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.
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