ANALYTICAL cHEMISTRY - iv

Experiment 2

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 an electromagnetic 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. Spectrophotometer analyzes the absorption compared to an existing standard/blank. 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.

• Fe²⁺ can be extracted from aqueous solution with 1% oxine in chloroform by double extraction when pH is around 2 to 2.5. At this point Ni²⁺, Co²⁺, Ce³⁺, Al³⁺ does not interfere. Fe (III)-oxinates is dark coloured in CHCl₃ and absorbs at 410 nm.

• 8-Hydroxyquinoline (oxine) is a versatile organic reagent and it has tendency to form chelate with Fe²⁺ very fast.

• The chelate of doubly or triply-charged metal ions having formula M(C₉H₆OH)₂ and M(C₉H₆OH)₃. Oxine is generally used as 1 % solution in CHCl₃.

• Oxine has both phenolic and basic nitrogen atom. So, it has an amphoteric nature.

• It is completely extracted from aqueous solutions using CHCl₃ between pH < 5 and pH > 9 range.

• Distribution co-efficient of compound between water and CHCl₃ is 720 at 18 ^oC.

• The structures of Fe (III)-oxinate and Fe (II)-phenanthroline complexes are shown below in the reference section. Fe (II)-phenanthroline and Fe (III)-oxinate complexes are colored species, which imparts reddish-brown color to the aqueous solutions. These complexes can also be extracted into the chloroform layer (organic layer) due to the organic nature of the ligand itself. The spectrophotometric analysis of these complexes in either aqueous or chloroform solutions allow determination of concentration from the respective absorbance values through Beer-Lambert's law.

Apparatus required

Spectrophotometer instrument, Funnel, Beakers, Pipette, Burette, Separating funnel (25 mL), etc.

Chemicals required

Ferrous ammonium sulphate (FAS), 1 % oxime (8-hydroxyquinoline) solution, Dilute ammonia solution, etc.

Preparation of reagents

1. Standard solution (100 ppm) of Fe (II):

Weigh 0.8635 g of FAS and dissolve it in 3 mL of concentrated H₂SO₄. The solution is made up to 1000 mL using double distilled water. This is 100 ppm standard solution of Fe (II).

2. Working standard solutions of Fe (II):

From the 100 ppm Fe (II) standard solution, transfer 1.0 mL, 1.2 mL, 1.4 mL, 1.6 mL, 1.8 mL, and 2.0 mL of solution in a beaker and then add respectively, 9.0 mL, 8.8 mL, 8.6 mL, 8.4 mL, 8.2 mL, and 8.0 mL of dilute sulphuric acid solution (0.005 M). The pH of the prepared working standards should be 2 - 2.5.

3. 1% oxine (8-hydroxyquinoline) solution in chloroform:

Dissolve 1 g of 8-hydroxyquinoline in 100 mL of freshly distilled chloroform. Cover the prepared solution to prevent the evaporative loss of chloroform.

Procedure

1. Take the prepared working standard solutions and transfer them quantitatively into the separating funnel (25 mL). Use dilute sulphuric acid solution (0.005 M) to transfer the working standard solutions quantitatively.

2. Add 10 mL of 1% oxine solution, prepared in chloroform and shake the contents of the separating funnel vigorously.

3. Allow to settle the contents of the separating funnel for about 10-15 min and the organic and aqueous layers should separate during this period of time.

4. Isolate the bottom organic layer into the clean dry test tubes. Add small quantities of anhydrous sodium sulphate to absorb the residual water content.

5. Measure the absorbance of each of the solution at 410 nm using spectrophotometer.

6. Prepare a blank sample as well by transferring 10 mL of dilute sulphuric acid solution (0.005 M) into the separating funnel and extracting it using 10 mL of 1% oxine solution, prepared in chloroform. Isolate the bottom organic layer into the clean dry test tubes. Add small quantities of anhydrous sodium sulphate to absorb the residual water content. Measure the absorbance of the blank solution at 410 nm using spectrophotometer.

7. Prepare the calibration curve by plotting the absorbance values on the Y-axis and the solution concentration (either in mL of standard solution or in ppm of Fe (II)) on the X-axis.

8. Similarly, take 1.0 mL of the sample solution and add 9.0 mL of dilute sulphuric acid solution (0.005 M) and transfer it quantitatively into the separating funnel (25 mL). Use dilute sulphuric acid solution (0.005 M) to transfer the solution quantitatively.

9. Add 10 mL of 1% oxine solution, prepared in chloroform and shake the contents of the separating funnel vigorously. Isolate the bottom organic layer into the clean dry test tube. Add small quantities of anhydrous sodium sulphate to absorb the residual water content. Measure the absorbance of the sample solution at 410 nm using spectrophotometer.

Find the concentration of Fe (II) in the given unknown sample solution by extrapolating the absorbance value of sample solution on the calibration curve.

Result

The unknown solution of Fe³⁺ have concentration of _________ ppm.

Questions

1. What other complexing agents can be used for spectrophotometric determination of Fe (II) and Fe (III)?

2. What will be the structure of Fe (II)-oxinate complex?

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