Chemistry Qualitative Analysis
Analytical chemistry deals with qualitative and quantitative analysis of the substances. In 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:
(а) Preliminary tests
- Physical appearance (colour and smell).
- Dry heating test.
- Charcoal cavity test.
- Charcoal cavity and cobalt nitrate test.
- Flame test.
- Borax bead test.
- Dilute acid test.
- Potassium permanganate test.
- Concentrated sulphuric acid test.
- 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 12.1 on next page)
- If you have touched any salt, wash your hands at once. It may be corrosive to skin.
- Never taste any salt, it may be poisonous. Salts of arsenic and mercury are highly poisonous.
- Salts like sodium sulphide, sodium nitrite, potassium nitrite, develop a yellow colour.
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 characteristic changes in the colour of residue. These observations are tabulated in Table 12.2 along with the inferences that you can draw.
Table 12.2. Dry Heating Test
- 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.
- 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.
- 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.
While performing charcoal cavity test, make a small cavity on a charcoal block with the help of borer as shown in Fig. 12.2. 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. 12.3. Heat strongly for sometime and draw inference according to the Table 12.3.
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. 12.4) 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:
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
- Perform this test only if the residue in the charcoal cavity test is white.
- 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.
- Use dilute solution of cobalt nitrate.
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 cfan 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. 12.5).
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 mixture with cone. HCl on a deem 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.
Borax bead test
This test is performed only for coloured salts.
Borax, Na2B4O7.10H2O, on heating gets fused and loses water of crystallisation. It swells up into a fluffy white porous mass which then melts into a colourless liquid which later forms , a clear transparent glassy bead consisting of boric anhydride and sodium metaborate.
Borax, Na2B4O7.10H2O is heated in the loop of platinum wire, it swells and forms transparent colourless glassy bead. When this hot bead is touched with small amount of coloured salt and is heated again, it acquires a characteristic colour. The colour of bead gives indication of the type of the cation present. The colour of the bead is noted separately in oxidising and in reducing flame (Fig. 12.6).
Table 12.6. Borax Bead Test
To remove the head from platinum wire, heat the head to redness. Tap the rod with finger stroke, till the bead jumps off (Fig. 12.7).
The identification of the acid radicals is first done on the basis of preliminary tests. Dry heating test is one of the preliminary tests performed earlier which may give some important information about the acid radical present. The other preliminary tests are based upon the fact that:
Take a small quantity of the salt in a test-tube and add 1-2 ml of dilute sulphuric add. Identify the gas and draw
- Do not treat the salt with a large quantity of dilute acid.
- Do not heat the salt with dilute acid.
Chemical Reactions Involved in Dil. H2SO4 Test
Dilute H2SO4 (or dilute HCl) decomposes carbonates, sulphides and nitrites in cold to give gases. These gases on identification indicate the nature of the add radical present in the salt.
Potassium permanganate test
To a pinch of salt in test tube add about 2 ml of dilute sulphuric acid. Boil off any gas evolved, add little more of dilute acid and then potassium permanganate solution dropwise. Note the changes as given in Table 12.8. This test helps in detection of Cl–, Br–, I–, C2O42- and Fe2+ radicals.
- As sulphides are oxidised by KMnO4 so they have to be completely decomposed by heating with dilute sulphuric acid before this test is performed.
- Potassium permanganate oxidises Fe2+ salts in cold. Oil H2SO4 acid is added to the salt and heated till sulphides, sulphites and nitrites are completely decomposed. Then KMnO4 is added dropwise to cold solution.
Chemical Reactions Involved
Concentrated sulphuric acid test
This test is performed by treating small quantity of salt with cone, sulphuric acid (2-3 ml) in a test tube. Identify the gas evolved in cold and then on heating. Draw inferences from Table 12.9
- Do not boil the salt with cone, sulphuric acid. On boiling, the acid may decompose to give SO2 gas.
- Nitrates give vapours of nitric acid (colourless) when heated with cone, sulphuric acid. When a paper pellet or copper chips is added, dense brown fumes evolve. Paper pellet acts as a reducing agent and reduces nitric acid to NO2 (Reddish brown gas).
Chemical Reactions Involved in conc. H2SO4 Test
Tests for independent radicals (SO42- and PO43-)
As already diseussed these radicals are not detected by dilute or concentrated H2SO4 .They are tested individually.
