Lead (II), mercury (I), and silver (I) ions are precipitated as chlorides

Tungsten and thallium may also be included

Forms precipitates in acid solution containing chloride ion

Form sparingly soluble compounds with chloride ion

Represented by the following equations:

Pb2+(aq) + 2 Cl- (6M HCl) → PbCl2 (s)

Hg2+(aq) + 2 Cl- (6M HCl) → Hg2Cl2(s)

Ag+ (aq) + Cl- (6M HCl) → AgCl(s)

Why is HCl used as precipitating reagent?

1. It does not introduce cations that would interfere with analysis

2. It creates/ maintains an acidic solution-the hydroxides or oxides would precipitate if

alkaline-if not strongly acidic, oxochlorides of bismuth

and antimony will also precipitate

What are the setbacks of using HCl as precipitating reagent?

1. Too much chloride ions lead to the formation of stable chloride complexes of other cations

2. Too much chloride ions lead to the formation of soluble chloride complexes

-Formation of chloro complexes that would redissolve precipitates

Separation of Lead (II) ion

Partially separated from mercurous chloride and silver chloride because of its increase in solubility in hot water.

Not all are precipitated, may remain as white residue

Identification of Lead (II) ion Detected by its reaction with chromate

ion in aq. HOAc forming the yellow ppt PbCrO4(s)

Acetic acid is added because it reduces the concentration of CrO4

2- so other chromates will not ppt

Addition of NaOH produces soluble HPbO2

- and the reaction reverses and PbCrO4(s) is reformed.

Reactions:Reaction1:Pb2+

(aq) + CrO42-

(aq) → PbCrO4(s)

Reaction2:PbCrO4(s) + 3 OH(aq) ↔ HPbO2

-(aq) + CrO4

2-(aq) +

H20

Identification of Mercury (I) ion

Relies on disproportionation of Hg2+(aq) to

elemental mercury and mercury (II)

Separation of Mercury (I) ion Concentration is lowered by formation

of sparingly soluble white mercury(II) amidochloride › due to the direct reaction of

mercury(I) [or mercury(II)] ions with ammonia molecules

Mercury(I) disproportionates in alkaline solutions› Finely divided black elemental mercury is

formed

Reactions:

Hg2Cl2(s) + 2 NH3(aq) → Hg0(l) + HgNH2Cl(s) +

NH4+

(aq) + Cl-(aq)

Identification of Silver (I) ion Silver ion is now present as the

diammine complex [Ag(NH3)2]+

This also has ammonium ion from the ionization of NH3 if acid is added, OH- decreases, lowering ammonia concentration.› This destroys the diammine complex and

increasing the concentration of Ag+

› AgCl reprecipitates

Aqua regia (1 part HNO3 + 3 parts HCl ) is needed to dissolve the ppt when there are large quantities of Hg2

2+

present› Provides both oxidizing agent (HNO3) and

complexing agent (HCl)

Reactions:Reaction1: 2 HgNH2Cl(s) + 6 Cl-(aq) + 2 NO-

3(aq) + 4 H3O+

(aq)

→ 2[HgCl4]2-(aq) + 2 NO(g) + 2 N2(g) + 8H2O

Reaction2:3 Hg(l) + 12 Cl-(aq) + 2 NO-

3(aq) + 8 H3O+(aq)

→ 3[HgCl4]2-(aq) + 2 NO(g) + 12H2O

Reaction3:3 Ag(s) + 6 Cl-(aq) + NO-

3(aq) + 4 H3O+(aq)

→ 3[AgCl4]-(aq) + NO(g) + 6 H2O

Barium, Strontium, Calcium, and Magnesium ions are precipitated as carbonates

Alkaline earth family Colorless except when anion is colored Ions are so chemically similar that

flame tests are used

Magnesium carbonate is somewhat more soluble than other carbonates› maybe kept in solution by increasing

ammonium concentration Represented by the following

equations:Ba2+

(aq) + CO32+

(aq) → BaCO3(s)

Sr2+(aq) + CO3

2+(aq) → SrCO3(s)

Ca2+(aq) + CO3

2+(aq) → CaCO3(s)

Mg2+(aq) + CO3

2+(aq) → MgCO3(s)

Since it is difficult to prevent precipitation of MgCO3, it is then encouraged. Its precipitation is formed in two ways:

1. Ammonium ion concentration is kept low by strong heating of the centrifuge remaining from the group 3 cations.

2. The group precipitate is formed from an alcohol-water solution in which all alkaline earth carbonates are soluble.

Carbonates are volatile compoundsAnalysis of Flame Test: Barium – lime green Strontium – crimson Calcium – brick red

Barium ion is precipitated from the HOAc solution as the yellow chromate:

BaCO3(s) + CrO42-

(aq) → BaCrO4(s)

Presence of HOAc decreases the concentration of the the chromate ion by converting it to the orange dichromate ion› Keeps chromate ion concentration low

enough so SrCrO4(s) does not ppt.

The presence of barium ion is confirmed observing green flame test color.

Reactions:Reaction1:

When sulfate ion is added to solution containing strontium and calcium ions, strontium sulfate ppt.› Calcium sulfate does not due to formation

of disulfatocalcate(II) ion.› Triethanolamine also forms a stable

complex with the calcium ion

strontium sulfate is treated with Na(CO3)2 to SrCO3

Solid SrCO3 is dissolved in HCl to produce

SrCl2 for flame testing.

The oxalate in the HOAc medium will ppt Ca oxalate, but not Mg due to the formation of the bisoxalatomagnesate(II) ion.› This does not necessarily mean that Ca is

present Ca ppt slowly

To enhance it :1. Adjust the volume of the solution

before (NH4)2C2O2 is added

2. The ppt is formed from a 50% alcohol solution

3. Cooling the solution

Ca oxalate is dissolved in HCl to form CaCl2 which forms a brick red flame

Mg ion is ppt from ammoniacal solution as Mg ammonium phosphate

This is dissolved with dilute HCl forming the Mg ion

The ion is reprecipitated as Mg(OH)2 › This is hard to detect so Mg reagent, para-

nitrobenzeneazoresorcinol is added to form a “blue lake”