@@ 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. With these variables Eq. (12) can phenanthroline, whichwas added to the solution after be rearranged to give the equation irradiation. But we found that 1,10-phenanthroline @@r=k-x(17) gave not only a complex compoundwith ferrous ion, If the kinetics model explains the experimental data, but also precipitates with ammoniummolybdate, a plot of Yversus x will give a straight line through the which madequantitative analysis impossible. In order origin. to eliminate molybdicion an excess of lead acetate was added to the irradiated solution and we obtained the Conversions were so low that concentration did not precipitates of both lead molybdate and lead sulfate. change significantly. This allowed the reaction rate to The further addition of 1,10-phenanthroline gave an be given by the simple formula orange color with ferric ion only. When the samples @@d[C]/dt = J[C]/Jt tabulated in Table 1 were centrifuged, the precipitates where z/[C] is the concentration of ferrous iron formed were collected in the bottom of the centrifuge tube. and At is irradiated time. The values of Y and x9 respec- The concentrations of the ferrous iron in the clear tively, calculated from the experimental results using liquids were measured in a spectrophotometer at Eqs.(14) and (15) are given in Table 3. The rate of 5 10 m^. The concentrations of the total iron in the clear incident radiation energy S-Io was measured with an liquids were determined by a similar method after re- ac-tionometer using the reaction in this paper at high duction, noted in Table 2. Conversion was given as the concentration of ferric iron. The ultraviolet spectra concentration ratio of the ferrous iron to the total iron were recorded efte a Shimadzuspectrophotometer, type in one sample. UV200, and the absorption functions of potassium ferrioxalate and ammoniummolybdate at 365/6 m^ Results and Discussion were found to obey Beer's law: @@ fiA= 1.45 X W.[A] HB= 1.70 x 105.[£] (18) Evaluation of rate constant ratio (19)
@@180 @@ JOURNALOF CHEMICAL ENGINEERING OFJAPAN @@Table 3 Experimental results for evaluation of rate constant
@@No*Run [cm]/ V[cm3] S-IoXlO*[ein/sec] [cm"!]fxB [cm"!]pa ~^~X1012[cml/sec.cm3] [-] x [ein/mol]y
1 1.13 5.00 2.81 0.860 0.860 7.20 1.00 32.7 2 2.81 0.335 0.730 14.8 0.458 17.3 5 2.06 0.0100 0.190 28.0 0.0528 1.98 6 2.06 0.0100 0.470 100.0 0.0213 0.86 7 2.06 0.00500 0.190 38.0 0.0263 1.26 8 2.06 0.00500 0.470 66.0 0.0106 1.74 9 2.06 0.0100 0.470 64.0 0.02 1 3 1.80 10 2.06 0.00670 0.470 62.0 0.0142 1.90 1 1 2.06 0.00500 5.58 40.8 0.000895 0. 1 7 12 2.06 0.0100 1.86 26.0 0.00538 0.56 13 2.06 0.0100 0.93 12.3 0.0108 0.81 14 2.06 0.00500 1.86 32.2 0.00269 0.29 29 5.00 25.0 2.22 0.800 0. 100 0.0283 8.00 345 31 2.22 0.800 0.800 0.607 1.00 72.5 32 2.22 0.800 1.50 1.10 0.532 52.0 33 2.22 0.800 3.00 2.58 0.267 25.4 30 2.22 0.800 5.00 4.00 0.160 18.3
@@where sa =.1.45 X 106 cm3/mol, sb = 1.70 X 105 cm^/mol, ^50 = 1.21 mol/einat365m^3)
@ 8\ Eq.(22)/_ \ ^^T^-^.\\ -l\ @ \ / \Eqs.(22) \\ W(2«> \ Eq.(23) \ &(23) \ ]\? \/ ? 6 \ >v \Eqs.(22>> L @@ E ^ \ ^V V8,(23) \ I.0 2- \Eq.(24).à" Eq.(24) - ^(24\^ V "
OQtf O / O (a) X. (b) (c) (d)\ o I-<-'-å >-»"-'-'-'-'-'---J*-'-'-'-'-M-'-I-:J 0 3 0 3 6 0 3 0 3
e (-)
, -2 .rt-1 10'3 10 10 1 10 (a) (b) (c) (d) x (") 7] 9.53 0.256 0.136 0.000 t 0.97 0.21 1 0.527 1.87 @@ Fig. 3 Effect of deactivation caused by ammoniummolybdate @@Fig. 4 Comparison of experimental with calculated values @@p* =M. PA _ eB £a = e-rj. Mo @@Fig. 3 shows a plot of Y versus x on logarithmic A= paper. The experimental data fit a linear relationship, allows Eq.(12), taking into account inner filter as well and the slope of the straight line is unity. Hence a plot as deactivation, to be written as of Y versus x on ordinary paper gives a straight line
