Question

Interfering radicals interfere the test of:
(A) III group radicals only
(B) III group radicals or downward
(C) Cations which are present in II group filtrate
(D) None of the above

Answer 1

The interfering radicals are anions that form complexes with reagent and precipitate. This should be removed carefully in order to get the correct result. This makes semi-micro qualitative inorganic analysis experiments more interesting.

Complete step by step answer:
The interfering radicals are anions such as Oxalate, tartrate, fluoride, borate and phosphate. These interfering radicals don't interfere till group II analysis because till group II all the reactions take place in acidic medium. From group III onwards almost all reactions take place in alkaline medium. When interfering radicals are in acidic medium they remain ionized thus remain inactive but when it is in alkaline medium it is insoluble so they actively make complexes with reagent resulting in precipitation with the actual yield. Therefore shows incorrect results.
Let me explain with the example: the interfering radicals make complexes with the regent of group III that is ammonium chloride and ammonium sulphide. As interfering radicals are insoluble in alkaline medium therefore affects the solubility product of cations which causes incomplete precipitation in group III and premature precipitation in group IV and V.
From the above explanation it is evident that interfering radicals interfere from group III and downwards.
In processes of separation of cation of group II, sometimes phosphate is precipitated, and silica is precipitated. Let me explain separation of cation from filtrate.
Firstly, filtrate boiled in porcelain dish till ${H_2}S$ is removed then add $HN{O_3}$in order to oxidize $F{e^{ + 2}}$ to $F{e^{ + 3}}$. Evaporate to dryness then again add a few drops of $HN{O_3}$ and heat gently this will remove organic acids.
1. If fluoride and borate are present, evaporate the residue with concentrated HCl. Even if borate is present in residue, add 5ml of methanol with 10ml of concentrated HCl.
2. If silicates are present, add 15ml of HCl and heat.
3. The phosphate can be found easily, if phosphate is present with ammonium molybdate reagent and few drops $HN{O_3}$ is added and heated at ${40^ \circ }C$. If yellow precipitate is obtained this indicates presence of phosphate.

Thus it is evident that interfering radicals interfere in separation of cation in group II.

Note: In order to get the correct answer the tests need to be done more carefully. Sometimes if the test is not performed correctly, detection of presence of interfering radicals will also be difficult as the residue obtained will neither match the exact colour of residue that is needed to be obtained nor that colour that indicates presence of interfering radicals. This indicates that the tests are not performed well.