Laboratory #7:
 Spectrophotometric Determination of Iron in Dietary Tablets*
Adapted from "Quantitative Chemical Analysis", D.C.Harris, 5th ed.

August 7, 2001
 

Introduction.  In this experiment, you will use Beer's Law to determine the iron content of a vitamin pill (Flintstones!). Using the relationship, A = ebc, where e is the molar extinction coefficient, b is the pathlength and c is the concentration (in mol/L), you can determine the concentration of a solution by measuring the absorbance. Typically this entails the generation of a calibration curve, which plots the absorbance of a series of solution with known concentrations. By describing the instrument response to the standard solutions, either graphically or mathematically, it is then possible to determine the analyte concentration in an unknown sample.
 

Procedure. Working in teams of two, follow the procedure outlined below.

Preparation of Tablet Solution.

1) Place one vitamin tablet in a 100 mL beaker and boil gently (in a fume hood) with 25 mL of 6 M HCl (CAUTION!) for at least 15 minutes. Do not "spatter" mixture as you will lose sample. Filter the solution directly into a 100 mL volumetric flask. Wash the beaker and filter several times with small portions of deionized water ro complete a quantitative transfer. Allow the solution to cool, dilute to the mark and mix well.

2) Dilute 5.00 mL (use a pipet!) of the above solution to 100.0 mL in a fresh volumetric flask. You will use this solution in step 6 below.

Preparation of Standard Iron Solutions.

3) Pipet 10.00 mL of standard iron solution (prepared by dissolving 0.0723 g of Fe(NH₄)₂(SO₄)₂^.6H₂O in a 250.0 mL volumetric flask) into a 100 mL volumetric flask and add 3.0 mL of sodium citrate solution. Also add 2.00 mL of hydroquinone solution and 3.00 mL of o-phenanthroline solution, dilute to the mark with deionized water, and mix well.

4) Prepare three additional solutions using the components listed in step 3. Pipet 5.00, 2.00 and 1.00 mL of the standard iron solution into 100 mL volumetric flasks. To each add sodium citrate in the same proportion as in step 3, i.e., use 1.5 mL, 0.60 mL and 0.30 mL, of sodium citrate in these three solutions. Also add 2.00 mL of hydroquinone and 3.00 mL of o-phenanthroline to each solution.

5) Prepare a "blank" solution by adding 2.00 mL of hydroquinone and 3.00 mL of o-phenanthroline to a 100 mL volumetric flask and diluting to the mark with deionized water.

Analysis of Solutions.

6) Pipet 10.00 mL of the tablet solution prepared in Step 2 to a 100 mL volumetric flask. Add 3.5 mL of citrate solution, 2.00 mL hydroquinone solution and 3.0 mL of o-phenanthroline. Dilute to the mark and mix well.

7) Allow all of the solutions to stand for at least 10 minutes before measuring their absorbance. Use the Ocean Optics spectrophotometer to measure the absorbance at 510 nm. Use deionized water as the reference.
 

Report.  Prepare a calibration curve of the iron standard solutions. Plot absorbance as a function of concentration (mol/L). Determine the equation for the "best-fit" line and use this to determine the concentration of iron in the unknown solution from its measured absorbance. From this value, calculate the amount of iron in the tablet (you will need to use the v₁c₁=v₂c₂ relationship for the two dilutions you made of your initial tablet solution).