Principle:

The operation of an atomic absorption spectrophotometer and the effects of different instrumental parameters will be studied. In addition, students will determine the concentration of unknown by the method of standard addition.

Solutions and Chemicals Required:

1. Provided: Ethanol, 500-ppm Ca stock solution from CaCl₂. Use the standard stock to prepare a 50-ppm working standard solution. Use the 50-ppm working standard to prepare the required solutions below, 2000 ppm Na stock solution from NaCl, 100-ppm phosphate solution from Na₂HPO₄ , and 100-ppm Al solution from Al₂(SO₄)₃*K₂SO₄*24H₂O.

Study of Instrumental Parameters:

Follow the instructor’s directions and provided operation manual for the operation of the instrument. ACETYLENE IS EXTREMELY FLAMMABLE! EXERCISE CAUTION WHENEVER THE ACETYLENE IS ON. The lamp required 5 to 10 minutes to warm up.

1. Burner height: Optimize the position of the lamp and burner to get the maximum abosrbance. The flame should have a slight yellow color. Now, using the software, increase the fuel proportion to impart a strong yellow glow to the flame (fuel rich). The yellow glow is due to unburned carbon particles in the rich flame. In a lean flame, and excess of oxidant is present and the flame appears blue. Prepare and aspirate a 5-ppm calcium solution and note its absorbance at 422.67 nm. The monochrometer is now set exactly at the calcium line. What other lines would also measure calcium? When would you use the 422.67nm line and when would you use the others? With the burner vertical adjusting knob, raise the burner so the light bean just passes over the base of the flame. Measure the absorbance of the 5-ppm calcium solution. Lower the burner in increments (six to eight steps) and record the absorbance at each height.  Plot the absorbance against burner height and select the optimum height.

2. Fuel/ air ratio: Hold the air pressure constant and adjust the fuel pressure in increments from a very fuel-rich to a lean flame. Record and report the absorbance of the 5-ppm Ca solution at each increment.

Select the optimum fuel pressure and vary the air pressure in a similar manner. Plot absorbance against gas pressure for both fuel and air, noting the pressure setting of one is held constant. Select the optimum fuel and air settings. Is this a rich, stoichiometric, or lean flame?

Solutions containing more than one analyte

1. Effect of phosphate: Prepare a solution containing 5 ppm Ca and 10 ppm phosphate. Record the absorbance of this solution using the optimum conditions determined above and compare them to results to the plain 5-ppm solution. Explain the results.

Prepare a solution containing 5-ppm Ca, 10 ppm phosphate, and 1% SrCl₂. Prepare also a solution of 5 ppm Ca and 1% SrCl₂. Record the absorbance of each solution, explain any differences.

2. Effect of sodium: Prepare a solution containing 5 ppm Ca and 1000 ppm Na. Record the absorbance and compare with that of 5 ppm Ca. Explain any differences.

3. Effect of aluminum: Prepare a solution containing 5 ppm Ca and 10 ppm Al. Record the absorbance and compare that with 5 ppm Ca alone. Suggest a possible reason for the observed results.

Organic Solvent Effect:

Prepare a solution of 5 ppm Ca in 50% ethanol. Record the absorbance using 50% ethanol as the blank. (When an organic solvent is aspirated, a lean flame must be used in order to burn the solvent. With the blank solvent aspirating, increase the air pressure until a blue flame is achieved.) Compare this with the absorbance of 5 ppm solution of Ca in water, explain the results.

Determination of Calcium in an Unknown: Method of Standard Addition

Determine the amount of calcium in milk. The method of standard additions is particularly useful when matrix effects in the sample may be important. Dilute your unknown, using distilled water to give a concentration of Ca in the milk of about 5 ppm Ca.  Transfer with a clean pipet separate 10.0 mL aliquots of the diluted unknown to three separate clean flasks. Add to these 0.5, 1.0, and 1.5 mL of the 500-ppm standard calcium solution. Dilute the solution to 100mL.  This results in an increase in the calcium concentration in the diluted unknown of about 2.5, 5.0, and 7.5 ppm respectively, depending upon the exact concentration of the standard and should bracket the concentration in the unknown.

Zero the instrument with distilled water and aspirate the diluted unknown and the standard addition samples. The absorbance increases in the latter are due to the added calcium. Prepare a plot absorbance against added concentration of calcium (starting at zero added, that is the sample). From the x-axis intercept of the plot, determine the concentration of calcium in the diluted unknown. Calculate the concentration in the original sample. How does this method account for phosphate or other interference?

Lab reports should be written with your lab partner.  Each individual should submit a contribution form.  The reports should be written up in the ACS format, suggested length, not including figures should be 2-3 times number of weeks spent on the lab in pages.  Be sure to include references.  Lab reports will be due one week after completion of the lab.  

For your report answer all the questions/ plot all the data for the portions of the lab procedure in bold. Be sure to answer them in a logical, easy to follow manner. Include directions for AA use in your notebook and in your material and methods section..

Pre-lab:  Determine the approximate concentration of calcium in milk. .Read pages 230 to 248 in your textbook and answer questions 9-1 and 9-3 from Chapter 9.