Question

Which of the following anion(s) is/are interfering radicals?
A). $BO_{3}^{3-}$
B). $F^{-}$
C). $PO_{4}^{3-}$
D). none of these

Answer 1

Hint: Interfering radicals are those which interfere in qualitative tests while performing salt analysis.
They form a complex with 3rd group reagents.Guess which one these would interfere with and precipitate with group three cations.

Complete step by step solution:
Interfering radicals are oxalate, tartrate, fluoride, borate and phosphate all of them are anionic radicals. They form a complex with third group reagents which are ammonium chloride and ammonium hydroxide. This leads to incomplete precipitation of third group cations and causes early precipitation of 4th and 5th group cations in alkaline medium. Let’s try to understand what happens.
Oxalate, tartrate, fluoride, borate, silicate and phosphate of the metals are soluble in acidic medium.
If you remember, for first and second group the analysis medium remains acidic (dilute HCl) that’s why they do not interfere then, but for the 3rd group analysis the medium becomes alkaline by group reagents ammonium chloride and ammonium sulphide. Here interfering radicals come into action and disturb the solubility product of cations which causes their premature or early precipitation.


In acidic medium these salts produce their corresponding acids like oxalic acid, phosphoric acid, hydrofluoric acid, boric acid and tartaric acid.

These interfering acids are weak acids so they do not dissociate completely and remain in solution in their unionised form. An equilibrium is developed between dissociated and undissociated acid.

Hydrochloric acid is a strong acid and is ionised completely.

Hydrogen ions act as common ions among them and higher concentration of $H^{+}$ suppresses the ionization of interfering acid. Therefore, the ionic product doesn't exceed the solubility product of which is why the ion remains in the solution as barium oxalate. That’s how interfering radicals do not interfere as long as the medium remains acidic enough. But when we make the medium alkaline by adding 3rd group reagent ammonium hydroxide $NH_4OH$, $OH^{-}$ ions combine with $H^{+}$ and neutralise the reagents. This decreases the concentration of $H^{+}$ ions which in turn shifts the equilibrium of dissociation of interfering acid forward and increases the concentration of the conjugate base. Thus the ionic product of the group 3 cations and interfering ions exceeds the solubility product and gets precipitated in the 3rd group, which actually belongs to the 4th group.

So, the correct answer is: Option A, B and C

Note: Check all the options, more than one option are possible. Also, if there is an interfering cation, the test may still give positive results depending upon conditions.
Also, Interfering radicals can be removed by some processes. One or more interfering radicals can be present in the given solution. They have to be removed in the following mentioned order: first we have to remove oxalate and tartrate, then borate and fluoride ions, then silicate and in the last phosphate ions must be removed.