J. R.Darley and J. I. Hoppi Medway and Maidstone College of Technology Chatham, Kent, United Kingdom
I
Thermal Decomposition of Potassium Bisoxalatocuprate(II) Dihydrate An inorganic-analytical experiment
The approach to the teaching of titrimetric analysis in the majority of introductory chemistry courses has changed in recent years from the repeated estimation of solution concentrations to the use of more purposeful exercises. Titrimetric methods are used, for example, to investigate reaction stoichiometries ( I ) , to investigate oxidation states of transition metals (8) and to estimate the purity of prepared compounds (3). Although such an a~nroachis eainful and stimulates much greater enthusiasm in the student, it presents the teacher with the problem of devising various types of exercises to include in a course which familiarize the student with as wide a range of titrimetric reactions as possible. I n the particular case where the determination of the purity of a prepared compound is carried out, the teacher has the additional problem of assessing the students' work where it is often d i c u l t to decide whether a poor final result is due to a badly prepared impure compound or to an inaccurate titrimetric analysis. Thus for this type of exercise it is important in introductory courses to select compounds which can be prepared in a high state of purity and which preferably can he used in some subsequent investigation. An exercise which we have successfully used, selected with the above criteria in mind, is the preparation of potassium bisoxalatocuprate(I1) dihydrate and the investigation of the thermal decomposition of the prepared salt. This investigation includes some elementary gravimetric analysis and qualitative analysis in addition to a simple extraction procedure and a range of titrimetric estimations. The preparation of the complex salt is not difficult and the method used (4) gives a pure product in good yield. The compound can he assessed for purity by the estimation of the oxalate by a permanganate titration and the copper by an iodine-thiosulfate titration on the same aliquot of solution as that first used to estimate the oxalate. The thermal decomposition of the compound is studied as follows
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..
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(1) Duplicttte samples of the hydrated salt are heated to constant weight in an oven at 105-110°C.
(2) The anhydrous salt from (1) is heated to constant weight in a mufflefurnace at 5 0 0 T (if a mufflefurnace is not available, a gentle Bunsen flame can be used).
+
K,CU(C~O,)~ 01-+KSCO. 60O0C
+ CuO + 3COz
(2)
A sample of the original hydrated salt is simultaneously heated as in (2) to constant weight and a qualitative analysis is carried out on the black residue produced. Part of the residue is water soluble leaving a black solid. Tests on the colorless aqueous solution indicate the presence of potassium and carbonate ions and the absence of oxalate ions, while examination of the black solid shows that it is completely soluble in dilute sulfuric acid to give a blue solution. The presence of copper ions and the absence of ions other than sulfate in the solution is readily confirmed. The color of the solid and the reaction with dilute sulfuric acid suggests that it is principally copper(I1) oxide with little, if any, copper(1) oxide present. On the basis of the gravimetric results and the qualitative analysis, the student is asked to suggest equac tions to represent the two stages of the thermal decomposition which occur under the conditions used (the detailed balancing of the equation for the decomposition of the anhydrous salt with respect to gaseous products is not expected), and then to confirm this overall stoichiometry by a quantitative analysis of the final residue using titrimetric methods which were not used in the estimation of the original compound. At this stage the methods to be used are discussed and particular attention is given to devising a simple experimental procedure to establish that copper is present in the residue as copper(I1) oxide. The quantitative examination of the residue is carried out by leaching with hot water, determining the potassium carbonate using hydrochloric acid, and then weighing the water-insoluble black solid before it is dissolved in dilute nitric acid and the copper(I1) ions in solution estimated using EDTA. Comparison of the experimental weight of black solid with the veight of copper(11) oxide calculated from the EDTA results confirms that the copper is present in the final decomposition products as copper(I1) oxide. A typical duplicate set of student results is given at the end of each of the relevant experimental sections below. Experimental Preparation of Potassium Bisoxalatocuprate(lI) Dihydrate Dissolve 4.1 g CuS0,.5Hn0 in 8 ml of water and heat the solution to 90°C. Add the hot solution, slowly and with stirring, to a solution (also at SOT) of 12.3 g KICIOI.H1Oin 35 ml of water. Allow the solution to cool to room temperature and then cool in an ice-bath to 10°C. Filter off the solid, wash with i c e cold water followed by ethanol and then acetone. Dry in air. The yield obtained is generally between 90 and 98% of the theoretical yield. Volume 49, Number 5, May 1972
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365
Estimation of Oxalate and Copper in the Prepared Compound
Dissolve the black solid in the sinter in 1 :1nitric acid and m&e up to 250 ml with water (soln B).