- Sulphate (SO42- )
Boil a small amount of salt with dilute HCl in a test tube. Filter the contents, and to the filtrate add few drops of BaCl2 solution. A white ppt. insoluble in cone. HCl indicates presence of sulphate.
- Phosphate (PO43-)
Add cone. HNOs to the salt in a test tube. Boil the contents and add excess of ammonium molybdate solution. A yellow precipitate indicates presence of phosphate.
Confirmation of acid radicals by wet tests
The acid radical indicated by dil. H2SO4 or cone. H2SO4 tests is further confirmed by wet tests.
Preparation of solution for wet tests of acid radicals
The confirmatory tests for acid radicals are performed with salt solutions. The solution used for the purpose is any one of the following:
- Aqueous solution or ‘water extract: Shake a little of the salt with water. If the salt dissolves, this aqueous solution obtained is used for the wet tests of acid radical and is called ‘water extract’ or ‘W.E.’. If the salt is not completely soluble in water, the salt is shaken with water and is filtered. The filtrate is treated as water extract.
- Sodium carbonate extract: This is prepared only if the salt is insoluble in water.
Preparation of Sodium Carbonate Extract. Mix about 1 g of the salt with about 2 g of pure sodium carbonate and boil it for 10-15 minutes with 20-25 ml of distilled water in a small conical flask having a funnel in its mouth (Fig. 12.8). The funnel acts as a condenser. This arrangement prevents the loss of water due to evaporation. Filter the solution, cool it and label it as sodium carbonate extract or S.E.
Alternatively, sodium carbonate extract can be prepared in a test tube. A pinch of salt is mixed with double the amount of sodium carbonate and is boiled with distilled water for sometime. The suspension obtained is filtered.The filtrate is sodium carbonate extract. p.g 12 8. Preparation of sodium carbonate extract.
Theory of Preparation of Sodium Carbon-ate Extract.
When the salts are boiled with strong solution of sodium carbonate, double decompo-sition takes place resulting in the formation of the carbonates of heavy metallic radicals and sodium salts of the acid radicals. The sodium salts of corresponding add radicals being soluble in water pass into the solution and carbonates of heavy metals are predpitated
How to Use Sodium Carbonate Extract
Sodium carbonate extract always contains unreacted sodium carbonate in solution which has to be destroyed before using the extract for various tests. To do this, the extract is addified with some suitable acid and is boiled to expel carbon dioxide. The selection of add used for destroying excess Na2CO3depends upon the radical to be identified.
Now we describe in detail the confirmatory tests for various add radicals discussed so far.
Confirmation of Carbonate, CO32-
(Indicated in dilute acid test by occurrence of brisk effervescence and evolution of carbon dioxide).
- Do not use sodium carbonate extract for performing the tests of carbonates because it contains sodium carbonate.
- Perform magnesium sulphate test only in case of soluble carbonates.
Confirmation of Sulphide, S2-
(Indicated in dilute acid test by the evolution of hydrogen sulphide).
Confirmation of Sulphide, S2-
(Indicated in dilute acid test by the evolution of hydrogen sulphide).
Confirmation of Nitrite, NO2–
(Indicated in dilute acid test by the evolution of brown vapours of nitrogen peroxide)
Chemical Reactions Involved in the Confirmation of Carbonate, Sulphide and Nitrite
Carbonate (CO32- )
1. Reaction with dil. HCl
Carbonates on reaction with dil. HCl give CO2 gas which turns lime water milky. In case of soluble carbonates this test is performed with water extract and in case of insoluble carbonates this test is performed with the solid salt.
Confirmation of Chloride,Cl–
(No action with dilute H2SO4 but decomposed by cone. H2SO4 with the evolution of HCl gas).
Confirmation of Bromide, Br–
(No action with dilute H2SO4 but decomposed by cone. H2SO4 with the evolution of bromine vapours).
Note. Chlorine water is prepared by adding dropwise cone. HCl to a small volume of KMnO4 solution till the pink colour is just discharged, the resulting solutio/i is chlorine water.
Confirmation of Iodide, I–
(No action with dilute H2SO4 but decomposed by cone. H2SO4 with the evolution of vapours
Confirmation of Nitrate, NO3–
(No action with dilute acids but decomposed by cone. H2SO4 with the evolution of brown vapours of nitrogen peroxide).
