VOLUMETRIC DETERMINATION OF IRON

FE 211

ANALYTICAL CHEMISTRY EXPERIMENT-4

VOLUMETRIC DETERMINATION OF IRON

Uğur ASİT

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PURPOSE: In this experiment we will focus on another type of volumetric analysis, oxidation -reduction titrations, and we will learn the main steps of this procedure. Also we will improve the laboratory skills in order to carry out such experiments. THEORY: Oxidation-reduction reactions or shortly redox reactions are the reactions where both oxidation state increase and decrease are seen together. It is possible use it in volumetric analysis if we know how to determine the end point. In this experiment we’ll determine the iron yield in a sample. 5Fe+2 + MnO4-  Mn+2 + 4H2O + 5Fe+3 As we can see in the equation theory is simple. If we titrate the ferrous ion solution with a known concentrated permanganate solution, at the end point the solution will turn pink. This is because permanganate can’t find any ferrous ion to react and because of its high color intense will dye the solution. Theorically looks simple. But practically it is not simple as it is seen. It is very important to know the concentration of permanganate so firstly solution must be standardized in order to be used in titration process. In the experiment sodium oxalate is used as primary standard solution. Sodium oxalate dissolves in acid and exists in the form of undissociated oxalic acid molecules. 2 MnO4- + 5 H2C2O4 + 6 H+  2 Mn+2 + 10CO2 + 8 H2O After standardization we face with another problem. How will we be sure that all the iron is in ferrous form because we know that iron ion can be found in two oxidation state. That is why we have to use another reagent that reduces all ferric ions into ferrous ions. (Remember from general chemistry that it is called reducing agent) That is why we used stannous chloride. In the presence of hydrochloric acid ferric ions reacts with tin ions and gives the reaction: 2Fe+3 + Sn+2  HCl 2 Fe+2 + Sn+4 Using an reducing agent causes another problem to solve: ”How can we remove the excess reducing agent?” It is best to remove reducing agent as a insoluble compound of it. So in the experiment mercury chloride is used which gives the reaction below: Hg+2 + Sn+2  HgCl2 + Sn+4 Before the titration solution must be treated with a special reagent in this experiment Zimmermann-Reinhardt reagent is used. Finally solution titrated and iron yield is calculated. This procedure is a good example for oxidation-reduction titrations for that it carries all the general steps of it.

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MATERIALS:

          

Hydrochloric acid solution. Sulfuric acid solution (diluted 1/18 from stock solution) Sodium oxalate solution (20 g/L) Potassium permanganate solution Mercuric chloride Stannous chloride Zimmermann-Reinhardt reagent Flask. Burette. Beaker. Graduated cyylinder.

PROCEDURE: Standartization of Permanganate Solution:  Take 10 ml of sodium oxalate solution (Na2C2O4) and add 90 ml of H2SO4 solution.  Heat the solution nearly to boiling.  Solution was titrated with permanganate solution until a pink coloration appears . Keeping the solution above 70oC throughout the titration.  Obtain a blank by heating 75 ml prepared H2SO4 solution to 80oC and titrating with permanganate.  Then from the corrected volume of permanganate , calculate the molarity of the permanganate solution . Iron Determination:  5 ml of sample was taken.  20 ml of distillated water was added.  Solution heated on the bunsen burner. ( Do not boil least iron be lost as volatile FeCl3).  Stannous chloride was added until the yellow color changes into light green. A few excess drop was added.  Solution was diluted to 100 ml and cooled.  10 ml of mercuric chloride, HgCl2, was added. Precipitation was observed. It was passed to next step in case precipitation was white; else previous steps were repeated with a new sample.  Solution diluted to 300 ml with distillated water and 25 ml of ZimmermannReinhardt reagent was added.  Solution was titrated with permanganate solution.  Calculate the percentage Fe in the sample.

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RESULT: 1. Standardzation of Permanganate Solution with Sodium Oxalate: 29.7 ml permanganate solution. Blank : 0.5 ml permanganate solution. 2. Iron determination: 12 ml permanganate solution.

CALCULATION:

1.)

10 ml

20 g 1L

*

Na2C2O4

2 moles MnO *4 5 moles H2C2O4  XM MnO4 - = 0,02 M

2.)

5 * XMFe

+2

=

29,7 ml MnO4-*

* 5 ml = 12 ml * 0.02 M  XMFe

5 ml Fe+2 * 9.6*10-3 M

*

1 mole H2C2O4 1 mole Na2C2O4

1 mole Na2C2O4 134 g Na2C2O4

*

1 L 1000 ml

*

+2

XM MnO4

-

= 9.6*10-3 M

55.847 g Fe 1 mole Fe

= 2.68 mg Fe

DISCUSSION: In this experiment we discussed the oxidation-reduction titrations and we carried out one procedure that was a very good example to these titrations. As I have discussed the steps and their reasons in the theory section I find no need to mention it again but to generalize this experiment, it contains the steps: Sampling Standardization of permanganate solution Reducing all ferric ions to ferrous ions Special reagent addition for the occurrence of one proper reaction during titration. Titration. Titration method mostly gives more accurate results than the precipitation method so this procedure can be used for more sensitive determinations.

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Oxidation-reduction is very important for analytical chemistry especially mineral determinations. There are a lot of task to mistake in that procedure. It is possible we’ve done some. Procedure must be carried out several times for mistakes. In this experiment first step standardization of permanganate solution. Firstly , we took sodium oxalate solution , Na2C2O4 , and add H2SO4 solution. Then we titrated with permanganate solution until pink coloration appear. We keep the solution above 70oC throughout the titration. Because titration became very fast. But we burned our hand. Then we made blank . we prepared 75 ml H2SO4 solution to 80oC and titrate with permanganate .From the corrected volume of permanganate. Finally, we calculated molarity. Second step we took sample and add distillated water. Then solution was boiled but do not boil least iron be last as volatile FeCl3. Then add stannous chloride until the yellow color change into light green. After that we added a few drops excess. Then add water and cooled. Then mercuric chloride and precipitation was observed. If precipitation was observed white, we passed next step. But observed dark, grey or turbid, we repeated again previous steps. At first trial was ruined because we observed grey color and second trial was very badly because we added mercuric chloride before we added distillated water. Therefore we observed black color. Third trial was made this step successfully. We observed white color. This color is similar milk color. Then this solution was diluted distillated water until 300 ml. After that we added Zimmermann-Reinhardt reagent. Finally solution was titrated with permanganate solution and calculate the Fe in the sample.

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