Isotopic Exchange in Prussian Blue A. K. Bonnette, Jr.
Baptist College at Charleston. Charleston, SC 2941 1
S. E. Gandy College of Medicine. Medical University of South Carolina. Charleston, SC 29403 T h e use of radioactive isotopes by chemistry students has long been limited to graduate courses or specialized courses a t the undergraduate level. This experiment is designed to demonstrate to undereraduate students the versatilitv of radhisotopes in differentiating two or three different cdemical environments of one element in a single compound. In addition, exchange reactions between these different chemical environments can be studied in a three-hour lahoratorv. . period. ~
a
~
Theory A classical example of a single compound containing one element in more than one chemical environment is Prussian hlue, K Fe Fe(CN)e, first prepared in 1704 ( 1 ) .The first reported crystal structure of Prussian blue was determined using powder crystal X-ray diffraction studies performed by Keggin and Miles (2). More recently Ludi, Gudel, and Ruegg (3)have published single-crystal X-ray diffraction work on Prussian blue analogs which disputes the original structure proposed by Keggin and Miles. Recent studies by other groups ( 4 , 5 ) support the Ludi Model. The major disagreement between the two models involves the chemical environment of cationic Few+. The crvstal structure. accordine" to Keeein and Miles. is a face-centered cubic lattice with iron ions a t the corner of a cube and cvanide ions bridnine - the iron atoms dona- the cube edges (Fig. i). Each lattice iron is honded either to six carbon atoms or six nitrogen atoms. To compensate for charge, the centers of the cubes. or interstitial uositions, are filled with various cations such as K+, Na+, ~e:i+,etc. One plane of the Prussian blue structure can be visualized as:
Structure of Prussian Blue as originally prepared by Keggin and Miles
-
I
N
111
C
I
C
111
N
I I -N~C-Fd"'-~~N-Fe"-N~C-FF(T1'-
I
The use of Arabic svmhols (Fe3+)indicates the oxidation state the oxidation state of "anionic" iron complex; that is, Fez+ bonded to the carbonend of thecyanide ligand in the [Fe(II)(CN)s14- anion. The first case (Few) is representative of a high spin octahedrally honded ion while the second case (Fe(I1)) is representative of a low spin octahedrally honded ion (I ). The K+ ion is found in the interstitial position. Prussian blue, itself, can exist in two forms, "soluble" and "insoluble," but the difference between these two forms is actually in the ease with which they can he peptized to form a colloidal dispersion. Soluble Prussian blue is represented generally by the formula MI+ Fe:'+ Fe(I1) (CN)CXHz0 where MI+ is usually K+ or Na+. Insoluble Prussian hlue may be represented as Fe"+ [F&'+Fe(II)(CN)& where, according to the model of Keggin
and Miles, one. iron out of seven is an interstitial iron. The Keggin and Miles model indicates that there are three different iron environments: (1) iron octahedrally bonded to six carbon atoms, (2) iron octahedrally honded to six nitrogen atoms, (3) interstitial iron for charge compensation. Conversely, the X-ray and density studies carried out by Ludi, et al. ( 3 ) on Prussian hlue analogs indicate that there are no interstitial Fe" ions. In this structure the two octahedral coordinating units are Fe(II)C6 and Fe"N4,4(H20)1.5, the formula of the latter complex showing an average composition. Thus, there are only two types of iron environments, one of which is variable. This crystal structure still retains the face-centered cubic structure of the older model though it differs in that one of every f ~ u r [ F e ( I I ) ( c N ) ~groups ] ~ - must he absent. Ludi states that the vacancies occur a t random throughout the crystal and that the holes are filled by water molecules. In performing selective isotopic labeling of one iron environment in Prussian hlue, it is imperative that no rapid exchange occur- among the various iron positions. R. C. Thompson (6)and Korshunov and Lehedeva (7) demonhas