828
C. F. GOODEVE .4ND M.
R. TAYLOR
T H E EFFECT OF TEMPERATURE AND IMPURITIES ON CERTAIS PHOTOCHEhlIC=IL REACTIONS I S SOLIDS C. F. GOODEVE
AND
RI. R. TAk’lrLOR1
r n i v e r s i t y College, London
Received September 29, 1947 IKTRODUCTI06
Certain oxides of metals are affected by light; for example, when antimony oxide is exposed to light of a wave length less than its absorption threshold it darkens, with the production of what is believed to be antimony metal (3). Titanium dioxide, on the other hand, has been shon-n by Goodeve and Kitchener (5) to be stable itself to light but to sensitize the bleaching of an organic dye, Chlorazol Sky Blue FF, which had been deposited upon it, the reaction having a quantum efficiency of 4 X lop3 when bleaching is brought about by light of a wave length in the absorption region of titanium dioxide. When exposed by itself, or on the surface of some non-absorbing material such as barium sulfate, the dye is stable to light. Although Goodeve and Kitchener noted that temperature affected the rate of the reaction, neither in the case of the bleaching of the dye, nor in the decomposition of antimony oxide, has the effect of temperature been carefully studied. The work of Hilsch and Pohl on the photolysis of the alkali halides has included the effects of temperature on the decomposition of KH and KD in potassium bromide (6), and they found that the quantum yield dropped from unity a t 500°C. to almost zero at - 100°C. The senzitivity of the phbtogrnphic proce,-s is 1cnon.n to be considerably decreased at the temperature of liquid air ( I ) , and some observations on some organic solids ( 7 , 10) have shon-n that the photochemical reaction rate is increased as the temperature is raised. Apart from these exceptions, little is known of the effect of temperature on photochemical reactions of solids. This report gives the methods and results of an investigation undertaken to determine the effect of temperature upon the darkening of antimony oxide and upon the bleaching of a dye deposited upon titanium dioxide. The results are put forward without discussion.2 BIATERIALS
One sample (H & W) of the antimony oxide used 71-as a commercially pure, stock reagent as supplied by Nessrs. IIopkins and Killiams. It was of a slight brown tinge and its x-ray pattern showed it to be the orthorhombic form o€ antimony oxide, valentinite. Another sample (Pr) of valentinite was prepared by adding crushed crystals of antimony chloride to boiling aqueous ammonia, filter1 Formerly a t the Sir William Rarnsap and Ralph Forster Labnratorie.; of Chemistry, University College, London. * This nnrk is described in more detail in the Ph D . thesis of R l . R. Taylor, University of London, 1916.
I'HOTOCH1;~lI('.\L ~~~~~~~~~~S I S SOLIDS
529
ing, ivashing, and drying. Thi. Lmple (Pi)\vas \\-hitel,and much IC t o light, having a reflection of 9'7 per cent over the \\-hole visililc spwtriini :is against '79 per cent for the sample (H & TI*). The :thorption antl pliotoc.hcmic~t1 threshold of these tn-o samples \\-ere tietei.minet1 arid found to agi'ec \vitli those reported by ('ohn and Goodeve ( 3 ) . 1he dyed titaniiim d i o d e \\-:is prepared as follmvs: I'iue tit:inium tetrachloride \vas dissolved in a large volume of ivater and tioiled for half :m lioiii~, \\.hen titanium dioxide began to precipitate in a semicolloidal form. TI'hile st ill at 100°C'. the solution \vas slou-ly neutralized \\-ith ammonia t o wiibe coagiilation. 1 his titaniiini dioxide \vas \\-aslied, and the \vet precipitate \vas tlyecl hy adding :I 7
.
