CHEMISTRY 425

Instrumental methods of analysis

                 ATOMIC ABSORPTION FLAME PHOTOMETRY OF CADMIUM

          Start the cadmium hollow cathode lamp as per instructions and allow about 30 minutes for warm-up. Adjust the lamp current setting to 5 mA.

          Use the stock Cd²⁺ solution provided to prepare - via dilution in 100 ml volumetric flasks - standard solutions of 0, 2, 5, 10, 15, 20 and 25 µg/ml Cd²⁺. Also dilute your unknown sample to 100 ml and mix thoroughly.

NOTE (This note will not be applicable after the repair is done) The digital readout on the instrument itself is not presently working. Instead, a voltmeter is connected to the output of the phototube amplifier. Calibration should be set such that minimum transmittance gives reading 0 V and maximum transmittance will read 5 V. Clearly, one will have to multiply the read potential by 20 to get % transmittance. The absorbance values will have to be calculated from transmittance. 

Cadmium vapor spectrum

          Ignite the acetylene-air burner and adjust the flame as instructed. Set the burner elevator and the gain control at intermediate position and the slit width at 0.5 nm. Set the wavelength selector at 228.8 nm and adjust for maximum meter deflection. Finally, adjust the gain control to give a meter deflection of about full scale.

(a) Lamp emission spectrum: While aspirating distilled water into the flame, scan manually from 600 to 200 nm and note the response. Determine  ?  for the lamp emission peaks and determine relative intensities.

(b) Flame absorption spectrum: Repeat part (a) but this time while aspirating your standard 25 µg/ml Cd²⁺ solution into the flame. Readjust the gain control so that the strongest emission peak remains on scale and repeat any portions of (a) and (b) where the response was off scale.

(c) Selection of wavelength. Using the results obtained above, calculate the transmittance and the absorbance at several selected values of wavelength. Explain our choice of 228.8 nm as the best wavelength for subsequent use. What other values could be employed?

Effect of slit width

          With the slit width at 2.0 nm, set the wavelength selector at 228.8 nm and adjust for maximum response. Readjust the gain control to give approximately full scale deflection. Sweep manually at low speed the wavelength through the 228.8 nm peak and note the maximum value and width of the response. Without changing the gain control, repeat at slit width settings of 2.0, 1.0, 0.5 and 0.20 nm. Interpret the results.

Optimum burner position

          Set the slit width at 0.5 nm, adjust the wavelength selector near 228.8 nm for maximum response, lower the burner elevator as far as it will go, then adjust the gain control to give about 90% of full scale deflection. Record the response while aspirating distilled water, then while aspirating your standard 20 µg/ml Cd²⁺ solution. Without changing the gain control, raise the elevator by 0.1 inch and repeat, continue to increments of 0.1 inch until the elevator has been raised 1.0 inch.

          Plot, on one graph, response vs. burner position for the water sample and for the magnesium-containing sample. For each burner position used, calculate transmittance and absorbance, plot absorbance vs. burner position and choose the optimum position for subsequent use. Interpret the results you obtained.

Empirical calibration data

          Set the slit width at 0.5 nm and - while aspirating distilled water - adjust the gain control to give precisely full scale response. Aspirate, sequentially, each of your standard cadmium solutions and your unknown solution and record the response. Take at least three readings for your unknown. Repeat using a slit width of 0.20 nm, then 1.00 nm. For each slit width used, plot absorbance vs. concentration of cadmium in the standard solutions. Determine the concentration of cadmium in your unknown; report the value in ìg/ml of Cd²⁺ and calculate standard deviation of your reported values. Mention in the discussion any variations from the original instructions.

          As per instructions, extinguish the flame and turn off the instruments. Clean all glassware and leave the area in a clean, orderly condition.

Materials

          Spectrophotometer: Model Varian Techtron 1200 equipped with a cadmium hollow cathode lamp.

Operating parameters:

Acetylene pressure 10 psi (do not use an acetylene cylinder with pressure less than 60 psi

Air pressure 60 psi

Cd hollow cathode lamp current 3 mA

Volumetric Flasks:   Five 100 ml

Pipets: One 5 ml, one 10 ml

Stock Solutions:      Cd²⁺ 100 µg/ml

Unknown Sample:   CdCl₂ in a 100 ml volumetric flask