shown by radioactive tracer work that the lattice irons in Prussian blue do not exchange with each other or with other ions in solution. In this experiment four samples of soluble Prussian hlue M+Fe3+Fe(II)(CN)6,will he prepared. Each of these precipitated samples will subsequently be placed in Few solntions where metal ion exchange will occur. T o the first non-radioactive precipitated sample of K+FeWFe(II)(CN)6[Sample One] will he added an aqueous solution of radioactive "Fe". Aliquots of this solution will he counted a t specific times to determine whether the 59F$+is exchanging with non-radioVolume 58 Number 4
April 1981
355
active Kt, Fi", andfor Fe(I1) in the Prussian blue. A decrease in the count rate of the aqueous aliquot a t this point does not identify which ion(s) (K+, Fe", or Fe(I1)) in t h e original precipitate exchange(s). Tests on t h e remaining three precipitates identify t h e exchanging ion(s): Studied Reaction: KtFe"+Fe(II)iCN)e 19pa3+
-Insoluble
Prussian blue
T h e second soluble Prussian blue sample [Sample Two] is prepared by labeling the cationic Fer'+ position with radioactive GgFe3+. I n t h e Keggin and Miles model, this Fe"+ is coordinated octahedrally to six nitrogens from t h e cyanide ligand while in t h e Ludi model this F e w environment is variable containing -NC and -OH2 coordinating ligands. T o t h e precipitated sample of K + 59FeS+Fe(II)(CN)fi will he added a n aaueous solution of non-radioactive Fe3+. Aliauots of this wlution m i l l i w counted at specirk times 11, d ~ t e r m i n e whether ,tnv of thv "'FeJ- in rhv oriainal - -r~recipirateexchange, with non-radioactive Few in the solution: Studied Reaction: KSgFeJ+Fe(II)(CN)n Pe)+
+Insoluble
Prusaan blue
T h e third soluble Prussian blue sample [Sample Three] will be prepared by labeling the anionic iron complex Fe(I1) position with radioactive "Fez+. T o the precipitated sample of K+Fe3+ 59Fe(II)(CN)6 will be added a n aqueous solution of non-radioactive Fe3+. Aliquots of this solution will be counted a t specific times t o determine whether any of t h e 59Fe(II) in t h e original precipitate exchanges with non-radioactive iron in t h e solution: Studied Reaction: K Fe SSFe(lI)iCN)~ P13A
+Insoluble
Prussian blue
T h e fourth soluble Prussian hlue sample [Sample Four] will he prepared using only non-radioactive iron a n d radioactive '"Na for the interstitial position. ("Na is more economical and more readily avuilnl,le that) mdioisutopei of K., li, the radioactrw solublr Prussian bhle precipitate of 2'Na Fe -FdIIl(CNI,. ~ . ,, ~ will be added a n atrueous solution of no,,radioactive F e " + . " ~ l i ~ u o tofs this so1;tion will be counted a t specific times t o determine whether any of the radioactive W a i n t h e original precipitate exchanges with non-radioactive Fe3+ in solution. T h i s series of experiments identifies t h e ion(s) in t h e precipitate which exchange($ with Fe" in solution. Several of these steps could he consolidated by using double labeling t o prepare compounds such a s 22Na59Fe3+Fe(II)(CN)6 or 22Na Few 59Fe(II)(CN)s.
'+
Objectives T h e objectives of this experiment are 1) Prepare radioactive compounds using selective labeling techniques. 2) Compare two models (Keggin and Miles versus Ludi) for Prussian blue. 3) Follow exchange reactions between ions in solution and ions in a precipitate. 4) Demonstrate that iron found in Prussian blue, K Fe Fe(CN)e, does not exchange with other cations at room temperature. 5) Demonstrate that only K+ (or Nat) in precipitated Pmssian blue exchanges with Fe3+ in solution. Chemicals and Equipment Gamma rav (single channel analvzer): 1.0 m l .soectrometer . . and 5.0 mi pipettes; test t l ~ h e icentrifuge; ; 10.0ml volumetric flask;O.l .\I KaFt.~CN)6:0.1.I1 FeCI. ur FeNH,{SO1)2;0.I A1 Na4Fc(('N)6; & m e ; IOU pC'i Fe-59 und Ka-$2 rad~oactive sources from New Knglantl Nuclear (:~~rporfition. Note: T h e 0.1 M FeC13 or F~NH~(SO.&m u s t he made slightly acidic with HCI t o prevent formation of colloidal Fe(0H)a.
