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Kinetics of Photoreduction of Ferri- Oxalat E - Deactivation and Inner Filter Caused by Ammoniummolybdate*

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Kinetics of Photoreduction of Ferri- Oxalat E - Deactivation and Inner Filter Caused by Ammoniummolybdate* @@ KINETICS OF PHOTOREDUCTION OF FERRI- OXALAT E - DEACTIVATION AND INNER FILTER CAUSED BY AMMONIUMMOLYBDATE* - Tadashi SHIROTSUKA and Hideo NISHIUMI Department of Applied Chemistry, Waseda University, Tokyo, Japan The kinetics of photoreduction of potassium ferrioxalate in the presence of ammoni- ummolybdate was studied at 365 6m//. Measurementswere madein a batchwise and parallel-beam reactor operated at atmospheric pressure and at room temperature. The concentrations of potassium ferrioxalate and ammoniummolybdate were changed from 6.17X10 8 to 3.45xlO"6 and from 2.94xlO"8 to 5.07X10"8 mol/cm3, respective- ly. The decrease in the reaction rate can be explained by the mechanism in which addi- tion of ammoniummolybdate causes both deactivation and inner filter. The ratio of the rate constant of deactivation, ko, to that of the forward reaction, k±9 was 12.5 at 365/6m//. The effect of the inner filter accompanied by addition of a substance on conversion is negligible when the optical thickness of all absorbing substances, r(c+//*), is very much smaller than 1. Undesirable wavelengths causing useless photo- that the addition of ammoniummolybdate caused a chemical side reactions or opaque tar from poly- decrease in the quantum efficiencies, which were en- chromatic light of a light source can be isolated by tirely dependent on the concentrations of both am- several methods: filter solution7), filter glass8), or a moniummolybdate and the reactant. fluorescent substance mixed in cooling water6). But The objectives of this paper are to present data on these are not the best industrial methods, because they the photoreduction of potassium ferrioxalate in the reduce not only undesirable radiation, but also a sig- presence ofammoniummolybdateand to obtain a rate nificant amount of effective radiation for a photo- equation based on reaction mechanismin order to chemical reaction. investigate the general characteristics of deactivation We are interested in a new method which reduces and inner filter. undesirable radiation by adding a substance causing Measurementswere madein a batchwise and deactivation or inner filter to a photochemical re- parallel-beam reactor operated at atmospheric pressure actant solution. As the first step to start the research we and at room temperature at 365/6 rn.fi. The concen- chose the photoreduction of potassium ferrioxalate to trations of potassium ferrioxalate and ammonium study the general characteristics of deactivation and molybdate were changed from 6.17x lO"8 to 3.45X inner filter. 10~6 and from 2.94xlO~8 to 5.07xl0"6 mol/crn*, It is well knownthat the rate of the photoreduction respectively. The analysis showed that the decrease in of potassium ferrioxalate in 0.1-N sulfuric acid is pro- reaction rate by addition of a substance was attri- portional to the absorption rate of radiation by the buted to both deactivation and inner filter. reactant, and that its quantum efficiencies of ferrous ion formation are independent of the concentrations of Deactivation and Inner Filter the reactant over a wide range of concentration1'^. On the other hand, our experimental results showed The mechanismfor the ferrioxalate decomposition in the absence of any added substance has not been Received on March 6, 1972 clarified4), but it is known that the ferric ion is re- Presented at the 36th Annual Meeting of the Soc. of Chem. Engrs., Japan, April 2, 1971. duced stoichiometrically to ferrous ion according to the T980 {ii]-&ffjnfiiLiMiSc39-i nut overall equation @@ 178 @@ JOURNALOF CHEMICALENGINEERING OFJAPAN @@ 2K3[Fe(C,O4)3] -> 2Fe(C2O4) + 3KaC2O4 + 2GO2 As stated above, the quantum efficiencies of ferrous ion formation by the photoreduction in the absence of @@Io C*Då Tl ammoniummolybdate are not only independent of the concentrations of the reactant but are also over unity1'3). On the other hand, its quantum efficiencies in the presence of ammoniummolybdate depend on the concentrations of both the added substance and the reactant. à" Wepropose the following mechanismconsistent with the experimental data for the decomposition of po- @@/: Path length, S: Incident cross-section area tassium ferrioxalate in the presence of ammonium V: Volumeof the reaction mixture Iq : Incident intensity of a parallel beam of radiation molybdate, although not as a complete description of @@Fig.1 Coordinate system and geometry the reaction. @@ A - Consider a completely mixed and batchwise photo- A*+A chemical cell with flat paralle windowstransparent to the wavelength regions concerned. This situation is %A* i%A+B* (1)(2)(3) A*+B- depicted in Fig. 1, where /, S, and Vdenote the path where A, A*, B, B*, and C are the ferric reactant, the length, the incident cross section area, and the volume excited ferric ion, the added substance, the excited of the reaction mixture, respectively. A parallel beam added substance, and the ferrous product, respectively. of radiation of incident intensity 70 propagates normal Eqs.