Photochemical Reaction between Sodium Formate and Iodine and a

PHOTOCHEMICAL REACTION BETWEEN SODIUM FORMATE AND IODINE AND A RELATION BETWEEN CHEMICAL REACTIVITY AND LIGHT ABSORPTION N. R. DHAR AND P. N. BHARGAVA Department of Chemistry, Allahabad University, Allahabad, India Received November 50, 1934

Amongst the photochemical reactions taking place in solutions, the iodine-formate reaction is easy to investigate as it is fairly rapid. N. R. Dhar (24) first showed that the reaction between sodium formate and iodine is markedly photosensitive; he studied the reaction only in visible light with iodine dissolved in an aqueous solution of potassium iodide, which considerably inhibits the reaction velocity. This reaction has now been investigated in detail, using an aqueous solution of iodine without any potassium iodide. I n addition to kinetic measurements we have tried to establish a relation between chemical reactivity and light absorption photographically with different concentrations of the reactants. EXPERIMENTAL PROCEDURE

A tilting-type quartz mercury-vapor lamp working at 220 volts was used in order to obtain ultra-violet radiations, whereas for visible and infra-red radiations, a 1000-watt gas-filled tungsten filament lamp was employed. All materials used were purified by recrystallization. Merck’s pure iodine was further purified by resublimation. All solutions were prepared in conductivity water. I n order to isolate different regions from the entire spectrum of the light source, solution filters were used. The light transmitted by these filters has been carefully determined and described in previous papers (1). The filters used for infra-red radiations cut off all the visible and ultra-violet light. For measurements of the energy absorbed, a Moll thermopile with a sensitive galvanometer was used. The velocity of the reaction a t a particular instant was ascertained by estimating the unchanged iodine against very dilute solutions of sodium thiosulfate. The reaction is monomolecular in the dark. The expression for a semimolecular reaction velocity is

1231

I

1232

N. R. DHAR AND P. N. BHARGAVA

where a denotes the initial concentration of the reactant and x its concentration after a period t . The reaction between sodium formate and iodine follows the semimolecular law when the system is illuminated by radiations of different wave lengths, as is clear from table l . KINETICS

The temperature coefficients have been calculated after deducting the dark reaction velocity (see tables 2 and 3). The results show that Einstein's law of photochemical equivalence is not obeyed. This is true for many exothermal photochemical reactions, and it appears that the energy given out in the transformation of molecules, caused by light absorption, may effect activation or loosening of the bindTABLE 1 The reaction between sodium formate and iodine CONDITIONS

TIME

minutes

Dark (T = 20°C.) ......................

0

HYPO PER 5 CC. OF THE RBACT- REACTION VELOCITY I N 0 MIXTURE

cc.

10

4.6 3'75

20

3.05

0.00888 (K,)

0.00892 ( K I )

I 0.00890

Mean K l . . ...................................................... X = 4295 A.U. (T = 15OC.) .............

0

4.6 3'7

0.04419 (K+) 0.04416 ( K i )

2.9 Mean K!. .....................................................

. / 0.04418

ing forces of molecules. The quantum yield increases with temperature and the frequency of the incident radiation. RELATION BETWEEN THE INTENSITY OF LIGHT (AMOUNT OF ENERGY ABSORBED) AND THE VELOCITY

OF REACTION

We have investigated the problem of the variation of the relation between intensity and velocity by changing the amounts of potassium iodide (a marked retarder) present in the iodine solution, and thus altering the dark reaction velocity. The results given in table 4 show that the relation between the velocity and intensity in the reaction between sodium formate and iodine a t 20°C. can vary from I' to It. From the results given in table 4 it is obvious that there are two important factors which are of consequence in the relation between intensity and velocity of the reaction: (1) the amount of absorption of the incident

1233

PHOTOCHEMICAL REACTION

radiation by the reacting system, and (2) the acceleration of the reaction on illumination. A photochemical reaction which follows less than direct TABLE 2 Reaction between sodium formate and iodine (aqueous without potassium iodide) N/25 sodium formate; N/1125 iodine; N/400 borax (used as buffer) PEMPERATURE

CONDITIONS

K (SEMIMOLECULAR)

TEMPERAU R E COEF FICIENTS

QUANTUM

YIELD

“C.

