Read Determination of iron in ore by titration with pot. dichromate text version

Rev. 4/96 GA/6 Analysis of Iron Ore by Titration with Potassium Dichromate


Purpose: A standardized solution will be prepared by the direct method by dissolving an accurate weight of primary standard potassium dichromate in deionized water up to the pre-calibrated mark of your volumetric flask. The percentage of iron in an unknown sample will be determined by an oxidation-reduction titration. Solutions and Chemicals Required unknown iron ore sample to be dried at 110°C for at least one hour predried primary standard K2Cr2O7 concentrated HCl SnCl2 solution HgCl2 solution syrupy H3PO4 diphenylamine sulfonate indicator Special Equipment: calibrated 250 ml volumetric flask PROCEDURE Preparation of K2Cr2O7 Precisely weigh by difference from a weighing bottle, between 0.55 to 0.65 g of the predried, primary standard K2Cr2O7. Carefully dilute the K2Cr2O7 solution with deionized water up to the calibration mark of the 250ml volumetric flask. The exact volume contained in the 250ml volumetric flask was determined in part II of Analytical Techniques and Measurements experiment. Write the balanced equation for the reaction of Fe2+ with K2Cr2O7. Calculate the molarity of the K2Cr2O7 solution. (FW of K2Cr2O7 = 294.19) Solution of the Iron Ore Sample Transfer the unknown iron ore sample to a clean weighing bottle and dry for one hour at 110°C. After the sample has cooled, weigh by difference, to the nearest 0.1mg, a 0.20 gm sample into a 250 ml Erlenmeyer flask. To dissolve this and subsequent samples, add 15ml of concentrated HCl (ALWAYS ADD ACID TO WATER) to the sample in the flask. Heat the solution to just below boiling on the hot plate until the ore dissolves. Make sure the exhaust fan is on. If the sample does not dissolve after 10-15 minutes, check with the instructor. This first 0.20gm sample is a trial to become familiar with the end point and to determine a sample weight which will result in the delivery of 30-45ml of K2Cr2O7 solution. Go through the entire procedure with this sample and then calculate an optimum weight for the remaining trials. Remember that three good trials must be done. 6-2 Reduction of Fe(III) to Fe(II) and Titration of Fe(II)

The reduction step must be done one trial at a time and the titration of that sample must be done immediately after the reduction. This is because the reduced Fe2+ is readily oxidized in air to Fe3+. Note the oxidized form of the iron does not react with K2Cr2O7. Heat each solution containing the iron sample almost to boiling. Carefully add SnCl2 solution dropwise until the yellow Fe(III) color just disappears. Then add only 2 drops excess of SnCl2 solution. Cool by running the outside of the flask under cold water (until the flask can be comfortably held, <40°C). Next rapidly add 10ml (measured in a graduated cylinder) of the HgCl2 solution. A small quantity of a white precipitate should appear. If no precipitate forms or if the precipitate is gray or black, the trial must be discarded. After waiting only about 2 to 3 minutes, add to the pre-reduced solution, about 5ml of concentrated sulfuric acid and 7ml of syrupy phosphoric acid (measured in a graduated cylinder). Dilute with distilled water to bring the volume to about 125ml. Again cool the solution to room temperature by running the outside of the flask under cold water. Add 8 drops of diphenylamine sulfonate indicator and slowly titrate with your standard K2Cr2O7 solution from a blue-green, through a greyish tinge to the first permanent violet, which is the end point. The titration should be conducted dropwise when the grey tinge is noted because the oxidation of the indicator is somewhat slow at this point. Do the trial run first to see the end point and determine the sample weight. Then do three good trials. The average relative deviation should not exceed 2 ppt. Blank There is a significant indicator blank for this titration. To run a blank, all the reagents except for the iron unknown should be added to a 250ml Erlenmeyer flask and the volume should then be brought up to the approximate volume at the end point using distilled water. Hence add 15ml of distilled water, 15ml of con HCl, 2 drops of SnCl 2, 10ml of HgCl2, 5ml con H 2SO4, 7ml of syrupy H3PO4, and dilute with deionized water to an approximate end point volume. Cool. Add 8 drops of diphenylamine sulfonate indicator and titrate with K2Cr2O7 through the greyish tinge to a permanent violet. Remember the actual end point change is slow. The blank should be no more than 1 to 4 drops (about 0.20ml) and its value needs to be subtracted from the volume of K2Cr2O7 required to titrate each iron sample. Do the blank twice.


1. 2.

Calculate the molarity of the K2Cr2O7 solution. Calculate the percentage of Fe in the iron ore sample for each trial. Calculate the average relative deviation for this experimental step. Formula Weights: Fe Fe2O3 K2Cr2O7 55.847 159.69 294.19


Determination of iron in ore by titration with pot. dichromate

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