Systematic Analysis of Cations
In this rather complex world of analytical chemistry, the systematic analysis of cations is the cardinal process that gives scientists and researchers the power to identify and quantify the positively charged ions in a given variety of samples. Cations occupy an important position in many significant chemical reactions and processes. Their identification assumes prime importance in fields ranging from environmental monitoring to forensic investigation and pharmaceutical quality control.
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One of the important preliminary tests in the systematic examination of cations is the addition of dilute sulphuric acid to the sample solution. The test thus helps in preliminary grouping and determines some chemical behaviour and interaction for the cations. Such a preliminary test with dilute sulphuric acid can, therefore, be regarded as imperative in systematic analysis and hence facilitate easy orientation of the chemist in the identification or differentiation of various cation groups with respect to their solubility and reactivity.
Analysis of Cations
Cations are positively charged ions that go to the cathode during electrolysis. Systematic analysis of cations is based on a sequence of tests and experiments aimed at the separation and later identification of the individual ions from any given mixture of salts. This is rather vital in forensic science, environmental analysis, and quality control in the pharmaceutical industry.
Preliminary Examination of the Salt for Identification of Cation:
Colour Test
Observe the colour of the salt carefully, which may provide useful information about the cations.
| Colour | Cations Indicated |
| Light green, Yellow, Brown | $F e^{2+}, F e^{3+}$ |
| Blue | $\mathrm{Cu}^{2+}$ |
| Bright green | $N i^{2+}$ |
| Blue, Red, Violet, Pink | $\mathrm{Co}^{2+}$ |
| Light pink | $M n^{2+1}$ |
Dry Heating Test
(i) Take about 0.1 g of the dry salt in a clean and dry test tube.
(ii) Heat the above test tube for about one minute and observe the colour of the residue when it is hot and also when it becomes cold.
Observation of changes gives indications about the presence of cations, which may not be taken as conclusive evidence.
| Colour when cold | Colour when hot | Inference |
| Blue | White | $C u^{2+}$ |
| Green | Dirty white or yellow | $\mathrm{Fe}^{2+1}$ |
| White | Yellow | $Z n^{2+}$ |
| Pink | Blue | Co2+ |
Flame Test:
The chlorides of several metals impart characteristic colour to the flame because they are volatile in non-luminous flame. This test is performed with the help of a platinum wire as follows :
(i) Make a tiny loop at one end of a platinum wire.
(ii) To clean the loop dip it into concentrated hydrochloric acid and hold it in a non-luminous flame
(iii) Repeat step (ii) until the wire imparts no colour to the flame.
(iv) Put 2-3 drops of concentrated hydrochloric acid on a clean watch glass and make a paste of a small quantity of the salt in it.
(v) Dip the clean loop of the platinum wire in this paste and introduce the loop in the non-luminous (oxidising) flame
(vi) Observe the colour of the flame first with the naked eye and then through a blue glass and identify the metal ion.
| Colour of the flame Observed by the naked eye | Colour of the flame Observed through blue glass | Inference |
| Green flame with blue centre | The same colour as observed without glass | $C u^{2+}$ |
| Crimson red | Purple | $\mathrm{Sr}{ }^{2+}$ |
| Apple green | Bluish-green | $\mathrm{Ba}^{2+}$ |
| Brick red | Green | $\mathrm{Ca}^{2+}$ |
Borax Bead Test
This test is employed only for coloured salts because borax reacts with metal salts to form metal borates or metals, which have characteristic colours.
(i) To perform this test make a loop at the end of the platinum wire and heat it in a flame till it is red hot.
(ii) Dip the hot loop into borax powder and heat it again until borax forms a colourless transparent bead on the loop. Before dipping the borax bead in the test salt or mixture, confirm that the bead is transparent and colourless. If it is coloured this means that the platinum wire is not clean. Then make a fresh bead after cleaning the wire.
(iii) Dip the bead in a small quantity of the dry salt and again hold it in the flame.
(iv) Observe the colour imparted to the bead in the non-luminous flame as well as in the luminous flame while it is hot and when it is cold.
(v) To remove the bead from the platinum wire, heat it to redness and tap the platinum wire with your finger.
| Heating in Oxidising (non-luminous) flame | Heating in Reducing (luminous) flame | Inference | ||
|---|---|---|---|---|
| Colour in cold | Colour in hot | Colour in cold | Colour in hot | Ion |
| Blue | Green | Red Opaque | Colourless | $C u^{2+}$ |
| Reddish Brown | Violet | Grey | Grey | $N i^{2+}$ |
| Light Violet | Light Violet | Colourless | Colourless | $M n^2$ |
| Yellow | Yellowish brown | Green | Green | $F e^{3+}$ |
Wet Tests for Identification of Cations
This preliminary test is a wet test carried out in an aqueous solution. The addition of dilute sulphuric acid to a solution containing various cations may result in the precipitation of some ions while the rest remain in the solution. This reaction helps group cations in an initial manner by their solubility in dilute sulphuric acid.
