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The third group of cations

3.4.1. Characteristic reactions of Copper cations Cu2+

 

1. Group reagent (NH4)2HPO4 forms blue crystalline precipitation with copper ions:

3Cu2+ + 2HPO42- + 2NH4OH = Cu3(PO4)2↓ + 2NH4+ + 2H2O

3CuSO4 + 2(NH4)2HPO4 + 2NH4OH = Cu3(PO4)2↓ + 3(NH4)2SO4 + 2H2O.

Test carrying out

Add to test-tube 10-12 drops of (NH4)2HPO4, 4-6 drops of NH4OH, 10-12 drops NH4Cl and 10-12 drops of copper salt.

Copper phosphate is soluble in acetic, mineral acids and ammonia.

2. Water ammonia NH4OH. The reaction is carried out in two stages:

the 1st: 2Cu2+ + SO42- + 2NH4OH = (CuOH)2SO4¯ + 2NH4+;

the 2d: (CuOH)2SO4 + 2NH4+ + 6NH4OH = 2[Cu(NH3)4]2+ + 8H2O

solution

For example,

2CuSO4 + 2NH4OH = (CuOH)2SO4↓ + (NH4)2SO4;

(CuOH)2SO4↓+(NH4)2SO4 + 6NH4OH = 2[Cu(NH3)4]SO4 + 8H2O.

Test carrying out

Add little by little NH4OH solution to 3-4 drops of copper salt solution up to basic salt formation (precipitation) and then the excess of NH4OH up to the dilution of precipitation and formation of complex compound which has dark blue color. This reaction is highly sensitive (c=1:500000).

 

3. Alkalis (NaOH, KOH). Alkali forms blue precipitation with copper ions:

Cu2+ + 2OH- = Cu(OH)2¯

For example, in molecular form:

CuCl2 + 2NaOH = Cu(OH)2¯ + 2NaCl.

Copper hydroxide has weakly amphoteric properties, may be dissolved in mineral and acetic acids.

Test carrying out

Add 2-3 drops of NaOH or KOH to solution of copper salt to forming of blue precipitation.

4. Potassium ferricyanide K4[Fe(CN)6] (yellow blood salt) forms red-brown precipitation with copper ions (pH<7):

2Cu2++[Fe(CN)6]4- = Cu2[Fe(CN)6]¯

For example, in molecular form:

2CuCl2 + K4[Fe(CN)6] = Cu2[Fe(CN)6]¯ + 4KCl.

This reaction is highly sensitive (c=1:2500000).

Precipitation Cu2[Fe(CN)6] is not dissolved in diluted acids, but is solved in excess of water ammonia.

Test carrying out

Add 1 drop of HCl and 2-3 drops of K4[Fe(CN)6] to 3-4 drops of Copper salt solution.

 

5. Potassium Iodide KI reacts with copper salt formed white precipitation CuI. But reactive mass has brown-red color due to free I2 isolation. This reaction is RedOx:

2Cu2+ + 4I- ® 2CuI¯ + I2.

For example, in molecular form:

2CuSO4 + 4KI = 2CuI + I2 + 2K2SO4

2I- - 2e ® I2
Cu2+ + 1e ® Cu+

Test carrying out

Add 2-3 drops of KI solution to 3-4 drops of Copper salt solution.

3.4.2. Characteristic reactions of Zinc cations Zn2+

1. Group reagent (NH4)2HPO4 forms white crystalline precipitation with zinc ions:

3Zn2+ + 2HPO42- + 2NH4OH = Zn3(PO4)2↓ + 2NH4+ + 2H2O

3ZnSO4 + 2(NH4)2HPO4 + 2NH4OH = Zn3(PO4)2↓ + 3(NH4)2SO4 + 2H2O.

Test carrying out

Add to test-tube 10-12 drops of (NH4)2HPO4, 4-6 drops of NH4OH, 10-12 drops NH4Cl and 10-12 drops of zinc salt.

