Question

Name the external indicator and reducing agent used in determination of hematite ore solution

Answer 1

Answer:

This method development lab explored the redox titration of potassium dichromate with an iron

oxide ore dissolved in a concentrated nitric acid medium. While this experiment differed by the use of

only the acid, it became well apparent that the chemistry of the nitric acid prevents the adequate

determination of iron in the hematite ore. The use of nitric acid in the dichromate method of iron

determination results in a significant difference between the same method using concentrated

hydrochloric acid.

Answer 2

Answer:

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Explanation:

This method development lab explored the redox titration of potassium dichromate with an iron

oxide ore dissolved in a concentrated nitric acid medium. While this experiment differed by the use of

only the acid, it became well apparent that the chemistry of the nitric acid prevents the adequate

determination of iron in the hematite ore. The use of nitric acid in the dichromate method of iron

determination results in a significant difference between the same method using concentrated

hydrochloric acid.

Introduction

The determination of iron in oxide ores is determined by either precipitation of hydrous ferric oxide

and ignition to Fe2O3 or by the reduction of dissolved Fe(III) to Fe(II) followed by the titration with a

standard oxidizing agent such as the dichromate ion, Cr2O72- (Rice, 27). This lab will utilize the swift

and accurate properties of the dichromate method in order to analyze the iron ore.

In the original dichromate method, hot concentrated hydrochloric acid dissolves iron in a hematite

ore according to the following reaction:

Fe2O3(s) + 6 HCl(aq) ---> 2 FeCl3(aq) + 3 H2O (1)

Lack of stable reducing agents suitable for the direct titration of the ferric ion in solution requires the reduction to the ferrous ion. Stannous Chloride (namely the stannous ion, Sn(II)) is utilized to reduce

the iron prior to titration. The iron from hematite is reduced from Fe(III) to Fe(II) upon forming the

halogenated iron complex. This reaction is described as follows:

Sn2+ + 2 Fe3+ ---> 2 Fe2+ + Sn4+ (2)

Excess Sn2+ in solution is removed by adding mercuric chloride producing insoluble Hg2Cl2:

Sn2+ + 2 HgCl2(aq) ---> Sn4+ + Hg2Cl2(s) + 2 Cl- (3)

The ferrous ion can now be titrated with the standard potassium dichromate solution to yield the

following net ionic equation:

6 Fe2+ + Cr2O72- + 14 H+ ---> 6 Fe3+ + 2 Cr3+ + 7 H2O (4)

An inside indicator, sodium diphenylamine sulfonate, is oxidized by the dichromate ion to produce a

purple product. This reaction, however, does not occur as quickly as that of the oxidation of ferrous ion (Rice, 27-28).

In designing this experiment to determine the iron in the oxide ore, the role that the hydrochloric acid plays in the reaction was looked upon with curiosity. In light of research and chemical intuition, it was

found that the role of excess hydrochloric acid is disputed by some chemists. Dr. James Carr from the

University of Nebraska believes that if the HCl method is used, the HCl must be removed so that the

Cr2O72- or MnO41- (depending on method) ions will not react with the chloride during the titration

(Carr, internet source). Carr suggests removing the HCl by adding concentrated H2SO4 and heating

until dense white fumes of HCl come off. Meanwhile, authors Skoog and West suggest that moderate

amounts of hydrochloric acid do not affect the accuracy of the titration (Skoog and West, 12