INSTRUMENTAL ANALYSIS

LAB.

Mr Haydar A.M.S

BSc, MSc

Faculty of Science

Chemistry Department

Email: Haydar.kovly@gmail.com

Exp. 3 Determination the Calibration Curve of Cobalt Nitrate by Spectrophotometer

ObjectivesIn this experiment, you will

• Prepare a set of Co+2 standard solutions of known concentrations,

• Measure the A or %T of the known solutions and construct a calibration curve, and

• Measure the A or %T for unknown solution and determine its concentration from the calibration curve.

Ultraviolet/Visible Molecular Absorption Spectrometry

The instrument used to measure the amount of light absorbed by the sample is a

spectrophotometer.

Components of a Spectrophotometer

4

5

Wavelength Selectors

Light source

Grating

Rotating the gratingchanges the wavelength going through the sample

slits

slits

Sample

filter

Phototube

Wavelength Selectors

When blank is the sample Io is determined

otherwise I is measured

Separates white lightinto various colors

detects light &measures intensity

- white light of constant intensity

6

7

8

Sample Containers

Range

Transmittance (T) is defined as the amount of light passing through the sample solution (I) divided by the amount of incident radiation (Io).

T = I/ Io

•Transmittance is often expressed as a percentage or

%T = I/Io × 100%

The Absorbance (A) of a medium is defined by the equation:

A = log Io/I = -log T

As the radiation is absorbed in the sample, the total intensity of radiation is reduced as it travels through the sample. This results in a non-linear relationship between transmittance and concentration

Absorbance (A) is based on the amount of light absorbed by the solution and is defined as the log of the inverse of the transmittance.

A = log10

(1/T)

Within limits, the relationship between absorbance and concentration is linear.

The blank

• The blank contains all substances except the analyte.

• Is used to set the absorbance to zero:

A blank = 0

• This removes any absorption of light due to these substances and the cell.

• All measured absorbance is due to analyte.

Analyte is a substance undergoing analysis.

Beer’s lawBeer’s law states that the amount of light absorbed by a solution (colored) is directly

proportional to the path length (b) through the medium and the concentration (c) of the absorbing species,

A (absorbance) = log Io/I = a b c

• Where (a) is the proportionality constant called the absorptivity

• b is the path length of the sample i.e. the inside cross section of the sample cuvette (cm).

• When the concentration is expressed in (mol/L), the absorptivity is called the molar absorptivity and is given the symbol (є)

A = є b c (units of є = L.mole-1.cm-1)

Determination the calibration curve of cobalt nitrate by spectrophotometer

Procedure

1. Obtain TWO matched cuvettes, the first cuvette is for distilled water and the second one is

for the cobalt solution.

2. Prepare the following solutions

From stock solution (0.2M)Co(NO3)2.6H2O

(M)

Absorbance

(A)

0.02

0.04

0.06

0.08

0

Part A Calibration Curve

Procedure Continued

3. Set the wavelength dial 505 nm

4. Measure the absorbance of the blank (water).

5. Measure the absorbance of the cobalt solutions and record the absorbance

6. Empty and rinse the cuvettes thoroughly with water.

7. plot the absorbance versus concentration

8. Measure LOD, LOQ and sensitivity

Procedure Continued

1. Measure the absorbance of the unknown concentration

2. use a calibration curve that you plot it in part A and determine the concentration of the unknown solution.

Part B Determination the absorbance of the unknown

20

1.00

0.20

0.60

Absorbance

0.30 0.40 0.50

0.80

0.40

0.10 0.20 Concentration in mol / Litre

0 0