The fourth group of cations

It is studied two representatives of this group – ions of Ag⁺ and Pb²⁺, formed slightly soluble precipitations of Chlorides, don’t soluble in diluted acids. Solubility of AgCl and PbCl₂ in water, excess of HCl, ammonia solution is of great importance for separation of these cations.

Group reagent – diluted HCl. HCl (and soluble chlorides) deposits Ag⁺ and Pb²⁺ in form of white precipitations of AgCl and PbCl₂, not soluble in diluted HNO₃ and H₂SO₄:

Ag⁺ + Cl^- = AgCl↓;

Pb²⁺ +2Cl^- = PbCl₂↓.

For example,

AgNO₃ +HCl = AgCl↓ + HNO₃;

Pb(NO₃)₂ + 2HCl = PbCl₂↓ + 2HNO₃.

Test carrying out

To 2-3 drops of Silver salt solution or Lead salt solution add the same quantity of diluted HCl. Formed white precipitations of AgCl and PbCl₂ divide into 4 tubes and study their solubility in hot H₂O, concentrated water ammonia, (NH₄)₂CO₃, concentrated HCl.

AgCl dissolves in:

- concentrated water ammonia:

AgCl↓ + 2NH₄OH = [Ag(NH₃)₂]⁺ + Cl^- + 2H₂O or

AgCl↓ + 2NH₄OH = [Ag(NH₃)₂]Cl + 2H₂O;

- excess of (NH₄)₂CO₃:

AgCl↓ + 2NH₄⁺ + CO₃^2- = [Ag(NH₃)₂]⁺ + Cl^- + 2H₂O +CO₂↑ or

AgCl↓ + (NH₄)₂CO₃ = [Ag(NH₃)₂]Cl + 2H₂O +CO₂↑.

- concentrated HCl or saturated solution of NaCl:

AgCl↓ +2H⁺ + 2Cl^- = 2H⁺ + [AgCl₃]^- or

AgCl↓ +2HCl = H₂[AgCl₃].

Sediment PbCl₂ in contrast to AgCl dissolves well in hot DW (near 10 g/L). This moment is used for separate off AgCl and PbCl₂.

PbCl₂ dissolves in:

- concentrated HCl or saturated solution of NaCl:

PbCl₂↓ + 2H⁺ + 2Cl^- = 2H⁺ + [PbCl₄]^2- or

PbCl₂↓ + 2HCl = H₂[PbCl₄].

If this solution dilutes of water, PbCl₂ will precipitate again.

3.6.1. Characteristic reactions of Silver cations Ag⁺

1. Water ammonia NH₄OH, reacting with Ag⁺, precipitates silver in form of Ag₂O, which dissolves in excess of reactant:

Ag⁺ + NH₄OH = AgOH↓ + NH₄⁺;

2AgOH↓ = Ag₂O↓ + H₂O;

Ag₂O↓ + 4 NH₄OH = 2[Ag(NH₃)₂]⁺ + 2OH^- + 3H₂O or

AgNO₃ + NH₄OH = AgOH↓ + NH₄NO₃;

Ag₂O↓ + 4 NH₄OH = 2[Ag(NH₃)₂]OH + 3H₂O.

Test carrying out

Add by drops ammonia solution to 1-2 drops of silver nitrate solution till precipitation formation. Observe dissolving of turbidity at adding of excess of ammonia.

3.6.2. Characteristic reactions of Lead cations Pb²⁺

1. Potassium Iodide KI isolates yellow sediment of PbI₂from lead(II) solutions. PbI₂ dissolves in boiling water. After cooling PbI₂ is precipitated again in form of goldish-yellow crystals (sometimes this reaction is called reaction of “gold rain”). This reaction is used for identification of Pb²⁺ in mixtures and substances.

Condition of this reaction realization: pH less than 7 (add a few drops of diluted acetic acid), fast cooling.

Pb²⁺ + 2I^- = PbI₂↓.

For example,

Pb(NO₃)₂ + 2KI = PbI₂↓ + 2 KNO₃.

Test carrying out

Add 1-3 drops of potassium iodide solution of 2 drops of lead nitrate or lead acetate solution acidified by 1 drop of acetic diluted acid. Add DW to formed yellow sediment (5-7 mL) and boil mixture in water bath. After complete dissolving of precipitation cool quickly tube at stream of cool running water. For acceleration of re-crystallization rub tube sides by glass stick.

2. Potassium Chromate (Dichromate) K₂CrO₄ (K₂Cr₂O₇) forms yellow precipitation PbCrO₄ with ions Pb²⁺. This sediment doesn’t dissolve in acetic and hydrochloric acids, but solutes in alkalis and nitrate acid. Conditions: pH less then 7 (in presence of acetic acid), absence of reducing agents, reduced Cr⁶⁺ till Cr³⁺ (for example, Fe²⁺), absence of cations, formed with CrO₄^2- sediments:

Pb²⁺ + CrO₄^2- = PbCrO₄↓;

or 2Pb²⁺ + Cr₂O₇^2- +H₂O = 2PbCrO₄↓ +2H⁺.

For example:

Pb(NO₃)₂ + K₂CrO₄ = PbCrO₄↓ +2KNO₃;

or

2Pb(CH₃COO)₂ + K₂Cr₂O₇ + H₂O = 2PbCrO₄↓ + 2KCH₃COO + 2CH₃COOH.

Test carrying out

Add 1-2 drops of potassium chromate or dichromate solution to 1-2 drops of lead salt solution. Yellow precipitate formed.

Note, that presence of reducing agents (Fe²⁺, Mn²⁺) prevents reaction owing to reduction of Cr⁶⁺ till Cr³⁺. In presence of Ag⁺ brick-red sediment Ag₂CrO₄ or dark-red Ag₂Cr₂O₇ is precipitated.

3. Diluted Sulfate Acid H₂SO₄ and soluble sulfatesprecipitate Pb²⁺ cation in the form of white crystalline precipitation PbSO₄. this sediment doesn’t dissolve in excess of diluted sulphate acid, but dissolves in alkalis, ammonia acetate and ammonia salt of wine acid (tartrate), concentrated H₂SO₄ and HCl. Conditions of reactions: pH less than 7, because lead sulphate dissolves in alkalis, absence of ammonia salts of acetic or wine acids, absence of Ca²⁺ and Sr²⁺ ions in examined solution:

Pb²⁺ + SO₄^2- + PbSO₄.

For example:

Pb(NO₃)₂ + H₂SO₄ = PbSO₄↓ +2HNO₃.

Test carrying out

Add 1-2 drops of diluted sulphate acid to 2-3 drops of lead salt solution. Observe formation of the white crystalline precipitation. Content of tube divide into two tubes and examine its solubility in excess of H₂SO₄ (diluted) and excess of alkali NaOH.

Date: 2016-01-03; view: 1183