2.2.4 Testing for some common ions
Frequently, chemists need to identify the ions present in a certain substance. Some of the reactions we have encountered in the previous section can be used as a test for some ions. The various tests will be demonstrated to you in class. Here are some tests that you must know:
Test for Cl–, Br–, I–
Test for Cl–, Br–, I–: If Cl–, Br– or I– ions are found in a substance to be tested, then add a few drops of silver nitrate solution (and a few drops of nitric acid, to make sure that if a precipitate forms it is not a hydroxide) to an aqueous solution of the sample.
i) A white precipitate will form if Cl– is present:
Ag+(aq) + Cl–(aq) → AgCl(s)
The solid is white. Upon exposure to light it will turn to violet then black within a matter of minutes.
On addition of ammonia solution the white precipitate dissolves immediately.
ii) A cream-colored precipitate will form if Br– is present:
Ag+(aq) + Br–(aq) → AgBr(s)
The solid is light cream. Upon exposure to light it will turn to darker within a few minutes.
On addition of ammonia solution, the cream-colored precipitate dissolves after some time
iii) A yellow precipitate will form if I– is present:
Ag+(aq) + I–(aq) → AgI(s)
The solid is yellow.
On addition of ammonia solution the precipitate does not dissolve.
Figure 2.11 (A) AgCl precipitate, (B) AgBr precipitate, and (C) AgI precipitate.
A few drops of silver nitrate solution were added with a couple drops of nitric acid to an aqueous solution of a sample studied. A yellow precipitate formed.
Identify the nature of ions found in this substance.
The yellow precipitate indicates the presence of
ions.
Test for SO42–
Add a few drops of Ba(NO3)2 solution to a solution of the substance being tested. If SO42– ions are present, a white precipitate will form, and this precipitate will not dissolve in acid (e.g. HCl).
Ba2+(aq) + SO42–(aq) → BaSO4(s)
Test for CO32–
Two steps that work together as one test are used to test for the CO32– ion:
- Step 1: Add a few drops of an acid (e.g. dilute nitric acid, because all nitrates are soluble) to a sample of the solid or solution to be tested. If the sample effervesces (you see bubbles), then a gas is formed. This gas could be carbon dioxide coming from the carbonate ion:CO32–(aq) + 2H+(aq) → H2O(l) + CO2(g)
If no gas forms, then there are no carbonate ions in the sample. - Step 2: If you see effervescence, to check if the gas is CO2, pass the gas through lime-water, which is a saturated solution of Ca(OH)2. If a white precipitate forms, (CaCO3(s)) then the gas is CO2 and the original sample contains CO32–Ca2+(aq) + 2OH–(aq) + CO2(g) → CaCO3(s) + H2O(l)
Test for SO32–
Two steps that work together as one test are used to test for the sulfite ion, SO32–:
- Step 1: Add a few drops of an acid (e.g. dilute nitric acid, because all nitrates are soluble) to a sample of the solid or solution to be tested. If the sample effervesces (you see bubbles), then a gas is formed. This gas could be sulfur dioxide coming from the sulfite ion.SO32–(aq) + 2H+(aq) → H2O(l) + SO2(g)
- Step 2: To check if the gas is SO2, place a piece of wet, blue litmus paper just above the liquid layer, without touching the sides of the tube. If it turns red then the gas is SO2. While CO2(g) is odorless, SO2(g) has a “chocking” smell (it makes you choke); the smell of burning sulfur. Later in the course you will learn about other tests for SO2(g).
Test for NO3–
Two steps that work together as one test are used to test for the ion, NO3–:
- Step 1: Add a few drops of aqueous sodium hydroxide to a sample of the solution to be tested, then add some aluminum powder and warm gently. If the sample effervesces (you see bubbles), then a gas is formed. This gas could be ammonia coming from the nitrate ion.
- Step 2: To check if the gas is NH3, place a piece of wet, red litmus paper just above the liquid layer, without touching the sides of the tube. If it turns blue then the gas is NH3.
A few drops of dilute nitric acid were added to a sample of a white solid in a test tube. The sample formed bubbles of gas. To identify the gas evolved, a piece of wet damp blue litmus paper was placed just above the liquid layer, without touching the sides of the tube. The blue litmus paper turned red. What ion is found in the white solid?
The white solid contains
ions.
A few drops of dilute nitric acid were added to a sample of a white solid in a test tube. The sample formed bubbles of gas. To identify the gas evolved, a piece of wet damp blue litmus paper was placed just above the liquid layer, without touching the sides of the tube. The blue litmus paper turned red. What ion is found in the white solid?
A sample of Na2CO3 was dissolved in water to which few drops of acid is added. The gas formed is
which turns wet blue litmus paper red.
A few drops of dilute nitric acid were added to a sample of a white solid in a test tube. The sample formed bubbles of gas. To identify the gas evolved, a piece of wet damp blue litmus paper was placed just above the liquid layer, without touching the sides of the tube. The blue litmus paper turned red. What ion is found in the white solid?
A few drops of Ba(NO3)2 solution is added to a solution of the substance being tested. A white precipitate that does not dissolve in acid forms. The name of the ion that is possibly present is
ion.
2.2.4 Testing for some common ions
Test for Ba2+
Add a few drops of Na2SO4 solution to a solution of the substance being tested. If Ba2+ ions are present, a white precipitate will form, and this precipitate will not dissolve in acid (e.g. HCl).