through the origin. From the intercept ofy at x=l in 1 +hrjfc 1 +e-rjfc Fig. 3 the value ofKcan be obtained; @@dc_X (22) [1 -exp(- t-(c+£.97))] @@K=88.0 ein/mol Eq.(22) showsWthat the relation between ~c and 0 can be Eq.(16) can be rewritten as clarified with11parameters e, k, rj, and r. @@ (20) Putting £=0 in Eq.(22), we obtain W=-T+W'[1"eXP(~r")] (23) Substituting numerical values for the quantities K> 0O, sB, and eA mto Eq.(20), we have which represents the effect of deactivation only. Furthermore, setting yj=O in Eq.(22) in the ab-
@@=12.5 (21) *1 sence of any added substance, we get Comparison of deactivation with inner filter @@dc On the assumption that absorption functions obey = - [1(24)-exp(-T--c)] Beer's law, use of dimensionless parameters The relation between c and d calculated from the @@ C := experimental data are shown in Table 4. Both the [A] __ t.
@@ VOL6 NO.2 1973 @@ 181 @@Run @@Table 4 Experimental results for c vs. 0 curves* No.M»x107 [nlol/m/][B]X107 [mol/m/] ^=[fl/Mo r [-] [-]
105.32 1.15
2.8 50.7
0.294
0.392 9.53
0.256
0.136 0.97
0.211
0.527 2.30 3.84 0.587 0.782 0.978 1.96 5.87 0.360
0.980 0.951 0.4320.503 0.957 0.911 4.32 0.900 0.892 0.867 0.928 0.908 0.869 0.646
0.000 0.000 1.87 10.20.707 0.502 1.46 0.143
e=(U'17 for Eq.(22) and s=0 for Eq.(23); £=12.5
@@ (a) ' /' (b) ' ' """ @@ fl^s^ Eq.( 27) 2.0" Eq.(25) /\ /o © Eq.(25) / /o
? 6 1*U ^N. ^=0.05 ^^
0.5 X ^ X CB""° V^ CA=0 v^ 0K . 1 »- JIZ- 1 1 1 l_ 0 50 100 0 5 10 CB (ppm) Ca^107 (mol/cm3)
@@Fig. 5 Comparison of experimental and calculated 0 3 6 9 12 values of absorption functions e (-) cause the close agreement between the solutions of Fig. 6 Effect of optical thickness on inner filter Eq.(22) and those of Eq.(23) is recognized. Therefore, the differences in the solutions are attributed to de- 10~7 mol/cm3, and in Fig. 5(b) the concentrations of activation. The values of 37, which are related to the A are changed when [2?]=50.0 ppm. In this case, the magnitude of deactivation, decrease from Fig. 4(a) to observed values of mixtures agreed approximately with Fig. 4(d) in turn. Comparison between these figures the theoretical values represented by the dotted lines in shows that the deactivation has a large effect on con- Fig. 5. Generally speaking, it is necessary to test versions for the reaction system. On the other hand, whether an absorption function ofa mixture is additive the calculated values from Eq.(22) agree well with the ornot. observed values represented by open circles in Fig. 4 If the absorption of radiation by a mixture is ad- within a maximumerror of0.05 in conversion. ditive, the ratio of radiation energy absorbed by a Absorption of radiation by a mixture componentA to that by a mixture is given by Fig. 5 shows the absorption functions of potassium @@ t*A ferrioxalate A, ammoniummolybdate B, and their Pa -r f*B mixtures in 0.1N sulfuric acid recorded on a Shimadzu (26) spectrophotometer, type UV200, at 365/6 m^. ptA and fiB, which are represented by solid lines, are found to Effect of Inner Filter obey Beer's law as expressed in Eq.(18) and Eq.(19), respectively. Setting k=0 in Eq.(22)? and considering ju^=S'7j, we Assuming that the absorption of radiation is additive, have we have @@ dc__ 1 Pa+b = ffA + Pb (25) where [iA+B is an absorption function of a mixture [l-e (27)c=1 .-r(C+^)l I.C. 0=0, composed of A and B whose respective absorption functions are jua and ^tB. Open circles in Fig. 5(a) This is the equation in the presence of inner filter only. indicate observed values for a mixture in which the Eq.(24) in the absence of inner filter can be solved concentrations of B are varied when [A]=5.00X analytically as follows :