Confirmation of Acetate, CH3COO–
(No action with dilute acids but decomposed by cone. H2SO4 with the evolution of CH3COOH vapours
Confirmation of Oxalate, C2O4–
(No action with dilute acids but decomposed by cone. H2SO4with the evolution of CO2 and CO gas)
Chemical Reactions Involved in the Confirmation of Chloride, Bromide, Iodide, Nitrate Acetate and Oxalate
Confirmation of Sulphate, SO42-
(Not indicated in dilute and concentrated H2SO4 acid tests).
Confirmation of Phosphate, PO43-
(Not indicated in dilute and concentrated H2SO4 acid test).
Chemical reactions involved in the confirmation of SO42- and PO43-
Wet tests for basic (cations)
Preliminary tests such as dry heating test, charcoal cavity test, flame test and borax bead test may give us some indication about the cation present in the salt. However, the cation is finally detected and confirmed through a systematic analysis involving wet tests. For the sake of qualitative analysis the cations are classified into the following groups (Table 12.10).
Before carrying out the wet tests for the analysis of cation, the salt has to be dissolved in some suitable solvent to prepare its solution.
Preparation of solution for wet tests of basic radicals
The very first essential step is to prepare a clear and transparent solution of the salt under investigation. For this purpose, the under noted solvents are tried one after another in a systematic order. In case the salt does not dissolve in a particular solvent even on heating, try the next solvent. The following solvents are tried:
- Distilled water (cold or hot).
- Dilute HCl (cold or hot).
- Cone. HCl (cold or hot).
Procedure for the preparation of solution
Take a small quantity of the given salt in a test tube. Add some suitable solvent into it and shake. If it does not dissolve even after heating for sometime, take the fresh quantity of the salt again and treat it in a similar manner with next solvent. The clear solution thus obtained is labelled as Original Solution (O.S.).
- In case some gas is evolved during the preparation of solution, let the reaction cease. Gas must be completely expelled by heating.
- In case solution is prepared in dilute HCl, group I is absent. Proceed with group II.
- If the salt is soluble in hot water, and on cooling white precipitates appear, lead chloride is indicated.
- It is necessary to dilute the solution if it is made in concentrated acid before proceeding with the analysis.
The following table will help the students in the choice of a suitable solvent:
The separation of cations into various groups by making use of suitable reagents (known as group reagents) is based on the differences in chemical properties of cations. For example, if hydrochloric acid is added to a solution containing all cations, only the chlorides of lead, silver and mercury (ous) will precipitate, since all other chlorides are soluble. Thus, these cations form a group of ions which may be precipitated from solution by addition of group reagent HCl. Similarly, H2S is a group reagent for group II. The following Table 12.11 clearly shows the group reagents for different groups and the form in which cations of the particular group are precipitated out.
Theory of precipitation of different groups
The classification of cations into different groups in the inorganic qualitative analysis is based upon the knowledge of solubility products of salts of these basic radicals. For example, chlorides
of Hg22+, Pb2+ and Ag+ have very low solubility products. On the basis of this knowledge these radicals are grouped together in group-I and are precipitated as their chlorides by adding dilute HCl to their solutions. For adjusting the conditions for precipitation, another concept called common ion effect plays very important role. Before we consider the precipitation of radicals of other groups, let us discuss in brief the concept of common ion effect.
Common ion effect
Weak acids and weak bases are ionised only to small extent in their aqueous solutions. In their solutions, unionised molecules are in dynamic equilibrium with ions. The degree of ionisation of a weak electrolyte (weak acid or weak base) is further suppressed if some strong electrolyte which can furnish some ion common with the ions furnished by weak electrolyte, is added to its solution. This effect is called common ion effect. For example, degree of ionisation of NH4OH (a weak base) is suppressed by the addition of NH4Cl (a strong electrolyte). The ionisation of NH4OH and NH4Cl in solution is represented as follows:
Due to the addition of NH4Cl, which is strongly ionised in the solution, concentration of NH4+ ions increases in the solution. Therefore, according to Le-Chatelier’s principle equilibrium in equation (12.1) shifts in the backward direction in favour of unionised NH4OH. In this way, addi¬tion of NH4Cl suppresses the degree of ionisation of NH4OH. Thus, the concentration of OH– ions in the solution is considerably reduced and the weak base NH4OH becomes a still weaker base.
The suppression of the degree of ionisation of a weak electrolyte (weak acid or weak base) by the addition of some strong electrolyte having a common ion, is called the common ion effect.