Zinc phosphate is diluted in acetic, mineral acids, ammonia and in alkali excess.

 

2. Water ammonia NH4OH forms white precipitation with Zn ions (Zn(OH)2) which is diluted in excess of NH4OH with the formation of complex compound:

Zn2+ + 2NH4OH = Zn(OH)2↓ + 2NH4+;

For example,

ZnCl2 + 2NH4OH = Zn(OH)2↓+ 2NH4Cl;

Zn(OH)2 + 4NH4OH (in excess) = [Zn(NH3)4](OH)2 + 4H2O.

colorless solution

Test carrying out

Add 2-3 drops of water ammonia to 2-3 drops of zinc salt solution to forming of white precipitation. Add excess of ammonia up to solving of precipitation.

 

3. Alkalis (NaOH, KOH). Sodium hydroxide forms white amorphous precipitation with Zn ions - Zn(OH)2 which is the amphoteric hydroxide soluble in acid and alkali:

Zn2+ + 2OH- = Zn(OH)2;

Zn(OH)2 + 2H+ = Zn2+ + 2H2O;

Zn(OH)2 + 2OH- = ZnO22- + 2H2O;

For example,

ZnCl2 + 2NaOH = Zn(OH)2¯ + 2NaCl;

Zn(OH)2 + 2HCl = ZnCl2 + 2H2O;

Zn(OH)2 + 2NaOH = Na2ZnO2 + 2H2O.

Test carrying out

Add slowly 10-12 drops of zinc salt solution to 2-3 drops of NaOH or KOH solution up to forming of white precipitation Zn(OH)2.

4. Potassium ferrocyanide K3[Fe(CN)6] (red blood salt) forms precipitation of mustard color with Zn ions (pH<7):

3Zn2+ + 2[Fe(CN)6]3- = Zn3[Fe(CN)6]2¯;

For example,

3ZnCl2 + 2K3[Fe(CN)6] = Zn3[Fe(CN)6]2¯ + 6KCl.

Precipitation is solved in alkalis:

Zn3[Fe(CN)6]2 + 18OH- = 3ZnO22- + 2Fe(OH)3 + 12CN- + 6H2O.

Test carrying out

Add 1 drops HCl and 2-3 drops of K4[Fe(CN)6] to 2-3 drops of zinc salt solution.

3.4.3. Characteristic reactions of Cobalt cations Co2+

1. Group reagent (NH4)2HPO4 forms lilac crystalline precipitation with cobalt ions:

3Co2+ + 2HPO42- + 2NH4OH = Co3(PO4)2↓ + 2NH4+ + 2H2O

3CoCl2 + 2(NH4)2HPO4 + 2NH4OH = Co3(PO4)2↓ + 6NH4Cl + 2H2O.

Test carrying out

Add to test-tube 10-12 drops of (NH4)2HPO4, 4-6 drops of NH4OH, 10-12 drops NH4Cl and 10-12 drops of cobalt salt.

Cobalt phosphate is diluted in acetic, mineral acids, ammonia.

2. Water ammonia NH4OH. The reaction is carried out in two stages:

the 1st: Co2+ + Cl- + NH4OH = CoOHCl¯ + NH4+;
  lilac  
the 2d: CoOHCl¯+7NH4OH =[Co(NH3)6]2+ +2OH-+NH4++Cl-+6H2O;
  lilac solution  
         

For example,

CoCl2 + NH4OH = CoOHCl↓ + NH4Cl;

CuOHCl↓+ 7NH4OH = [Co(NH3)6](OH)2 + NH4Cl+ 6H2O.

Test carrying out

Add little by little NH4OH solution to 3-4 drops of copper salt solution up to basic salt formation (precipitation) and then the excess of NH4OH up to the dilution of precipitation and formation of complex compound which has lilac color, changed to yellow-brown a few moments after in the result of oxidation Co2+ → Co3+.

