Robert J. Balahural and Nita A. Lewis Unwersiiy of Guelph Ontario, Canada
I I
Purifying and Identifying Transition Metal Complexes An experiment using cation exchange chromatography
T h e techniaue of ion-exchange chromatomaphy is well known and is Lsed in many are& of chemistry ( I ) . Cationto separate exchange chromatography may he and purify the mixtures of transition metal complexes that can result from preparative procedures (2).The method offers distinct advantages over other purification techniques in that one can usually be certain of the purity as well as the charge of the complex. This is particularly valuable when the compounds are being used for kinetic studies in which small amounts of impurities can complicate the rate data. Experiments which illustrate the method and uses of cation-exchange chromatography to undergraduate stufound (3). We have such an dents are not experiment which employs readily available equipment and a resin which is easily regenerated for re-use. I n this experiment, a weakly acidic ionic group is used to facilitate separation of two cationic complexes of Co(II1). The resin, Rexyn 102 (H)is converted from the hydrogen ion form, R-COO-H+ to the sodium ion form RCOO-Na+. When a mixture of cations is added to the resin, it will replace some of the Na+ ions and hecome hound. An eouilihrium hetween the added cations and Na+ will be established
R-COO-Naf
+ M+ a R-COO-M+ + Na'
As is indicated by the equation, the position of the equilibrium will depend upon the relative concentrations of M+ and Na+ ions in the solution, In general, the trick is to find a resin which will moderately hind all of the ions of interest. If the ions are hound too tightly, excessive concentra(NaCI in our experiment) will be re. tion of eluting quired to move the ions down the column. On the other hand, if they are not hound tightly enough, a good separation of the ions will not be We have found that a mixture of any 2+ and 3+ complexes of Co(II1) is easily separated on this resin. In most ion-exchange separations, the individual ions are eluted from the column ( 4 ) . However, the solutions ~ b . tained are dilute and isolation of the product as a solid (if possible) is difficult. We avoid this problem by "cutting" the column as outlined in the experimental section and ob. taining a concentrated solution of complex addition of hydrogen ions, Rexyn 102 (Na) has a very strong affinity for H + and thus addition of acid effectively displaces the complex from the resin almost quantitatively, Exoerlmental Conversion of Resin to Sodiom-Ion Form
The resin, Rexyn 102 (H) (Fisher Scientific Company, Mesh size 100-200) is converted from the.hydrogen-ion form to the sodiumion form, Rexyn 102 (Na) by adding 2 M NaOH to an aqueous suspension of the resin with stirring until the solution has a pH of about 12 to pH paper. The solid resin is then filtered off and washed repeatedly with water until the washings ire neutral. This conversion may be done in a batch process by the instructor prior to the laboratory period. (One pound of resin will make about 20-24 columns.) Preparation of Column
Fill a 250-ml beakei with a slurry of Rexyn 102 (Na). Stir and pour about 100 ml of this solution into a column which has been plugged with glass wool as illustrated in the figure. Allow the resin to settle. Keep adding mare of the resin slurry until the settled resin has reached a height of 12 cm in the column. Add water to the column until the level of the water is about 5 em from the top. Stopper the column, pinch the Tygon tubing, and shake this assembly until the resin and water are homogeneous. Clamp the column to the retort stand keeping the apparatus as perpendicular as possible so that the resin will settle evenly. (This is the most important step since if the resin settles unevenly, the bands will separate in a "candy-cane" fashion and it will not he possible to cut the column.) Quickly remove the stopper, remove the pressure on the Tygon tubing, and add water in a circular motion around the top of the column using a wash bottle until the resin has settled. During this procedure the water should be flowing through the whole assembly into a receptacle. After the resin has finished settling, allowthe water level to drop to about 5 cm from the top of the ealumn (so that an air-space will be present). Put the separatory funnel in place and clamp the tubing at the appropriate level as shown by the dotted line in the figure. Apparatus for column chmmatography.Campanents include: 250-ml separatory funnel fined with a #2 rubber stopper, glass column (largetubing 30 cm lorn. 22 mm o.d. with small flare at too. coned at bottom for 3 cm and ioined to ;ma I tmng 6 mm o d . 4 cm an&, glass w w l p u g trmly p r e ~ ~ eontdo in. uatl. 50 cm long atono om of column and ~ ~ g too a nng 's n i d by tamed to small end of co umn. 324 / Journal of Chemical Education
