C. G. Ramsay University of Aberdeen
I 1
EDTA Titration of Cadmium and Mercury
Cadmium(11) and mercury(I1) can he determined by reartiun with excess of ethvlenediaminetetra-accticwid (EDTAI and hack-titration with standard zinc(I1) solution, followed by masking of the mercury(II), e.g. with iodide ( I ) , and continued hack-titration of the released EDTA. The principle of the method is illustrated in Figure 1. These reactions are of interest in the undergraduatr nnnlytical lahoratorysince they demunstratc the use of masking toobtain drlectivitv in cumnlexometrv. and narticularlv since exnerimental results aeree most impresiveiy with theutheoretical predictions fromihe "conditional" ("apparent," "effective") stability constants, a concept introduced by Schwarzenbach ( 2 ) and developed by Ringhom (3,4).
.~ ~
~
The Conditional Constant
The degree of completeness (i.e. the quantitative nature) of a reaction can be evaluated from the conditional stahilitv constant. The thermodynamic equilibrium constant for a l:i complexation reaction, as occurs with EDTA
J
Figure 1. Stickdiagram representing the cadmium(li)and mercury(l1)titration. (1)Amition of excess of EDTA. (2)back-tivation with zinc(ll),(3)second backtiration after masking mercury(1l) to release EDTA from HgEDTA complex. Cd = (1H2H3):Hg = (3).
+ [MA] + .. . + [MA,O = [MI (1 + f: [ A I V ~ )
[M'] = [MI
I=,
MtLtML
is defined by the law of mass action as
However, the equilibria in real aqueous systems are never so simple as to involve only metal and complexant, since protons and hydroxide ions are always present, and since competing ligands (e.g. buffer components and masking agents) and iuterferine" metals mav also "eive rise to side reactions. Consequently, the conditional constant which takes into account all competing equilibria is defined as
= [MI UM(A1
where aMcA) is the side reaction coefficient or "alpha coefficient" for the reactions of A with M, and ,3i is the overall formation constant of the comolex MAG'
. .. . If these formation constants and the concentration of A are known, a can be evaluated. Corresponding alpha coefficients for side reactions involving the ligand and the metal complex can he defined. Consequently
. .. .
where [M'I the total concentration of all species . . represents . containing M nut complexed tu I., where [L'Irepresents the total concentration of L not hound tu the metal ion M, and where lh(l.'l is a function of the total concentrution of all species'con.9). Even if theconcentrat~onof ammonia is raised considerablv. the errors should remain nealicible. In the case of mercuG,it is the added stability of the mixed Volume 54. Number 11, November 1977 / 715
Figare 6. Cond tional olabilily canslants tar EOTA complexes u Ihcadmium(l1) in aosence ol bromlde (1). in 0.1 Morom de (2). and in 2.0 Moromlae (31:and w i n mercurylll) n absence ol orom de (4). in 0.1 M oromos (5).and in 2.0 M bromide (6).
EDTA-ammine complex which permita quantitative reaction. If formation of this mixed complex is ignored, a value of log K' = 2 a t p H 10 (i.e. only 8% reaction) is obtained in 2 M ammonia; the falseness of this prediction is readily checked experimentally. Quantitatiw reaction with all three metals is ~ l s nossiblr o at DH5.5 in hexamine buffer. assumine that the effects of the lhffer are negligible. The predicted influence of iodide, bromide, and chloride on the EDTA complexation reactions is now examined (Figs. 5-71, Since zinc forms verv weak halide comulexes, the reduc;ion in the conditional constant of the zinc-~DTAcomplex can safelv he nealtvted. even in 2 hi halide. A \,slue of lop. K' = 0 correipondsto 0.1% reaction for a metal concentration of 0.001 M a t the equivalence point. At pH 10, mercury(I1) is quantitatively masked by 0.1 M iodide (log K' = -3.6) whereas EDTA still reacts quantitatively with cadmium(II), even in the presence of 2 M iodide. Consequently, i t should be possible to determine cadmium in the presence of mercury by masking the latter as tetraiodomercurate, [HgI4]2-: there is considerable latitude in the choice of pH and iodide concentration. With bromide or chloride as masking agent, very high concentrations (>2 M ) would be necessary to mask mercury quantitatively a t p H 10. At pH 5.5, however, mercury should be quantitatively masked by bromide a t concentrations greater than 0.1 M , while the complexation of cadmium by EDTA remains ouantitative a t bromide concentrations less than 0.5 M. Complete masking of mercury at pH 5.5 requires a chloride concentration of at least 2 M (Fig. 