The cations indicated by the preliminary tests given above are confirmed by systematic analysis. The first essential step is to prepare a clear and transparent solution of the salt. This is called original solution. It is prepared as follows:
Preparation of Original Solution (O.S.):
To prepare the original solution, the following steps are followed one after the other 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:
- Take a little amount of the salt in a clean boiling tube add a few mL of distilled water and shake it. If the salt does not dissolve, heat the content of the boiling tube till the salt completely dissolves.
- If the salt is insoluble in water as detailed above, take fresh salt in a clean boiling tube and add a few mL of dil.HCl to it. If the salt is insoluble in cold, heat the boiling tube till the salt is completely dissolved.
- If the salt does not dissolve either in water or in dilute HCl even on heating, try to dissolve it in a few mL of conc. HCl by heating.
- If salt does not dissolve in conc. HCl, then dissolve it in dilute nitric acid.
- If salt does not dissolve even in nitric acid then a mixture of conc. HCl and conc.HNO3(3:1 ratio). This mixture is called aqua regia. A salt not soluble in aqua regia is considered to be an insoluble salt.
GROUP ANALYSIS:
Analysis of Zero group cation ($\left(N H_4^{+}\right.$ion):
- Take 0.1 g of salt in a test tube and add 1-2 mL of NaOH solution to it and heat. If there is a smell of ammonia, this indicates the presence of ammonium ions. Bring a glass rod dipped in hydrochloric acid near the mouth of the test tube. White fumes are observed.
- Pass the gas through Nessler’s reagent. The brown precipitate is obtained.
Chemistry of Confirmatory Tests for $\mathrm{NH}_4^{+}$ion
Ammonia gas evolved by the action of sodium hydroxide on ammonium salts and reacts with hydrochloric acid to give ammonium chloride, which is visible as a dense white fume.
$\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4+2 \mathrm{NaOH} \rightarrow \mathrm{Na}_2 \mathrm{SO}_4+2 \mathrm{NH}_3+2 \mathrm{H}_2 \mathrm{O}$$\mathrm{NH}_3+\mathrm{HCl} \rightarrow \mathrm{NH}_4 \mathrm{Cl}$$\mathrm{NH}_3+\mathrm{HCl} \rightarrow \mathrm{NH}_4 \mathrm{Cl}$
$\mathrm{NH}_3+\mathrm{HCl} \rightarrow \mathrm{NH}_4 \mathrm{Cl}$
On passing the gas through Nessler’s reagent, a brown colouration or a precipitate of basic mercury(II) amido-iodine is formed.
$2 \mathrm{~K}_2 \mathrm{HgI}_4+\mathrm{NH}_3+\mathrm{KOH} \rightarrow \mathrm{HgO} \cdot \mathrm{Hg}\left(\mathrm{NH}_2\right) \mathrm{I}+7 \mathrm{KI}+2 \mathrm{H}_2 \mathrm{O}$
For the analysis of cations belonging to groups I-VI, the cations are precipitated from the original solution by using the group reagents according to the scheme shown in the flow chart given below: The separation of all the six groups is represented as below:
Group reagents for precipitating ions:
| Group | Cations | Group Reagent |
| Group zero | $\mathrm{NH}_4^{+}$ | None |
| Group-I | $\mathrm{Pb}^{2+}$ | Dilute HCl |
| Group-II | $\mathrm{Pb}^{2+}, \mathrm{Cu}^{2+}, \mathrm{As}^{3+}$ | $\mathrm{H}_2 \mathrm{~S}$ gas in presence of dil.HCl$\mathrm{NH}_4 \mathrm{OH}$ |
| Group-III | $A l^{3+}, F e^{3+}$ | $\mathrm{NH}_4 \mathrm{OH}$ in presence of $\mathrm{NH}_4 \mathrm{Cl}$$\mathrm{H}_2 \mathrm{~S}$ |
| Group-IV | $\mathrm{Co}^{2+}, \mathrm{Ni}^{2+}, \mathrm{Mn}^{2+}, \mathrm{Zn}^{2+}$ | $\mathrm{H}_2 \mathrm{~S}$in presence of $\mathrm{NH}_4 \mathrm{OH}$ |
| Group-V | $\mathrm{Ba}^{2+}, \mathrm{Sr}^{2+}, \mathrm{Ca}^{2+}$ | $\left(\mathrm{NH}_4\right)_2 \mathrm{CO}_2$ in presence of $\mathrm{NH}_4 \mathrm{OH}$ |
| Group-VI | $\mathrm{Mg}^{2+}$ | None |
Grouping of Cations Based on Test with Dilute Sulphuric Acid
The addition of dilute sulphuric acid to a sample solution may result in some cations precipitating out while others remain soluble. This reaction is the basis for the classification of the cations into a number of groups.
