Question

Ex: Separation of Group 1 Cations

(Pb+ and Ag+)

10 drops of unknown was place along with 6 drops of 6M HCl, centrifuge. 2 more drops of 6M HCl, centrifuge. Decant supernate, use precipitate and add 5 drops of cold water, centrifuge.

Test for Pb2+

15 drops of water to precipitate & put in hot water, centrifuge then decant supernate. repeat x2 Combine supernates and save precipitate for Ag+ analysis. Add 3 drops of 1M K2CrO4 to supernate. yellow precipitate forms PbCrO4, which confirms Pb2+.

So what is happening here? Why is the ion present? I want to understand this. So that i’m able to properly explain what is happening for these ions.

Ag+

Pb2+

Fe3+

Ni2+

Ba2+

Ca2+

NH4-

Na-

Answer 1

First of all you should understand the condition of precipitation. A precipitate will form only when the ionic product of the of concentrations cation and the precipitating reagent exceed the Ksp, the solubility product.

NH4+ comes under Group Zero- the only way you can selectively and exclusively detect it is with NaOH. When NaOH is added NH3 gas is released from the NH4+ cation which is detected as white fumes when a glass rod dipped in HCl is introduced near the mouth of the testtube.

Group1 contains Pb2+, Ag+ and Hg+2

The precipitating reagent for this is 6M HCl. When HCl is present All ions in this group will get selectively precipitated.however Pb2+ if present can be selectively separated as its precipitate will be soluble in hot water.

Similarly for Group 2 the precipitating reagent is HCl and H2S. Here only Cu 2+ ions get precipitated as in an acidic pH the ionic product exceeds Ksp which is why Ni2+, Co2+, Zn2+ and Mn2+ will not get precipitated as they will require an alkaline pH for precipitation with H2S. Similarly Ca, Ba, and Sr will be precipitated by ammonium phosphate in an alkaline medium.