Lab Overview Investigation #19: "Iron in Pyrite"

Introduction

The ability to determine concentrations of metals in industry has become extremely important. Even more important though is the determination of metals for environmental purposes. The Environmental Protection Agency sets standards, which must be met, for drinking water, air, and soil.

An example of a metal, which has become a great concern, is lead. Lead can be found in extremely high concentrations in paint. Some old house paints contained these extremely high concentrations and a lot of industrial paints still do. Levels above 5000 ppm are common. The limit for lead in drinking water is about 0.015 ppm while the limit for mercury is 0.0002 ppm. The unit, ppm, means parts per million. It is a common term used to describe concentration. You can think of 1 ppm as being 1 white marble in a container with 1 million marbles of other colors. (One in a million huh!!) So in the case of mercury, the EPA requires that drinking water contain less than 0.2 mg in a kg. In our marble example, that would be less than 2 white marbles in 10 billion marbles. How is ppm related to ppb, mg/L, and mg/kg? (Answer at the bottom of the page.)

In this lab you will determine the concentration of a metal, iron, using a very easy and cheap method. Iron poses a much smaller health hazard and is easier to determine in terms of concentration than either lead or mercury. The method of determination will be by titration, which you should all know well.

  Concepts of the Experiment

In previous labs, I have said that the endpoint of a titration means that the number of moles of acid = the number of moles of base. In the last lab, we found out that this wasn't quite true, but that it was a close enough approximation. In this lab, we will again modify the meaning of endpoint to mean that the number of moles of one reacting chemical = the number of moles of another chemical which it is reacting with if the reaction is a 1:1 ratio. In this lab you will be titrating Iron molecules with Potassium Permanganate in the following balanced formula unit equation:

10FeSO₄ + 2KMnO₄ + 8 H₂SO₄ ® 5Fe₂(SO₄)₃ + 2MnSO₄ + 8H₂O + K₂SO₄

This equation can be better expressed in its net ionic form.

5Fe²⁺ + MnO₄^- + 8H⁺ ® 5Fe³⁺ + Mn²⁺ + 4H₂O

Note that in this equation, 5 moles of iron react with one mole of permanganate. So the ratio at the endpoint of the titration will be 5 moles of iron = 1 mole of permanganate. THIS IS NOT A 1:1 ratio. For example, say you titrate an iron solution with permanganate. At the endpoint you have added 4 moles of permanganate. This would mean that the beaker contained 20 moles of iron.

The potassium permanganate will also serve as an indicator in this titration. The first drop, which is in excess, will cause the solution to turn a faint pink. The oxidized form of Fe will cause a faint yellow color. By adding a reagent, the faint yellow color will be turned clear so you can better see the endpoint. The reagent will have the added effect of catalyzing the oxidation reduction reaction.

  Oxidation/Reduction Equations

All balanced equations must meet the following rules:

1. The mass must balance. If you have XCa on one side then you must have XCa on the other side.

2. The charge must balance. In most cases the charge on each side is zero. But if you have an ion of 2+ on the reactant side, then you must also have a 2+ on the product side.

When balancing Oxidation/Reduction Equations you must assign the oxidation numbers correctly or all else fails. Follow the directions and examples in your book on page 369. Don't forget that in a acid solution, you can add H⁺ or H₂O if needed. In a basic solution, you can add OH^- or H₂O.

Procedure

As in our previous acid/base titrations you will have to standardize the titrant. The balanced equation for the standardization is:

5Na₂C₂O₄ + 2KMnO₄ + 8H₂SO₄ ® 10CO₂ + 2MnSO₄ + K₂SO₄ + 5Na₂SO₄ + 8H₂O

You will be using sodium oxalate to perform the standardization. Note that in the above balanced equation, 5 moles of Na₂C₂O₄ react with 2 moles of KMnO₄.

At the endpoint of your standardization titration, calculate the number of moles of sodium oxalate titrated.

# moles of Na₂C₂O₄ = mass_{sodium oxalate} x [1 mole sodium oxalate/ FW of sodium oxalate]

The number of moles of KMnO₄ can then be calculated.

X moles Na₂C₂O₄ x [2 moles KMnO₄/ 5 moles Na₂C₂O₄] = # moles of KMnO₄

From this you can calculate the molarity of the KMnO₄ solution.

M KMnO₄ = # moles of KMnO₄/ Volume of Titrant (L)

Similar calculations will be used when determining the amount of iron. Remember to be careful what ratio you use though.

Ppm means parts per million. When talking about a dry sample, 1 ppm is equal to 1 mg/kg. When we talk about a liquid the convention is to express the concentrations as mg/L. It is generally assumed that the liquid is water, which has a density of 1 g/mL. Notice that if we multiply 1 mg/L by 1 g/mL x 1 kg/1000 g x 1000 mL/1L we end up with mg/kg which is the expression of the dry concentration. Parts per billion, ppb, is simply a smaller concentration. 1000 ppb are equal to 1 ppm.