Question

Conc.\[HN{O_3}\]​ is added before proceeding to test for group III members. This is to
A. Oxidise any remaining\[{H_2}S\]
B. Convert ferrous ions into ferric ions
C. Form nitrates which give granular ppts
D. Increase ionisation of ammonium hydroxide

Answer 1

Hint: Cations belonging to group III precipitate out as their corresponding hydroxides. Concentrated nitric acid (\[HN{O_3}\]) is a strong oxidising agent.

Complete Step by Step Solution:
In qualitative inorganic analysis, a given salt (or a mixture of salts) is analysed to determine its elemental composition. The analysis is conducted in an aqueous medium, usually.
The given salt sample is first dissolved in water. The aqueous solution of the sample is then treated with reagents and the constituent anions and cations are detected through their characteristic reactions. These reactions, most commonly, involve precipitation, colour changes or some other form of visible changes.

Cations are divided into six groups based on their properties. Each group has a common reagent that can be used to separate the cations of that group from the aqueous solution of the salt sample.

For meaningful results, the analysis of cations must be performed in sequential order of the groups i.e., tests for group I cations must be performed first, group II cations come after group I, group III comes after group II, and so on. This is done because some cations of earlier groups may react with certain reagents of a later group which can produce unreliable results.

Some of the important group III cations are\[F{e^{2 + }},F{e^{3 + }},A{l^{3 + }},C{r^{3 + }}\] etc. They precipitate out as hydroxides. \[F{e^{2 + }}\] precipitates out as iron(II) hydroxide \[Fe{\left( {OH} \right)_2}\] and\[F{e^{3 + }}\] precipitates out as iron(III) hydroxide\[Fe{\left( {OH} \right)_3}\]. Out of these two, iron (II) hydroxide is more soluble in an aqueous medium than iron(III) hydroxide. So, if the sample contains iron, it will be much more detectable when it is in the\[F{e^{3 + }}\]form. Thus, it becomes desirable to oxidise any\[F{e^{2 + }}\]ions into\[F{e^{3 + }}\]ions. This oxidation is carried out by adding concentrated nitric acid (\[HN{O_3}\]) as it is a strong oxidising agent.

Oxidising \[F{e^{2 + }}\]ions into\[F{e^{3 + }}\]ions also allows for more proper separation of iron from the aqueous solution of the sample for the testing of later group cations.
Thus, option B is correct.

Note: All cations of previous groups must be removed from the aqueous solution before proceeding with the tests for later groups. Not doing so properly may cause some cations of earlier groups to precipitate with the cations of later groups causing false positive results.