1st analytical group of cations consists of ions that form insoluble chlorides. As such, the group reagent to separate them is hydrochloric acid, usually used at a concentration of 1–2 M. Concentrated HCl must not be used, because it forms a soluble complex ion (2-) with Pb2+. Consequently the Pb2+ ion would go undetected. NH3 is also included in ZERO group of cation (according to NCERT textbooks). The most important cations in 1st group are Ag+, Hg2+
2, and Pb2+. The chlorides of these elements cannot be distinguished from each other by their colour - they are all white solid compounds. PbCl2 is soluble in hot water, and can therefore be differentiated easily. Ammonia is used as a reagent to distinguish between the other two. While AgCl dissolves in ammonia (due to the formation of the complex ion +), Hg2Cl2 gives a black precipitate consisting of a mixture of chloro-mercuric amide and elemental mercury. Furthermore, AgCl is reduced to silver under light, which gives samples a violet colour.
PbCl2 is far more soluble than the chlorides of the other two ions, especially in hot water. Therefore, HCl in concentrations which completely precipitate Hg2+
2 and Ag+ may not be sufficient to do the same to Pb2+. Higher concentrations of Cl− cannot be used for the before mentioned reasons. Thus, a filtrate obtained after first group analysis of Pb2+ contains an appreciable concentration of this cation, enough to give the test of the second group, viz. formation of an insoluble sulfide. For this reason, Pb2+ is usually also included in the 2nd analytical group.
This group can be determined by adding the salt in water and then adding dilute hydrochloric acid. A white precipitate is formed, to which ammonium hydroxide is then added. If the precipitate is insoluble, then Pb2+ is present; if the precipitate is soluble, then Ag+ is present, and if the white precipitate turns black, then Hg2+
2 is present.
Confirmation test for lead:
- Pb2+ + 2 KI → PbI2 + 2 K+
- Pb2+ + K2CrO4 → PbCrO4 + 2 K+
Confirmation test for Silver:
- Ag+ + KI → AgI + K+
- 2Ag+ + K2CrO4 → Ag2CrO4 + 2 K+
Confirmation test for dimeric mercury ion:
- Hg2+
2 + 2 KI → Hg2I2 + 2 K+ - 2 Hg2+
2 + 2 NaOH → 2 Hg
2O + 2 Na+ + H2O
Read more about this topic: Qualitative Inorganic Analysis, Detecting Cations
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