3. Alkalis (NaOH, KOH). Sodium hydroxide forms blue amorphous precipitation of basic salt with Co2+ ions:

Co2+ + OH- + Cl- = CoOHCl↓;

For example,

CoCl2 + NaOH = CoOHCl¯ + NaCl;

Excess of alkali converts CoOHCl at heating in Co(OH)2 of pink color:

CoOHCl¯ + OH- = Co(OH)2¯ +Cl-;

CoOHCl¯ + NaOH- = Co(OH)2¯ +NaCl.

Oxiding by air oxygen, Co(OH)2 converts to Co(OH)3 of bark-brown color:

4Co(OH)2 +O2 +2H2O = 4Co(OH)3

Co2+ - 1 → Co3+
O20 + 4 → 2O2-

If add alkali and hydrogen peroxide H2O2 simultaneously to cobalt(II) salt solution, Co(OH)3 forms at once;

2 CoCl2 +4NaOH + H2O2 = Co(OH)3¯ + 4NaCl;

Co2+ - 1 → Co3+
2O1- + 2 → O20

Test carrying out

Add 1 drop of NaOH to 3-4 drops of Cobalt(II) salt solution. Note color of sediment. Add excess of alkali. Observe change of color of sediment.

 

4. Ammonium or Potassium Thiocyanide (NH4SCN or KSCN) - crystalline or solution in organic solvents (foe example, acetone ) forms the solution of dark blue color:

Co2+ + 4SCN- = [Co(SCN)4]2-.

For example,

CoCl2 + 4NH4SCN = (NH4)2[Co(SCN)4] + 2NH4Cl.

Reaction is very sensitive. Complex compound solves better in the layer of organic solvent, that in water solution.

Test carrying out

Add 4-5 drops of saturated solution of ammonium or potassium thiocyanide in organic solvent to 3-4 drops of Cobalt (II) salt. Shake up content of tube and wait a few seconds. The upper layer is colored in dark-blue color.

Presence of Fe3+-ions in analyzing mixture prevents to determination of cobalt(II), because complex ion [Fe(SCN)6]3- is more stable that Co(SCN)4]2-. In such case it is necessary to mask Fe3+-ion added solid NaF, where F- ions form with Iron (III) ions stable colorless complex ion [FeF6]3-, when cobalt (II) ions don’t compexing with this masking agent. As masking agent orthophosphoric acid and wine acid H2C4H4O6 might be used.

 

5. Potassium Nitrite KNO2in presence of acetic acid oxidizes Co2+ to Co3+ forming with excess of KNO2 sediment K3[Co(NO2)6].

In general this reaction is presented as:

CoCl2+7KNO2+2CH3COOH=K3[Co(NO2)6]↓+2KCl+2CH3COOK+NO+H2O.

Co2+ - 1 → Co3+
N3+ + 1 → N2+

Test carrying out

To 5-7 drops of cobalt(II) salt solution add crystalline KNO2 to saturation. Ad to this mixture concentrated acetic acid by drops up to formation of yellow-orange precipitation K3[Co(NO2)6].

 

6. α-nitrozo-β-naftol C10H6(NO)OH (Il’inski’s reactant).At the action into ion Co2+ this reactant oxidizes Co2+ to Co3+, leaved reddish voluminous sediment of intra-complex salt Co[C10H6(NO)O]3 simultaneously. Reaction is realized in neutral or weak acidic medium.

Test carrying out

Add 1-2 drops of diluted acetic acid and 2-3 drops of fresh-prepared solution of Il’inski’s reactant to 3-4 drops of Cobalt(II) salt solution. Heat mixture in water bath. If necessary, to rub side of tube by glass stick.

Presence of Fe3+ ions in analyzing mixture prevents to determination of cobalt(II), because iron(II) ions form brown-reddish sediment with this reactant too.

3.4.4. Characteristic reactions of Nickel cations Ni2+

1. Group reagent (NH4)2HPO4 forms light-green crystalline precipitation with nickel ions:

3Ni2+ + 2HPO42- + 2NH4OH = Ni3(PO4)2↓ + 2NH4+ + 2H2O

3NiSO4+2(NH4)2HPO4+2NH4OH=Ni3(PO4)2↓+3(NH4)2SO4 + 2H2O.