r 7
F r ( ; , 1 , T h e progress of t h e photodecoinpositioii of a dye oil titanium dioxide. Sariiple C , 1.87 per cent dye tiy weight; sample D , 0.3s per cent d y r hy weight; samplc 1,:. 0.57 per eiit tlye by ]\-eight, Broken line. the quantum rfficiericy cleducrd from t h c iiica11 (solid 1 curve. C
solution of ('hlorazol Sky Blue dissolved in alcohol, and then an excess of harium chloride. The dyeti suspension \\-as then washed, filtered, clrid. groiintl, :mcl sieved. One sample (E) \vas digested for 24 hr. at 80°C'. prior t o heing \\.axlied etc. The dyed powders \\-ere light hliie, \vere sensitive t o light, nntl pos>esscd :i photochemical threshold and absorption corresponding to that determined 11y Goodeve antl Kitchenel. for pori-der; prepared in a similar manner. *Inindependent check of the determination of the quantum efficiency of l)lex*hing \\-as made using the previous procedure ( 5 ) , hut the concentration of tlie tlye \vas varied over a wider range. The results of the quantum efficiency of lileaching are sho\\-n in figure 1. The quantum efficiency curve follo\vs the s:ime t i x m l as found 11y Goodeve and Iiitchener, falling off at long exposures t o a constant value, although the maximum value obtained vas 1 X 10V3ah agnin-t 4 x 10P obtained 11.v them.
PHOTOCHEMICAL REACTIONS IX SOLIDS
83 1
figure 34. The pon-der was placed as a smooth film about 0.5 mm. thick on a glass disc D, 1 in. in diameter. The whole system was rigidly mounted in position. The tube T containing the sample mas 12 in. in diameter and 8 in. long. It could be sealed at the top by an optically plane quartz plate, and a side arm permitted its evacuation. The tube was EO mounted that, when swung into position, it could be immersed in a cooling mixture, or in a thermostat at the desired temperature. A duplicate tube (TI),in which the position of the sample was occupied by a thermopile, could be swung into exactly the same position to check the constancy of incident light. The light source was a quartz mercury arc
Q “1 F
FIG. 3. Apparatus for exposure of samples and iiieasureiiient of reflection
of the high-pressure type, run in a vertical position and fed from a constant D.C. source. The light from this m s focussed by means of quartz lenses (L) on to a quartz 45” prism Tvhich reflected the light down the tube and on to the sample. The inner walls of the tube were coated ivith dead-black paint to avoid reflection. A cell containing a 1 per cent solution of cupric sulfate (plus a Koods glass filter in the case where the titanium dioxide-dye system \vas being bleached) was placed at F. In the normal procedure the samples were l o ~ e r e dto the bottom of the tube in a fitted holder and allowed to stand for 1 hr. to come to equilibrium temperature before exposure n-as made. All the exposures of antimony oxide n-ere carried out in vacuum to prevent condensation on the sample during long exposures at low temperatures. These conditions should not influence the experiment, it having been shon-n (3) that the darkening of antimony
832
C. F. GOODEVE AND M. R. TAYLOR
oxide is not affected by drying in vacuum for at least 24 hr. With dyed titanium dioxide, however, the bleaching is dependent upon the immediate presence of moisture and all the exposures were made under atmospheric pressure. The majority of the exposures with dyed titanium dioxide were made above O'C., but during a few exposures a t - 60°C. little condensation was apparent. The arrangement for comparing the amount of bleaching is shown in figure 313. The method is based on measurements of the diffuse reflecting power of the sample after exposure. The light source was a tungsten filament Point-0-Lite lamp, which was run from batteries and was focussed by a lens (L) to a parallel beam. A diaphragm allowed a small beam of light to fall on to the sample, which was placed at an angle of 45" to the light source. h copper oxide photocell (P) was placed close to the sample and at an angle of 45" t o it and 90" to the light source. The photocell was screened from any direct light from the lamp. The samples were mounted by simply gluing the glass disc to a stiff section of cardboard, and they could be clipped into position without disturbing the rest of the apparatus, which was rigidly fixed. The photocell was connected directly to a sensitive galvanometer and its deflection read directly from a scale. The reflections of the various samples were referred to an absolute standard,-namely, smoked magnesium oxide, which has been found t o have a constant reflecting power down to 254 mp. Its absolute reflecting poiver has been determined (8, 11) and has been taken as 97 per cent over the range measured in these experiments. RESULTS