Dark

15 20 25

0.03393 0.04166 0.06012

1.77

X = 3125 A.U. (range of transmission = 3290-2961 A.U.)

15 20 25

0.06101 0.07214 0.09921

1.45

32 46 64

X = 3340 A.U. (range of transmission

15 20 25

0,05282 0.06485 0.08835

1.52

31 40 54

X = 3452 A.U. (range of transmission = 3307-3598 A.U.)

15 20 25

0.04865 0.06120 0.08329

1.57

26 36 42

X = 3512 A.U. (range of transmission =

15 20 25

0.04865 0.06120 0.08329

1.57

4295 A.U. (per cent of transmission = 20.4; range of transmission = 40004590 A.U.)

15 20 25

0.04418 0.05688 0.07669

1.61

X = 5700 A.U. (per cent of transmission = 16.2; range of transmission = 52006200 A.U.)

15 20 25

0.04059 0.05187 0,07119

1.66

X = 6640 A.U. (per cent of transmission = 58.8; range of transmission = 6280. 7000 A.U.)

15 20 25

0.03859 0.04696 0.06809

1.71

h = 8500 A.U. (range of transmission =

15 20 25

0.03662 0.04417 0,06483

1.75

=

4063-2618 A.U.

3290-2961 A.U., 4023-4063 A.U.)

X

=

8000-9000 A.U.)

relationship, becomes directly proportional or even greater than proportional to the intensity of incident radiation, by increasing the dark reaction velocity and exposing it to radiation, slightly absorbed by the system. On

1234


the contrary, a photochemical reaction which is nearly proportional to the square of the incident radiation or is directly proportional can be made t o be proportional to the square root of the incident radiation by decreasing the dark reaction velocity. This is certainly one of the many reactions which have been proved by us to show a variable relation between the velocity and the light intensity or amount of energy absorbed (compare reference 4, pp. 3 4 0 4 ) . TABLE 3 Reaction between sodium formate and iodine (aqueous solution containing a little potassium iodide) N/6.25 sodium formate; N/6.7 sodium acetate; N/204 iodine; N147.4 potassium iodide CONDITION ~

