Characteristic reactions of Aluminium cations Al3+ Characteristic reactions of Zinc cations Zn2+
Characteristic reactions оf сhromium cations Cr 3+
2. qualitative reactions of cations of analytical group 5:
Characteristic reactions of Magnesium cations Mg2+ Characteristic reactions of Ferrous cations Fe2+ Characteristic reactions of Ferrous cations Fe3+ Characteristic reactions of manganese cations Mn2+
Group reagent (NH4)2HPO4. The reaction is carried out in presence of mixture NH4OH + NH4Cl which is the buffer mixture, which keeps pH of solution equal to 9. At pH equal to 9 the cation Ca2+, Ba2+, Fe2+, Fe3+, Al3+ are deposited as orthophosphates:
Ca3(PO4)2↓
|
Sr3(PO4)2↓
|
Fe3(PO4)2↓
|
FePO4↓
|
AlPO4↓
|
white
|
white
|
White
|
Yellowish
|
White
|
and cation Mg2+, Mn2+ are deposited as double salts of Magnesium ammonium phosphate and Manganese ammonium phosphate:
MgNH4PO4↓,
|
MnNH4PO4↓
|
White
|
white
|
For example,
3CaCl2 + 2(NH4)2HPO4 + 2NH4OH = Ca3(PO4)2↓ + 6NH4Cl + 2H2O;
3Ca2+ + 2HPO42- + 2NH4OH = Ca3(PO4)2↓ + 2NH4+ + 2H2O;
MnCl2 + (NH4)2HPO4 + NH4OH = MnNH4PO4↓ + 2NH4Cl + H2O;
Mn2+ + HPO42- + NH4OH = MnNH4PO4↓ + H2O.
In contrast to phosphates of the third group, all above mentioned precipitates are not dissolved in excess of NH4OH. As related to solubility of phosphates in acetic acid, cations of the second group subdivide into two groups:
II-А sub-group - includes cations Mg2+, Ca2+, Sr2+, Mn2+, Fe2+, phosphates are soluble in acetic acid.
For example,
Ca3(PO4)2 + 4CH3COOH = 2Ca(CH3COO)2 + Ca(H2PO4)2;
Ca3(PO4)2 + 4CH3COOH = 2Ca2+ + 4CH3COO- + Ca2+ + 2H2PO4-.
ΙΙ-B sub-group - includes cations Fe3+, Al3+ – phosphates of these cations are insoluble in acetic acid.
Phosphates of both sub-groups are soluble in mineral acids. For example,
FePO4↓ + 3H+ = Fe3+ + H3PO4;
FePO4↓ + 3HCl = FeCl3 + H3PO4.
Aluminum phosphate due to amphoterity is soluble in alkalis:
AlPO4↓ + 4OH- = AlO2- + PO43- + 2H2O;
AlPO4↓ + 4NaOH = NaAlO2 + Na3PO4 + 2H2O.
Test carrying out
Add 3-5 drops of NH4OH and 6-8 drops of NH4Cl to 6-8 drops of (NH4)2HPO4 into test-tube and then add 6-8 drops of Calcium salt solution. Make the same action with all the rest cations of second group.
Phosphates of II-A sub-group are soluble in acetic acid and the phosphates of II-B sub-group are soluble in acetic acid.
Characteristic reactions of Aluminium cations Al3+
1. Alkalis (NaOH, KOH). Sodium or Potassium hydroxide forms white amorphous precipitation with Al3+ ions - Al(OH)3 which is the amphoteric hydroxide soluble in acid and alkalis:
Al3+ + 3OH- = Al(OH)3↓.
For example,
AlCl3 + 3NaOH = Al(OH)3↓ + 3NaCl,
In excess of alkali Al(OH)3 is solved with forming of aluminates:
Al(OH)3 + OH- = AlO2- + H2O;
Al(OH)3↓ + NaOH = NaAlO2 + H2O.
Test carrying out
Add solution of Aluminium salt (by drops) to 1-2 drops of alkali as long as white translucent precipitation is formed.
2. Alizarin (1, 2-dioksiantrahinon) with ions Al3+ in weak acidic medium forms red complex of Aluminium alizarinate Al(OH)2[C14H6O3(OH)] which is not dissolved in acitic acid. It is called as an aluminium varnish.
Test carrying out
Reaction are executed in the drop way. On a strip of a filtering paper put a drop of solution K4[Fe(CN)6], and in its centre a drop of an investigated solution. After that a strip of a filtering paper are keeped above ammonia, and put on it some drops of alizarin solution. After that a strip of a filtering paper are keeped above ammonia. After that paper are dried, and put on it some drops of 2 mol/L CH3COOH solution. If there are ions Al3 +, there is a pink ring.
Characteristic reactions of Zinc cations Zn2+
1. 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.
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.
