Question

Develop an analysis scheme that would allow you to take a solution that was said to contain any or all of Ag+,Ba2+,Co2+,Pb2+, and Zn2+ cations and would confirm or deny the presence of each of these ions in the solution.

Answer 1

Group I – Pb2+ (lead): OS + dil HCl = white ppt; add water, boil, and divide into three parts

1. Leave OS undisturbed = white crystals formed on cooling

2. Potassium iodide test: OS + KI = yellow ppt

3. Potassium chromate test: OS + K2CrO4 = yellow ppt

1. Co2+ / Ni2+: Black ppt; dissolve ppt in aqua regia 10 and evaporate sol to dryness to get residue

1. Co2+: Blue residue; turns pink / purple when dissolved in water; divide into two parts

1. Part 1 + dil acetic acid + KNO2 + warm = yellow ppt

2. Part 2 + ether (1 mL) + solid NH4CNS 11 = blue colour in ether

Co2+ (cobalt), Ni2+ (nickel), Mn2+ (manganese), Zn2+ (zinc)

OS + solid NH4Cl + excess NH4OH + pass H2S gas

3. Zn2+: Greyish white ppt; divide into two parts

1. Part 1 + excess NaOH = white ppt dissolves

2. Potassium ferrocyanide test:

Part 2 + K4[Fe(CN)6] = white / bluish white ppt

Ba2+ (barium), Sr2+ (strontium), Ca2+ (calcium): OS + (NH4)2CO3 + NH4Cl + NH4OH = white ppt; add dil acetic acid, divide sol into three parts and test for following IN ORDER

1. Ba2+: Part 1 + excess K2CrO4 13 (aq) = yellow ppt

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 [Ag(NH₃)₂]⁺), 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.

Confirmation test for Silver:

Ag⁺ + KI → AgI + K⁺

2Ag⁺ + K₂CrO₄ → Ag₂CrO₄ + 2 K⁺