The Mg2+ test with magneson is carried out in alkaline solution. For 0.010 M Mg^2+ (K_sp of Mg(OH)2 = 5.6x10^-12), Mg(OH)2 just starts to form at a pH of about, and the blue colour arises because

9.37; magneson is adsorbed on Mg(OH)2 forming a blue lake

4.63; magneson is adsorbed on Mg(OH)2 forming a blue lake

9.37; magneson forms a soluble red complex with free Mg^2+

12.0; magneson oxidises Mg^2+ to Mg^3+

JEE Advanced levelSingle correct optionDifficulty 3 of 3qc216

The Mg2+ test with magneson is carried out in alkaline solution. For $0.010 M$ $Mg X^{2 +}$ ( $K_{s p}$ of $Mg (OH) X_{2} = 5.6 \times 1 0^{- 12}$ ), $Mg (OH) X_{2}$ just starts to form at a pH of about, and the blue colour arises because

[OH X^{-}] = 5.6 \times 1 0^{- 12} /0.010 = 2.37 \times 1 0^{- 5} M

pOH = 4.63

pH = 14 - 4.63 = 9.37

. Beyond this pH

Mg (OH) X_{2}

is present and adsorbs the azo dye magneson, giving a blue lake; 4.63 is the pOH, not the pH.