~~

~~~

~

~~

TEMPERATURE

Kt

bm-

MOLECULAR

!IYPERATURI :OPFFICIENTFJ

~

"C.

Dark. .....................................

X = 4295 A.U...............................

X = 5700

A.U..............................

X = 6640 A.U..............................

{ ' ii 1 ii i ii

1

20

::

0.00883 0.03760 0.15686

4.26 4.17

0.01216 0.05076 0.20736

3.95 3.84

0.01104 0.04676 0.19391

4.14 4.04

0.010529 0.04472 0.18547

4.19 4.02

0.00928 0.03948 0.16461

4.22 4.12

Temperature coeficients The results recorded in table 2 show that the temperature coefficient df the dark reaction velocity has the value 1.77 between 15°C. and 25"C., when aqueous solutions of iodine are employed. On the contrary the temperature coefficient has the value 4.26 between 20°C. and 30"C., and 4.17 between 30°C. and 40"C., when a little potassium iodide (N/47.4) is added to the aqueous iodine solution. Potassium iodide markedly retards the reaction. This is in agreement with the observations of N. R. Dhar (3), who showed that the temperature coefficient of a reaction falls off when

1235


-

TABLE 4 Relation between reaction velocity and light intensity K1 (MONOMODIAMETER

EXPT.

At$$utl

S O U R C E OF LIGHT

REACTANTS

DARK ZEACTION VELOCITY

I N CM.

__

LECULAR F T E R DBDCCT[NG THE DARK REACTION VELOCITY

A

N/6.25 sodium formate; N/6.7 sodium acetate; N/204 iodine; K I , 1.0464 g. in one liter

Total light from 1000-watt lamp

2.2 0.5 1.o

0.00561 0.00566 I 0.00035 111 0,001293 I1

B

N/6.25 sodium formate; N/6.7 sodium acetate (as buffer); N/204 iodine; N/47.4 potassium iodide


2.2 1.o 0.5

0.0018( 0.00472 I 0,00132 I1 0.00044 111

C

N/25 sodium formate; N/1125 iodine without K I ; N/26.8 sodium acetate


2.2 1.o 0.5

0.0082t 0,00764 I 0.00166 I1 0.00060 111

D

N/25 sodium formate; N/1125 iodine; N/400 borax (as buffer)


2.2 1.o

0.0089C 0,00483 I 0.00091 I1

E

Same as in D

Mercury vapor lamp, X = 3340 A.U.

2.2 1.o

0.01954 I 0.00288 I1

F

Same as in D

X = 3125 A.U.

2.2 1.o

0.02083 I 0.00372 I1

-

IA DIRECTLY PROPOR-

EXPT.

RATIO OF VLLOCITIES

A

1/11 = 4.37 II/III = 3.67 I/III = 16.07

4.84 4.0 19.36

B

1/11 = 3.57 II/III = 3 . 0 I/III = 10.7

4.84 4.0 19.36

11 /I

1)

EXPT.

RATIO OF VELOCITIES

I F DIRECTLY PROPORTIONAL TO CHANGE I N INTENSITY

C

1/11 = 4 . 6 II/III = 3 . 8 I / I I I = 17.5

4.84 4.0 19,36

D

1/11 = 5 . 3

4.84

E

1/11 = 6 . 7

4.84

1/11

4.84

F

I

=

5.6

1236

N . R. DHAR AND P. N . BHARGAVA

it is accelerated and rises when it is retarded. Moreover, it will be observed that the temperature coefficient in ultra-violet light, which markedly accelerates the reaction, varies from 1.45 to 1.57 between 15°C. and 25°C. Hence it is clear that the greater the velocity of the reaction in light, the smaller is the temperature coefficient of the reaction, When the reaction is accelerated by infra-red radiations of mean wave length 8500 A.U., which affect the velocity much less than ultra-violet light, the difference between the temperature coefficients of the thermal and photochemical velocities is much less. I n radiations of wave length 8500 A.U. the temperature coefficient between 25°C. and 15°C. is 1.75. In other words, the greater the acceleration of the reaction by light absorption, the smaller is the temperature coefficient. CHEMICAL REACTIVITY AND LIGHT ABSORPTION

We have measured the light absorption of the reacting substances separately and in mixtures by photographing their absorption spectra with Hilger quartz spectrographs Ei and EDusing a copper arc as the light source. The effect of increase in the concentration of sodium formate on light absorption on the entire spectrum, with a constant concentration of iodine and vice versa, has also been investigated photographically. The exposure for taking the photographs was varied from 7 to 45 seconds. The amount of chemical change during the period of exposure was negligible, and hence the products formed as a result of the chemical change from the reacting mixtures did not play any important part in effecting the light absorption. We have tested this by taking the photographs with 20 FIG. 1. VISIBLEAND ULTRA-VIOLET REGION From 5220 to 3247 A.U. Exposure = 45 seconds. Illford rapid plates Light absorption

(1) Copper arc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2) N/700 iodine ..................................................

5220-5105 5105-4580 4573-4441 3577-3426 3426-3282

A.U.

A.U. A.U. A.U. A.U.