Test carrying out

Add to test-tube 10-12 drops of (NH4)2HPO4, 4-6 drops of NH4OH, 10-12 drops NH4Cl and 10-12 drops of nickel salt.

Nickel phosphate is diluted in acetic, mineral acids and excess of ammonia:

Ni3(PO4)2↓+6HCl = 3NiCl2 + 2H3PO4;

Ni3(PO4)2↓+6CH3COOH = 3Ni(CH3COO)2 + 2H3PO4;

Ni3(PO4)2↓+18NH4OH (excess) = [Ni(NH3)6]3(PO4)2+18H2O.

 

2. Alkalis (NaOH, KOH). Sodium hydroxide forms green amorphous precipitation of Ni(OH)2:

Ni2+ + 2OH- = Ni(OH)2↓;

For example,

NiSO4 +2 NaOH = Ni(OH)2↓ + Na2SO4.

Nickel(II) hydroxide dissolves in mineral acids, excess of ammonia, solutions of ammonia salts, but doesn’t soluble in alkali excess.

For example:

Ni(OH)2↓ +6NH4OH= [Ni(NH3)6](OH)2 + 6H2O.

In presence of chloric or bromic waters in alkaline medium Ni(OH)2 is oxidized to Ni(OH)3 of black-brown color:

2Ni(OH)2↓ + Cl2 +2NaOH = 2 Ni(OH)3↓ +2NaCl.

Ni2+ - 1 →Ni3+
Cl20 + 2 → 2Cl-

Test carrying out

Add 20-25 drops of NaOH to 8-9 drops of Nickel(II) salt, heat mixture. Observe color of precipitation. Divide precipitation into 5 tubes. Study solubility of Ni(OH)2 in HCl, NH4Cl, NaOH excess, NH4OH. Add 3-4 drops of fresh-prepared chloric or bromic water to precipitation in the fifth tube. Observe change of precipitation color.

 

3. Water ammonia NH4OH. The reaction is carried out in two stages:

the 1st: 2Ni2+ +2SO42-+2NH4OH = (NiOH)2SO4¯ +2NH4+;
  light-green
the 2d: (NiOH)2SO4¯+(NH4)2SO4+10NH4OH=2[Ni(NH3)6]SO4+12H2O;
  light-blue solution
     

Ni2+ doesn’t precipitate by NH4OH in presence of ammonia salts.

Test carrying out

Add by drops NH4OH solution to 3-4 drops of nickel salt solution up to basic salt formation (precipitation) and then the excess of NH4OH up to the dilution of precipitation and formation of complex compound which has light-blue color. Ions of Cu2+ and Co2+ prevent to this reaction of Ni2+.

 

4. Dimethylglioxime (Chugaev’s reagent) C4H8N2O2 in ammonia medium (pH=9) forms bright pink precipitation of intra-complex salt with Ni2+ ions [Ni(C4H7N2O2)2]↓. Precipitation solves in acids, alkalis, but doesn’t dissolve in diluted ammonia solution.

Reaction is very sensitive, concentration limit ñ=1:300000:

Ni2++2C4H8N2O2+2NH3=[Ni(C4H7N2O2)2]¯+2NH4+.

For example:

NiSO4 +2C4H8N2O2+2NH3=[Ni(C4H7N2O2)2]¯+(NH4)2SO4.

Test carrying out

Add 12-16 drops of ammonia solution and 8-10 alcohol solution of Chugaev’s reagent to 4-5 drops of nickel salt solution. Study solubility of prepared precipitation in mineral acids, alkalis and ammonia solution. For this to divide prepared precipitation into 4 tubes and add corresponding reactant in every doze of precipitation.

Ions of Fe2+, Fe3+, Cu2+ prevent to determination of nickel(II) ions with Chugaev’s reagent.

Date: 2016-01-03; view: 1323

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