated its exact temperature. It was found possible to maintain a constant temperature of 2 5 O i I . j O , which was close enough A paper read before the Eighth International Congress of -4pplied Chemistry in X e w I-ork, September, 1 9 1 2
as temperature coefficients of photochemical reactions are small. The solutions used were 0 .I normal. I n titrating the weight burette was used. 'The water from which the solutions were prepared IT-asspecially distilled from the city water supply; potassium permanganate was added to the n-ater before distillation. and the first third of the distillate rejected. Sodium Sulphite ; iliter of 0 . I normal sodium sulphite \vas divided into two portions of j00 cc. each and placed in glass-stoppered liter flasks. One of these was placed in the dark and the other placed in daylight. The solutions \\-ere titrated a t intervals \\-ith a 0 . I normal solution of resublimed iodine. The solution in the light oxidized completely in four days' time, irhile t h a t portion of the same solution which was kept in the dark for the same period of time \\-as hut about half oxidized. From this it \vas very evident that the oxidation of sulphite solutions is greatly accelerated by light. Separate experiments in Trhich the containing vessels \\-ere entirely filled ivith the solutions shoived that in the absence of air the oxidation is h u t \-ery slight. This slight oxidation is, of course, due to the oxygen contained in the ivater. For the study of the effect of ultra-s-iolet light on the oxidation of sodiuni sulphite solutions, a 0 .I normal solution contained in a quartz flask was placed under the light. It was found t h a t the ultra-violet light has a decidedly accelerating influence. on the reaction. The oxidation proceeded about twenty times as rapidly in this light as in the ordinary light of the laboratory. I n order t o determine whether there might not be an autoxidation of the sulphite in the ultra-s-iolet light, a 0 . I normal solution prepared from boiled, water \vas placed in a quartz flask, care being taken to completely fill the flask. Exposure to the light for six hours produced no appreciable oxidation.
Photochemical Eflects by C'ltia- I'iolet Light
213
Potassium Permanganate Solutions of potassium permanganate are usually considered quite stable, although it is known that upon long standing they do change somewhat. -1 rather strong solution of potassium permanganate was subjected to the action of ultra-violet light for seven hours. It was titrated before and after exposure with a standard solution of oxalic acid, in presence of sulphuric acid. -1slight reduction of the strength of the solution was evident, and a deposit of oxide appeared on the walls of the vessel.
Potassium Bichromate Six hours exposure of a o . I normal solution of potassium bichromate to the ultra-x-iolet rays produced n o change ivhatex-er in the strength of the solution
Decomposition of Oxalic Acid Solutions of oxalic acid when placed in ultra-violet light are b u t very slon-ly decomposed. Hon-ever, if a uranium salt be present the reaction is greatly accelerated. series of determinations of the rate of decomposition of oxalic acid solutions with x-arying amounts of uranium salts and with salts of uranium containing different acid radicles was made. The nitrate, sulphate and acetate were used. The oxalic acid solutions were o I normal, and were titrated it-ith o I normal solutions of potassium permanganate. In the first experiment, a solution containing approximately o 5 gram of uranium nitrate per 100 cc of o I normal In the first series of titrations conoxalic acid was used siderable trouble was experienced in keeping the intensity of the light constant. This fact accounts for the irregularity shown in the curxre [Fig I ) . During the first two hours the pressure was low. after which it increased while the batteries were being charged until a t the third hour the maximum was reached. -1t the end of the fifth hour the pressure began to fall off again, and the rate of decomposition of the acid decreased correspondingly, all of which is n-ell shown on the graph.
J . Howard -\.lathews and Leon H . Dewey
214
In a later series, using the same concentration of uranium nitrate, no difficulty was experienced in the regulation of the light, and the curve plotted from the data obtained was perfectly linear.
Fig
I
Oxalic acid with varying amounts of uranium nitrate were then subjected to the action of the ultra-violet rays, with the results shown in the accompanying tabulations and the graphs plotted therefrom S E R I E S N O . I -cOST;\ISING 0 j GRAMc R . I S I C h I SITR.4TE I N I O 0 cc OF SOLUTIOS Titne oftitration
SERIESKO. ~.-COSTAISISG
Millirnols of acid present pel- liter
0
z j GRAMUR.VWX KITRATEIS
100
CC OF SOLUTIOK -~
Time of titration II
,0o A i . l I .
I2
.oo 11.
I , 0 0 P.M. 2
.oo P.M.
3 .oo P.11. 4 . 0 0 P.M. j . O O P.31.
AIilliniols of acid present per liter 100.0
96.0 90.5
85.5 79.4 73.5 65.0
Photochemical El'jects by l'ltva- I'iolet L i g h t S E R I E S NO. 3 . - c O N T A I S I S G
GRAMOF
0 I
215
c R A S I T ? K NITRATE IN I O 0
CC OF S O L P T I O S ~~
8 9
__
Millimols of acid per liter
Time of titration
100.0
00 A . ~ I
I 00 P.31.