(l), (2), and (3) represent excitation of the re- to the plane of the windows. It is necessary to empha- actant A, reaction of one excited ion with one reactant size the absorption coefficient as a function of concen- molecule to give two molecules of the ferrous product, tration because of the change in concentration through and deactivation of an excited ion by one molecule of a reaction. In the present paper we call it an ab- the added substance, respectively. k± and k2 are rate sorption function ^ of an absorbing species i. Assuming constants for the corresponding reactions. Beer's law for components A and B, we have The rate equations for components A, A*, and C are as follows : @@ ftA = SA-l-A] (9) (IB(10) = SB-[B] @@dt= - $Q.a -d[A]kx[A*].[A] + k,[A*].[B\_ where ^A and pB represent absorption functions of the componentsAand B, respectively. eAand sB are their d[A*] _ respective molar absorption coefficients. The ab- ~dt sorption rate of radiation energy of the reactant Ain the irradiated part of the cell is given by5) d[C] _ VA *Q.A2k1 -ki[4*WllA*].[A\ (4)-W-[B\ (5)(6) dt @@dA= P-A + f*B à"[1 - exp (- {pA + /tB).l)] where [A], [A*], [B~\9 and [C] are concentrations of (ll) respective components. 0 is the primary quantum ef- where it has been assumed that the absorption of radi- ficiency in Eq.(l). QArepresents the absorption rate ation energy by a mixture is additive. of radiation energy by the reactant A. Substituting Eq.(ll) into Eq.(8), and taking into Assuminga stationary state concentration for A*and account the effect of the shaded part on the reaction putting Eq.(5) =0, we have cell, we find @@ [A*] @@ dt dt V 9o' k^A] + k2[B] (7) k,[A] + kt[B] X ^ [1-exp(-(^+^)/)] Substitution ofEq.(7) into Eqs.(4) and (6) gives (12) [^]=[A]a, [C]=0, when ^=0 d[A] kx[A\kt[A]+ k2[B] @@dtå Qa (8) d[C]_ where [A], [B] and [C] are their respective concen- where 0O=20. By putting [B]=0 in Eq.(8), we trations to be measured. The effects of both inner filter obtain the quantum efficiency in the absence of an and deactivation by the added substance are involved added substance, 0O, whose value is 1.21 mol/ein at in this equation. 365/6 m/i1'^. The effect of deactivation by the added species on the reaction rate is represented in Eq.(8). Experimental A suitable expression for the material balance involving the inner filter caused by the added substance as well as A lamp, a housing, a filter system and a reactor were deactivation is developed below. placed in line. The ultraviolet light source was a Ushio @@VOL6 NO.2 1973 @@ 179 @@Table 1 Comparison of centrifuged sample @@ Components Volume [cm3] Sample a 0.1-N sulfuric acid 5.00-a 0.8-N lead acetate 1.50 0.1 % 1, 10-phenanthroline* 1,00 Buffer solution* * others Total 10.00 * Added after precipitants by lead acetate formed. ** Mixed a 100m/ of cone sulfuric acid with a 822g of sodium @@fy"in:: ( C® ) ) acetate and diluted to 10/ @@Table 2 Determination of total iron concentration 1. Reactantsolution 2. Window 3. Stirrer @@ Components Volume[cm3] clear liquid ft 4. Incident light 5. Irradiated section 0. 1 %-phenanthroline 1.00 @@Fig. 2 Schematic diagram of reaction chamber 1 % -hydroquinone 1.00 pure water others USH-500Dhigh-pressure mercury 500wpoint-source Total 10.00 d-c lamp. The parallel beam housing for the lamp was of type Ushio UI-501C. The filter system composed of a Corning glass color filter CS7-51 and a 1-inch It is convenient to use the following variables to quartz filter cell containing 0.2M nickel sulfate aque- obtain the rate constant ratio k^\kx from the experi- ous solution was used to isolate a 365/6 m^ beam from mental data : the spectrum of the lamp. The schematic diagram of the reaction chamber is @@y V.(d[C]ldt) pA+pB L J shownin Fig. 2. The reactor tube was madeofacrylic (13) plastic, 25 mm.i.d., 94 mm.o.d. with optical path Y=y-±- (14) lengths of 1 and 5 cm. The reactor windowswere made 00 of quartz. The stirrer was driven at moderate speed by Furthermore, an electric mixer so as to achieve good mixing. @@ (15) Experiments were made in a dark room. The ferrio- xalate solutions with ammoniummolybdate in 0. 1-N which is proportional to [B~\j[A\ in accordance with sulfuric acid were irradiated at 365/6 m^on an optical Beer's law. And Kis defined as @@ jy- bench with periodic mixing during the photolysis. 1 k^A The concentrations of the ferrous ion produced by (16) the ultraviolet light ^?er§ measured with a spectro- where eA and eB represent respective molar ab- photometer in a complex compoundwith 1,10- sorption coefficients.
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