Introduction to qualitative analysis, test for cation using H2S, NaOH and HN4OH, confirmatory test for the cations

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INTRODUCTION TO QUALITATIVE ANALYSIS

• Introduction to Qualitative Analysis

Test for carbons using H₂S, NaOH and NH₄OH, Confirmatory Test for the cat ions.

 

Introduction to Qualitative Analysis

Qualitative analysis involves examination of colour, flame test, effect of heat and confirmatory test for cat ion and anions.

Cations are metallic ions e,g. Ca²⁺, Pb²⁺, Al³⁺, Cu²⁺, Fe²⁺ , Fe³⁺, etc

 

Rules in Qualitative Analysis

1. Your test solution should not be diluted too much
2. Use only small quantity of reagents

Examination of colour and physical state of specimen

 

Substance                      colour                              physical state

1. Sulphur       yellow                              solid
2. Copper(ii)oxide       black                               solid
3. Iodine     dark brown                      solid
4. Nitrogen iv oxide      reddish brown               gas
5. Mercury      white                                 liquid

 

Flame test

1. Deep green colour of flame indicate the presence of copper
2. Deep yellow colour indicate the presence of sodium

• Brick red indicate calcium.

 

EVALUATION

1. State ten (10) examples of cations
2. Give the colour of the following substance.

1. Distilled water
2. Iron filling
3. Manganese(iv) oxide
4. Benzoic acid.

 

Test for Cations

The Cations are Ca²⁺, Zn²⁺, Al³⁺, Pb²⁺, Fe²⁺. Fe³⁺, Cu²⁺, NH4 ⁺

 

Test for Ca²⁺

          Test                       Observation                             Inferences

Sample + H₂O                     dissolve to give a                           soluble

Colourless solution

+ NaOH in drop                  white powdery precipitate

in excess                               insoluble in excess                          Ca²⁺ present

soln + NH₃ soln                    No noticeable reaction               Ca²⁺ present

 

Test for Zn²⁺

Sample + H₂O                     dissolve in water to                        soluble

Give a colourless solution

Soln  + NaOH in                  white gelatinous precipitate       Zn²⁺, pb²⁺,

Drop in excess                     soluble in excess                 Al³⁺ may be present

Soln  +  NH₃ soln                  white gelatinous precipitate

In drop

In excess                               ppt soluble in excess                     Zn²⁺ present

 

 

Test for Al³⁺

Sample +H₂O                      dissolve in water to give   soluble

a colorless solution

soln + NaOH

in drop                                  white gelatinous precipitate     Zn²⁺,pb²⁺ Al³⁺ may be

in excess                               soluble in excess                             present

 

Soln + NH₃OH                      white gelatinous ppt

In drops

In excess                               insoluble in excess                          Al³⁺ or pb²⁺

 

Test for Pb²⁺

Sample + H₂O                     dissolve in water to give soluble

Colourless solution

Soln + NaOH

In drop                                  White gelatinous ppt                     Zn²⁺,Pb²⁺, Al³⁺

In excess                               soluble in excess                             may be present

Soln+ NH₃ soln

In drop                                  white gelatinous ppt                     Al³⁺ or Pb²⁺

In excess                               insoluble in excess                          may be present

Soln + dil HCl                         white ppt                                        Pb²⁺ present

Ppt + heat                            ppt dissolve when hot and

Reappear when cool                     Pb²⁺ present

Soln + K₂CrO₄                      yellow precipitate                           Pb²⁺ present

 

 

Test for Fe²⁺

Test                        Observation                             Inferences

Sample + H₂O         dissolve in water                             soluble

Soln +NaOH soln

In drops                     dirty green gelatinous ppt           Fe²⁺ present

In excess                   insoluble in excess

Soln +NH₃ in             Dirty green  gelatinous

Drop                          precipitate                                       Fe²⁺ present

In excess                   insoluble in excess

 

Test for CU²⁺

Test                       Observation                             Inference

Sample +H₂O          dissolve in water                             soluble

Soln +NaOH in

Drops                         Blue gelatinous ppt

In excess                   insoluble in excess                          Cu²⁺ present

Soln+NH₃ in drop    pale blue gelatinous ppt

In excess                   soluble in excess to give a

Deep blue solution                        Cu²⁺ present

 

Test for NH₄⁺

Test                       Observation                                       Inferences

Sample + H₂O         dissolve in water to give a

Colourless solution                                     soluble

Soln +NaOH             No ppt, but effervescence occur

In drop + warm      with libration of colourless gas

with choking smell

Gas + moist              It turns moist red litmus paper                 Alkaline gas

Litmus paper            blue.

Gas  + con HCl       It gives white fumes with con HCl          NH₃ gas from NH₄⁺

 

GENERAL EVALUATION

1. Give the common reagents used for confirmatory test for cations.
2. State the colour of the solution when the sample that contains the following dissolve in water:

1. Fe ²⁺ b. Cu ²⁺ c. Fe ³⁺

 

WEEKEND ASSIGNMENT

1. The following give white gelatinous precipitate in NaOH except? (a) Al ₃₊ (b) pb₂₊ (c) CU ₂₊

(d) Zn₂₊

2. One of the following gives ‘pop’ sound when contact with lighted splint. (a) O₂ (b) H₂ (c ) NH₃

(d) CO₂

3. Example of cations that gives gas during its confirmatory test is (a) SO₃^2- (b) SO₄^2- (c) CO₃^2-

(d) NH₄⁺

4. Deep green colour of flame indicate the presence of (a) sodium (b) calcium (c) copper (d) iron
5. The only alkaline gas that changes moist red litmus paper to blue is (a)NH₃ (b) HCl            (c) NaOH (d) NH₄OH

 

THEORY

1. Explain the confirmatory test for the following cations
2. State the flame test for the following :

1. Calcium b. Copper c. sodium        d. iron

 

READING ASSIGNMENT

School Chemstry by O.Y Ababio pages 165 – 183

Practical chemistry by R. I. Makanjuola pages 31-36.

Practical Chemistry for Schools and Colleges by Godwin O. Ojokuku pages 30 – 98.

 

 

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