Qualitative Analysis

Analytical chemistry deals with qualitative and quantitative analysis of the substances. In inorganic qualitative analysis, the given compound is analyzed for the radicals, i.e., cation and the anion, that it contains. Physical procedures like noting the colour, smell or taste of the substance have very limited scope because of the corrosive, poisonous nature of the chemical compounds. Therefore, what one has to resort to is the chemical analysis of the substance that has to be carried out along with the physical examination of the compound under consideration.
The common procedure for testing any unknown sample is to make its solution and then test this solution for the ions present in it. There are separate procedures for detecting cations and anions, therefore qualitative analysis is studied under cation analysis and anion analysis. The systematic procedure for qualitative analysis of an inorganic salt involves the following steps :
(a) Preliminary tests
1. Physical appearance (colour and smell).
2. Dry heating test.
3. Charcoal cavity test.
4. Charcoal cavity and cobalt nitrate test.
5. Flame test.
6. Borax bead test.
7. Dilute acid test.
8. Potassium permanganate test.
9. Concentrated sulphuric acid test.
10. Tests for sulphate, phosphate and borate.
(b) Wet tests for acid radical.
(c) Wet tests (group analysis) for basic radical.

Physical Examination Of The Salt

The physical examination of the unknown salt involves the study of colour, smell and density. The test is not much reliable, but is certainly helpful in identifying some coloured cations. Characteristic smell helps to identify some ions such as ammonium, acetate and sulphide. (See Table 9.1 on next page)

Note :
1. If you have touched any salt, wash your hands at onte. It may be corrosive to skin.
2. Never taste any salt, it may be poisonous. Salts of arsenic and mercury are highly poisonous.
3. Salts like sodium sulphide, sodium nitrite, potassium nitrite, develop a yellow colour.

Table 9.1. Physical Examination

Experiment Observations Inference
1. Colour

 

 

 

Blue or Bluish green

Light green

Dark brown

Green

Pink

Light pink, flesh colour or earthy colour

 

White

 

 

Cu2+

Fe2+

Fe3+

Ni2+

Co2+

Mn2+

 

Shows the absence of Cu2+,

Fe2+, Fe3+, Ni2+, Mn2+, CO2+

2. Smell

Take a pinch of the

salt between your

fingers and rub with

a drop of water

Ammoniacal smell

Vinegar like smell

Smell like that of rotten eggs

NH4+

CH3COO

S2-

3. Density

 

(i) Heavy

(ii) Light fluffy powder

 

Salt of Pb2+, or Ba2+

Carbonate

4. Deliquescence Salt absorbs moisture and

becomes paste like

(i) If coloured, may be

Cu(NO3)2, FeCl3.

(ii) If colourless, may be

Zn(NO3)2, chlorides of

Zn2+, Mg2+ etc.

Dry Heating Test

This test is performed by heating a small amount of salt in a dry test tube. Quite valuable information can be gathered by carefully performing and noting the observations here. On heating some salts undergo decomposition thus evolving the gases or may undergo characteris ¬tic changes in the colour of residue. These observations are tabulated below (Table 9.2) along with the inferences that you can draw.

Table 9.2. Dry Heating Test

Observations Inference
1. Gas evolved

(a) Colourless and odourless gas

CO2 gas turns lime water milky

(b) Colourless gas with odour

(i) H2S gas—Smells like rotten eggs, turns

lead acetate paper black.

(ii) SO2 gas—Smells like burning sulphur,

turns acidified potassium dichromate paper green

(iii) HCl gas—Pungent smell, white fumes with

ammonia, white ppt with silver nitrate solution.

(iv) Acetic acid vapours—Characteristic vin-

egar like smell.

(v) NH3 gas—Characteristic smell, turns

Nessler’s solution brown.

(c) Coloured gases—Pungent smell

(i) NO2 gas—Reddish brown, turns ferrous

sulphate solution black.

(ii) Cl2 gas—Greenish yellow, turns starch io-

dide paper blue.

(iii) Br2 vapours—Reddish brown, turns starch

paper orange yellow.

(iv) I2 vapours—Dark violet, turns starch paper

blue.

 

CO32- or  C2O42-

 

 

Hydrated  S2-

 

SO32-

 

 

Cl

 

 

CH3COO

 

NH4+

 

 

NO2- or NO3-

 

Cl

 

Br

 

I

2. Sublimate formed

(a) White sublimate

(b) Black sublimate accompanied by violet

vapours

NH4+

I

3. Decrepitation

The salt decrepitates.

A salt having no water of crystallisation. For example,

Pb(NO3)2, NaCl, KBr.

4. Swelling

The salt swells up into voluminous mass.

PO43- indicated
5. Residue

(i) Yellow when hot white when cold

(ii) Brown when hot and yellow when cold

(iii) White salt becomes black on heating

(iv) White residue, glows on heating

(v) Original salt blue becomes white on heating

(vi) Coloured salt becomes brown or black on

heating.