The effect of temperature on the photochemical darkening of antimony oxide and the bleaching of dyed titanium oxide was investigated, using the procedure described above. The percentage reflertion changes of antimony oxide (H C! W) for a series of exposures at temperatures of 50", 0", -2O", -5O", and -78°C. are shown graphically in figure -1, as wcll as a few observations on samples (Pr) at 50°C. The curve obtained with (I'r) i i similar t o that of (€1 & W), but the former is much less sensitive. The results for the bleaching of the dyed titanium dioxide pon-der for a series of exposures a t 50", 30", lo", and 0°C. are sho\m in figure 5 . Some exposures mere made at -60°C. and showed no measurable bleaching after some hours. The preliminary experiments with the pon-der spread dong a bar had shon-n that there was negligible thermal bleaching up to 50°C. I n both the bleaching of dyed titanium oxide and the darkening of antimony oxide the percentage reflection approaches a constant value a t long exposures. The reaction rate, R , is taken as the slope of the curves in figures 4 and 5 in the units given, and the relation between log R and 1 T is plotted in figure 6. I n the case of the dyed titanium dioxide system, the relation between bleaching and time was, except for the very long exposure, effectively linear (figure 5 ) , R therefore being a constant for each temperature. With antimony oxide however, the relation was not linear; accordingly, R has been taken as the slope of the darkening curves at points corresponding to both 60 per cent and 70 per cent reflection, giving the other two curves shon-n in figure 6. It will be seen that all three curves
833
PHOTOCHEMICAL REACTIOSS IS SOLIDS
z W
I
4"Tt4E
I
I
60
120
I
I
160 OF EXPOSURE (Mi$
200
t
740
FIG.4. The effect of exposure a t different temperatures on the reflection of antimony oxide, referred t o R-hiteness of smoked magnesium oxide.
TIME O F E X POSURE 50
100
150
(MIN~
200
I
250
300
FIG.5 . The effect of exposure a t different temperatures on the reflection of dyed titanium dioxide, referred t o whiteness of smoked magnesium oxide.
are approximately linear, showing that the energy of activation is independent of the temperature. In the case of antimony oxide the parallelism of the two curves indicates that the energy of activation is independent of the percentage reaction
834
C. F. GOODEVE AXD 31. R. TAYLOR
over the range measured. The numerical values of the energy of activation as calculated from the slopes of these curves are 5.5 kg.-cal. per mole for the darkening of antimony oxide and 8.2 kg.-cal. for the bleaching of dyed titanium dioxide. A calculation from the results of Hilsch and Pohl, and of Padoa, give values of about 2, and 1-10, kg.-cal. for the energies of activation for the decomposition of KH in potassium bromide and of certain organic reactions, respectively. The temperature coefficients for the reactions studied here are higher than the coefficient for the photographic process. Results of various workers (1, 2, 9) are consistent and give a value of the energy of activation around 0.7 for the temperature interval from room temperature to the temperature of liquid air.
FIG.6. The effect of temperature on the photochemical reaction rate of antimony oxide and dyed titanium dioxide.
The temperature-dependent reaction in this case is considered to be the ionic conduction, follo\ying the quantum absorption and photoconduction processes. T H E EFFECT O F T H E ADDITIOS OF I J I P U R I T I E S UPON THE BLEACHIKG
O F DYED TITANIUM DIOXIDE
During the exposure of dyed titanium dioxide on a metal bar it was noted that traces of metallic salts had a very strong inhibiting effect upon the photochemical bleaching. Cohn (4) has pointed out that in most photochemical reactions of solids it is apparently necessary to have moisture present and that certain other agents tend to increase the reaction rate. The necessity of moisture is known
PHOTOCHEMICAL HEACTIOSS I S SOLIDS
835
