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J o h n J. Farrell Franklin and Marshall College LanCBSter. Pennsylvania 17604
Physical and Chemical Properties of the .. . mpper-~lannesysrem 1
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An advanced laboratory project
An integrated physical-analytical-inorganicproject has been developed as part of a laboratory program for junior level biology majors enrolled in a one-semester course entitled "Physical Chemistry for t h e Life Sciences." T h e project is intended to introduce t h e student to t h e various physical and chemical properties of metal-amino acid complexes using a copper(I1)-alanine system. T h e oroiect includes acid-base standardizations.. notentiometric titrations of polyprotic acids, thedeterminationof pK,'s, potentiometric determination of stability constants, electrodeposition, synthesis of some inorganic complexes. s ~ e c t r o ~ h o t o m e t r determination ic of crvstal-field solittines. a i d computer usage.
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Procedure Standardizations Preoare 500 ml of carhonate-free 0.1 M sodium hvdroxide. Standardize hvdroeen ohthalata -~ with ~otassium Prepare 560 ml of 0.1 if nitric acid. Standardize with the 0.1 M sodium hydroxide. tor, if the mtructor prefers, with Na2C03.j 5 H1O in 200 ml of water. Filter Dissolve ahout 5 g of Cu(N031~2 to remove traces of Cu(0H)z. Use aliquots of the resultant solution and standardize by electrodeposition. The standardized copper solution should be 4.1M. ~~
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Fqwe 1 The llnatlon of 0 0999 g of alanlne in 100 rnl ot 0 05 MKNOsand 10 ml of 0 1068 MHN03 wlth 0 1183 M haOH T = 21°C
Determination of the pK.k of Alanine The equilibria between the acid form, t b zwitterion, and the basic form of alanine can be expressed as follows
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Titration of Alanine Dry 0.3-0.4 g of alanine for 1hr a t llO0C and cool in a desiccator. Weigh (to the nearest 0.1 mg) a0.1-0.15-g ssmpleintoa 200-ml tallform beaker. Pipet 100 ml of 0.05 M KNOs into the beaker, mix, and record the p H of the solution. Pipet 10 ml of standardized 0.1 M nitric acid into the beaker. Record the p H of the solution. With continuous stirring, titrate with the standardized 0.1 M sodium hydroxide solution, recording the p H at appropriate increments. Titrate until the moles of base added is equal to the moles of nitric acid plus the moles of alanine present. Plot the p H versus ml of base added. Titration of the Copper-Alanine System Weigh (to the nearest 0.1 mg) a dry U.1-0.15-g sample of alanine into a 200-ml tall-form beaker. Pipet IWml of 0.05M KNO3inm the beaker and record the pH of the solution. hoe1 2 ml of 0.1 M standardized coooer(~~)&rate into the solutionand record the DH. With continuous srirring, titrate usmg -0.2-mI inrrempnts oithe standardized 0.1 M v d u m hydroxide. Record thepH untd the mules of base addtd 1s twice the moks of copper present.
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" , From eqns. (3)and (4) we find at the 0.5 and 1.5 equivalence points, respectively, pK1 = p H (0.5 eq. pt.) and pKz = p H (1.5 eq. pt.). Typical student results are shown in Figure 1. The results are in agreement with reported ~ a l u e s . ~
Determination of the Stability Constants for the CopperAlanine System In aqueous s d u t ~ o ncontaining ropperrll) ions and alanine, the following equilibria must hold in addnion to eqns (31and (4,
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Preparation and Spectral Measurements of Copper(l0 Complexes1 Bis(alaninat0) copper(lI), dried at 1 0 5 T to remove water, and bis(2,4-pentanedionato) copper(I1) are prepared. The visible spectra of the following species are determined: C U ( H ~ O ) ~Cu(acac)2, ~+, Cu(ala)~(H20)~, Cu(NHs)dH~0)2~+, and Cu(en)dHz0)z2+. Treatment of Data and Typlcal Results
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Standardizations The usual treatment of data should yield 4 . 1 M solutions of acid, base, and copper(I1).
,- - ,. - where ii = the formation function = the ratio of the total hound chelate to the total metal present
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' Potts, R. A., 3. CHEM. EDUC., 51,539(1974). ~.
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If the pH of this solution is adjusted by addition of sodium hydroxide, eqns. (8) and (9) can he shown to apply
Sillen. L. G.. and Martell..A. E.. "Stahilitv Constants.'', 2nd Ed... The chemical Society. Hurlington Hduue. London. 1951. p. 398. 'Trapp, C., and Johnson, R., .I. CHEM. EDUC., 44,527(1967). >
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where [Hala] = alanine concentration, [HalaIt = total alanine concentration (assuming no reaction but including the dilution factor caused by the addition of base), and [Cu2+]r= total copper ion concentration (assuming no reaction hut including the dilution factor) and i [ala-1. Plot ii versus Using eqns. (8).(91, and (4), calculate ? -log[ala-I, pala-. A computer program is available from the author Volume 54. Number 7, July 1977 1 44!i
Wavelengths of Maximum Absorbance and Crynal Field Parameters for Several Copper(ll) Complexsr
upon request to perform this calculation. From eqn*. (fi) and 17) we see that at ii = 0.Sand 1.5 lauuurning a large separation between k l and k ? pk~ = log kl = pala- (at ii = 0.5) pk? = log kZ= pala- (at ii = 1.5) Typical student results are shown in Figure 2. Note that there is not a large separation between pkl andpk?, and, as a result, there is no inflection point in the curve. In spite of this. however. the results are in agreement with reported value^.^ A computer program is available from the author upon request that uses the exact solutions for pkl and pkp and treats the data by the method of Least squares. Determination of Crystal-Field Splinings and the Spectrochemical Series
Typical yields for the two cumplexes prepared were 5 0 8 . Wavelenflh maximaand crystal field splrttmgs. A's, areshown in the tahle. A pseudo-ortahedrsl field was assumed. The results arc in agreement with reported values7and yield the folluwinp.spertrochemical wries: H1O < arac < ala < NH?
