Donald K. Sebera
Wesleyan Univers~ty Middletown, Connecticut
Preparation and Analysis of a Complex Compound
Two trends are evident in the teaching of freshman chemistry: increased emphasis on principles in the lecture material and a greater number of quantitative exercises in the laboratory. As the theoretical content of the lectures increases, the amount of time available for descriptive chemistry decreases; and more and more dependence is placed upon student reading and laboratory work to provide the facts of chemical behavior. Experiments simply taken from quantitative analyses laboratory-valuable as they are in other ways-and introduced in the f r e s h n laboratory to make it quantitative rarely provide the kind of interaction between principles, chemical behavior described in the text, and chemical behavior observed first hand that is so desirable. The quantitative laboratory exercise described here has been successful in integrating these three areas. Inclusion of an exercise dealing with complex ions is almost mandatory in any modern freshman course. Certainly if the lectures reflect the "renaissance of inorganic chemistry," tramition metal chemistry and crystal field theory will occupy a prominent position among the principles and descriptive chemistry covered in lectures and reading. I n recent years a number of experiments have been described for undergraduate laboratory courses1 but none has been suggested which is primarily directed toward meeting the goals of a freshman course. The synthesis of the complex described is a modification of the procedure of Hynes, Yanowski, and Shiller.2 -
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476
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Journal o f Chemical Education
The experiment is scheduled near the close of the second semester (it is usually followed by a 3-4 week period for an independent project) and requires three full four hour laboratory periods. The students are familiar with quantitative techniques from earlier experiments which include acid-base titration, spectrophotometry, Mohr titration of a chloride, ion-exchange resin techniques, and the turbidimetric determination of the formula and stability constant of a labile complex ion. Five lecture periods and additional readings are devoted to the chemistry of the transition metals and the structure of complexes from both the valence bond and crystal field points of view. Each student synthesizes and purifies his own preparation during the first laboratory period, but it has been found most expedient to have students work in pairs for the analysis. There are several steps-e.g., alkaline distillation-which require long waiting periods but cannot be left completely unattended; by sharing the work of analysis it can be completed in two periods. Because his grade in part depends upon the accuracy, each student actively participates in each phase of the analyses, taking aliquots which his partner titrates, correcting his partner's technique when needed, etc. Them are many natural breaks in the procedure so that rigid scheduling is not required. A discussion period held during the first week deals with the techniques of gravimetric analysis and questions from the students.
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H. F., Jn., THIS JOURNAL., 29, 95 (1952); GAYER,K. H., AND ELKIND,M. J., THIS JOWNAL,30, 90 (1952); DODSON, V. H., rnrs JOURNAL, 32,422 (1955); and KIRSCHNER, S., THIS JOURNAL, ~
The Experiment
L. K., SEILLER, M., J. Am. Chem. HYNES,W. A,, YANOWSEI,
The compound is prepared and purified using the following procedure:' Dissolve 8.0 g of CoCI2.6Hz0 in 10 ml of water. Add this solution to an evaporating dish containing a slurry of 25 g of NHICl in 25 ml of concentrated aqueous ammonia and stir well. Add, in one or two drop portions, 5 ml of 30% H20z stirring thoroughly after each addition (CAUTION: keep HIOc off of skin and eyes-it causes very painful burns!). After all the peroxide has been added, place the evaporating dish on the steam bath in the hood. Heat, with occasional stirring, for 15-30 min. The contents should be a paste or thick slurry-do not allow the contents to become dry! Rinse the paste into a 250 ml beaker containing 100 ml of 3 M HCI, and heat the mixture to 60" (steam bath) for 10 min. Allow to cool and filter off the precipitate using a Biichner funnel. Wash the precipitate with three 10 ml portions of ice-cold water, then with three 10 ml portions of acetone, and suck dry. Transfer the impure precipitate