ANALYTICAL cHEMISTRY - III

MSCM301

Experiment 1

Aim of the Experiment

Determination of iron content (as Fe3+) in the given salt/tablet solution using colorimeter/spectrophotometer.

Principle

  • The experiment is based on Beer-Lambert's law, according to which the absorption of an electromagnetic radiation (light) transmitted through the medium is directly proportional to the medium concentration.

  • When a monochromatic radiation (light) passes through a colored solution via series of lenses, which navigate the colored light to the measuring device, some part of the incident radiation (light) is absorbed by the colored solution which is related to the color intensity of the solution. Colorimeter analyzes the color compared to an existing standard. A microprocessor then calculates the absorbance (A) or percent transmittance (%T). If the concentration of the solution is greater, more light will be absorbed, which can be identified by measuring the difference between the amount of light at its origin and that after passing the solution.

  • To determine the concentration of an unknown sample, several sample solutions of a known concentration are first prepared and tested. The concentrations are then plotted on a graph against absorbance, thereby generating a calibration curve. The results of the unknown sample are compared to that of the known sample on the curve to measure the concentration.

  • Colorimeter contains filters. Filters prevents passage of radiation at all wavelength but accept a fixed wavelength regions. The filters are classified as:

  1. Absorption filters: Absorbs radiations of all wavelengths beyond a fixed wavelength.

  2. Interference filters: (a) Wedge filter: Path length is varied. (b) Dichoric filter: Path length is fixed.

Apparatus required

Colorimeter, Nessler’s Tubes (6 Nos.), 100 ml standard measuring flask (2 Nos.), Evaporating dish, Test tubes, etc.

Chemicals required

Concentrated HNO3, HNO3 solution (4 M), H2SO4 solution (2 M), KMnO4 solution (0.02 N)

Reagents preparation

Standard solution of Fe (II): Dissolve 0.7022 g ammonium ferrous(II) sulphate in water and dilute it to 1000 mL with water and mix thoroughly.

Working standard of Fe (II): Pipette out 5 mL of the standard solution of Fe(II) into a 100 mL measuring flask using a 10 mL graduated pipette. Add 5 mL of 2 M sulphuric acid and run in cautiously a dilute solution (1:5) of potassium permanganate until a slight pink coloration remains after stirring well. Make up the volume to 100 mL with de-ionized water.

Potassium thiocyanate solution: Dissolve 10 g of potassium thiocyanate (KSCN) in 100 ml de-ionized water.

Procedure

  • Part I : Construction of calibration curve: Place 2 mL, 4 mL, 6 mL, 8mL and 10 mL of the working standard solution into cleaned, dried and labeled Nessler’s tubes. Add 3 mL of 1:5 nitric acid. Dilute each of the solution to 25 mL with de-ionized water. Prepare a blank using the same quantity of reagents. Measure the absorbance of each of the solution at 480 nm against blank after adding 5 ml of thiocyanate solution. Construct calibration curve by plotting the absorbance → concentration.

  • Part II : Pipette out 10 mL of the given sample solution of iron tablet or salt into a Nessler’s tube, add 3 mL of (1:5) nitric acid and make up the volume to 25 mL with de-ionized water. Measure the absorbance of the solution at 480 nm after adding 5 mL of potassium thiocyanate (KSCN) solution. Determine the amount of iron in the solution from calibration curve.

Determination of concentration in ferric ion in ferric salicylate or thiocyanate complex 1.mp4

Observation

Calculation

Iron Content_Calculation Part

Result

The amount of iron in the given salt/tablet is ____________ ppm and __________ mg.

Reference Material

  1. G H Jeffery, J Bassett, J Mendham and R C Denney, Vogel's Textbook of Quantitative Chemical Analysis, 5th Edition

  2. S. Suzanne Nielsen, Food analysis, 4th Edition

  3. Working principle of colorimeter, https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Kinetics/02%3A_Reaction_Rates/2.01%3A_Experimental_Determination_of_Kinetics/2.1.05%3A_Spectrophotometry

Working principle of Colorimeter

Questions

  1. What is the working principle of colorimeter?

  2. What is the major difference between colorimeter and spectrophotometer?

  3. Discuss the limitations of Beer Lambert's law.

  4. What type of colored species is being formed during the analysis? Discuss the role of potassium thiocyanate. What other reagents can be used to form the colored species with Fe(III) ions?

Developed by

Dr. Viraj Bhanvadia,

Assistant Professor, Chemistry,

viraj.bhanvadia@gsfcuniversity.ac.in