3290
Inorg. Chem. 1985, 24, 3290-3297
iodosomethane, while the remainder may rearrange or dissociate. No evidence was found for further reaction of the O2byproduct. During the second period of irradiation, this time at 290-420 nm, the iodomethane-ozone molecular complex is already depleted and iodosomethane is photodecomposed. Since neither uncomplexed ozone nor iodomethane is affected by this radiation, iodosomethane must be a common precursor to all the photoproducts. Excited iodosomethane may eliminate H I directly or rearrange to methyl iodate or iodomethanol, which can eliminate H I (Scheme 11). While these experiments do not provide incontrovertible evidence for the detection of methyl hypoiodite and iodomethanol compounds in the photodecomposition of iodosomethane, several absorptions have been observed in the relevant "fingerprint" regions of their infrared spectra. Either could also be a suitable precursor to formaldehyde and hydrogen iodide, and with regard to the formation of two complexed formaldehyde species on photolysis, it may be significant that methyl hypoiodite would give a formaldehyde-hydrogen iodide molecular pair in the wrong orientation for hydrogen bonding to occur. When the matrix is warmed, rotation of H I could occur, and as the formaldehyde and hydrogen iodide are formed in such close proximity, it is hardly surprising that all of the less stable formaldehyde complex is depleted. While the occurrence of two different H I and formaldehyde complexes after photolysis is not, of course, proof that methyl hypoiodite and iodomethanol are acting as precursors to differently oriented molecular pairs (either precursor could produce excited hydrogen iodide with enough energy to rotate into or away from the correct orientation for hydrogen bonding to occur), the possibility exists. Work on chemical laserz0 systems provides supporting evidence that iodomethanol might be expected to yield formaldehyde; Lin proposed the reaction of 'D oxygen atoms with chloromethanes Cl,CH, to give chloromethanols, C14_,CH,10H (n = 1-3), which serve as precursors for hydrogen chloride and chlorinated formaldehyde H,C12-,C0 ( x = 0-2). Furthermore, elimination of hydrogen iodide from iodomethanol places the acid hydrogen adjacent to the carbonyl oxygen and facilitates formation of the hydrogen-bonded complex 5. Finally, the above photochemical pathway has distinct analogies to that reported for C H 3 N 0 2and CH,ONO, which photolyze to give the H,CO--HNO complex.21,22 (20) Lin, M. C. J . Phys. Chem. 1972, 76, 811. (21) Jacox, M. E.; Rook, F. L. J . Phys. Chem., 1982, 86, 2899.
The small yield of CH3IOZcan be accounted for by a similar mechanism to that proposed for formation of C1102,swhich involves further reaction between iodosomethane and oxygen atoms released by decomposition of excited iodosomethane. Conclusions Codeposition of iodomethane and ozone in argon matrices leads to the formation of a molecular complex between iodomethane and ozone, which exhibited a 395-nm absorption band and photodissociated with 360-470-nm mercury-arc radiation presumably involving a charge-transfer mechanism. The iodosomethane photoproduct, CH310, is probably formed initially with excess internal energy; this excited intermediate species is either quenched by the matrix or it rearranges to iodomethanol (ICH,OH) or methyl hypoiodite (CH,OI) or it eliminates HI. The infrared spectrum of iodosomethane is closely related to that of the parent iodomethane, and the 1-0 bond is weaker than that found in iodosyl chloride. Photodecomposition of the quenched iodosomethane can be initiated by irradiation at 290-420 nm. The yields of both iodomethanol and methyl hypoiodite are thereby increased as are those of two formaldehyde-hydrogen iodide molecular complexes. Both iodomethanol and methyl hypoiodite can also serve as precursors to the photoproducts involving formaldehyde, the latter formed by elimination of hydrogen iodide from the precursors. Warming the matrix leads to a reduction in several photoproducts; however, the formaldehyde-HI hydrogen-bonded complex increased at the expense of the other formaldehyde-HI complex. Indeed depletion of the latter is complete under these conditions and is a consequence of the proximity of the hydrogen iodide molecule on photodecomposition of the precursors. The infrared spectrum of the complex can be measured without interference from that of either parent and is consistent with a hydrogen-bonded structure. Acknowledgment. The authors gratefully acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research and S. R. Davis for preparing the figures. Registry NO.CHpI, 74-88-4; 03,10028-15-6;" 0 9 , 21424-26-0;CDpI, 865-50-9;CH,IO, 97551-34-3; ICHzOH, 50398-30-6; CH301, 2646604-6; HI, 10034-85-2;CH20, 50-00-0; Ar, 7440-37-1. (22) Muller, R. P.; Huber,J. R. J . Phys. Chem. 1983, 87, 2460.
Contribution from the Department of Chemistry and Institute of Colloid and Surface Science, Clarkson University, Potsdam, New York 13676
Thermodynamics and Kinetics of Aqueous Ferric Phosphate Complex Formation7 RICHARD B. WILHELMY,' RAMESH C. PATEL,* and EGON MATIJEVICI Received September 15, 1983
The equilibria and kinetics of complexation of iron(II1) with phosphoric acid (at pH + FezHP044t FeZ(0H)P0?+ Fez(0H)HP043* FezH3(PO4)?+ Fe3H6(PO4),'+
ref 12, 15 4, 15, 21 7, 11 11, 15, 16, 21 21 20 20
species could be found. Recently, ferric phosphate hydrosols of narrow size distribution were generated by aging solutions containing ferric and phosphate ions at 40-50 0C.23In order to clarify which complexes play a role in the formation of the solid phase, UV/visible and stopped-flow spectroscopies were employed to identify the species and to determine the relevant thermodynamic and kinetic constants. This paper presents the results of this investigation under experimental conditions similar to those leading to hydrosol formation.
z
g
0.10
200
300
400
500
WAVELENGTH (nm) Figure 1. Change in absorbance as a function of the wavelength of a M in Fe(C104),, 2.9 X M in H3P04,0.20 M in solution 8.0 X HCIO,, and 2.5 M in NaC10, at 25 OC measured against a reference cell containing a solution 8.0 X M in Fe(CIO,),, 0.20 M in HCIO,, and 2.5 M in NaC104.