@@ 182 @@ JOURNALOF CHEMICALENGINEERING OFJAPAN @@e=-ln[l + (f- !).«-'à"«] (28) addition of ammoniummolybdate causes both deacti- vation and inner filter. From Eq.(28) c approaches 1 -0 when T becomes infi- The ratio of the rate constant of deactivation k2 to nite. that of the forward reaction k± was 12.5 at 365/6 m^. Fig. 6 shows the comparison of the calculated values The effect of the inner filter accompanied by ad- in the absence of inner filter [Eq.(28) or (24)] with dition ofa substance on conversion is negligible when those in Eq.(27) when /*#=1. From Fig. 6 the inner the optical thickness of the absorbing reactant, filter by an added substance can be seen to be neglected r(£+/**) is much smaller than 1. within a maximumerror of 0.05 conversion when r^0.2. In our previous paper5) we have mathemati- @@Nomenclature cally proved that the solution of Eq.(24) is identical with that ofEq.(27) when rf+^X1* = M/Mo [-] = incident intensity of radiation [ein/cm2 -sec] The calculated results indicating that addition of an = rate constant represented in Eq.(2) [cm3/sec-mol] absorbing species little affects the rate of the photo- = rate constant represented in Eq.(3) [cm3/sec-mol] chemical reaction seem strange. The reason lies in two = *2/*l [-] = value defind in Eq.(16) [ein/mol] opposing effects compensating each other; one is the = path length [cm] decrease in available radiation energy of A by ab- = absorption rate of radiation energy [ein/cm3 -sec] sorption ofB and the other is the increase in the total = incident cross-section area [cm2] radiation energy absorbed by a mixture. The calcu- = irradiated time [sec] = volume of the reaction mixture [cm3] lated results indicate that the effect of the former is of = value denned in Eq.(15) [-] the same degree as that of the latter. y = value denned in Eq.(13) [ein/mol] Similar results can be obtained not only in the case Y = value denned in Eq.(17) [ein/mol] ofk=0 as stated above, but also in the case of other sa - molar absorption coefficient of a reactant A values of A:. Therefore, the reason for the small effects sb = molar absorption coefficient of an [cm2added /mol] of inner filter in Fig. 4 lies in the small values of species B [cm2/mol] £ = Sb/sA [-] v = [B]I[A]O [-] Recently, newtransparent paints utilizing photo- o __ t , overall quantum efficiency in the absence of an added species [mol/ein] paint are 0. 1 mol//, 50^, respectively, the optical thick- < Subscripts> ness of the initiator T=(23.0)(0.1)(50x 10"4)=0.0115. = reactant Then, even if the absorption function of impurities, ad- = excited ion of reactant A ditives or alkyd resin is 10 times as big as that of the = added species = product initiator, r(c+fi+)^(0.0115)(1+10)=0.127, which = initial value is much less than 1.0. So, according to our discussion, = concentration wecan foretell that absorption by substances other than the initiator have little effect on the photopoly- Literature Cited merization reaction rate. 1) Hatchard, G. G. and C. A. Parker: Proc. Roy. Soc. (London), A235, 518 (1956) 2) Murata, K. : Shyokuzai-Kyokai-shi, 44, 70 (1971) Conclusions 3) Parker, C. A.: Proc. Roy. Sue.,(i«Qpdon), A220, 104 (1953) 4) Parker, G.A. and C. G. Hatchard: /. Phys. Chem., 63, 22 (1959) 5) Shirotsuka, T. and H. Nishiumi: Bull. Sci. Eng. Res. Lab., The kinetics of photoreduction of potassium ferri- Waseda Univ. 54, 43 (1972) oxalate in the presence of ammoniummolybdate was 6) Tokkyo Koho, Showa 39-10336 studied in batch reactors. The decrease in the reaction 7) Tokkyo Koho, Showa 39-17917 rate can be explained bv the mechanismin which 8) Tokkyo Koho, Showa 39-22959 @@VOL6 NO.2 1973 @@ 183