LAB REPORT FOR RESOLUTION OF TWO COMPONENT MIXTURE

ABSTRACT Spectroscopy is the study of the interaction of matter and radiation. There are many types of spectrochemical methods, but the one that is applied in the experiment is absorption spectroscopy, because it measures light as a function of wavelength. Chromate and Dichromate ions both absorb visible light though their absorbance maxima are fairly well separated. A spectrophotometer was used to determine the absorbency at certain wavelength of the chromate and dichromate solutions, which in this experiment, are 545 and 440 nm. By measuring the absorbance at two different wavelengths of a solution containing both ions, it is possible to simultaneously determine the concentration of each ion in the solution. An unknown

solution

containing

these

species

was

analysed

using

a

spectrophotometer. The concentration of cobalt in the mixture calculated using Beer’s law was 0.0131 M, while chromium was 0.0185 M.

INTRODUCTION The spectrophotometer is an important instrument for scientists in analysing chemical compounds. Spectrophotometers take advantage of the EM radiation absorption of chemical compounds. Specifically, it detects absorbing EM radiation of selected wavelengths and produces a value corresponding to its absorption. Spectroscopy is an easy method to use in determining the concentration of an unknown species since it only needs the molar absorptivity of the sample, the absorbance, and the cell path length to calculate the concentration of the analyte. Beer’s Law states that there is a directly proportional relationship between the absorbance and concentration of a substance A=ebc. Where A is the absorbance, e is the molar absorptivity, b is the path length through the cuvette, and c is the concentration. Absorbance values measured at the same wavelength can be added and subtracted. This is important because it allows corrections to be applied and also allows for the analysis of two colored compounds in a mixture. The Lambert – Beer law has an additive property such that if the sample comprises several light-absorbing molecular species. Beer’s law is valid simultaneously for all absorbers in a solution. There are instances when the presence of one species in a sample does not influence the measurement of another species in the same sample; i.e., they do not interfere. If several species in a solution absorb at the same wavelength without chemically interacting with each other, then the total absorbance is the sum of the individual absorbances for each species. In this experiment two analytes are present and neither affects the light-absorbing properties of the other. Thus, the absorption of light by the components of the sample solution is additive; that is, the total absorption of light at any given wavelength is just the sum of the absorbances the two substances would show if measured individually under the same conditions. Beer’s Law applies to each absorbing species

independently; thus we may express the absorbance at any wavelength as Aλ= eλ1bC1 + eλ2bC2 + … This additive property may be used to determine the concentration of both components of a binary mixture by measuring the absorbance of the mixture at two different wavelengths. For a two component system (say species present are: X & Y) when each component absorbs appreciably at λ max of one another, one can write: A1λ1= e1λ1bC1 + e2λ1bC2, A1λ2= e1λ2bC1 + e2λ2bC2 where λ1 and λ2 are two different wavelengths and C1 and C2 are concentrations of two species. Once e and b are known, the concentration of an unknown solution can be determined by measuring the absorbance and using the Beer-Lambert Law given above. Usually the path length, b, is known; thus, we can solve these simultaneous equations for the concentrations of species A and B if the absorptivity coefficients of both species at both wavelengths are known. In this

experiment,

the

absorptivity

coefficients

of

the

chromate

and

dichromate ions will be determined at two different wavelengths using Lasany UV – Vis Spectrophotmeter. The absorbance of the sample mixture will then be obtained for the same two wavelengths; solution of simultaneous equations will thus enable us to obtain estimates of the unknown concentrations. In this experiment, a two-component mixture will be resolved in this manner. This spectroscopic analysis method is simpler for the simultaneous determination of multi-component mixtures without separation and degradation of samples. The purpose of this experiment is to introduce the proper operation of a spectrophotometer and how its use relates to chemical analysis. This will be achieved by recording absorbance measurements of a two component mixtures, and calculating its concentrations using Beer’s Law.

CONCLUSION Spectroscopy

is

an

easy

method

to

use

in

determining

the

concentration of an unknown species since it only needs the molar absorptivity of the sample, the absorbance, and the cell path length to calculate the concentration of the analyte. An absorption law called the Beer’s Law governs the overall process of spectroscopy since it gives the relationship of the quantities obtained from spectrophotometric data in order to know the quantity of unknown in the sample. The calculation basically focuses on one formula – the Beer’s law. Beer's law states that absorbance of electromagnetic radiation is directly proportional to concentration: if there is more than one absorbing species in solution, the total absorbance is the sum of the individual absorbance of all the absorbing species, provided there is no interaction among the various species. By measuring the absorbance at different wavelengths (510 and 575 nm), different absorbing components could theoretically be measured by this technique. The absorptivity directly measured from the spectrometer or derived from the Beer’s Law can be used to derive the transmittance of the substance at certain wavelengths.

In a simultaneous determination of the concentration of two species within a mixture, a direct Beer’s Law cannot be applied but instead a combined Beer’s Law must be used since the absorbance measured is the total absorbance of both ions. It is highly advised to keep the cuvettes clean, and wipe them before placing them inside the spectrometer so as not to make discrepancies in the results.