In: Science

Submitted By ernestmagier
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Cations are divided into six groups. A specific reagent for each group is used for their separation from the solution. This separation is done in a particular sequence because some ions of prior groups can also react with the reagent of later groups.

This can be cause of difficulty in their separation. The qualitative analysis of cations depends on the solubility products of the ions. The cations get precipitated on the optimum needed concentration and easily detected.

1st group of cations

The group (I) cations form the insoluble chlorides. The dilute hydrochloric acid (generally 1-2 M concentration) is used as the group reagent. The Concentrated HCl is not used because it forms a soluble complex ion ([PbCl4]2-) with Pb2+ and would not be detected. The most important cations of 1st group are Ag+, Hg22+, and Pb2+. Color detection test is not suitable for their chlorides because all are white solid compounds. The analyzing of group is done by adding the respective salt into water and follow it with dilute HCl. This results in a white precipitate in which NH4OH is added. The insoluble precipitate shows the presence of Plumbous (Pb2+) and if precipitate is soluble, then it shows the presence of Argentous (Ag+).If the white precipitate turns black, it shows the presence of Hg22+.

The obtained filtrate after first group analysis is used for the test of second group. Usually Pb2+ is also included in the 2nd analytical group because the solubility of PbCl2 is more than the chlorides of other two ions.

Confirmation Test

For lead

Pb2+ + 2KI → PbI2 + 2 K+
Pb2+ + K2CrO4 → PbCrO4 + 2K+

For Silver

Ag+ + KI → AgI + K+
2Ag+ + K2CrO4 → Ag2CrO4 + 2K+

For mercury ion

Hg22++ 2KI → Hg2I2 + 2K+
2Hg22+ + 2NaOH → 2Hg2O + 2Na+ + H2O

2nd analytical group of cations

The ions of this group form acid-insoluble sulfides. Cations of the 2nd group…...

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