Qualitative Analysis of Anions
, , , ,
Qualitative analysis of anions
• Anions are negatively charged ions. When only one anion is present in sample, it can be identified with a chemical test .
• When they are present in a mixture, first, they are separated into groups by their precipitating agents, and then the qualitative analyses are performed. The main
principle of the anion groups is the difference between their solubility products, just like the cation groups.
• Hovewer, the differences between the solubility products of the anion groups are low. So they cannot be separated easily, hence the systematic analysis of anions is difficult to perform. Also, the anions are much more active than the cations.
• The most abundant anions, which are , , , and , can be identified in precence of each other because they have specific reactions.
• Since they have specific reactions, the identification tests of , , , and can be performed without separation or systematic analysis.
•
1. Reactions of chloride ion ()
1.1. With AgNO3 ile
• ion reacts with AgNO3 to precipitate as white AgCl. AgCl does not dissolve when HNO3
is added to the medium.
• In neutral solutions, , and all react with AgNO3 to precipitate as silver salts. (AgCl, Ag2CO3, Ag3PO4, Ag2SO4)
• When HNO3 is added to them, all these silver salts except for AgCl dissolves. Which means AgCl is the only silver salt which does not dissolve with HNO3.
• That is why, HNO3 is added to the test tube for identification test. If present, the other anions cannot be precipitated as silver salts when HNO3 is added.
• If the acidification is performed by adding HCl, the chloride anion in HCl precipitates with AgNO3 even though the sample does not contain chloride. So the selection of the acidifying agent is important.
•
4
��−+����3→ ����(�)+��3− 1.1. With AgNO3 ile
1. Reactions of chloride ion ()
1. Reactions of chloride ion ()
(continued)• AgCl is dissolved by forming a complex with NH3, cyanide and thiosulfate ions.
• Diamine silver complex () is destroyed with the addition of HNO3 , and AgCl re- precipitates.
•
2. Reactions of sulfate ion ()
• 2.1. With BaCl2 ile
• ion reacts with BaCl2 to precipitate as white BaSO4. BaSO4 does not dissolve in acidic medium.
• In neutral solutions, , ve all react with BaCl2 to precipitate as barium. (BaSO4, BaCO3, Ba3(PO4)2, BaSO3 )
• When an acid is added to them, all these barium salts except for BaSO4 dissolves.
Which means BaSO4 is the only barium salt which does not dissolve in acidic medium.
• That is why, HNO3 or HCl is added to the test tube for identification test. If present, the other anions cannot be precipitated as barium salts when acid is added.
• If the acidification is performed by adding H2SO4, the sulfate anion in H2SO4 precipitates with BaCl2 even though the sample does not contain sulfate. So the selection of the acidifying agent is important.
•
2. Reactions of sulfate ion ()
2.2. With AgNO3
• When AgNO3 is added to concentrated solutions of sulfate, Ag2SO4 precipitates as white crystallines.
• Ag2SO4 dissolves in mineral acids (HNO3, HCl).
•
3. Reactions of nitrate ion ()
3.1. With Diphenylamine
• Diphenylamine, which is a colorless reactant, oxidizes to diphenyl benzidine violet, which has a blue-violet color, in presence of an oxidant such as , HNO2, FeCl3. This reaction occurs in acidic medium.
• Diphenylamine solution which is prepared by dissolving solid diphenylamine in concentrated H2SO4 is used for the identification of nitrate.
• When one or two drops of diphenylamine solution is added to nitrate solution, a blue-violet colour appears on the contact surface of two liquids. This test is
performed in a watch glass because it is a exotermic reaction.
•
3. Reactions of nitrate ion ()
3.2. With Fe+2 (FeSO4) (Brown ring test)
• In acidic medium, FeSO4 reduces nitrate ion to nitric oxide(NO) .
• NO forms a brown complex with the excessive Fe+2. The name of this complex is Iron II nitrosyl (??).
+
•
(Brown) ??
(Iron II nitrosyl sulfate) ??
4. Reactions of carbonate ion ()
4.1. With acid (HCl, H2SO4, CH3COOH) addition
• ion reacts with dilute acids to give water, CO2 and salt (or anion).
• The releasing gas is introduced to Ca(OH)2 or Ba(OH)2 solution. Formation of the cloudy-milky precipitate proves that the releasing gas is CO2.
• In this laboratory, the formation of colourless gas bubbles is enough for the identification of .
•
11
��(��)
2+ ��
2(�)→ ����
3(�)+ 4H
2O
4. Reactions of carbonate ion ()
4. Reactions of carbonate ion ()
4.2. With BaCl2
• ion reacts with BaCl2 to precipitate as white BaCO3 çöker.
• BaCO3 dissolves in mineral acids (HNO3, HCl) by releasing CO2.
•
4. Reactions of carbonate ion ()
4.3. With AgNO3
• ion reacts with AgNO3 to precipitate as white Ag2CO3 çöker.
• Ag2CO3 dissolves in mineral acids (HNO3, HCl) by releasing CO2.
•
5. Phosphate ()
• There are three type of phosporic acids.
1. H3PO4 Orthophosphoric acid 2. H4P2O7 Pyrophosphoric acid 3. HPO3 Metaphosphoric acid
• H3PO4 is a tribasic acid and can produce 3 different salts with NaOH.
I. NaH2PO4 Primary orthophosphate
II. Na2HPO4 Secondary orthophosphate (orthophosphate) III. Na3PO4 Tertiary orthophosphate
5. Reactions of orthophosphate ion ()
5.1. With ammonium molybdate (molybdate ion):
• ion reacts with molybdate to precipitate as yellow ammonium phosphomolybdate in precence of HNO3.
• In order to speed up the formation of ammonium phosphomolybdate precipitate the solution may be heated to 40 °C in water bath , or ammonium nitrate may be added to the test tube.
•
5. Reactions of orthophosphate ion ()
5.2. With AgNO3
• Orthophosphate ion reacts with AgNO3 to precipitate as yellow .
• dissolves with the addition of NH3 or HNO3.
•
5. Reactions of orthophosphate ion ()
5.3. With BaCl2
• Orthophosphate ion reacts with BaCl2 to precipitate as white amorphous BaHPO4.
• BaHPO4 dissolves with the addition of acetic acid and mineral acids.
• If H2SO4 is used for the acidification, the sulphate in the acid reacts with barium to precipitate as BaSO4 . Because both BaSO4 and BaHPO4 are white precipitates, the identification of orthophospate cannot be performed correctly.
•
References
• Analitik Kimya Pratikleri Kalitatif Analiz, F. Onur (Ed.), A.Ü. Eczacılık Fakültesi Yayınları No. 103, 2012.
