Redox determination of Iron

The purpose of this lab is to teach you about:

Ø Redox chemistry

Ø Quantitative analysis

Ø Error propagation

“Theory” behind the lab

Iron commonly exists in the +2 and +3 oxidation states.

FeO Fe2O3

If we want to determine the total iron content in a

sample how do we do it?

1. dissolution of the sample

q Fe compounds are often insoluble in water

q They will dissolve in hot, concentrated HCl.

q Silica which does not dissolve in hot, concentrated HCl will appear as a white

residue.

2. quantitative conversion of iron(III) to iron(II) with a suitable reducing agent

2Fe3+ + SnCl42− + 2Cl− → 2Fe2+ + SnCl6

2−

Ø Chloride complexes of Fe(III) are yellow while complexes of Fe(II) are colorlessØ Chloride complexes of Fe(III) are yellow while complexes of Fe(II) are colorless

Ø Add Sn(II) DROPWISE until the yellow color just disappears. Then add ONE

DROP of EXCESS Sn(II).

3. removal of the excess reducing agent

Remove excess Sn(II) by adding HgCl2

SnCl42− + 2HgCl4

2− → SnCl62− + Hg2Cl2(s) + 4Cl−

Mercurous chloride (white preciptate)

If you have TOO MUCH Sn(II) (i.e. you screwed up Step 2) you will get the reaction

SnCl42− + HgCl4

2− → SnCl62− + Hg(l) + 4Cl−

colloidal, grey-black

preciptate DISCARD

SAMPLE

4. Addition of special reagents to aid in detection of the endpoint and to

ensure that the proper reaction occurs during the subsequent titration

6Fe2+ + Cr2O72− + 14H+ → 6Fe3+ + 2Cr3+ + 7H2O

The reaction we want to study is:

Ferrous

ion Dichromate

ion = orangeFerric ion Chromic ion

(green)

Since dichromate ion can’t serve as its own indicator, we need an indicator for the

dichromate-chromic ion redox. WHY cant we use dichromate ion can’t serve as its

own indicator?

N

H

N

H

N

H

irreversible

diphenylamine (colorless) diphenylbenzidine (colorless)

+ 2H+ +2e−

diphenylbenzidine violet (violet)diphenylbenzidine (colorless) ç Ł + 2H+ +2e−

The diphenylbenzidine/diphenylbenzidine violet system is reversible a

good characteristic of an indicator.

5. Quantitative titration of Fe(II) to Fe(III)

Oxidizing agent: a species that oxidizes something else, in this case

Fe2+Ł Fe3+

Potassium dichromate is an excellent oxidizing agent for iron(II) for three reasons:

a) dichromate and iron(II) react quantitatively and with a known stoichiometry;

b) the reaction is sufficiently fast to be practical for a titration; and

c) the ∆E is large enough to produce a well-defined endpoint.

Potassium dichromate is a primary standard

i) it can be purchased as a high-purity solid.

ii) Standard solutions of potassium dichromate can be prepared from a weighed

quantity of the dried solid and need not be standardized

iii) the prepared solutions are very stable.

Procedure

A. You will be provided by dried K2Cr2O7 that has already cooled.

B. Weigh the sample to the nearest mg. Then transfer the contents to a

CLEAN BEAKER.

C. Now re-weigh the “empty” sample bottle to determine how much

K2Cr2O7 has been transferred to the beaker.

D. Dissolve the K2Cr2O7 by adding a small amount of distilled water.

E. Clean the volumetric flask thoroughly so that you do not get “beading”.

F. Transfer the solution to a 500 mL volumetric flask using a funnel.

1) Preparation of the standard potassium dichromate solution

F. Transfer the solution to a 500 mL volumetric flask using a funnel.

G. Rinse the beaker 3 times making sure to get ALL of the solution into the

volumetric flask.

H. Add distilled water to the volumetric flask up to the calibration mark.

I. Stopper the flask and mix thoroughly.

J. Calculate the concentration of your K2Cr2O7 solution and label the

standard solution with BOTH the concentration and your name.

2) Preparation of the sample solution (Fe)

A) Transfer 2.0 – 2.5 g of the iron ore containing sample into a

weighing bottle.

B) Dry at 100 °C for one hour. Allow to cool

C) Divide into three roughly equal portions. Weigh each portion and

place in a 500 mL Erlenmeyer flask.

D) Add 10 mL of distilled water to each flask

E) Add 10 mL of concentrated (12M) HCl to each flask.

F) In a ventilation hood gently heat each flask to dissolve the F) In a ventilation hood gently heat each flask to dissolve the

sample. The silica residue can be ignored.

From this point on, treat each flask all the way through

the experiment before working up the next flask.

3. Adjusting the oxidation state of iron (Fe3+ Ł Fe2+ )

A) In a ventilation hood, heat to boiling the sample solution in the Erlenmeyer

flask.

B) Add dropwise the 0.5 M SnCl2 solution until the yellow color that signals the

presence of Fe3+ disappears. Add ONE or TWO additional drops.

C) Cool the flask to a temperature below 25 °C.

D) Add ALL AT ONCE 10 mL of 0.18 M HgCl2. A silky whit preciptate should

form.

E) IF A GREY-BLACK preciptate forms you had too much SnCl from Step B) E) IF A GREY-BLACK preciptate forms you had too much SnCl2 from Step B)

and you should discard the sample and start with a new sample.

F) Afetr 2-3 minutes (but not longer) add 150 mL of distilled water to the

Erlenmeyer flask.

You are now ready to add the special reagents,

indicator and start titrating.

4) Addition of special reagents, indicators, and titration.

A) Add 10 mL of the sulfuric acid – phosphoric acid solution to the flask

B) Add 8 drops of barium diphenylamine sulfonate (indicator)

C) Titrate with your potassium dichromate until the appearance of a deep

violet color throughout the solution.