Inorganic Chemistry, Vol. 9, No. 9, 1970 2019
PRUSSIAN BLUEAND TURNBULL’S BLUE solute ions such as TiFe2- and SiF6’- rapidly exchange 19Fwith the solvent anhydrous H F (at a rate independent of added F- ion) via a low activation energy path. For the latter complexes, facile nucleophilic displace-
CONTRIBUTION FROM
ment of lgFby HF is unlikely, but an HF-assisted dissociation of F- ion seems reasonable. Acknowledgment.-This work was performed under the auspices of the U. S. Atomic Energy Commission.
THE
DEPARTMENTS OF CHEMISTRY A N D CHEMICAL ENGINEERING No. 5 FROM THE CENTERFOR THERMOCHEMICAL STUDIES, BRIGHAM YOUNG UNIVERSITY, PROVO, UTAH 84601
AND
A Calorimetric Study of Prussian Blue and Turnbull’s Blue Formationla BY R E E D M. IZATT,’b GERALD D. WATT,’08d CALVIN H . BARTHOLOMEW,
AND
JAMES J. CHRISTENSEN”)
Received January 28, 1970 Values of A H o valid a t zero ionic strength and 25’ have been determined calorimetrically for the formation of “soluble” Prussian blue and Turnbull’s blue from their constituent ions in dilute aqueous solution. The calorimetric data are shown to be consistent with literature data for the Fe3+-Fe(CN)e4- redox reaction. The results are discussed in terms of the energy relationships for two proposed mechanisms of complex formation.
Introduction The “soluble” forms2a3of Prussian blue and Turnbull’s blue have been known for many years and present interesting examples of the intense colors exhibited by species containing the same element in different oxidation states. For many years Prussian blue and Turnbull’s blue were thought to be different, primarily because of the different method by which they are formed as shown by the reactions Fez+
+ Fe(CN)sa- = Turnbull’s blue + Fe(CN)s4- = Prussian blue
Fe3+
@)
Keggin and Miles4found the structures of the crystalline forms of Prussian blue and Turnbull’s blue to be identical and to consist of one group - of iron atoms surrounded by an octahedral arrangement of carbon atoms while another group of iron atoms is surrounded by an octahedral arrangement of nitrogen atoms. A theoretical treatment6 of the electronic structure of Prussian blue based on ligand field theory considerations is consistent with the compound having the formula KFe”I[FeII(CN)6]. Mossbauer spectra6 of solid Prussian blue and Turnbull’s blue indicate they are identical and these spectral data are in complete agreement with (1) (a) Supported by U. S. Atomic Energy Commission Contract AT(04-3)-299. (b) T o whom inquiries should be directed. (c) Supported by a Public Health Service predoctoral fellowship (No. GM20, 545) from the Division of Research Grants, Public Health Service. (d) Taken in part from the Ph.D. dissertation of G. D. Watt, Brigham Young University, Provo, Utah, June 1966; see Diss. Abstv. B, 2 1 , 1406 (1966). (2) G. D. Parkes, Ed., "Melior's Modern Inorganic Chemistry,” revised edition, Wiley, New York, N. Y., 1961, p 927. (3) There appear to be two forms of Prussian blue and Turnbull’s blue. One form “dissolves” in water giving a deeply colored blue “solution.” This form is referred to as the “soluble” form and results when 1 equiv of Fez+ The is added to F e ( C N ) @ - or 1 equiv of Fe3+ is added t o Fe(CN)&. “insoluble” form results when nonstoichiometric of the above salts are mixed.2 This paper deals primarily with the “soluble” form of Prussian and Turnbull’s blue which is represented a s KFeFe(CN16. (4) J. F. Keggin and F. D. Miles, Nature (London), 137, 577 (1936). (5) M. B. Robin, Inovg. Chem., 1, 337 (1962). (6) E. Fluck, A d v a n . I n o i g . Chem. Radiochem., 6, 433 (1964).
