VOLUMETRIC DETERMINATION OF IRON IN AN ORE

OBJECTIVES:

1. The student will use chemical principles to dissolve an iron ore.

2. The student will pre-treat the sample to obtain the iron in the reduced(+2 oxidation) state.

3. The student will use a strong oxidant as the titrant.

4. The student will use a solution of primary standard as the titrant.

PROCEDURE:

Preparation of 0.016 M Potassium Dichromate

Weigh out approximately 0.008 mole of dry K2Cr2O7 (100o C for drying) on the analytical balance. Dissolve in water and dilute to 500 ml. in a volumetric flask. Preparation of Preventive Solution Preventive solution is a strong acid solution which is used to prevent bad results. It is 4-5 M in H2SO4 and a 1-1.5 M in H3PO4. In a pyrex beaker add in order with stirring 400 ml. distilled water; 60 ml. 18M sulfuric acid; 40 ml. 15 M phosphoric acid. After cooling to room temperature, store in a glass bottle. Determination of the Iron Weigh a 0.7 g samples of ore into 500 mL erlenmeyer flasks. Since the iron ore is very hygroscopic, weigh the sample on the same day you dry it. Plan to run 4 or 5 samples so that you will have 3 good ones to report. Have DRY erlenmeyer flasks for ease of weighing. Dissolving the Ore Sample Add 10 ml. conc. (12 M) HCl and 3 ml. stannous chloride. Heat on a hot plate until no colored ppt. remains. This should be under a hood. NOTE: From this point on do only one sample at a time. Each sample will probably take from 30 to 45 min. The sample must be taken to completion today if it is carried past this point.
 
 
Reduction of Iron (III) to Iron (II) If the solution is yellow in color, add stannous chloride to the hot (nearly boiling) solution. Add until the solution is colorless, then add 2 to 3 drops in excess. Immediately place in ice bath. IF MORE THAN 4 DROPS OF EXCESS STANNOUS WAS ADDED AND If the solution is colorless, all of the iron is reduced and excess Sn2+ may be present. Add one or two crystals of KMnO4. Be sure that they all dissolve completely. Continue to add the crystals until one crystal turns the solution yellow. All of the excess stannous chloride has been destroyed. Add stannous chloride drop-wise until the solution is colorless. Add two drops in excess.

Cool in an ICE BATH until the solution is from 5-10o C. Add 75 ml. of ice cold (5 - 10o C) preventative solution and 200 ml. of cold water. Add 10 ml. of saturated (you should see particles in bottle) mercuric chloride fast, using a graduated cylinder. Swirl for 1/2 to 1 minute. Let the solution stand for 5 min. in the ice bath. If you have no silky ppt., note this on your results. Add 6 drops of Barium Diphenylamine sulfonate indicator. Titrate to the first violet or purple color. In calculating subtract 0.05 ml. for an indicator blank.
 
 

CALCULATION

Calculate percent iron in ore. Include computer print-out for the calculation. Check pages 372-3 of your text to see the chemical reaction.
 
 

CAUTION: The iron ore should be dried on the day it is weighed.

This may mean re-drying the sample.

DO NOT OVER DRY THE SAMPLE!


SAFETY AND DISPOSAL INFORMATION: Dispose of all test solutions and precipitates in the "IRON EXPERIMENT WASTE" container.

CAUTION: Remember to treat all chemicals with respect.

Return to Table of Contents