Spectrophotometric Techniques for Investigating Metal Speciation – Copper(I) Chlorides

The thermodynamic stability of aqueous metal complexes can be used for predicting aqueous speciation and metal solubility in the environment. As a graduate student at Monash University I used UV-Vis spectrophotometric techniques (Figure 1) to study the formation of aqueous copper(I) complexes. Copper(I) complexes exhibit no visible absorbance due to a full d-valence band ([Ar] 3d10), however, they absorb strongly in the UV region of the spectrum due to charge transfer between the metal and associated ligands. Therefore, the structure of the UV-Vis absorbance bands are indicative of changes in aqueous metal complexation. Since UV-Vis spectrophotometry is a quantitative technique, where absorbance is described by Beer-Lamberts law (equation 1), a systematic series of absorbance spectra with varying ligand concentration can be deconvolved using principal component analysis coupled with a chemical speciation model to determine the thermodynamic stability of aqueous metal complexes forming. Figure 2 plots a series of copper(I) absorption spectra as a function chloride concentration and temperature.

Figure 1:. Picture of glove box used to prepare oxygen free aqueous copper(I) chloride samples. Sealable quartz cuvettes containing copper(I) chloride solutions and a Varian UV-Vis spectrophotometer used for measuring absorbance spectrum.

(Equation 1)

where, λ = wavelength, l is pathlength, CMi is the concentration of aqueous metal species (i) and εiλ is the molar absorbance (or extinction coefficient, a constant) for species i at wavelength λ.

Figure 2. Copper(I) chloride charge transfer spectra as a function of chloride concentration (LEFT) and temperature (RIGHT). [Cu]total ~ 1x10-3 m.

!=

""=n

i

iMi

ClA

1

,## $

!

"[CuCln ]

n#1(aq )

=m[CuCln ]

n#1( aq)$[CuCln ]

n#1(aq )

mCu

+$Cu

+ (mCl

# $Cl

# )n

A chemical speciation model describing aqueous complex formation in the copper(I) chloride solutions is defined by a series of mass action, mass balance and a charge balance equation:

e.g. mass action equations: Cu+ + Cl- = [CuCl]0

(aq) Cu+ + 2Cl- = [CuCl2]-

(aq) Cu+ + 3Cl- = [CuCl3]2-

(aq) Cu+ + 4Cl- = [CuCl4]3-

(aq) The formation constants (β) of absorbing species are described by their aqueous activities

(a = m * γ = molal concentration * activity coefficient) and are optimized by fitting Beer-Lamberts law to the measured absorbance spectra. This deconvolution (Figure 3) is performed using the software BeerOz [see Brugger, J. (2007) Computers & Geosciences, 33, 2007, p. 248-261] for MATLAB, by optimizing the component species concentration, CMi (Eq. 1) or [C], and molar absorbances, εiλ (Eq. 1) or [E] by non-linear least squares regression to find the minimum value of a residual defined as Σ(AbsCalculated - AbsMeasured)2 (Figure 4):

[A] = [C] * [E]

Figure 3. Deconvolved absorbance spectra showing the quality of fit (LEFT), species distribution given the optimized formation constants (βs) (CENTRE) and the optimised molar absorbance spectrum (RIGHT) of the absorbing species considered in the chemical speciation model fit to the data.

Figure 4. Diff

Figure 4. Residual maps showing the absolute minimum in residual space where optimum β values describing the formation of aqueous metal complexes are found.

Once a suitable chemical speciation model that fits the data has been optimized, the thermodynamic properties for the aqueous species forming in solution can be calculated from the optimized βσ obtained as a function of temperature (see Arrhenius plot below).

!

log"CuCl2

#

!

log"CuCl3

2#

!

log"CuCl3

2#

!

log"CuCl2

#

!

Residuals = (AbsCalc

" AbsMeas)2#