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Mechanism of Formation of Hydrogen Chromatophosphate(2-) Sven-Ake Frennesson, James K. Beattie, and G. P. Haight, J r . Contribution f r o m the Department of Chemistry and Chemical Engineering, Unicersity of Illinois, Urbana, Illinois 61801. Received M a y 17, 1968 Abstract: T h e kinetics of the formation of [H03P-O-Cr03]2- from HCr04- a n d HzP04- were examined by the spectrophotometric stopped-flow method at 25” in aqueous solution of ionic strength 3.0 M ( N a C 1 0 4 ) in the p H range 3.5-6.5. A spectrophotometric study of the equilibrium was also carried out ( K = 6 f 2 M - l ) . Similar condensations have been proposed earlier as a first step in many Cr(V1) oxidations. A postulated reaction mechanism involves a five-coordinated intermediate, [H203P-O-Cr04H]2-, to which the steady-state approximation can b e applied. T h e transient complex is proposed to lose water by the catalytic action of H30+, H3P04, a n d HzP04-. General acid-base catalysis is confirmed by rate studies with added buffers. An alternative mechanism, involving a direct formation of the product, is ruled out on the basis of the kinetic data.
he system of dichromate ion and mononuclear chromium(V1) species has been investigated extensively both by thermodynamic and kinetic methods. Several related reactions, involving dinuclear oxo or hydroxo species, have been reported recently. Spectrophotometric studies have been performed on equilibria involving complex formation between HCr04- and other oxoanions or Evidently, similar condensations occur between Cr(V1) and hydrated metal ions.6 Complexes between molybdate and oxo ions have also been proposed.’ The formation of such simple heteropolyanions has been postulated as a first step in several chromium(V1) However, oxidations of alcohol^^^^ and oxo ions. these condensations have not been previously studied kinetically without the complication of succeeding redox processes. Therefore, this investigation was undertaken with the aim of obtaining information, by the stopped-flow technique, on the mechanism of the reaction
T
HCr04-
+ H2P04-I_ [HO3P-0-CrO3I2- + HzO
(1)
The most reliable measurements for the reaction given by eq 1 may be made around pH 4,4n13where H2P04is the predominant P(V) species and HCr04- the predominant mononuclear chromium(V1) species. Experimental Section Chemicals. The source of chromium(V1) was potassium dichromate, analytical grade. Sodium perchlorate, sodium acetate, and
( I ) Presented at the 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1968. Supported by Public Health Service Research Grant No. 14348 from the National Institute of General Medical Sciences. (2) R . H. Holyer and H . W. Baldwin, Can. J . Chem., 45, 413 (1967), and earlier references cited therein. (3) N . Sutin, Ann. Reo. Phys. Chem., 17, 160 (1966). (4) F. Holloway, J. Amer. Chem. Soc., 74, 224 (1952). ( 5 ) G . P. Haight, Jr., D. C. Richardson, and N. H. Coburn, Inorg. Chem., 3, 1777 (1964). (6) J. C. Sullivan and J. E. French, ibid., 3, 832 (1964). (7) G. P. Haight, Jr., and W. F. Sager, J . Amer. Chem. Soc., 74, 6056 ( 1952). (8) F . H. Westheimer, Chem. Rec., 45, 419 (1949). (9) R . Stewart, “Oxidation Mechanisms: Applications to Organic Chemistr),” W. A. Benjamin, Inc., New York, N. Y., 1964, p 37. (10) 3. G . Mason and A. D. Kowalak, Inorg. Chem., 3, 1248 (1964). (1 1) G . P. Haight, Jr., E. Perchonock, F. Emmenegger, and G . Gordon, J . Amer. Chem. Soc., 81, 3835 (1965). (12) G. P. Haight, Jr., M. Rose, and J. Preer, ibid., 90, 4809 (1968). (13) L. G . Sillen and A. E. Martell, “Stability Constants of MetalIon Complexes,” Special Publication No. 17, The Chemical Society, London, 1964, pp 89, 180.
sodium phosphates were of analytical grade, as was the perchloric acid (G. Frederick Smith Chemical Co.), the acetic acid (Baker and Adamson), and the pyridine (Baker and Adamson). Apparatus. A stopped-flow technique was used for kinetic measurements. The apparatus, based on a design by Dulz and Sutin,14 was supplied by the Atom-Mech Machine Co., Patchogue, N. Y . It has an eight-jet Teflon mixing chamber. The detector system was composed of a Beckman DU monochromator with a tungsten lamp, an EMI-6256B photomultiplier, and a Tektronix Type 564 storage oscilloscope with a Polaroid camera. The power supply for the tungsten lamp consisted of four 6-V batteries, connected in parallel and maintained on trickle charge. An additional lens of 25-mm focal length was mounted between the monochromator slit and the observation tube, which improved the signal-to-noise ratio by at least a factor of 2. A Fluka high-voltage dc power supply (Model 412 B) was used to bias the photomultiplier tube. The slit being fully open (2.0 mm), the photomultiplier voltage was about 460 V. The trigger circuit and the variable filter were similar to the ones described by Dulz, but the zero-suppression unit was modified; in this study a balancing potential circuit, containing a 1.35-V mercury battery, was employed. The performance of the apparatus was tested by a study of the reaction between hydrochloric acid and hydrogen carbonate according to the method described by Dalziel.16 The rate constant 1 sec-l at 25.0”, obtained for the dehydration of H2C03,24 is in agreement with previous results,*7 which range from 10 to 17 sec-l at 18”, corresponding to about 27 i 7 sec-l at 25” according to the value of the activation energy given by Dalziel. Measurements. One of the two reactant solutions contained dihydrogen phosphate and, to obtain the desired pH value, perchloric acid or hydrogen phosphate; the ionic strength was adjusted to 3.0 M by addition of sodium perchlorate. The other solution consisted of (1-3) X 10-4 M chromium(V1) in 3.0 M NaCI04. In some experiments the background salt of the latter solution was partly replaced by a buffer of the pyridine system or by an acetic acid-acetate buffer. At high buffer concentrations efficient mixing was obtained only when this buffer was present in both the solutions. In a few experiments equilibrium was approached from the opposite direction; i.e., an equilibrium solution containing the complex was diluted with 3.0 M NaCIOa at the moment of mixing. In the formation experiments a decrease in absorbance was observed at wavelength 350 nm, where both HCr04- and the complex have an absorption maximum. The internal cell breadth was 0.2 cm, and the change in absorbance was generally less than 0.010. Therefore, the recorded trace could be treated as a concentration L’S. time curve without significant error. The hydrogen-ion concentrations were determined with a Radiometer pH meter (Type PHM26c), equipped with a calomel electrode (Type K401), in which 4 M NaCl had been substituted for the potassium chloride to prevent precipitation of KClOa. Solutions
*
____ (14) G . Dulz and N. Sutin, Inorg. Chem., 2, 917 (1963). (15) G . E. H . Dulz, Ph.D. Dissertation, Columbia University, 1963, pp 90, 92. (16) I
