Determination of iron in iron tablets

Lack of iron in the diet is a common cause of anaemia. This can be corrected by taking "iron" tablets, which contain ferrous sulfate, now known as iron(II) sulfate. In this experiment, we analyze the iron content of two such ferrous sulfate tablets. We use a titration method with potassium manganate(VII) as the self-indicating titrant. Watch the video, in which I have assumed you are aware of the standard titration technique. If you are not sure, you may wish to look again at the calcium hydroxide titration met on the AS course.

Try to calculate the mass of iron(II) sulfate in each tablet, and compare this with the value quoted by the manufacturer. Also express this as a percentage of the mass of the tablet. Don't be surprised if the percentage is quite small. Most tablets contain a large percentage of fillers and binders. You can check your answers in the link below.

It is important to use sufficient sulfuric acid in this experiment. Failure to do so leads to incomplete reduction of the manganate(VII) ion. Rather than being reduced to the virtually colourless manganese(II) ion, it is often reduced to the brown solid, manganese(IV) oxide. This is not a quantitative reduction, and will prevent the calculation from being accurate. Sulfuric acid is often added to iron(II) sulfate solution as it inhibits its oxidation by the air to iron(III).

incomplete manganate(VII) reduction

Remember, that although I have only shown one titration on the video, it is always essential that at least two concordant results are obtained in titration work. It is also worth observing carefully in the video when the volumetric flask is being made up to the mark. The meniscus is poor and not level. This is caused by dirty or greasy glassware. This is quite common in school and college laboratories, where aggressive cleaning agents are not often used. The only sure solution is to start again and clean the glassware thoroughly. However, the error will be small, and the time allowed to you in class practicals will generally not allow you to start again.

Video - analysis of iron tablets

Calculation of results