Ion-Exchange Procedure
Transfer the column set-up to a cold room if one is available ipreferahly maintained at ahnut 5 ' C ) Ifrhr eompl~xesI,, h r qeparated are suff~ricnrlyitnhle townrd hydrolysis, thii step will no1 be necerwry. Wnrh nhour 100 ml of aarrr r h r n q h the column. IXr-
solve the mixture of compounds to be separated (about 1g of material) in approximately 150 ml of water and charge the column. After the separatory funnel is empty, add 50 ml of water and allow this to run through the column as well. Then add sequentially solutions of increasing concentrations of NaCl until a separation is obtained. Ion-exchange of a large number of pentaamminecahalt(II1) derivatives bas shown that a 2+ ion can be moved slowly down a column of Rexyn 102 (Na) with 0.2-0.4 M NaCI. At these concentrations a 3+ ion remains at the top of the column. A concentration of 0.6-0.8 M is required to move 3+ ions. Comparable ehelated complexes of bis(ethylenediamine)cohalt(III) of the same charge require concentrations of about one-half of these amounta. Transfer the set-up hack to the laboratory and remove the separatory funnel. Use a 10-ml transfer pipet and propipet as outlined below to physically separate the bands. Attach the propipet to the delivery end of the pipet (pointed end) and suck the band up into the pipet. Do not allow the water level in the column to fall below 3 em above the resin during this procedure or it will not be possible to add more water without disturbing the resin. Water should always he added using a wash bottle in a circular motion around the top of the column. Filter, wash with water and put each band into a 50-ml beaker. Add about half as much water as there is "dry" resin in the beaker and gently swirl the solution while adding concentrated HCI dropwise until the supernatant is acidic (pH 1-2). Filter off the resin and wash once with a small portion of water. Transfer the filtrate to a small beaker and add solid NaCIO4 (or other precipitating agent suitable for the compound being used) with stirring until the complex precipitates out. Filter off the complex, wash with a suitable solvent (e.g. ether, ethanol), and dry at the pump. Regeneration of Used Resin A concentrated solution of NaOH is added to a rapidly stirring aqueous slurry of the contaminated resin. The solution is allowed to stir for a few minutes after which time the resin is filtered off and washed thoroughly with distilled water. Then concentrated HCI is added to the resin with stirring until there is about three times as much liquid as solid when the resin is allowed to settle. The resin is again filtered off and thorouehlv " .. . washed with water. If the rciin is uot whiw, ,he process i~repented. N'hen the resin is clan, it is ronvrrled ro the sodium-ion iorm ns described above. The purification strp is most effioientlv dune in n hnteh prucess by the instructor. Dlscusslon
Identification of a Known and Unknown Mixtore Our students in the introductory inorganic chemistry course have used the cation-exchange chromatography method outlined above to senarate the comoonents of a known mixture of two cobalt(i11) compoundLcontaining a 2+ and a 3+ snecies. The students isolate the comoounds and run the infrared and visible spectra of them. The values obtained are comoared aeainst the accepted results, with which the students are supplied. This gives them some idea of the magnitude of error they may expect from their spectra. The known complexes we have been using are acetatooentaamrninecobak(III). (I) (5) and N-cyanomanidi-
cn) These complexes were carefully selected for ease in separation and identification. After isolating the components of the known mixture, the students are presented with a mixture of two unknowns which they separate and characterize in the same manner. A list of all unknowns used and their spectral data is supplied. Each student is given a different unknown mixture as far as possible. We have used by-products and "retired"
