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Experiment
If the equipment is set out and the students have done their dilution calculations beforehand, the lab takes approximately 4 hours.
The students are provided with 0.005 M Na2 EDTA and 0.02 M KSCN solutions (labeled with their concentrations and respective uncertainty) and asked to make up 2.0 M HC104 from concentrated HC104; 0.5 M HC104 from the 2.0 M HC104; 0.002 M KSCN from the provided 0.02 M KSCN; and approximately 0.1 M Fe(NO3)39H20 in 0.5 M HCI04. This last solution is standardized by titration with 0.005 M Na2 EDTA, using variamine blue as indicator (7): 1 mL of 0.1 M Fe(NO3)3 in HC104 solution is diluted with distilled water to 50 mL to bring the pH up to 2, one drop of indicator is added, and approximately 20 mL of Na2EDTA is added via buret until the yellow endpoint is reached.
The spectrometer is zeroed using the iron solution as the blank. A 0.0002 M thiocyanate solution in 0.5 M HC104 is prepared by adding 10 mL of the 0.002 M KSCN to a 100mL volumetric flask along with 25 mL of 2.0 M HC104 and distilled water to the mark. This is mixed well and poured into a 250-mL beaker. Successive 1-mL portions of the iron solution are added by pipet, the solution is mixed, and the absorbance is measured. After each absorbance measurement the solution is returned to the 250-mL beaker and the next portion of iron is added. Ten absorbance measurements at different concentrations are generally enough to obtain good results.
Results
The data are best analyzed by a computer spreadsheet program because there are several calculations and the concentration for each absorbance measurement must be corrected for dilution. We illustrate such a spreadsheet here with 4 points from a typical run: 1-, 3-, 5-, and 7-mL iron solution additions.
The variables for the linear plot are calculated as shown in Spreadsheet 1. A graph of CfCS/A vs. Cf + Cs is made, the best-fit line is found, and the slope/intercept ratio is used to calculate Kobs. In this particular run, the observed equilibrium constant was found to be 146, a value that agrees well with recent measurements made by other methods (8).
The next step is to relate...