to a 600 ml beaker containing 300 ml of 2 M aqueous ammonia and, with constant stirring, heat to 5040°C-no higher! After a short time the precipitate will dissolve giving a deep red solution which is filtered hot (Biichner funnel) through fine filter paper. Transfer the filtrate to a one liter beaker and reheat the solution to 50-60DC. At 5 miu intervals slowly add three 75 ml portions of 12 M HC1. The solution should be stirred continuously between and especially vigorously during addition of acid. Continue to heat and stir for a n additional 15 mio, then cool to mom temperature. Filter and wash as before. Transfer precipitate to a weighing bottle and dry a t 110" C for a t least one hour. The reactions involved in the preparation are:
The reactions involved in the purification are:
Details of the analyses will not be given here since they are the standard procedures. First, some qualitative tests are made on a n aqueous solution of the preparation: chloride by Ag+(aq); free ammonia by odor and pH paper, followed by odor upon heating strongly basic (add NaOH) solution; cobalt(I1) ion by thiocyanate test in acetone solution: Aoetone
+
C o ( c ~ ) ~ + 4 SCNpink
[Co(NCS).IZ-
water colorless
deep blue
Ammonia is determined by distilling a weighed sample from strongly alkaline solution, collecting the distillate in boric acid solution, and titrating it with standard acid:
Copiea of the complete laboratory procedure as distributed to the students are available from the author upon request.
Cobalt is determined using the same sample. Solid KI is dissolved in the solution remaining in the distilling flask which is then made strongly acidic with HCI. The triiodideformed:
is titrated with standard thiosulfate. The total chloride in a weighed sample is determined by heating in an acidic Ag+(aq) solution and weighing the AgCl(s) formed:
A solution of the preparation is passed through a Dowex-50 cation exchange resin in the acid form. The [Co(NHJsC1IZ+is held on the resin heads; the ionic chloride-i.e., chloride ion of the ionic latticein the eluent is determined by Mohr titration. Results The results of most student preparations are a very pure, fine-grained, purple-red crystalline product. Occasionally NH&l is present; but if the product is crystallime, this is easily seen. Too rapid addition of H202will decompose the product without oxidizing the cobalt(I1) ion; this can be recognized by the blue crust which forms on the sides of the evaporating dish. The preparation sometimes can be salvaged a t this point by adding aqueous ammonia and HzOz,but it is best to start afresh. Dissolution of the complex in the purification step is often slow especially if acid remains in the solid because of poor washing; if the precipitate does not dissolve in 10-15 min, add 15 ml of concentrated aqueous ammonia. The analytical results obtained by careful workers differ from the calculated values by less than 0.5%, with the ammonia tending to be low. The good analytical results are probably partly fortuitous; e.g., the low results expected for the Co(II1) analyses are compensated by air oxidation of iodide ion during dissolution of CopOs. Questions I n the course of the laboratory work the laboratory instructors ask the simple questions concerning technique, reactions occurring, etc., and the standard laboratory report contains a section where sources of error are discussed and estimated. In addition, there are several questions which are part of the written laboratory report. Three types of questions are asked. The first is directly concerned with the details of the experiment, e.g., technique, stoichiometry, etc. The second draws upon principles from lecture and reading to interpret or extend the experimental results. The third goes beyond the rather straight forward application of theory and demand original interpretation or speculation. These latter questions (indicated by an asterisk) are "not required" but are attempted by 10-20% of the class with perhaps half of them giving reasonably good answers. To indicate their type and scope the questions are given in full. The list of questions is about twice as long as that for a one period experiment. Volume 40, Number 9, September 1963
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477
( b ) What it the pK, of this substance? Is this value consistent with the pK.'s of HxO and HIO+ and what you know of the bonding in complexes? (c) What is the formula of the complex ion in aqueous ammoniscal solution? 6. The visible and near ultraviolet spectra. of equimolar concentrations of hexsmminecobalt(II1) ion, ICo(NH&Ia+, aquopentamminecobalt(III) ion, [ C O ( N H ~ ) ~ H ~ Oand I~+, chloropentamminecobaIt(II1) ion, [CO(NHS)~CII~+ are:
