CHM3120L

Introduction to Analytical Chemistry: Laboratory

 
 Spectrophotometric Determination of Iron in Drinking Water: Procedure

Preparation of Stock Solution.

1. To prepare a 50 ppm stock solution of Fe, accurately weigh 175 mg of ferrous ammonium sulfate into a clean 500 mL volumetric flask.
2. Dissolve in 100 mL of H₂O and then add 1 mL of concentrated H₂SO₄. Dilute to the mark and mix thoroughly.
3. Calculate the concentration of your stock solution, which should be close to 50 ppm.
   

Preparation of Standard Solution.

1. To determine the number of drops of sodium citrate needed to bring the pH of standards to a value between 3 and 4, use a pipet to transfer a 5 mL aliquot of the iron stock solution to a beaker and add a drop of bromophenol blue indicator.
2. Add sodium citrate drop-by-drop until the indicator turns blue, noting the number of drops required.
3. Discard this sample, and calculate the number of drops of citrate needed per milliliter of solution.
   

Determination of the Unknown.

1. Obtain an unknown sample from the TA.
2. Exchange a 100-mL volumetric flask for one that contains the unknown.
3. Dilute to volume with deionized water.
4. Record the unknown code from the label.
5. Find the number of drops of citrate needed to bring the sample to pH 3-4:
   • Transfer a 10 mL aliquot of the sample to a beaker and add a drop of bromophenol blue.
   • Add either 25% sodium citrate (if the water is acidic) or 0.1 F H₂SO₄ (if the solution is basic) dropwise until the indicator reaches its transition color.
   • Calculate the number of drops required per milliliter of sample and then discard this sample.

All standard and sample solutions should be prepared at the same time so that when the spectrophotometric measurement is made all solutions will have had equal time for color development, which is quite slow for this procedure. It is recommended that the actual absorbance measurement not be made until the following lab period.

To prepare colored Fe(II)-o-phenanthroline standards.

1. Transfer 1, 2, 5, and 10 mL aliquots of the stock solution to 100 mL volumetric flasks using a Mohr pipet.
2. Add 1 mL of hydroxylamine solution, 3 mL of o-phenanthroline solution, and the appropriate amount of citrate.
3. Prepare a blank in the same way, omitting the stock iron solution and citrate. Dilute all solutions to the mark.
4. Transfer with a transfer pipet a 50 mL aliquot of the unknown water sample to a 100 mL volumetric flask.
5. Add 1 mL of hydroxylamine solution, 3 mL of o-phenanthroline solution, and adjust the pH to 3-4 as described above.
6. Dilute to the mark.
   

Absorbance Measurements.

1. Turn on the ThermoSpectronic Genesis 20. The instrument must warm up for at least 20 minutes before measurements are made.
2. Set the wavelength to 510 nm by using the nm up/down keys.
3. Use the A/T/C key to put the instrument in the absorbance mode. The instrument cycles through modes, so press the button repeatedly until the absorbance mode is reached.
4. Fill a cuvette with the blank solution and insert it into the spectrometer. Be sure to line up the mark on the cuvet with the arrow on the instrument so that the cuvet can be inserted in the same position for each measurement.
5. Press the 0 ABS key. The display should read zero absorbance.
6. Using the same cuvette, measure the absorbance of the standard and sample solutions. It is recommended that the measurements be made on the lowest concentration standard first and then on sequentially higher concentration solutions. Do this by removing the cuvette from the spectrometer, rinse it with the next solution to be measured, fill it with the solution to be measured, and place it back into the spectrometer.
7. Using Excel, prepare an analytical curve of absorbance vs. ppm iron.
8. Have Excel add a trendline and report the equation of the line:

where A is absorbance, C_Fe is the Fe concentration, a is slope and b is intercept of the linear regression line.Use the equation to determine ppm Fe of your drinking water sample. Don't forget the 50:100 dilution!