7) while this concentration permits very nearly quantitative reaction (99.7%) with cadmium. but the DH and the chlorideconcentration arecritical. At such high ionic strengths, the data upon which the calculations of K' are hased are not strictly applicable. Moreover, the indicator color change may be adversely affected, as is the case with xslenol orange where the maximum p H value for a clear color change decreases as the ionic strength increases (9). Nevertheless, by back-titration of EDTA with zinc at p H 5.5, it is ~ o s s i b l invractice e to determine cadmium and mercury error when mercury is masked by 2 M with'less than i% chloride. If the concentration of chloride is decreased to 0.2 masking of mercury is incomplete and the endpoint is nrotracted and inaccurate, while at D H 5.0 and in 2 M chloride medium, some of the cadmium is aiso masked. These findings 716 / Journal of Chemical Education
Conditional stability constants fn EOTA complexes with cadmium(l1) in absence of chloride (1). in 0.2 Mchloride (2). and in 2.0 Mchloride (3):and wilh mercury(l1)in absence of chloride (4). in 0.2 M chloride (5).and in 2.0 M Figure 7
chloride (6). Dstermination of Cadmium and Mercury (0.2 mmole in totall in a Mixed Solution Relatwe to the Determmation of Each in t h e Absence of the Other (molar rafio)
Masking Agent
1:3 1:l 3:l 1:3 1:l 3:l
iodide iodide iodide bromide bromide bromide
Cd: Hg
Standard deviations are given in
Recovery of Cd 1%)
Recovery of Hg 1%)
(0.1) 100.4 (0.1) 100.1 (0.1) 99.1 (0.4) 99.6 (0.21
100.1 (0.1) 100.0 (0.1) 100.1 (0.1) 100.2 (0.1) 100.3 (0.2) 100.3 (0.11
100.5
99.7 (0.11
parentherer.
are in complete agreement with the data in Figure 7. The titration conditions are, therefore, too critical for chloride to be recommended as a selective masking agent. Experimental L)isrillcd water and analytical grade reagents (apart from the in. dicators) should be used throughout. Prepare a mixture of cadmium(lll and mercuryill, tn dissolving the metal nitrates in 0.05 M nitric acid: the total metai concentration should be approximately 0.01 M. Dissolve pure zinc metal in the minimum volume of8Mnitric acid, and dilute to volume to give a standard 0.01 M zinc(I1)solution. Standardize a 0.01 M solution of the disodium salt of EDTA against the zinc solution. Dissolve NH&l (70 g) in concentrated ammonia solution (600 ml, sp.gr. 0.88), and dilute to 1I. Prepare a 1%dispersion of eriochrome black T in powdered KC1, and a 0.5%aqueous solution of xylenol orange. Add35ml ofEDTAsolution to25mlofCdlHgsolution.After5min (the reaction between HefII) and EDTA is relativelv slow) add amdonia buffer (about 2 mfi'kd eriochrome black T iddicator. Titrate with zinc to the first perceptible permanent red-purple, then add K1 (2 g) and titrate to the second endpoint. Instead of ammonia buffer, solid hexamine (1g) may be added to raise the pH to 5.5, in which case xylenolorange isused as indicator. The solution is titrated with zinc to the first permanent pink, then KBr (1.2 g) is added and the titration continued as before. To prevent discharge of heavy metals into the environment, the residues from the titration should he collected and concentrated (ex.. by evaporation to dryness) prior to appropriate disposal. Conclusions Both bromide and iodide yield satisfactory results (the table), but iodide is the preferred masking agent in analytical applications owing to the greater permissible latitude in the reaction conditions.
The close correlation found in these systems between experimental results and the predicted reaction feasibility is an excellent demonstration of the usefulness of conditional stability constants.
Ed., F.Enke, Stutfgart,
,.XL
9,w
Acknowledgment
The author is indebted to Drs. R. A. Chalmers, I. L. Marr, and M. R. Masson for helpful discussion. Literature Cited (11 "KampleromelrischeBe~limmungsmelhw'on mif nlripkr:3.Aunage,E. Darmstadt, p. 73.
(2) Schwareenbseh.G.. "Die komplerometrisehe Turofian:2nd
(3) Ringbom. A., J. CHEM. EDUC.. 35.282 (1958). (41 Ringbom, A,. "Complexation in Analytical Chemistry: Interaeienee, New York. 1963. (5) Ringbom, A., in "Indicators: (Editor: Bishop, E.), Pergamon Preas, Oxford, 1972, p. (6) Rpilley, C. N., Schmid, R. W., and Sadek, F. S., J. CHEM. EDUC.. 36, 555, 619, (1959). (71 van dor Linden, W. E..and Beers, C., Talonto, 22.89 (1975). (8) Grassino, S. L..and Hume, D. N., J.Inorg. Nuci Chem., 32,3112 i1970l. (9) Flaschka, H., and Butcher,J., Microehem. J.,7.407 (1963): Chomiat-Anolyat, 54.36 (1965).
Merck AG,
Volume 54. Number
11. November 1977 / 717