Group I Cations:
The cations which form insoluble sulphates with dilute sulphuric acid ar$\mathrm{Ba} 2+, \mathrm{Sr} 2+$, and $\mathrm{Ca} 2+$.They are white precipitates.
Group II Cations:
Cations that do not form insoluble sulphates with dilute sulphuric acid, such as $\mathrm{Mg} 2+, \mathrm{Mn} 2+, \mathrm{Fe} 2+, \mathrm{Co} 2+, \mathrm{Ni} 2+, \mathrm{Cu} 2+, \mathrm{Zn} 2+$, and $\mathrm{Al} 3+$ remain in the solution.
Significance and Applications
Systematic analysis of cations, including the preliminary examination by dilate sulphuric acid,
Finds its application in many fields:
Forensic Science: The identification of some of the elements present in samples from a crime scene, for example, gunshot residues or explosives.
Environmental Analysis: Determination of the cations in water, soil, and air helps to assess the level of pollution and hence the quality of the environment.
Quality Control in the Pharmaceutical Industry: The rationale for the cation analysis states the purity and safety of products in the pharmaceutical industry and, more particularly, the development of new drugs.
Geological Studies: Cation analysis performed on rock and mineral samples helps in identifying and classifying different types of rocks and minerals.
Biological Research:
This analytical method could be applied to biological research into the functioning of some elements in biological processes, in particular, the functionality of enzymes and the structurality of proteins.
Some Solved Examples
Example 1
Question:
The metal that does not give the borax-bead test is
1) Chromium
2) Nickel
3) Lead
4) Manganese
Solution:
The borax bead test is generally given by transition elements. Lead, which is not a transition element, will not respond to the Borax bead test. Hence, the correct answer is option (3) Lead.
Example 2
Question:
The alkaline earth metal that imparts apple green colour to the Bunsen flame when introduced in it in the form of its chloride is
1) Barium
2) Strontium
3) Calcium
4) Magnesium
Solution:
Barium ((text{Ba}^{2+})) imparts a green colour to the flame. Therefore, the correct answer is option (1) Barium.
Example 3
Question:
The metal that does not give the borax bead test is
1) (text{Cr})
2) (text{Ni})
3) (text{Na})
4) (text{Mn})
Solution:
Sodium ((text{Na})) does not give a distinctive colour in the borax bead test, typically producing a colourless or nearly colourless bead when heated with borax. Therefore, the correct answer is option (3) (text{Na}).
Example 4
Question:
Which of the following gives violet coloured bead in borax bead test in oxidising flame under hot conditions?
1) (text{Fe}^{2+})
2) (text{Cu}^{2+})
3) (text{Co}^{2+})
4) (text{Mn}^{2+})
Solution:
(text{Mn}^{2+}) gives a violet-coloured bead in the borax bead test when heated in an oxidising flame under hot conditions. Therefore, the correct answer is option (4) (text{Mn}^{2+}).
Example 5
Question:(text $\left.\{\mathrm{Al}(\mathrm{OH})\}_{-} 3\right)$ and (text $\left.\{\mathrm{Fe}(\mathrm{OH})\}_{-} 3\right)$ can be separated by the reagent?
1) (text \{NH\} _4text\{Cl\}) and (text\{NH\}_4text\{OH\})
2) (text $\{\mathrm{NaCl}\}$ ) solution
3) (text \{NaOH\}) solution
4) Conc. (text\{ \{HC1\})
Solution:
(text{Fe(OH)}_3) is insoluble when dissolved in (text{NaOH}) solution, however, (text{Al(OH)}_3) is soluble in (text{NaOH}). Therefore, (text{Al(OH)}_3) and (text{Fe(OH)}_3) can be separated by (text{NaOH}) solution. Hence, the correct answer is an option (3) (text{NaOH}) solution.
Summary
The systematic analysis of cations, especially by preliminary tests with dilute sulphuric acid, is the simplest analysis done in analytical chemistry. This provides information on establishing the identification and grouping of cations with respect to their solubility and reactivity, hence giving information that is critical about their chemical properties. Their grouping into classes, such as I and II, in relation to their response towards dilute sulphuric acid, enables one to proceed in a much more orderly fashion in the analysis of cations.
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