1. I n this experiment hydrogen peroxide is used as an oxidizingagent. How many liters of air would have to he bubbled through the reaction mixture t o effect the oxidation? 2. Why would you expect the ionic chloride analyses to give high rather than low results? 3. The most likely impurity in the preparation is ammonium chloride. (a) Supposes. preparation (certainly not yours) contains 5% ammonium chloride. Compare the theoretical analysis for this mixture with that of the pure complex. Which individual analysis is most "sensitive" t o the presence of this impurity? (b) Assuming a relative uncertainty of 1% in the analytical procedures and your technique (except 2Y0 in the ionic chloride analysis), what is the minimum eoncentration of ammonium chloride you can determine by the most "sensitive" of the analyses? (c) Describe procedure by which YOU can detect and determine the concentration of ammonium chloride impurity t o two significant figures in a sample containing about 1% impurity? 4. The blue color formed in the acetone solution of potassium thiocyanste and cobaltous ion results from the presence of the labile complex ion [Co(iYCS)d-. (a) Briefly describe how you would experimentally determine the formula of the complex ion in solution. (b) Using Lewis structure and/or m and 7 pictures describe the binding in thiocyanate ion. Predict which is the negative end of this dipolar ion. ( e ) Predict whether the N or the S end of the ion points toward the metal atom in the complex. (d) How could your prediction be confirmed experimentally?
Wavelength, m#+ Figure 2.
-O-[Co(NH31jCI11+.
Vilible and near ultraviolet rpectr.:
-+-[Co(NHals]
a+;
-++-[ColNHslsH20]
=+;
---cell
correction;
(a) Arrange the ligands in order of their crystal field strength, stating your reasoning. (b) If NHs has a 10 Dq "due of 58.7 keal/mole estimate the 10 Dq values of NH3 and CI. i.c .) Account for the relative magnitudes of 10 Dq in terms of ligand structure. *7. Three compounds of molecular formula [CO(NH~)~CI?ICI are known. Two are exactly the same color violet, the third is a. bright green. (a) Explain the existence of the three compounds accounting for the number of each color. ( b ) How could you distinguish between the two violet compounds? ( e ) How many different compounds of molecular formula [Co(NH&Cl8I can exist? Would they have different colors? '8. I t has been proposed that the intense greenish-black color obtained upon first adding the HnOzsolution is associated with the compound: Figure 1.
5.
[(H3N)5Co--Oa-Co(NHa)s16+
Titrationcurve of George 8. Wildered.
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What evidence e m you cite to support this proposal? What. exueriments can be used to determine the oxidation state(s) of the cobalt?
George B. Wildered wanted to leave laboratory early and so modified the purification procedure. He dissolved his preparation in warm 2 M aqueous ammonia, filtered the solution, and then-to save tim-cooled the filtered solution in an ice-bath, added the oonoentrt~ted hydrochloric acid all a t once, and filtered off the precipitate which formed immediately. The solid he obtained was an orange-red color instead of the purplish-red obtained by others; he also obtained s. slightly higher yield of precipitate. H y Ieue, his laboratory instructor, suggested that he carefully titrate an aqueous solution of his solid with standard base using a glass electrode to measure the pH of the solution. The titration curve he obtained (titrsting 1.3425 g of sample with 1.156 M NaOH) was: (a) What is the formula. of the precipitate obtained under the modified conditions? Cite 811 the experimental evidence which supports your formulation.
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Journol o f Chernicol Education
Conclusionr
Students find the experiment interesting and challenging. They are delighted when the analyses confirm the identity and purity of their compound and most will repeat discordant analyses. Many are motivated to continue work with complexes in the independent project period. Some problems which have been studied are: preparation of the nickel(I1) ammine cyanide-benzene clathrate, preparation and resolution of the isomers of cis-trans [Co(en),Clp]C1,and the rate of aquation of [Co(NHa)aC1I2+.
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