97.0 92.0 58.8 86.5 85.0
2 00 P.M.
77.5
00 .\.?VI. I O 00 A . M .
I I 00 -4.31, I 2 00 >I.
Fig. 2 shows, graphically, the results given in the above tables. I t will be observed t h a t the curves are linear, the rate of decomposition being constant.
100
80
60 40
20
0
1
2
3
4
Fig.
s
e
7
8
2
The next series of determinations were made with uranium sulphate as catalyzer. Since it is the uranium which is the active agent in accelerating the reaction, care was taken to use equimolecular amounts of the different salts. SERIES SO. 4.-cOST.%ISISG
0
4 2 7 G R a h l URAXILX SULPH.iTE I N
100 CC O F S O L T T I O S
Time of titratioii
~ l i l l i m o l sof acid per liter
10.00 A . M .
100.0
I 1 j0 A.M. I 2 5 0 P.X. I j 0 P.M. 2 50 P.M.
93 . 0
3 50 3 .j 0
P.X. P.X.
76.8
65.8 j0.7 39.9 2j.8
J . Howard Mathews and Leon H . Dewey
2 16
SERIESNo. ~.-CO~YTAINIKG o 214 GRAMCRAXIUM SULPHATE IN 100 CC OF S O L U T I O X
__-
_ _-
i'dillimols of acid per liter
Time of titration
,oo A . M .
100.0
1 2 . 0 0 A.X.
95.0 89.0 81.8
II
I .00 P.M. 2 .oo P.M. 3 .00P.M. 4 . 0 0 P.M. j .00 P.XI.
SERIES
*-
13.2
68.2 59 5
so. ~.-COSTAINIKG
o 085 GR.\x
100 C c
UR.\SILX
SKLPHATE IS
OF S O L U T I ( Z S
T i m e of titration
h11lliniols of acid present per liter 100.0
95.2 91 4 87.3 82.j
80.2
The curves for the sulphate solutions, Fig. 3, show the same regularity in rate of decomposition as previously found ivith the nitrate of uranium. The more uranium present the faster is the decomposition
100
80 60 40
20
0
1
2
3
4
5
6
7
8
Fig. 3
The final series of measurements was made with ura nium acetate as the catalyzer, the amounts used being varied as before.
Photochemical Efjects by Ultra-Violet Light
8 9
I j A.M.
IO0 0
I j A.M.
90 0 77 0 69 8 59 0 48 5 38 9
IO Ij I1 I j I 2 Ij I Ij 2 15
SERIES S O
A.M. A.M. P >I. P.31. P.M.
8.-cOST.XISISCr
0
217
084 GRAMOF URAXIU31 -kCETr\TE IK
100 CC O F S O L C T I O N -
~~~
Time of titration I I 00 A.31. I 2 .0011.
I00 0
96 0 93 0 89 8 84 9 78 5 72 6
I 00 P.X. 2 00 P . X .
3 4 5
Milliinols of acid present per liter
00 P.M. 00 P.M. 00 P.31.
0 2 I I GRXI O F URASIC3I A%CETATEIS 100 CC O F S O L C T I O S
SERIES YO. 9 -cOST.\IXISG ~
Time of titration
Rlillimols of acid present per liter 100.0
90.5 81.8 73 0 '
6j.0
57.5 46.8
The decomposition of the oxalic acid with uranium acetate as the catalyzer proceeds regularly, as with the other salts of uranium. Other radio-active salts should be tried to ascertain whether this property is limited to uranium compounds, or whether i t is common t o those possessing radio-activity. I t may be t h a t the rays given off from the
218
J . Howard 1rllathew.s and Leon H . Dewey
uranium throw the molecules into an unstable condition, which favors their decomposition by the light waves.
too 80 60 40
20 o
i
a
a
4
5
e
.
7
8
F%. 4
From the experiments described in this paper i t is concluded t h a t : I . The rate of decomposition of a solution by ultra-violet light is dependent upon the amount of light photochemically absorbed. 2 . The rate of decomposition of the oxalic solutions in the presence of the catalytic uranium salts varies directly with the amount of uranium salt used. 3. The salts potassium permanganate and potassium bichromate are very stable toward ultra-violet light. 4. Sodium sulphite solutions do not oxidize (autoxidation) when air is kept away from them, even under the influence of ultra-violet light, but the oxidation in air is greatly accelerated by such light. j. Possibly other radio-active elements might behave similarly to uranium in its accelerating action on the decomposition of oxalic acid. 1.nt:eiszty o j TI'tsconszii
