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I'm investigating the concentration of iron in bananas. After heating a sample in concentrated sulphuric acid, clearing it with hydrogen peroxide I've to then adjust the pH with sodium hydroxide before adding hydroxyammonium chloride followed by phenanthroline reagent. What is the function of the hydroxyammonium chloride in this experiment?

Igloo replies ...
 
This analysis depends on the ability of Fe²⁺ ions to form an intensely red complex with phenanthroline. In contrast, Fe³⁺ ions do not form a complex with phenanthroline. Your extraction of the iron from the bananas almost certainly produces iron in the +3 oxidation state, since you used hydrogen peroxide (a strong oxidising agent) to clarify your solution. Thus it is necessary to reduce all the Fe³⁺ ions to Fe²⁺ ions before adding the phenanthroline.
 
Hydroxylammonium chloride (also known as hydroxylamine hydrochloride) is an excellent reducing agent and accomplishes this reduction rapidly, evolving gaseous nitrogen in the process:
2Fe³⁺(aq) + 2NH₂OH.HCl(aq) + 4OH^-(aq) ----> 2Fe²⁺(aq) + N₂(g) + 6H₂O(l) + 2Cl^-(aq)
 
You can see from this equation that hydroxide ions are also used up. This is why you had to neutralise your acidic solution with sodium hydroxide before adding the reducing agent.
 
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Risk assessment
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updated: 16 December 2006