Experimental Procedures I. Standardization of Gamma Ray Spectrometer (Single Channel Analyzer) 1) Locate the 1.292 Mev gamma peak ofan Fe-59 standard on the single channel analyzer. The samples from Parts (111, (111).and (IV) of this procedure will be counted at mid-point of this oeak.
11. Preparation and Reaction of Soluble Prussian Blue [Sample One]. 1) Add 5 ml of 0.1 M FeCls or Fe NH4(SOh to 5 ml of 0.1 M K4Fe(CN)sin a test tube. The blue precipitate, K Fe3+Fe(II)(CNk will form. 2) Centrifuge, pour off supernatant, and wash the remaining precipitate with 5 ml acetone,then 5 ml H20,and then 5 ml acetone, centrifuging after each wash. (The acetone helps prevent formation of a colloidal suspension of Prussian blue.) 3) Spike 10 ml of the 0.1 M Fe:'+ with enough Fe-59 lo give 10,000-20,000 counts per minute (cpm). 4) Count exactly a 1.0 ml aliquot of the spiked 0.1 M Felt solution. 5) Add 5.0 ml of the spiked solution t~,the blue precipitate in the tesl tube. Record the time, stir for 330 see. 6) Every 10 min for 1 hr, centrifuge the solutim and arunt a 1.0 ml aliquot ofthe supernatant solution for 30 see. Record the count rateand the time. Return the counted solution t~ the teat tuhe containine.. the Prussian blue. (Note: It is imoortant that the Fttrrln: twhn~queh.wnststent thrrmyhout rlw detrrnun;ttion I t ~ s a c . . n \ ~ n ~ e n t tt ~ , imr ~c r u h t n r h e a i,~.arereturnl''irm i l l cdulitm is essentially ~ o & ~ l e t exchanged el~ with ion(s) in the precipitate in approximately one-half hour. This step does not identify the specific ion or inns (K+,Few+,Fe(I1)) that do exchange, only that exchange does occur. Parts 111, IV, and V, of the experimental procedure attempt to determine exactly which ion in the precipitate (interstitial K+ or Na+, anionic
i l l Cotton, F.A, and Wiikinson, Geoffrey. "Basic inorganic Chemistry" John Wiley and Sun%,NewYurk. 1976. (21 Keggin. J. F. and Milas. F.D.,Naturc 137,517 (19961. I:,) h d i , A . . Cudel, H.. and Ruegg, M . . I n o r ~ o n uC h o r n r r w 9.222d 118701. (41 Eeail,G., Milligan, W. D.,Kork,.li.. and Bernal. 1.. Inorganic C h ~ m u l r y 16,2715 .
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151 ~ d lG.,, Millipan. W. D., Patrick, J.. and Swanrun. B.,inorzanic Chrrnistry, 17.2978
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is) Thompson.R. C.. J
A r n w C h r m S o ? , 70. 1045 119461. ( 7 ) Kllrahunly I. A. md l.elredeva, % M.. H u w u n J i n w ~l'hrax , 1.227 IlYSfi!. (8) Edwrrdi, E. H., Marter's ' l h r i i ~Cleylsm . University. Clemwn. South l'andina. 1970.
I81 Bannette.A.K.andAilen,.loeF.,lnurz. C h u m , 10.1613119711. %and Bonnetie, A. K., J. lnuip Nurl C h m . . 36. 1011
(101 Ailen..Joe li., Edwards, B. (iY741.
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