comoounds from o w research laboratories in making up these unknown mixtures. Any stable species of CO(III) havine a 2+ or 3+ charge has been found to be suitable. Two representative clas& are the pentaamminecohalt(III), his-ethylenediaminecobalt(III), (NH&Co--X and enzCoX2 complexes. These compounds are highly colored and a few qualitative generalizations are possible. For example, ligands coordinated through nitrogen are usually orange to yellow in color. On the other hand, ligands bonded through oxygen are pink or red. The experiment emphasizes that the strength of the eluting agent used gives information regarding the charge on the complex. In addition t o the separation of 2+ and 3+ species already mentioned it was found that 1+ compounds of pentaamminecobalt(II1) can he moved slowly down a column with 0.05 M NaC1. The analogous 1+ compounds of enaCo(II1) are not retained by the column. I t was also found that two complexes of the same charge could be separated by very slow elution. For example2 e n Z C ~ ( p t d n ) ~ + and en2C0(3-formyl-ptdn)~+(7) have been successfully separated using this procedure. Finally, the eluting agents, solutions of NaC1, (others that can be used include sodium acetate and sodium bicarbonate) are easily prepared and safelv handled offering a definite advantage over the 3-5 M acids which are necessary when using st;ongly acidic cation-exchange resins. The experiment is performed over three laboratory periods, each of which is three hours long. The first week the students nrenare their NaCl solutions and the column for ~a~~~ their known mixture. During the week, a t their own convenience, they put the salt solutions on the column (the apparatus is set up so that the column will not go dry when the separatory funnel is empty). The following week they isolate their known mixture of compounds and prepare the column for the unknown mixture. The final week, they isolate their unknown complexes and run the spectra first on their known comoounds for practice and finally on their unknowns. If c o m ~ o u n d sare not available for the unknowns, this procedure may he adapted for use as a purification technique. For example, the students might prepare enzCo ptdn2+ which is usually contaminated with a small amount of en&03+ (8).Rexyn 102 (Na) may easily be used to separate these compounds giving hright red and yellow bands on the column. Another possibility that we are considering implementing is to have classes of non-honors students prepare a different Co(II1) complex,each time the course is offered since many Co(II1) compounds are easily synthesized. These could then he used to make up unknown mixtures for a class of honors students who require a more challenging experiment. Student response to the experiment has been very favorable with 84% of the students over three semesters indicating in a laboratory evaluation survey that they considered i t to be a valuable contrihution to their education. In this time interval, 63% of the unknowns were identified correctly. (This figure could easily be raised to 80% by eliminating very difficult unknowns.) We do not give a heavy penalty for an error in identification since we prefer t o grade the students on the reasoning they used to make their decisions. Lists of unknowns and their spectral data are available from the authors upon request. Acknowledgment T h e authors would like to thank T. Anne Hostetter, University of Guelph, for making the chromatographic col-
'Author to whom correspondence should he addressed. 2The abbreviation ntdn is used for 2.4-nentanedione,commonly called aeetylaeetone k c a d Volume 53.Number 5. May 1976 / 325
umns and also the students of Chemistry 261 (Spring 1974, Fall 1974, and Winter 1975) for their cisms which have assisted greatly in modifying the original design of this experiment. Literature Cited (11 Wheaton. R. M., and Seamstar, A. J., "Ene~dopedisof Chemical T s b n o l w , " 2nd Ed.. 11.871 (1966).
326 / Journal of Chemical Education
(21 Bdah-, R.~.,a+dJonlan, R. B., J. Amer Chem. Soe.. 92,1533 (1970). (3) A "&ble exapt80" ia experiment 7, Angelici. Rokrt J.. "Synthesis and Technique in I n o r p i e Chbmiatry: W. B. Savndera Co.. Philadephi. 1969, p.58. (41 M ~c.., and h k e t t . B. w., '.hadied lnorganie chemistry,van~ ~ t ~~e i nm- d hold Co., London. 1912.p.68. (5) Jackman, L.M., 3mtt.R. M.,nndPortman,R.H., Chem. Commun., 1338 (1968). (61 Bdehma, R. J., and Jordan, R. B., J ~ m e them. r Sac., 93,625 119711. (7) Bdahura, R. J., and Lewis, N. A , Can. J. Chem., 53,1154 (1975). (81 Reid, I. K.. and Sargeaon. A. M ,Inorg. Synfh.9 167 (1967).