Application of concept of common ion effect in the qualitative analysis is illustrated as follows:
The cations of group II (Pb2+, Cu2+, AS3+) are precipitated as their sulphides. Solubility products of sulphides of group II radicals are very low. Therefore, even with low concentration of S2- ions, the ionic products (Qsp) exceed the value of their solubility products (KSp) and the radicals of group II get precipitated. The low concentration of S2- ions is obtained by passing H2S gas through the solution of the salts in the presence of dil. HCl which suppresses degree of ionisation of H2S by common ion effect.
It is necessary to suppress the concentration of S2- ions, otherwise radicals of group IV will also get precipitated along with group II radicals.
Radicals of group IV (Ni2+, CO2+, Mn2+, Zn2+) are also precipitated as their sulphides. But solubility products of their sulphides are quite high. In order that ionic products exceed solubility products, concentration of S2- ions should be high in this case. High concentration of sulphide ions is achieved by passing H2S gas through the solutions of the salts in the presence of NH4OH. Hydroxyl ions from NH4OH combine with H+ ions from H2S. Due to the removal of H+ions the equilibrium of H2S shifts in favour of ionised form.
Hence, concentration of S2- ions increases. With this increased concentration of S2- ions ionic products exceed solubility products and radicals of group IV get precipitated.
Radicals of group III (Fe3+, Al3+) are precipitated as their hydroxides by NH4OH in the presence of NH4Cl. The purpose of NH4Cl is to suppress the degree of ionisation of NH4OH by common ion effect in order to decrease the concentration of OH– ions.
The solubility products of hydroxides of group III radicals are quite low. Therefore, even with this suppressed concentration of OH– ions their ionic products exceed solubility products and hence they get precipitated. If the concentration of OH– ions is not suppressed, the radicals of groups IV, V and Mg2+ will also be precipitated along with radicals of group III.
Radicals of group V (Ba2+, Sa2+, Ca2+) are precipitated as their carbonates by the addition of (NH4)2 CO3 in the presence of NH4Cl and NH4OH. NH4Cl suppresses the degree of ionisation of (NH4)2 CO3 by common ion effect and hence decreases the concentration of CO32- ions.
But solubility products of carbonates of group V radicals are quite low and hence even with the suppressed concentration of CO32- ions their ionic products exceed solubility products and they get precipitated whereas Mg2+ and other radicals of group VI having relatively high solubility products are not precipitated.
Analysis of group Zero(NH4+)
This group includes NH4+ cation. During the analysis of cations NH4Cl and NH4OH are added in many steps. Therefore, H4+ ion is detected in the beginning using solid salt.
The solid salt is heated with concentrated solution of sodium hydroxide. In case, ammonia gas is evolved, NH4+ is present. Evolution of NH3 gas is confirmed by the following tests:
- Characteristic ammoniacal smell.
- The gas gives white fumes when a glass rod dipped in dil. HCl is brought near the mouth of the test tube.
- When the gas is passed through Nessler’s reagent, it would give brown ppt. in case of NH3.
Chemical Reactions Involved in Group-Zero Analysis
Analysis of group I (Silver Group)
This group includes Pb2+, Ag+ and Hg22+. But in the present context, we shall study only Pb2+. Group reagent for this group is dil. hydrochloric acid.
- To the original solution add dil. hydrochloric acid. If a white precipitate is formed, first group (Pb2+) is present.
- Filter and wash the ppt. with cold water and examine as in Table 12.12.
- If the original solution is prepared in cold dilute hydrochloric acid, first group is absent.
- If the original solution is prepared in cone, hydrochloric acid, simply add water. White ppt. shows the presence of first group.
Chemical Reactions Involved in Group I Analysis
Analysis of group II (copper group)
This group includes Pb2+ and Cu2+ in IIA group and As3+ in IIB Group. These are precipitated as their sulphides. If group I is absent, the tests for radicals of group II are carried out. Group reagent for this group is H2Sgas in the presence of dil. HCl.
Take about 2 ml of the original solution in a test tube’. Make it acidic with dil. HCl and warm the contents. Through this solution pass H2S gas from the Kipp’s apparatus by turning
the stop cock as shown in Fig. 12.10, Formation of the black or yellow precipitates indicates the presence of group II radical. If this is observed, pass more of H2S gas to ensure complete precipitation of the radical sulphide. Centrifuge and separate the precipitates.
