Experiment6_plus apparatus

Experiment6_plus apparatus - EXPERIMENT 6 ABSORPTION...

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1 EXPERIMENT 6 ABSORPTION SPECTROPHOTOMETRY: MULTI-COMPONENT DETERMINATIONS USING ABSORBANCE AND FIRST-DERIVATIVE DATA This is a group experiment. All groups in each laboratory will share one set of standard solution s. I. INTRODUCTION This experiment involves the use of absorption spectrophotometry to quantify concentrations of three metal ions, Co(II), Cu(II), and Ni(II) based on differences in the absorption spectra of EDTA complexes of the ions. Principal purposes of the experiment are to illustrate: the use of chelating agents to develop colored complexes with metal ions, the use of solid-state imaging detectors for quantitative spectrophotometry, the use of first-derivative spectroscopy to compensate for possible instrumental drift, and the combined use of multiwavelength data with matrix algebra to resolve multi-component samples without a separation step. II. OVERVIEW A. Options for mixtures Most practical quantitative determinations require the quantitation of two or more components in each sample. Two general approaches used to quantify different components in common samples are a) to use procedures with built-in selectivity for the individual components and b) to separate the individual components prior to the measurement step. This experiment illustrates the first option, namely the use of a procedure with built-in selectivity for the three components of interest. Specifically, a complexing agent, ethylenediaminetetraacetic acid (EDTA), is used to form complexes with three metal ions that have different absorption spectra. Then, differences in the absorption spectra are used to resolve the three components from one another. B. Rationale If different components in a sample have different absorption spectra and if there are no interactions among the components, then the absorbance at each wavelength will be the sum of the absorbances of the individual species. By measuring the absorbances at a number of wavelengths equal to or greater than the number of components in the sample, it is possible to write a series of simultaneous equations in terms of the absorbances, path lengths, molar absorptivities, and concentrations of the different species. If the path length and molar absorptivities are known, it is possible to solve the simultaneous equations for the concentration of each of the species in the sample. The process works best for wavelengths involving maximum differences among the absorptivities of the different species. C. Specific example Metal ions such as Co(II), Cu(II), and Ni(II) react with dihydrogen form of EDTA to form complex ions as follows: M 2+ + H 2 EDTA 2- MEDTA 2- + 2H + (1) Many of the complex ions produced by such reactions are colored and absorb light in the visible region of the spectrum. The absorption of light can be used to quantify concentrations of the metal ions in solution. The solid curve in Fig. 1A is the spectrum of a mixture of the EDTA complexes of Co(II),
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Experiment6_plus apparatus - EXPERIMENT 6 ABSORPTION...

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