During qualitative analysis, what factor is used for the preliminary separation

Question

During qualitative analysis, what factor is used for the preliminary separation of metallic cations into groups by addition of four group reagents? An aqueous solution of a salt (one cation and one anion) is thought to contain the Group III metal cation Mn^+2. To test this hypothesis, a student added H_2S in NH, /NH_4Cl buffer to 1 mL of the salt solution and obtained a precipitate. The student then concluded that the aqueous solution contained a Group III metal cation. What is wrong with this student's reasoning? To what groups could the cation belong? what is the proper procedure for testing for the presence of a Group III metal cation?

Explanation / Answer

1. The factor is their relative Ksp values

2.(a)

If the student directly add H2S in buffer then if it is a group 1 or 2 metal it will precipitate. In or der to do a proper and accurate nalysis, the group reagents should be added in order.

(b)

The metal could be from group 1 2 or 3.

(c)

When analyzing for the cation in a general unknown, the four group reagents must be added in the sequential order: 1) cold dilute HCl, 2) H2S in 0.3 M HCl, 3) H2S inNH3/NH4Cl buffer, If any precipitate aprrears then it could be either of NiS, MnS,FeS or Al(OH)3.

To confirm Mn^2+ add NaBiO3 , if pink color of permanganate is observed, then presence of Mn^2+ is confirmed

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