2. Dithizon (diphenylthiocarbasol) in solution CCl4 (or chloroform CHCl3) with ions Zn2+ forms complex bright red colour. Zinc dithizoate, which is extracted by ССl4 or CHCl3.
Test carrying out
The ions of Ag+, Bi3+, Pb2+, Cu2+ interfere with the exposure of ions of Zn2+. So before deremination of Zn2+ ions their linkage in complexes with 0,5 mol/L Sodium thiosulphate.
.To 2-3 drops of an investigated solution add 1 mL acetic buffer (pН=5) and 1-2 mL of 10 % of dithizon solution in CCl4 (or CHCl3). If there are ions Zn2+ the organic layer is painted in red colour.
Characteristic reactions оf сhromium cations Cr 3+
Reaction with hydrogen peroxide in alkaline conditions: H2O2 + NaOH - oxidizes Cr3+ to Cr6+, forming a yellow solution of sodium chromate (СгО4 2-).
Test carrying out
To 2 – 3 drops of a solution containing chromium ions, was added dropwise 2M NaOH to form a precipitate of chromium hydroxide and further to its dissolution and the formation of hydroxocomplexes. Then add 1 drop of 30% or 6 to 8 drops of 3% solution H2O2. Forms a yellow solution of chromate.
Characteristic reactions of Ferrous cations Fe2+
Alkalis (NaOH, KOH). Alkali deposits Ferrous ions as precipitation of grey-green color:
Fe2+ + 2OH- = Fe(OH)2
For example,
FeSO4 + 2NaOH = Fe(OH)2 + Na2SO4.
But color of this precipitation was changed quickly to color of rust as the result of oxidation of Fe2+ to Fe3+ by air oxygen:
4Fe(OH)2 + O2 +2H2O = 4Fe(OH)3
-
Fe2+-1e→Fe3+ O2 + 4e→ 2O2-
|
4
1
|
Add 3-4 drops of sodium hydroxide to 2-3 drops of Ferrous ion salt solution.
Characteristic reactions of Ferric cations Fe3+
1. Alkalis (NaOH, KOH). Alkali deposits ferric ions as precipitation of brown color:
Fe3+ + 3OH- = Fe(OH)3
For example,
FeCl3 + 3NaOH = Fe(OH)3 + 3NaCl.
Test carrying out
Add 3-4 drops of sodium hydroxide to 2-3 drops of ferric ion salt solution.
2. Potassium ferricyanide K4 [Fe(CN)6] (yellow blood salt)- forms precipitation of dark blue color which is called Prussian blue with Fe3+ ions in acid medium. Reaction is highly specific and very sensitive (minimum concentration c= 1:1000000):
4Fe3+ + 3[Fe(CN)6]4 - = Fe4[Fe(CN)6]3↓.
For example,
4FeCl3 + 3K4[Fe(CN)6] = Fe4[Fe(CN)6]3↓ + 12 KCl.
The precipitation is not soluble in acids but is decomposed in alkalis:
Fe4[Fe(CN)6]3 + 12OH - = 4Fe(OH)3 + 3[Fe(CN)6]4-;
Fe4[Fe(CN)6]3 +12NaOH = 4Fe(OH)3 +3Na4[Fe(CN)6].
Test carrying out
Add 1 drop of HCl and 2-3 drops of potassium ferricyanide to 2-3 drops of iron (III) salt solution.
Characteristic reactions of Magnesium cations Mg2+
1.Magnezon I(p-nitrobenzol-azo-rezortsin) in the basic medium is painted in red-violet colour. At presence Magnesium hydroxide is formed the adsorbed compound which paints a solution in dark blue colour.
Test carrying out
To 2-3 drops of an investigated solution add 1-2 drops of a basic solution of a reagent. Depending on quantity of Magnesium in an investigated solution the dark blue precipitate or dark bluesolution is formed.
2.Sodium or ammonium hydrogenphosphate (pharmacopeia’s reaction) with ions Mg2+ in presence of ammonia and NH4Cl, form white crystal precipitate MgNH4PO46H2O:
Mg2+ + HPO42- + NH3 + 6H2O = MgNH4PO46H2O.
Test carrying out
To 1-2 drops of an investigated solution add 2-3 drops of 2 mol/L HCl and 1-2 drops Na2HPO4. After that add some drops of 2 mol/L aqueous ammonia solution, slowly mixing it after addition of each drop. After ammonia will neutralise acid and is formed NH4Cl, characteristic crystal precipitate MgNH4PO4×6H2O forms. Ammonia is necessary adding to pН 9-10. Necessary rub the wall-side of test tube a glass stick.
4-LW Qualitative reactions for cations of analytical group 6
Task:
In notebook to prepare the report on laboratory work that includes the General characteristics of this group of ions, reaction with group reagent and private reactions of ions. To perform laboratory work (experiments, see below), to complete the report observations.
Еxperiments:
Достарыңызбен бөлісу: | 
