SPECTROPHOTOMETRIC DETERMINATION OF Mn IN STEEL


Obtain your unknown from the instructor. Weigh out duplicate 1.0g samples (to the nearest 0.1 mg) in 150 mL or 250 mL beakers. Add about 50 mL of 6 M HNO3 to each beaker and boil gently in the hood. Add carefully about 1 g of ammonium persulfate, and boil gently for an additional 10-15 min. If the solutions are pink or show a brown residue due to MnO2 add 0.1g of sodium bisulfite and heat for 5 minutes. You will probably notice a very dark residue at this point - it is probably carbon from the steel. It will not interfere with the experiment. Cool these solutions and transfer quantitatively to 500 mL volumetric flasks, dilute to the mark with DI water, and mix well. Use a 25.00 mL pipet to transfer 3 aliquots of each sample to individual beakers. Treat the beakers as follows: [NOTE: Be sure to pipet your standard solution!]
Solution Description 85% H3PO4 Standard Mn KIO4
Blank 10 mL 0.00 mL 0.0 g
Sample 1 10 mL 0.00 mL 0.8 g
Sample 2 10 mL 10.00 mL 0.8 g

Boil each solution gently for 5 minutes, cool, and transfer quantitatively to 100 mL volumetric flasks. Dilute to the mark, mix well, and determine the %T for each sample with the spectrophotometer adjusted to 525 nM. Convert these %T values to absorbance, and calculate the percent Mn in the original sample.


Return to the Experiments Page