Zn2+

Pb2+

CH3COO indicated

Ba2+, Sr2+, Ca2+, Mg2+, etc.

Hydrated CuSO4 indicated

CO2+, Cu2+, Mn2+ indicated.

Note:
1. Use a perfectly dry test-tube for performing this test. While drying a test-tube, keep it in slanting
position with its mouth slightly downwards so that the drops of water which condense on the upper cooler parts, do not fall back on the hot bottom, as this may break the tube. ,
2. For testing a gas, a filter paper strip dipped in the appropriate reagent is brought near the mouth of the test tube or alternatively the reagent is taken in a gas-detector and the gas is passed through it [Fig. 9.1].
qualitative-analysis-1
3. Do not heat the tube strongly at one point as it may break.

Charcoal Cavity Test

This test is based on the fact that metallic carbonates when heated in a charcoal cavity decom ¬pose to give corresponding oxides. The oxides appear as coloured incrustation or residue in the cavity. In certain cases, the oxides formed partially undergo reduction to the metallic state producing metallic beads or scales.
Examples :
qualitative-analysis-2
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Procedure

While performing charcoal cavity test, make a small cavity on a charcoal block with the help of borer as shown in Fig. 9.2. Mix small amount of salt with double its quantity of sodium carbonate. Place it in the cavity made on the block of charcoal. Moisten with a drop of water and direct the reducing flame of the bunsen burner on the cavity by means of a mouth blowpipe as shown in Fig. 9.3. Heat strongly for sometime and draw inference according to the Table 9.3.
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Table 9.3 Charcoal Cavity Test

Observations Inference
Incrustation or Residue Metallic bead
Hot Cold
Yellow White None Zn2+
Brown Yellow Grey bead which marks the paper Pb2+

 

None None Red beads or scales Cu2+
White residue which glows None None Ba2+, Ca2+, Mg2+

 

Black None None Nothing definite—generally coloured salt

To obtain a reducing flame with the help of a mouth blow pipe, make the bunsen burner flame luminous by closing the air holes of the burner. Keep the nozzle of the blow pipe just outside the flame (Fig. 10.4) and blow gently on to the cavity.

Cobalt Nitrate Test

This test is applied to those salts which leave white residue in charcoal cavity test.
The test is based on the fact that cobalt nitrate decomposes on heating to give cobalt oxide, CoO. This combines with the metallic-oxides, present as white residue in the charcoal cavity forming coloured compounds. For example, when a magnesium salt undergoes charcoal cavity test, a white residue of MgO is left behind. This on treatment with cobalt nitrate and subsequent heating forms a double salt of the formula MgO.CoO which is pink in colour. In addition to metallic oxides, phosphates and borates also react with cobalt oxide to form  Co3(PO4)2  and Co3(BO3)2 which are blue in colour.
Some of the reactions involved are given below :
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Procedure

Put one or two drops of cobalt nitrate solution on the white residue left after charcoal cavity test. Heat for one or two minutes by means of a blow pipe in oxidising flame. Observe the colour of the residue and draw inferences from Table 9.4.

Table 9.4. Cobalt Nitrate-Charcoal Cavity Test

Color of the Residue Inference
Green Zn2+
Pink Mg2+
Blue PO43-
Black It is due to the formation of CoO. No definite

indication.

Note:
1. Perform this test only if the residue in the charcoal cavity test is white.
2. Do not put more than 2 drops of cobalt nitrate on the white residue. Excess cobalt nitrate may decompose to give cobalt oxide which is black in colour.
3. Use dilute solution of cobalt nitrate.

Flame Test

Certain salts on reacting with cone. HCl from their chlorides, that are volatile in non-luminous flame. Their vapours impart characteristic colour to the flame. This colour can give reliable information of the presence of certain basic radicals.
For proceeding to this test, the paste of the mixture with cone. HCl is introduced into the flame with the help of platinum wire (Fig. 9.5).
qualitative-analysis-7

Procedure

Clean the platinum wire by dipping it in some cone. HCl taken on a watch glass and then heating strongly in the flame. This process is repeated till the wire imparts no colour to the flame. Now prepare a paste of the salt with cone. HCl on a clean watch glass. Place small amount of this paste on platinum wire loop and introduce it into the flame. Note the colour imparted to the flame with naked eye and through blue glass.

Table 9.5. Flame Test

Color of the Flame Inference
With naked eye Through blue glass    
1. Brick-red (not persistent) Light yellowish green Ca2+
2. Crimson-red (persistent) Crimson Sr2+
3. Persistent grassy-green

(appears after prolonged

heating)

Green

 

Ba2+

 

4. Bright-bluish green Visible Cu2+
5. Green flashes Zn2+ and Mn2+ salts
6. Dull bluish-white White Pb2+

Note: Sodium salts impart golden yellow colour to the flame while potassium salts impart pink violet colour. However, in the present context Na+ and K+ salts are not in the syllabus.

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