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INSTRUMENTAL ANALYSIS

LAB.

Mr Haydar A.M.S

BSc, MSc

Faculty of Science

Chemistry Department

Email: Haydar.kovly@gmail.com

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Simultaneous determination of chromium and cobalt in a solution by

visible spectroscopy

Lab 5

Aim of the experiment

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Simultaneous analysis of two-components mixture using Beer’s Law.

Introduction

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This experiment is concerned with the simultaneous spectrophotometric determination of two solutes in a solution.

These solutes absorbs light in the visible region

The absorbance’s are additive, provided there is no reaction between the two solutes. Hence we may write

A = A1 + A2 + A3 ........

and

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Introduction

Absorption Spectrum

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Introduction

and

A = ɛ b c If we set b = 1 cm then

and

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Introduction

• C1 is the concentration of Co(NO3)2

• C2 is the concentration of Cr(NO3)3

• ɛ1λ1 is the molar absorptivity for Co(NO3)2 at 510 nm

• ɛ1λ2 is the molar absorptivity for Co(NO3)2 at 575 nm

• ɛ2λ1 is the molar absorptivity for Cr(NO3)3 at 510 nm

• ɛ2λ2 is the molar absorptivity for Cr(NO3)3 at 575 nm

• Aλ1 is the absorbance of the mixture at 510 nm

• Aλ2 is the absorbance of the mixture at 575 nm

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Introduction

The values of molar absorption coefficients ɛ1 and ɛ2 can be deduced from measurements of the absorbances of pure solutions of substances 1and 2.

By measuring absorbance of the mixture at wavelengths λ1 & λ2, the concentrations of the two components can be calculated.

Molar absorption coefficients (ɛ)

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Experimental

Reagents list

Prepare a stock solution of Cr(NO3)3 (0.05 M)

Prepare a stock solution of Co(NO3)2 (0.188 M)

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Procedure

1. Perform a series of serial dilutions of the Cr(NO3)3 and Co(NO3)2 as a table below:

[Cr(NO3)3] (M) A at 510 nm A at 575 nm [Co(NO3)2] (M) A at 510 nm A at 575 nm

0.01 0.0376

0.02 0.0752

0.03 0.1128

0.04 0.1504

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Procedure

2. Measure the absorbance of the solutions at 510 and 575 nm and record the absorbance of each concentration.

3. Plot A versus concentration of Cr(NO3)3 and Co(NO3)2

4. Determine the slopes (ɛ) for Cr(NO3)3 at 510 and 575 nm and the slopes for Co(NO3)2 at

510 and 575 nm.

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Procedure

5. Calculate the concentration (in mole/L) of Cr(NO3)3 and Co(NO3)2 in the mixture using equation 5 and 6, where C1 is the concentration of Co(NO3)2 and C2 is the concentration of Cr(NO3)3.

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Questions

•What is the difference between the absorbance and absorbtivity?

•Why is Beer Lambert's law not obeyed for high and low

concentrated solutions?

•On what the wavelength depends?