Ba2+(aq) + SO42–(aq) → BaSO4(s)
Test for Ca2+
Add a few drops of Na2CO3 solution to a solution of the substance being tested. If Ca2+ ions are present, a white precipitate will form, and this precipitate will dissolve in acid (e.g. HCl) producing CO2(g).
Ca2+(aq) + CO32–(aq) → CaCO3(s)
CaCO3(s) + 2H+(aq) → Ca2+(aq) + H2O(l) + CO2(g)
To confirm, we try the flame test for the calcium ions. Obtain a platinum wire and clean it in concentrated HCl solution several times until when placed in the blue part of the flame of a Bunsen burner it does not change the color of the flame. Place the platinum wire in a solution of the sample then place it in the flame again. If the flame becomes momentarily brick red, then Ca2+ ions are present in the solution.
Add a few drops of NaOH solution to a solution of the substance being tested. If Ca2+ ions are present, a white precipitate will form, that does not dissolve in excess NaOH solution:
Ca2+(aq) + 2OH–(aq) → Ca(OH)2 (s)
Test for Zn 2+
Add a few drops of NaOH solution or ammonia solution to a solution of the substance being tested. If Zn2+ ions are present, a white precipitate will form, that dissolves in excess NaOH solution:
Zn2+(aq) + 2OH–(aq) → Zn(OH)2 (s)
To confirm, add few drops of ammonia solution to the substance being tested, a white precipitate is formed, that dissolves in excess ammonia solution.
Test for Ag+
To a sample of the solution to be tested add a few drops of Sodium chloride solution; If Ag+ ions are present, a white precipitate forms and this precipitate will turn black upon exposure to light.
On addition of ammonia solution, the precipitate will dissolve immediately; but it will not dissolve in HNO3.
Test for Pb2+
Add a few drops of HCl solution to an aqueous solution of the substance being tested; if Pb2+ ions are present, a white precipitate will form:
Pb2+(aq) + 2Cl–(aq) → PbCl2(s)
Upon heating, the white solid dissolves, and will reappear if the solution is cooled again.
Test for Cu2+
Add a few drops of NaOH solution to a solution of the substance being tested, in a test tube. If Cu2+ ions are present, a pale blue precipitate will form, that does not dissolve in excess NaOH solution:
Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s)
To confirm, add few drops of ammonia solution to the substance being tested, a pale blue precipitate is formed, that dissolves in excess ammonia solution to form deep blue solution
Cu(OH)2(s) + 4NH3(aq) → Cu[NH3]42+(aq) + 2OH–(aq)
Deep blue
Test for Fe2+
Add a few drops of NaOH solution to a solution of the substance being tested. If Fe2+ ions are present, a dirty-green precipitate will form, that is insoluble in excess NaOH solution:
Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s)
To confirm, add few drops of ammonia solution to the substance being tested, a dirty green precipitate is formed, that does not dissolve in excess ammonia solution.
Test for Fe3+
Add a few drops of NaOH solution to a solution of the substance being tested. If Fe3+ ions are present, a reddish-brown precipitate will form, that does not dissolve in excess NaOH solution:
Fe3+(aq) + 3OH–(aq) → Fe(OH)3(s)
To confirm, add few drops of ammonia solution to the substance being tested, a reddish brown precipitate is formed, that does not dissolve in excess ammonia solution.
Test for Al3+
Add a few drops of NaOH solution to a solution of the substance being tested. If Al3+ ions are present, a white gelatinous precipitate will form, that dissolves in excess NaOH solution:
Al3+(aq) + 3OH–(aq) → Al(OH)3(s)
Add a few drops of ammonia solution to a solution of the substance being tested. If Al3+ ions are present, a white gelatinous precipitate will form, that does not dissolve in excess ammonia solution.
Test for NH4+
All ammonium (NH4+) salts are soluble, so we cannot identify ammonium salts through a precipitation test. We can however, test for the ammonium ion by dissolving some of the solid to be tested in water in a test tube and then add NaOH solution. If NH4+(aq) ions are present, NH3 gas will be produced. NH3 gas will turn a red litmus paper placed just inside the mouth of the test tube (without touching the liquid or sides of the tube) into blue. It can also be recognized by its pungent smell:
NH4+(aq) + OH–(aq) → H2O(l) + NH3(g)
2.2.4 Testing for some common ions
Flame Test for Li+, Na+, K+
All lithium, sodium, and potassium salts are soluble, so we cannot identify lithium, sodium or potassium through precipitation test.
A platinum wire is cleaned in HCl solution and placed in the blue part of the flame of a Bunsen burner. The process is repeated until the color of the flame remains unchanged upon inserting the platinum wire in it. Next, dip the wire in some of the substance and place it in the flame. If the flame turns red then lithium ion is present. If it turns yellow/orange then sodium ion is present. If it turns lilac-violet then potassium ion is present.
A platinum wire is cleaned in HCl solution and placed in the blue part of the flame of a Bunsen burner. The process is repeated until the color of the flame remains unchanged upon inserting the platinum wire in it. Next, dip the wire in some of the substance and place it in the flame. If the flame turns red then lithium ion is present. If it turns yellow/orange then sodium ion is present. If it turns lilac-violet then potassium ion is present.
Figure 2.22 (A) Flame test used to identify Li+ ions,
(B) Flame test used to identify Na+ ions,
(C) Flame test used to identify K+ ions.