Weigh out accurately between 0.16 and 0.18 g of the salt into a 250-ml conical flask and dissolve it in about 25 ml of water. Add 20 ml of 2.5 M sulfuric acid, herut to about S O T and titrate with standardized 0.02 M potassium permanganate solution. Record the titer at the end-point. To the titrated solution, add solid sodium carbonate until a precipitate first appear8 and then add dilute acetic acid until the pH is P 5 . Add about 1 g of solid potassium iodide and titrate the liberated iodine with standardized 0.02 M sodium thiosulfate solution using freshly prepared starch as indicator (% sharper end-point is obtained by the addition of 1-2 gpotassium thiocyanate as theend-point is approached
Estimation of the &COa Solution A
(5).
KMnO,: Weight of complex (g)
0.02020 M Titer (KMnO,) (ml)
Na&Oa: 0.01980 M Titer (N%SzOs) % oxalate (ml) % Cu
Titrate 50 ml aliquots with standardized 0.05 M hydrochloric acid using methyl orange as indicator.
Estimation of the Cu Solvtim B ( 6 ) Neutralize 8.20-ml aliquot of the solution by the gradual addition of solid sodium carbonate, then add 10% acetic acid dropwise until the pH is approximately 4. Add 8-10 drops of aliaarin complexone indicator' and 10 ml of an acetate buffer solution (pH = 4.3). Titrate the red-magenta colored solution with 0.01 M EDTA to a ereen end-mint. The indicator solutionuis preparid by dissolving 0.25 g of alizarin complexone in 50 ml of 0.1 M ammonium acetate. EDTA: 0.0100oM Wei ht Wei ht HCI: 0.0503 M Titer c cd Titer
Samole
Theoreticalvalues:
%oxdate = 49.8; D/o Cu = 17.9s.
Thermal Decomposition of the Compound
(i) Weigh out accurately into each of two crucibles approximately 1.3 g of the salt and heat in an oven at 10.5-llO°C until there is no further change in weight. (ii) Transfer the crucibles to a muffle furnace and heat to constant weight at 500°C. Weight of Weight of O/, loss Weight of yo loss hydrated residue in weight residue in weight (ii) Sample salt (g) (i) (g) (i) (ii) (g) (a)
(h)
1.2116 1.2675
1.0872 1.1378
10.2, 10.28
0.7429 0.7821
31.6, 31.2s
Theoretical weight loss: Eqn. (1) 10.18%; eqn. (2) 31.4&. Quantitative Analysis of the Residue [Duplicates ( a ) and ( b ) ]
Thoroughly leach the residue from (ii) above with hot water and filter through a, sintered-glass crucible previously dried to constant weight. Wash the solid on the sinter with hot water, collect the filtrate and washings and make up to 250 ml with water (soln A). Dry the crucible and contents to constant weight at 105°C.
The a.li.liemin complexone is avd.ilshle from BDH Chemicals Ltd.. Poole. Eneland or Gallard-Schlesineer Chemical Mfe. Corpor&ion, 584 $neola Ave., Carle ~ l a e e , ~ N eYork w 11514
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Journal of Chemical Education
(73
Moleso of K"CO2
(EDTA) (,"I,
Molean oi
cut+
(Tit& (Gravimetric) metric) 1.71
in1
a Total number oi moles extracted from the residue.
The results given above lead to the following molar ratios for the thermal decomposition sequence in good accord with eqns. (1) and (2).
(&)
(b)
Hydrated salt
Anhydrous salt
KXO.
CuO
1.00 1.00
1.00 1.00
1.01 1.01
1.01 0.99
Conclusion
We find that the inclusion of the suggested experiment in a course is profitable in that it adds variety to the laboratory program and it enables the student to gain experience in simple gravimetric analysis, qualitative analysis and four different types of standard titrimetric estimations, all of which are involved in a purposeful investigation. Furthermore, it is an exercise which students appear to enjoy. Literature Cited C O O P EJ. ~ ,N., A N D RAMBTTE, R. W., J. CREM.EDOC.,46. 872 (1969). ST~RX J. .G.. J. CHEM.Eouc., 46, 505 (1969). F o w ~ e sG. , W. A,, J. Cmenr. Eouc.. 39,401 (1962). KrRscxmR, S.. "lnorpanio Synthesis V r ' (Edilor: R o c ~ o r a ,E. G.) MoGraar-Hill Rook Co.. Near York, 1960, p. 1. 15) A. I.. VOOEL. "Quantitative Inoreanic Analvris" (3rd ed.). Lonnmans. Green and Co. i t d London. 196L. p. 358. (6) Weal, T. S.. "Complexometry wlth EDTA and Related Reazents" (3rd ed.), BDH Chemicals Ltd., Poole, England, 1969, p. 182.
(1) (2) (3) (4)
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