their formulation as a ferric ferrocyanide species. The Mossbauer spectra6 further indicate that very weak ionic bonding exists between the nitrogen end of the bonded cyanide ion and the ferric ion. A more complete discussion of the evidence for the identity and structure of solid Prussian blue and Turnbull’s blue has recently appeared.’ A search of the literature reveals that no thermodynamic data have been reported for the formation of soluble Prussian blue and Turnbull’s blue. Such data would be useful in understanding the energy relationships involved in such formation. In this paper are presented AHo values for the formation of the soluble forms of Prussian blue and Turnbull’s blue from their respective ions. Experimental Section Materials.-Reagent grade FeS04.7H20(Baker and Adamson), KaFe(CN)& (Mallinckrodt), KsFe(CN)s (Mallinckrodt), and standard iron wire (Baker and Adamson) were used in the preparation of solutions for this study. An Fe(C10i)3 solution was prepared by dissolving a weighed amount of standard iron wire in a known volume of hot concentrated HClOl and diluting with an appropriate volume of water. All solution preparations and manipulations were carried out under a nitrogen atmosphere to minimize oxidation of air-sensitive compounds. Procedure.-The calorimeter and supporting equipment have been described.* The calorimetric procedure consisted of adding 10-ml portions of 0.1 Mpotassium ferrocyanide and potassium ferricyanide solutions to 200 ml of M ferric and ferrous solutions, respectively. The time interval from when the solutions were mixed to when the calorimetric measurement was finished was always less than 5 min. The reagents were mixed in stoichiometric amounts in order to avoid formation of any insoluble prussian or ~ ~ ~ ~blue,2 b ~ corrections l l * ~were made for the dilution Of Potassium ferricyanide and potassium ferrocyanide (7) L. D. Hansen, W. M. Litchman, and G. H. Daub, J. Chem. E d u c . , 46, 46 (1969). (8) J. D. Hale, R. M. Izatt, and J. J. Christensen, J . Phys. Chem.. 67, 2606 (1963).
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Inorganic Chemistry, Vol. 9, No. 9, 1970
IZATT, WATT,BARTHOLOMEW, AND CHRISTENSEN
using the heat of dilution data reported for these compounds by Lange and M i e d ~ e r . I~n the 10-3 JI ferric ion solutions some hydrolysis of the ferric ion had occurred. A thermal correction for this hydrolysis was made as follows. A 10-nil portion of 2 11.1 He104 was added to the ferric ion solution and the total heat of interaction was measured. The difference between this measured heat and the heat of dilution of the HC1O4l0was taken to be the correction for the ferric ion hydrolysis. This correction was about -2 cal. The time required to reach thermal equilibrium after reagents were mixed was approximately 2-3 min. The intense blue color of both Prussian and Turnbull's blue was observed to form instantly after mixing. Spectrophotometric measurements a t 680 mp showed that a reasonably constant absorbance value was reached within 2-3 min after mixing. Although kinetic parameters have not been reported for the redox reaction
Fe3'
+ Fe(CiY)s4- = Fez+ + Fe(CN)e3-
(31
one would expect the second-order reverse rate constant to be of the order of magnitude of the forward rate constants reported for the Fe2+-FeSC and Fe(Cx)63--Fe(CS)64- redox reactions of 3.2 x lo3 l1 and 1 X lo6 ~V1-1 sec-l,l2 respectively. I n fact, this value could be somewhat larger since one expects that a reaction between unlike ions would occur more rapidly than that between like i0ns.1~ Furthermore, since the reportedI4 equilibrium constant for reaction 3 is very large (1.82 X lo8), the forward rate constant will be extremely large (ki > 10"). For these reasons i t is concluded that under the conditions of the experiment chemical equilibrium was attained well within the 2-3 min required to reach thermal equilibrium.
Results The calorimetric data for the reactions represented in (1) and ( 2 ) are given in Table I. Values for AH" were calculated from the data in Table I by extrapolating a plot of AH ZIS. total iron concentration, [ F ~ T ]to , [ F ~ T=] 0. It was necessary to make the extrapolation against [ F ~ Trather ] than ionic strength because the uncertain nature of the "soluble" iron cyanide species made the calculation of the ionic strength impossible. The AH" values calculated in this manner for reactions l and 2 are - 15.15 & 0.35 and 0.72 i: 0.18 kcal/mol of Fez+ and Fe3+, respectively, where the uncertainties are estimated from the plots of AH" 'us. [FeT].
Discussion The difference between the reactants in reactions 1 and 2 is given by eq 3. The consistency of the AH' values reported for reactions 1 and 2 can now be checked by comparing their difference with the AH" value for reaction 3 since AH2" - AH," = AH3'. The agreement between the AH3"value calculated from reactions 1 and 2, 15.87 It 0.35 kcal/mol, and that reported earlier, l4 15.63 kcal/mol, is good. The AHo values alone for reactions 1-3 do not provide structural or mechanistic information regarding Prussian and Turnbull's blue. However, when viewed in light of spectral results, these values lead to interesting energy relationships. For instance, the earlier (9) E. Lange a n d W. Miederer, 2. Elekliochem., 60, 34 (1956). (10) C. E. Vanderzee and J. A Swanson, J . Phys. Chem., 67, 285 (1963). (11) F. Basolo and K. G. Pearson, "Mechanisms of Inorganic Reactions," 2nd ed, Wiley, New York, N. Y., 1968, p 476. (12) A . Loewenstein and G. R o n , I n o i g . Chem., 6, 1604 (19137). (13) P. Geoi-ge and D. H. Irvine, J . Chem. Soc., 587 (1954). (11) G. D. W a t t , J. J. Christensen, and I
