Estimation of Iron in TMT Bar by Diphenyl Amine/External Indicator Method

Exp9

What are the constituents of hematite ore?

The main constituents of hematite ore are ferric oxide and silica. The other ores of iron are magnetite, siderite, iron pyrite, etc.

How is the dissolution of hematite ore carried out?

When an accurately weighed amount of hematite ore is digested with concentrated HCl, Fe₂O₃ is insoluble and silica is filtered off, and the filtrate is the iron solution.

Why is stannous chloride added?

Since iron in FeCl₃ is already in the highest possible stable oxidation state (Fe³⁺), it cannot be oxidized further. Therefore, it is reduced to the ferrous state and then titrated with potassium dichromate solution, which is an oxidizing agent. Ferric chloride is reduced to ferrous chloride by stannous chloride in the presence of hydrochloric acid, and a slight excess is added to ensure complete reduction of ferric to ferrous.

Why is mercuric chloride added?

Mercuric chloride is added to remove excess stannous chloride. Mercuric chloride reacts with the excess stannous chloride to form a silky white precipitate of mercurous chloride.

What happens when excess stannous chloride is not removed?

Since stannous chloride can reduce potassium dichromate, the excess stannous chloride is destroyed by adding a strong solution of mercuric chloride.

Saturated HCl₂ is added after the reduction of Fe³⁺ to Fe²⁺ is effected by the addition of SnCl₂ solution. In light of the above, interpret the following cases:Case A: No precipitate.Case B: Grey precipitate.Case C: Silky white precipitate

Case A: No precipitate.
Answer: The trial should be discarded if no precipitate is obtained upon the addition of HCl₂, because complete reduction of Fe³⁺ to Fe²⁺ is not ensured.

Case B: Grey precipitate.
Answer: A black or grey precipitate of finely divided mercury may be produced if too much SnCl₂ is present or if HCl solution is added too slowly.

Case C: Silky white precipitate.
Answer: A silky white precipitate of HgCl₂ should be obtained because HgCl₂ removes the excess SnCl₂ as a silky precipitate. The reaction is SnCl₂ plus two HgCl₂ to give SnCl₄ plus Hg₂Cl₂, and this silky white precipitate is suitable for the experiment.

hy is the reaction mixture diluted with distilled water?

This is to minimize the quantity of the reaction mixture taken out during testing with the indicator drops.

Why is K₃Fe(CN)₆ complex used as an external indicator?

It cannot be used as an internal indicator in the determination of iron in hematite because potassium ferricyanide combines irreversibly with Fe2+ ions to form a deep blue derrous ferricyanide complex. These fe2+ ions involved in compelx formationa re nto avaialble for reaciton with potassium dichromate. Furhter, the ned point cannot be detected as there is no colour change.

Why does the color of the indicator drop remain the same at the endpoint?

At the endpoint, there are no more Fe²⁺ ions available to react with the indicator, as they have been oxidized to Fe³⁺. Since Fe³⁺ ions do not produce any color with the indicator, the color remains unchanged at the endpoint.

What is the reaction occurring during the titration?

Potassium dichromate, in the presence of hydrochloric acid, oxidizes Fe²⁺ ions present in the hematite solution to Fe³⁺ ions, and the potassium dichromate itself is reduced to chromic ions.

What is an ore?

An ore is a mineral from which a metal can be conveniently and economically extracted.

What is a mineral?

A mineral is a naturally occurring inorganic solid with a specific chemical composition and a crystalline structure.

Which acid is used to convert hematite into Fe³⁺ ions in solution?

Concentrated hydrochloric acid is a good solvent for dissolving hematite and obtaining iron in the Fe³⁺ state.

Why is the FeCl₃ solution heated prior to the addition of SnCl₂?

Higher temperature speeds up the rate of conversion of Fe³⁺ to Fe²⁺ by the reducing agent SnCl₂.