Experimental Section Materials. Ferric perchlorate stock solutions were prepared by dissolving the anhydrous salt (G. F. Smith; anhydrous, nonyellow reagent) in doubly distilled water and by passing the resulting solution through 0.20-pm pore size Nuclepore membranes before use. These stock solutions were found to be stable over extended periods of time either in concentrations greater than 1 M or in the presence of a tenfold excess of perchloric acid. The solutions were analyzed spectrophotometrically as the iron-a,d-dipyridyl complex.z4 Perchloric acid (G. F. Smith; doubly distilled) and phosphoric acid (Baker) were used as supplied. The acid content of the stock solutions was determined by pH titrations with high-punty carbonate-free NaOH. Phosphate ion concentration was analyzed colorimetrically as an antimonyl phosphomolybdenum blue complex.2s Solutions of sodium perchlorate (C. F. Smith; anhydrous reagent) were prepared with doubly distilled water and filtered by using a 0.20-pm pore size membrane. Sample Preparation. The components were added in the following order: ferric perchlorate or phosphoric acid solution, HC104, NaC104, and water. The mixture was allowed to equilibrate overnight at room temperature prior to any measurements. Solutions used in the spectroscopic study were mixed in the stopped-flow apparatus. The ranges of to 8.0 X 10-4 M in Fe(CIO4),, concentrations investigated were 8.0 X 0 to 2.8 X M in H3P04, and 0.030 to 0.20 M in HCIO,. The concentration of NaC10, was maintained at 2.5 M in order to assure a constant ionic strength. Spectrophotometric Measurements. All measurements were made on a Perkin-Elmer Model 553A UV/visible double-beam recording spectrophotometer. The temperature of the cell holder was controlled to kO.1 OC by a Haake Model FS circulating water bath. Data were taken at 25 and 50 OC. The samples, contained in matched 1.0-cm or 1.0-mm stoppered quartz cells, were allowed to equilibrate for at least 15 min in
Filatova, L. N. Russ. J. Inorg. Chem. (Engl. Transl.) 1974,19, 1827. Kim, M. S.; Kim, C. H.; Sohn, Y. S. Taehan Hwahakhoe Chi 1975, 19, 325. Filatova, L. N.; Shelyakina, M. A.; Plachinoda, A. S.; Makarov, E. F. Russ. J. Inorg. Chem. (Engl. Transl.) 1976, 21, 1494. Salmon. J. E. J. Chem. SOC.1952. 2316. Salmon; J. E. J. Chem. SOC.1953; 2644. Holroyd, A,; Salmon, J. E. J. Chem. SOC.1956, 269. Jameson, R. F.; Salmon, J. E. J. Chem. SOC.1954, 28. Holrovd. A.: Salmon. J. E. J. Chem. SOC.1957. 959 Filato;a,.L. N.; Shelyakina, M. A. R w s . J. Phys. Chem. (Engl. Trawl.)
-.
1974., 48. 1700. - --
Filatova, L. N.; Galochkina, G. V.Russ. J. I t w g . Chem. (Engl. Trawl.) 1974, 19, 1675; 1978, 23, 369. Jensen, K. Z . Anorg. Allg. Chem. 1934, 1 , 221. Faucherre, J.; Msika, D. Bull. SOC.Chim. Fr. 1962, 1824. Ciavatta, L. Ann. Chim. (Rome) 1974, 64, 667. Holroyd, A,; Jameson, R. F.; Odell, A. L.; Salmon, J. E. J. Chem. Soc. 1957, 3239. Ram, A,; Bose, A. K.; Kumar, S. J. Sci. Ind. Res. 1954, 133, 217. Wilhelmy, R. B.; MatijeviE, E., submitted for publication in Colloids
w
0
z
3
1.2
a 0
v)
3
0.8
E
w (3
0.4
U I
0
0 0
40
tH,PO,'I
80
Figure 2. Change in optical absorbance at 272 nm and 25 OC as a function of the initial phosphate ion concentration to ferric ion ratio. The four curves are for different ferric ion concentrations. The pH of the samples varied between 0.7 and 1.5. The solid lines represent the predicted values from a computer modeling program for the species FeHzPO?+.
the spectrophotometer before recording absorbances. The spectra of dilute ferric salt solutions in the absence of phosphate ions were obtained by using water in the reference cell. In all cases in which the sample contained ferric and phosphate ions, the reference solution was comprised of a similar concentration of ferric perchlorate, perchloric acid, and sodium perchlorate. Since phosphoric acid alone did not absorb light under the experimental conditions studied, the measured absorbance difference (AA)was due to the formation of the ferric phosphate complex(es). Stopped-Flow Measurements. An improved version of a combined stopped-flow temperature-jump apparatus already described in detail was used to obtain thermodynamic and kinetic parameters of the ferric phosphate complexation reactions.
Results Equilibrium Studies. Absorbance measurements were used to determine the stability constants of complexes of ferric ions with phosphoric acid. A clearly discernible change in absorbance (as determined in a Perkin-Elmer spectrophotometer) due to the presence of the phosphate ion in aqueous solutions containing the ferric ion is shown in Figure 1. The maximum change in absorbance occurred a t 278 nm and was a function of iron(III), phosphate, and hydrogen ion concentrations. Even though the experiments were performed at pH
