Determination of Iron in an Ore by Titration with Dichromate

 

 

 

 

In this experiment, the percent iron by weight in an ore sample will be determined by performing a redox titration using potassium dichromate (K2Cr2O7). That this reagent is available as a primary standard and does not require standardization is a significant advantage. In acidic solution, dichromate is reduced to Cr3+ in a 1:6 molar ratio

 

Cr2O72- + 6 Fe2+ + 14 H+ 2 Cr3+ + 6 Fe3+ + 7 H2O

 

This is a rapid, quantitative reaction with a very large equilibrium constant. The endpoint is readily detected using diphenylamine indicator, which turns from colorless to violet-purple-blue in the oxidized form.

 

The basis of use of this reaction for determination of iron is that all of iron is in the +2 oxidation state when the sample is titrated. This can be accomplished by prior reduction with stannous ion

 

2 Fe3+ + Sn2+ 2 Fe2+ + Sn4+

 

In the concentrated HCl solution used in the procedure the metal ions are actually present as chloro complexes, e.g. FeCl4- and SnCl62-. The slight excess of stannous chloride used in this step would react with the dichromate in the titration reaction and must be removed before the titration. This is accomplished by reaction with a saturated solution of HgCl2

 

2 Hg2+ + Sn2+ + 2 Cl- Sn4+ + Hg2Cl2 (solid)

 

The silky white precipitate of calomel (Hg2Cl2) does not react with dichromate under the experimental conditions.

 

The experiment requires good technique in order to get accurate values. The prior reduction steps need to be performed without wasted time. In addition a major source of error is air oxidation of the ferrous ion before the titration is started.

 

2 Fe2+ + O2 + 2 H+ 2 Fe3+ + H2O2

 

This reaction is slow enough that if the titration is started immediately after addition of the HgCl2 solution, accurate results can be obtained.

 

 

 

 

 

 

Procedure

 

Preparation of Dichromate Solution

 

Dry about 3 g of primary standard K2Cr2O7 (fwt 294.2) in a weighing bottle at 120 oC for at least 2 hours. Drying for longer periods will do no harm. Cool the potassium dichromate in a dessicator for 30 minutes. Weigh accurately to the nearest milligram, about 1.3 g in a weighing dish and using a buret funnel transfer quantitatively to a 250 mL volumetric flask. Dilute to volume and mix thoroughly. Rinse a clean 1 liter bottle with three small portions of the solution and transfer the remainder of the solution to the bottle for storage. Calculate the molarity of the solution.

 

Sample preparation

 

Weigh out three samples of unknown between 0.6 and 0.7 grams into two clean labeled 400 mL wide mouth flasks. Add about 30 mL of a 3:1 con HCl/H2O solution, cover the beaker with a watch glass and heat on a hot plate in the hood until the sample dissolves. Do NOT let the solution boil or go to dryness. Do not boil the samples because FeCl3 can be lost to volatilization and only causes excessive evaporation of the acid. Add more HCl if necessary. Adjust the final volume to about 50 mL. It should not take longer than 30 minutes for the sample to dissolve. A white residue of insoluble silica, SiO2, is due to the silicate rocks that are the primary waste material of the commercial ore. The insoluble silica does not interfere in the titration. After samples are dissolved, they may be cooled and stored covered. In a cabinet, not a drawer.

 

Prior reduction

 

At this point each sample must be handled individually, one at a time, until the titration is complete.

 

Heat the sample solution to nearly boiling and add 0.25 M SnCl2, dropwise while stirring the solution by rotating the flask until the solution changes to colorless or a very pale green. This signals the disappearance of the yellow color due to the FeCl4-. The add 2 or 3 drops in excess, but no more.

 

Cool the sample and dilute to approximately 150 mL with water. While swirling the solution rapidly add 15 mL of 5% HgCl2 solution to oxidize the excess Sn2+. Formation of a white, silky precipitate indicates that the prior reduction step was performed successfully. However, if a grey precipitate is formed indicating the presence of metallic mercury, the sample must be discarded. Likewise if no precipitate forms, there was probably no excess of SnCl2 and the sample must also be discarded.

 

 

 

 

 

As a practice sample, approximately 10 mL of 0.10 M FeCl3 solution in 6 M HCl is combined with about 50 mL of water. Follow the above procedure for heating and reduction with stannous to become familiar with the color change. If the sample solution is too concentrated, the pale green of Fe2+ will make detection of complete reduction more difficult.

 

 

 

Titration

 

Complete the titration as soon as possible from this point. Add 100 mL of the phosphoric acid/sulfuric acid solution. Add about 5 drops of diphenylamine sulfonate indicator solution. Titrate with dichromate until the solution remains The phosphate forms a nearly colorless complex with the Fe3+ titration product and sharpens the endpoint. Distilled water with 2-3 drops of SnCl2 may be used as a blank but is generally unnecessary.

 

 

 

Calculations

 

Report the percent iron in the ore sample, the standard deviation of the replicates and the confidence interval of the results.