Purpose: To determine the concentration of Calcium and Magnesium in water using the Perkin Elmer 1100B Spectrophotometer.
Materials:
Procedure:
1. Select four concentrations of each standard metal solution to bracket the expected metal concentration of a sample by creating serial dilutions of stock solutions. Prepare the dilutions using distilled water containing 1.5 mL conc HNO3/L
a. Stock Calcium: To 2.497 g oven-dried CaC03 add 50 mL of water. Add dropwise a minimum volume of conc HCl (about 10 mL) to complete solution. Dilute to 1000 mL with water; 1.00 mL = 1.00 mg Ca.
b. Stock Magnesium: Dissolve 4.952 g oven-dried magnesium sulfate, MgSO4, in 200 mL water. Add 1.5 mL conc HNO3 and dilute to 1000 mL with water; 1.00 mL = 1.00 mg Mg.
2. Prepare a Lanthanum solution by dissolving 58.65 g lanthanum oxide, La2O3, in 250 mL conc HCl. Add acid slowly until the material is dissolved and dilute to 1000 mL with water.
3. For both Ca and Mg mix 100 mL of standard or sample with 10 mL lanthanum solution before aspirating.
4. Follow the manual which accompanies the spectrometer for operating directions. Different spectrometers have different operating directions, and consequently universal operating procedures can not be made applicable to every instrument.
YOU ARE RESPONSIBLE FOR UNDERSTANDING THE PURPOSE OF EVERY STEP IN THE MANUAL.
5. Prepare a calibration curve by plotting on linear graph paper absorbance of standards versus their concentrations or by printing the graph produced by the computer. Plot calibration curves for Ca and Mg based on original concentration of standards before dilution with lanthanum solution.
6. Rinse nebulizer by aspirating water containing 1.5 mL HNO3/L. Atomize blank and zero instrument. Atomize sample and determine its absorbance. Determine the concentration of the unknown (tap water). Use replicates. You may need to make serial dilutions of the tap water to bring the results within the range of your standards.
7. Check standards periodically during a run. Recheck calibration curve by aspirating at least one standard after completing analysis of a group of instruments.
8. Run a blank or solvent between sample or standard readings to verify baseline stability.
QUESTIONS:
1. (10 points) What is the hardness of tap water in terms of ppm Ca + Mg? Show graphs and calculations.
2. (10 points) What is the hardness of tap water in mg/L CaCO3? How does your value for hardness generated by the AAS compare to the data you got using the EDTA method?
3. (5 points) Why does rotating the flame ninety degrees help with high concentration samples?
4. (10 points) Why is there more than one useable wavelength per element? Why would you want to use a wavelength other than the first recommended one?
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