for the photochemical reactions of zinc sulfide, silver salts, cadmium iodide, cadmium bromide, and antimony oxide, and these reactions are sensitive to the presence of impurities. Some qualitative experiments were made to investigate this effect as applied to dyed titanium dioxide. To a water suspension of the powder a small amount of the reagent was added and, after being allowed to dry, the sample was exposed with a blank and comparisons made. It was found that : (a) Bleaching was increased by the presence of mercurous chloride, lead sulfate, antimony chloride, thorium hydroxide, and barium hydroxide in ascending order of effectiveness. ( b ) Bleaching was inhibited by cadmium chloride, copper sulfate, magnesium sulfate, aluminum chloride, and zinc sulfate, in ascending order of effectiveness. (c) The addition of potabsium hydroxide and sodium hydroxide tended to remove the dye from the surface of the titanium dioxide, and appeared usually to increase the bleaching. The addition of sulfuric acid and hydrochloric acid inhibited the reaction slightly. (cl) In general, acidic materials tend to decrease and basic materials increase the bleaching, although the maximum effect was shown by metallic salts rather than acids or bases. (e) The effectiveness of the impurity in inhibiting or increasing bleaching increases as the concentration of the impurity is increased, although it was found that a concentration as low as four molecules of copper to one of dye ~ a sufficient s to keep the powder stable to 15 min. exposure, while a pure bample was completely bleached in 2 min. if) The effect of moisture on the bleaching was tested by exposing samples in a vacuum, in the presence of water vapor with air excluded, and in the presence of air and moisture. The sample exposed in a vacuum did not bleach but both of the other samples bleached readily. ( V I Thermal bleaching, which takes place above 100°C., wa? found t o be dependent upon the presence of moisture. YUR.ZMIART
It has been shown that the photochemical bleaching of a dye, Chlorazol Sky Blue FF, deposited upon the surface of titanium dioxide is greatly increased by an increase in temperature, the reaction having an activation energy of about 8.2 kg.-cal. The darkening of antimony oxide when exposed to light is also affected by an increase in temperature, the activation energy being about 5.5 kg.-cal. The energy of activation for both these systems is independent of the temperature over the range measured. The bleaching of the dye on titanium dioxide is considerably influenced by the presence of small quantities of metallic salts, aqd neither the photochemical nor the thermal bleaching n-ill proceed in the absence of moisture.
836
D. R. MAY B S D I. 11. KOLTHOFF
The authors are indebted to Professor A. J. Allmand of King's College, London, for the use of laboratory facilities during the war-time evacuation of University College. REFERENCES (1) BERG: Trans. Faraday Soc. 35, 445 (1939). (2) BERGAND MESDELSOHN: Proc. Roy. Soc. (London) 168, 168 (1938). (3) COHNAKD GOODEVE: Trans. Faraday Soc. 36, 433 (1940). (4) COHNA N D HEDVAL:J. Phys. Chem. 47, 603 (1943). (5) GOODEVE AKD KITCHEXER: Trans. Faraday Soc. 34, 570 (1938). (6) HILSCHAXD POHL:Trans. Faraday Soc. 34, 883 (1938). (7) PADOA: Atti accad. Lincei, 1909-1916. (8) PRESTOS:J. Optical Soc. Am. 31, 15 (1930). (9) SHEPPARD, WIGHT.IIAS, AND QUIRK:J. Phys. Chem. 38, 817 (1934). (10) STOBBE:Ber. 40, 3372 (1907). (11) TAYLOR:J. Optical Soc. Am. 24, 192 (1934).
STUDIES OS THE -4GISG O F PRECIPIT.'i'I'ES A S D COPRECIPI'Ta\TIOS. XI,
THESOLUBILITY
O F I,E.iD
CHHO\f.%TE .\-i .%
FLITTIOXO F
THE
PARTICLE SIZE' D. R. K i Y 2 AXD I. 11. KOLTHOFF School of Chemistry, U n i v e w i t y of Minnesota, Minneapolis 14, Minnesota
Received October 23, 2947 ISTRODUCTIOS
Willard Gibbs (10) in 1878 was the first to relate the particle size of a solid to its solubility. Ostwald (20) somewhat later derived an expression which was improved by Freundlich (8) and which is generally known asIthe OstwaldFreundlich equation. -According to this equation the soluhility of a solid is B function of its particle size:
where R is the gas constant, 1' the absolute temperature, S and S , the solubility of large solid particles and of particles having small radii T , and M, u , and d are the molecular weight, surface tension, and density of the solid. The Ostn-ald-Freimdlich equation doe5 not take into acoount the possible ionic From a thesis submitted by D. It. May to the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the degrw of Doctor of Philosophy July, 1944. * Present address .inierican ('yanainid Companr, Stamford. Cnnnrct irut.
