QUANTITATIVE AND QUALITATIVE ANALYSIS Identification of ions

WEEK 2

TOPIC: QUANTITATIVE AND QUALITATIVE ANALYSIS

CONTENT:

1. Identification of ions
2. Test for gases
3. Identification of fats and oils

Sub-topic 1: IDENTIFICATION OF IONS

IDENTIFICATION OF CATIONS AND ANIONS

CATIONS: Zn²⁺, Al³⁺, pb²⁺ , Ca²⁺ ,Fe²⁺ Fe³⁺, Cu²⁺, NH₄⁺

1. THE USE OF NaOH_(aq): If a few drops of NaoH_(aq) is added to aqueous solutions of the salts of the above cations the following observations will be made. If it is:

a. Zn²⁺: There will be formation of a white gelatinous precipitate which will be soluble in excess NaoH_(aq).

b. Al³⁺There will be formation of a white gelatinous precipitate which will be soluble in excess NaoH _(aq).

c. Pb²⁺:There will be formation of a white precipitate which will be soluble in excess NaoH_(aq).

d. Ca²⁺: There will be formation of a white precipitate which will be soluble in excess NaoH_(aq).

e. Fe²⁺: There will be formation of a green gelatinous precipitate which will be insoluble in excess NaOH_(aq).

f. Fe³⁺ There will be formation of reddish- brown gelatinous precipitate which will be insoluble in excess NaoH_(aq).

g. Cu^2+: There will be a formation of a blue precipitate which is insoluble in excess NaoH_(aq).

h. NH₄⁺_: If NaoH_(aq) is added to the test solution and warmed, a colourless gas with irritating smell which will turn moist red litmus paper to blue will be evolved. Thus, showing that the test solution is containing NH₄⁺ i. e. the test solution is an NH₄⁺ salt solution.

ANIONS: Cl^–, NO₃^–, SO₄^2-, SO₃^2-, CO₃^2-:

The following tests help to identify these anions:

1. Cl^– : To the test solution add dilute HNO₃ acid (there will not be visible reaction), followed by AgNO_3(aq), (there will be formation of a white precipitate). To the precipitate add NH_3(aq) in excess and shake . If the precipitate dissolves then the test solution is containing Cl^– ions.
2. NO₃^– : To the test solution add FRESHLY prepared FeSO₄ solution followed by concentrated H₂SO₄ acid. Formation of a brown ring indicates the presence of NO₃^– ions.
3. SO₄^2- : To the test solution add dilute HCI acid,(there will not be any visible reaction, this is just to get rid of any other ions that may tamper with the experiment), followed by BaCl₂(aq). There will be formation of a white precipitate which will be insoluble in excess dil.HCI. This shows that SO₄^2- is present in the test solution.
4. SO₃^2-: To the test solution add BaCl_2(aq). There will be formation of a white precipitate which will be soluble in dil. HCl acid
5. CO₃^2-: The test for CO₃^2-, is also carried out exactly in the same way as in (d) above. The same result is obtained. To differentiate between SO₃^2- and CO₃^2- , add MgSO_4(aq) solution to solutions containing SO₃^2-and CO₃^2- ions in different test tubes. The one that gives a white precipitate is CO₃^2- . The example below shows how to report salt analysis.

Worked Example

1. Credit will be given for strict adherence to instructions, for observations precisely recorded and for accurate inferences. All tests, observations and inferences must be clearly entered in your work book, in ink, at the time they are made. C is a mixture of two salts. Carry out the following exercises on C. Record your observations and identify any gas (es) evolved. State the conclusion you draw from the result of each test. (a) Put all of C in a test tube and add about 10cm³ of distilled water. Shake thoroughly and filter. Keep both the filtrate and the residue. Divide the filtrate into three portions. (i) To first portion, add NaOH_(aq) in drops and then in excess. (ii) To the second portion, add NH₃ solution in drops and then in excess. (iii) To the third portion, BaCl_2(aq) followed by dilute HCl. (b) Divide the residue into two portions. (i) Heat the first portion strongly in a test tube. (ii) Add dilute HCl to the second portion.

Solution

EVALUATION

1. Credit will be given for strict adherence to instructions, for observations precisely recorded and for accurate inferences. All tests, observations and inferences must be clearly entered in your work book, in ink, at the time they are made. C is a mixture of two salts. Carry out the following exercises on C. Record your observations and identify any gas(es) evolved. State the conclusion you draw from the result of each test. (a) Put C into a beaker and add about 10cm³ of distilled water, stir the mixture and filter. Test the filtrate with litmus papers. Keep the residue and filtrate. (b)(i) To about 2cm³ of the filtrate, add few drops of aqueous HNO₃ followed by AgNO_3(aq) .

(ii) Add excess NH₃ solution to the resulting mixture. (c) To about half of the residue from 1(a) above, add about 5cm³ of dilute HNO₃ in drops. Divide the resulting solution into two equal portions. (d) (i) To the first portion, add ammonia solution in drops and then in excess. (ii) To the second portion, add dilute HCl.

Sub-topic 2: Test for gases

SO₂, H₂S, NH₃, CO₂, NO₂, HCI, O₂, CI₂ and H₂