(3) N/2 sodium formate.. ... (4) Mixture of N/350 iodine with N sodium formate.. . . . . . . . . . . . . 5220-5105 A.U. 5105-3247 A.U. (5) N/4 sodium formate No absorption 5105-4850 A.U. (6) Mixture of N/2 sodi 4441-3247 A.U. (7) N / 8 sodium formate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . No absorption (8) Mixture of N/4 sodium formate with N/350 iodine.. . . . . . . . . . . 5105-4441 A.U. 4441-3266 A.U. (9) N/12.5 sodium formate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . No absorption (10) Mixture of N/6.25 sodium formate with N/350 iodine.. .... 4441-3266 A.U.

1237


___s

3208

3194

,3247

3266 3 282 3379

.3512

3643 ,3775 -3907 -40 22

-41 77 ,4309

,4441

,4704 a

-4842

,4968 5105

,5220

5144

1238


seconds and 7 seconds exposure and found no perceptible difference in light absorption. Figures 1to 5 are the photographs takeii with different concentrations of sodium formate and a constant concentration of iodine (aqueous), separately and with mixtures. From the photograph takeii with N/2 sodium formate and N / 7 0 0 iodine separately and their mixtures, it is evident that the iodine solution alone shows complete absorption from 2369 A.U., while with sodium formate complete absorption begins from 2294 A.U., but in the mixture complete absorption starts from 5216 A.U. It appears, therefore, that the presence of sodium formate sensitizes the decomposition of iodine molecules, which becomes reactive in radiations of longer wave lengths 011 the addition of sodiuni formate. The addition of sodium formate weakens the binding forces of halogen molecules, which are readily broken u p either by increase of temperature or illumination, and thus increased light absorption has been observed with the ,mixture. Hence the reactivity of a mixture is preceded by the formation of an additive product with the weakening of the binding forces and increased light absorption. Our experimental observations further show that the increase in the concentration of the reducing agent leads to more light absorption by the mixtare. In other words, the higher the concentration of the reducing agent, the greater is the light absorption by the mixture of the reducing agents with the halogens. This may be due to the loosening of the binding forces of more of the iodine nioleciiles, leading to increase in light absorption, especially of visible and ultra-viold regions. I t has also been observed by lis that the decrease in the coilcentration of iodine, keeping the concentration of sodium formate constant, leads to a diminution of light absorption by the mixture, as is clear from the above photographs. It is interesting to note that the Jrariatioii of the concentration of iodine produces more marked effect on light absorption by the mixture than when the concentration of the reducing agent is altered.

FIQ.2. VISIBLE A N D ULTRA-VIOLET REGION From 5220 to 3274 A.U. Exposure = 45 seconds I.ight absorplion

(1) Copper arc.. . . . ................................... (2) N/1400 iodine.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5105-4573 A.U. 4573--4441A.U. (3) N/2 sodium formate,,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . No absorption . 4022-3266 A.U. (4) Mixture of N sodium formate and N/700 iodine.. . . . . . . . (5) N/4 sodium formate ............................... ormate and N/700 iodine., , , , , . , . . , . . 3907-3379 A.U. (6) Mixture of N/2 sodi ...................... No absorption (7) N/8 sodium formate.. . . . . . (8) Mixture of N/4 sodium formate and N/700 iodine.. . . . . . . . . . . .3775-3400 A.U.

1239


324 7

3379 3512

3643 3775

,3907

4022 ,4177 -

4307

-4441 - 4563 - 4575 - 4704 - 4842 - 3968

-5144 - 5105

- 5220

1240


The observations of Fajails and Karagunis ( 5 ) on increased light absorption by silver halides containing adsorbed silver and those of Weigert and Kellermann (7) on increased light absorption by a mixture of chlorine and hydrogen, and the increased light absorption observed by J. C. Ghosh and collaborators (6) with mixtures of organic substances and solutions of ferric and mercuric chlorides and uranyl nitrate, etc., can be interpreted on the viewpoint that the cheinical reactivity of these various systems is associated with the weakening of the binding forces and increased light absorption. Our conclusions are also supported by the observations of Franck and Victor Henri, who showed that light absorption or increased temperature enhances the reactivity and the power to absorb light of gaseous molecules. Just as the physical agencies like light absorption, increase of temperature, etc., can loosen the binding forces of the molecule, so the introduction of a reactive chemical substance into a light-absorbing system can weaken the FIG.3. ULTR.4-VIOLET REGION From 3247 to 2442 A.U. Exposure = 45 seconds Light absorption

(1) Copper arc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2) N/700 iodine.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(3) N/2 sodium formate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . (4) Mixture of N sodium formate and N/350 iodine..

