October 21, 2011
In this experiment the percent of iron in an unknown sample will be determined by using a redox titration and then compared to a different method. A primary standard which in this case is ferrous ammonium sulfate will be used to standardize potassium dichromate. The standardized potassium dichromate will then be used to titrate the unknown iron sample using the indicator p-diphenylamine sulfonate.
The standardized potassium dichromate which has a concentration around 0.017 M is then titrated into the unknown iron sample which is dissolved in deionized water. The unknown sample is titrated to its end point using the indicator p-diphenylamine sulfonate, the acid mixture helps to make the end point sharper which would make it more accurately and more easily determined. The equation of the reaction taking place is:
6 Fe2+ + Cr2O72- +14 H+ --------> 6 Fe3+ + 2 Cr3+ + 7 H2O
This equation can be used to convert moles of the dichromate ion to moles of the unknown iron to determine the percent of iron contained in the sample.
To prepare the acid mixture add 12.5 mL of both concentrated phosphoric acid and sulfuric acid to 500 mL of deionized water and mix well. To prepare the potassium dichromate solution dissolve about 1.25 g of potassium dichromate in deionized water and dilute to 250 mL.
To standardize the potassium dichromate weigh out about 0.5 g of ferrous ammonium sulfate, hexahydrate into a 250 mL flask and dissolve in 50 mL of deionized water and mix for two minutes. Then add 50 mL of the acid mixture previously made and add three to four drops of p-diphenylamine sulfonate indicator to the solution. Then titrate with the potassium dichromate until a change in color from green to purple occurs. Repeat these steps for three more trials. To titrate the unknown sample of Fe weigh out about 0.5 g of the unknown into a 250 mL flask and dissolve the sample...