Redox and redox equilibria
Read our general notes on Risk Assessment
I’m doing an experiment on the analysis of iron tablets. I haven’t done titration experiments before so I’m lost. I need to know why the iron tablets should not be heated more than necessary when dissolving in sulphuric acid, and why they are being dissolved in sulphuric acid and not water.
Igloo writes
The titration you are going to carry out almost certainly involves the redox reaction using potassium manganate(VII), and this technique relies on the fact that the iron is present in the form of Fe2+ ions. These will be oxidised to Fe3+ ions during the titration itself.
The tablets contain iron(II) sulphate, so when they are crushed and dissolved in water, iron(II) ions are released into solution. So far, so good.
Unfortunately, these ions are slowly removed in solutions with a relatively high pH (>7) for two reasons:
Firstly, they are hydrolysed by water molecules, forming insoluble iron(II) hydroxide:
Fe2+ (aq) + 2H2O(l) <=> Fe(OH)2 (s) + 2H+(aq)
Addition of acid (H+) will cause the equilibrium to adjust to the left, re-forming the Fe2+(aq) ions.
Secondly, Fe2+(aq) ions are rapidly oxidised (by the air) to Fe3+(aq) ions under alkaline conditions, and these hydrolyse even more easily to give a 'rusty' cloudiness (Fe(OH)3) to the solution. The use of sulphuric acid ensures that this process is prevented, so that your solution contains the original number of Fe2+(aq) ions from the tablet, prior to the titration with potassium manganate(VII).
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
back to Redox and redox equilibria
Rate this page or react
Share your views on this page, 11 ratings so far
, rated at 
updated: 14 December 2006