(5) N/1400 iodine.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(6) N/4 sodium formate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (7) Mixture of N/2 sodium formate and N/350 iodine.. . . . . . . (8) Mixture of N/2 sodium formate and N/700 iodine. . . . . . . . (9) N/8 sodium formate., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (10) Mixture of N/4 sodium formate and N/350 iodine.. . . . . . . (11) Mixture of N/4 sodium formatte and N/700 iodine.,

.. . . 3000-2961 A.U.

2961-2824 A.U. 2824-2766 A.U. 2766-2618 A.U. 2600-2492 A.U. 2492-2442 A.U. No absorption Total absorption 2961-2882 A.U. 2880-2824 A.U. 2824-2766 A.U. 2729-2618 A.U. No absorption Total absorption 3194-2618 A.U. 2618 onwards total No absorption 3247-2618 A.U. 2618 onwards complete 3012-2618 A.U. 2610-2492 A.U. 2492 onwards complete

1241


442

'492

2618

2766

2 824

2961 -3012 - 3063

31 14 .3165

3194 - 3247

3208

N. R. DIIAR AND P. N. BHARGAVA

FIGS.4 AND 5. ULTRA-VIOLET REGION From 2406 to 2104 A.U. Exposure = 45 seconds Light absorption

............................................

. . . . . . . . . . . . 2406-2398 A.U. 2398-2392 A.U. 2392-2369 A.U. and onwards complete (3) N/2 sodium formate.. . . . ............................ 2369 onwa.rds complete (4) Mixture of N/350 iodine and N sodium formate., . . . . . . . . . . . . . Total absorption (5) N/4 sodium formate., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . From 2369 total absorption (6) Mixture of N/2 sodium formate and N/350 iodiiic.. . . . . . . . . . . . Total absorption (7) Copper arc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (8) N/1400 iodine,, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2369 A . U . onwards complete absorption (9) Mixture N/700 iodine and N sodium formate.. . . . . . . . . . . . . . . . .Total absorption (10) Mixture N/700 iodine and N/2 sodium formate.. ... . . . Total absorption . . . . . . . . . . . . . . . . . .2369-2303 A.U. (11) N/8 sodium formate . . . . . . . . . . . . . . 2303 onwards complete absorption (12) N/4 sodium formate and N/350 iodine,. . . . . . . . . . . . . . . . . . . . . . . Total absorption (13) N / 4 sodium formate atid N/700 iodine. . . . . . . . . . . . . . . . . .Total absorption

1243


71

81

1244


binding forces of the molecule. In other words, the presence of a reducing agent like sodium formate sensitizes the decomposition of iodine molecules and niakes them reactive in radiations of longer wave lengths. It seems that chemical reactivity is associated with increased light absorption. SUMMARY

1. The reaction between sodiuni formate and iodine (aqueous) in the absence of potaqsiuin iodide is iinimolecular in the dark and semimolecular in light. 2. The temperature coefficient of the reaction velocity between 15OC. and 25°C. has the following values: 1.77 (dark), 1.75 (8500 A.U.), 1.71 (6640 A.U.), 1.66 (5700 A.U.), 1.61 (4295 A.U.), 1.57 (3512 A.U.), 1.57 (3452 A.U.), 1.52 (3340 A.U.), a i d 1.45 (3125 A.U.). The greater the acceleration due to light, the snialler is the temperature coefficient. 3. The relation between light intensity and velocity of the reaction varies from 12 to I+. 4. Einstein's law of photocheinical equivalence is not obeyed. The quantuni yield increases with the temperature and the frequency of the iiicident radiation. 5. It has been observed that the light absorption by a mixture of sodium formate and iodine is greater than the nbsorptioii by the ingredients. The increased absorption appears to be due to the activation of the iodine molecules by the presence of the molecules of the reducing agent. The activation of the molecules is associated with the weakening of the binding forces and consequent increased light absorption. The increase in the concentration of sodium formate or iodine causes an increase in the light absorption by the mixture of sodium formate and iodine. REFERENCES (1) BHATTACHARYA AND DHAR: Z. anorg. Cliem. 199, 2 (1931); J. Indian Chem. SOC. 6, 451 (1928); Z. anorg. allgem. Chem. 176, 357 (1928); 176, 372 (1928). (2) DHAR,N. R . : Proc. Acad. Sci. Amsterdam 16, 1097 (1916). (3) DHAR,N. R . : J. Chem. SOC.111, 707 (1917). (4) DHAR, N. R.: Chemical Action of Light, p. 156. Blackie and Son, London (1931). (5) F A J A N S A N D KARAGUNIS: Z. pliysik. Chem. SB,385 (1929). (6) GHOSH,J. C., A N D COLLABORATORS: J. Indian Chem. S O C . 4, 363 (1927); 6, 191, 569 (1928). (7) WEIGERTA N D KELLERMANN: Z . physik. Cliem. 107, 1 (1923).

Photochemical Reaction between Sodium Formate and Iodine and a

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