Identification of IIA and IIB Groups. Note the colour of the precipitate. If the precipitate is black in colour, it indicates Pb2+ or Cu2+. If the colour of precipitate is yellow this indicates As3+.
Chemical Reactions Involved in the Analysis of Group II
Analysis of group III (iron group)
The cations present in this group are Fe2+, Fe3+, Cr3+ and Al3+. Only Fe2+/Fe3+ and Al3+ are included in the syllabus of this class. These cations are precipitated as hydroxides by adding ammonium hydroxide in presence of ammonium chloride. Thus, group reagent for this group is NH4OH in the presence of NH4Cl.
In case, first and second groups are absent proceed for group III with the original solution. Take about 5 ml of the original solution and add 4-5 drops of cone, nitric acid. Boil the solution for sometime. Add to it about 2 g of solid NH4Cl and boil again. Cool the solution under tap water. Add excess of ammonium hydroxide to it and shake. A ppt. shows the presence of some cation of group III. Filter the ppt. and wash with water. Note the colour of the ppt. If the ppt. is reddish brown in colour, it indicates the presence of Fe3+ and if the colour is white, it indicates the presence of Al3+. Analyse the ppt. and draw inferences as in Table 12.14.
- Test of Fe2+. The addition of cone, nitric acid in the analysis of group III serves to oxidise Fe2+ions to Fe3+ ions. Add cone, nitric acid only if the cation is Fe2+ otherwise the addition of nitric acid may be avoided. To test this, add a few drops of potassium ferricyanide solution to the original salt solution. A deep blue colouration shows Fe2+.
- Use sufficient quantity of ammonium chloride, otherwise the hydroxides of higher group may be precipitated along with the radicals of third group.
- Add NH4OH until the solution gives the smell of ammonia.
Chemical Reactions Involved in the Analysis of Group III
Analysis of group IV (Zinc group)
The radicals present in this group are CO2+, Ni2+, Mn2+ and Zn2+. These are precipitated as sulphides by passing H2S gas through the ammonical solution of the salt.
The group reagent for this group is H2S gas in the presence of NH4Cl and NH4OH.
If there is no ppt. in the third group, then use the same ammonical solution for the fourth group. Pass H2S gas through the solution. If some ppt. is formed, presence of some radical of group IV is indicated. Filter the ppt. and wash it with water. Note the colour of the ppt. and analyse the ppt. according to the Table 12.15.
Chemical Reactions Involved in the Analysis of Group IV
Passing of H2S gas through the group III solution will precipitate the radicals CO2+, Ni2+, Mn2+ and Zn2+ as their sulphides. Formation of black ppt. (CoS or NiS) indicates cobalt or nickel. Formation of buff-coloured ppt. (MnS) indicates manganese and dirty white ppt. (ZnS) indictes zinc
2.Ammonium thiocyanate ether test
On addition of ether and a crystal of ammonium thiocyanate (shaking and allowing to stand), a blue colour due to the formation of ammonium cobhlti thiocyanate, is obtained in the ethereal layer.
Analysis of group V (calcium group)
Group V consists of three radicals: Ba2+, Sr2+ and Ca2+. These cations are precipitated as their carbonates.
Group reagent for this group is (NH4)2CO3 in the presence ofNH4Cl and NH4OH.
If the fourth group is absent, then proceed for radicals of group V.
To the O.S. add 2-3 gins of solid NH4Cl, boil, cool and add NH4OH till the solution smells of ammonia. Then add (NH4)2CO3 solution. Appearance of white ppt. indicates the presence of group V cation. Filter and wash the ppt. with water. Dissolve the ppt. in hot dil. acetic acid. Divide the solution into three parts and proceed as in Table 12.16.
- Proceed to test for group V cations in the order, Ba2+, Sr2+ and Ca2+. If Ba2+ is confirmed, do not test for Sr2+ or Ca2+ Similarly if Sr2+ is confirmed, do not test for Ca2+.
- Original solution can be preferably used for testing Sr2+ and Ca2+.
Chemical Reactions Involved in the Analysis of Group V Radicals
When (NH4)2CO3 is added to a salt solution containing NH4Cl and NH4OH, the carbonates of Ba2+, Sr2+ and Ca2+ are precipitated.
Analysis of group VI (Magnesium group)
Chemical Reactions Involved in Confirmation of Mg2+