Salt Analysis – Chemical Analysis, Salts, Analysis of Anions, Analysis of Cations, Practice Problems and FAQs

Regardless of our various religious beliefs, Diwali is a unique time of year for everyone in India. We all enjoy the Diwali season because there are sweets, new costumes, and firecrackers galore! We also promote happiness by giving our neighbours treats. We treasure the nighttime fireworks. Our spirits are raised by the sight of the various colours in the sky.

Have you ever wondered how these colours emerge from firecrackers?

Salts have a wide range of industrial and daily purposes in addition to being used to create different colours in firecrackers during festive celebrations.

There are particular chemical salts that can give the fireworks a certain colour. For example, SrCO3, NaNO3, etc are some of the metal salts used in crackers to produce red and yellow colours in firework displays. Salts are the product of the reaction between an acid and a base. Salts are mostly ionic-containing cations(positively charged) and anions(negatively charged). Though they are quite soluble in water, their solubility is determined by their solubility product.

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It is necessary to identify the ions present in an unknown salt. The tests performed for the identification is referred to as qualitatively salt analysis..

Without further ado, let's learn about the qualitative laboratory analysis of inorganic salts.

TABLE OF CONTENTS

Chemical Analysis
Salt
Analysis of Anions
Analysis of Cations
Practice Problems
Frequently asked questions-FAQs

Chemical Analysis

Knowledge of any substance or a chemical compound involves 1) knowing the constituents and 2) estimating the composition of the constituents, using both physical and chemical tests.

Thus the analysis of a chemical substance can be classified into two:

Qualitative analysis: Identifying the elements, ions or groups or radicals etc present in either a pure substance or in a mixture.
Quantitative analysis: Quantitative analysis (QA) is a technique for measuring the amount of either the substance in a mixture or the individual constituents of a pure substance.

Steps involved in qualitative and quantitative analysis

Step 1: The first step involves physical identification from their colour, odour, test and other physical properties and comparing with the known data. inspection of the solid salt and its solution is performed.

Step 2: Treat the sample with a set of reagents, following an established procedure to identify the unknown cations and anions in the solid or solution.

Step 3: Applying appropriate techniques like precipitation, spectroscopy, calorimetry, titration, etc, to determine the exact amount of the substance or constituents present in the sample

Salt

When an acid and a base react, salt and water are formed. A few examples of salt include sodium chloride, barium sulphate, sodium nitrate, etc.

Examples:

1. HCl(aq)+NaOH(aq)NaCl(aq)+H₂O(l)
2. H₂SO₄ (aq) +2KOH(aq) K₂SO₄ (aq) + H₂O(l)

Salts are a combination of oppositely charged ions-namely cations and anions. The cations and anions are also referred to as base radicals and acid radicals respectively.

General representation of salt: Axy+Byx-

Where, A-Basic radical or cations

B-Acidic radical or anions

In, NaCl, Na+ is the basic radical obtained from a base like NaOH and Cl- is the acidic radical obtained from acid like HCl.

In Axy+Byx-

Examples of Basic Radicals/cations Axy+ are: Cu²⁺, Fe²⁺, Fe³⁺, Al³⁺, Co²⁺, Mn²⁺, Ni²⁺, Zn²⁺, Ba²⁺, Sr²⁺, Ca²⁺, Mg²⁺, NH⁴⁺ and

Acidic radicals/anions Byx- are: Cl^-, SO₄^2-, NO^3-, SO₃^2-, NO^2-, ClO^4-, ClO^3-, ClO^2-, etc.

Salt Analysis

Salt analysis is the qualitative analysis of cations and anions present in a given pure inorganic salt or in a mixture. The salt analysis involves a series of chemical reaction steps that help in identifying a salt and its constituent ions. These chemical reactions are classified into two types- namely

1) Preliminary tests that suggest the presence of a particular substance or a constituent and

2) Confirmatory tests which confirm the presence of the suggested constituent in the same.

The preliminary test involves treating the substance with a series of chemicals consecutively one after the other. Chemicals added in each step can identify one or more constituents(referred to as a group) that have some common characteristics in a mixture, by virtue of say development of colour, formation of precipitates, or solubility of formed precipitates etc. And the constituents identified in an earlier step do not interfere in the subsequent step treatments. Inferred constituents in a step can be confirmed their presence by tests specific to that particular constituent.

Analysis of Anions:

Anions are classified into three groups as follows from the common properties of identification:

List of some anions and their groups for analysis:

Group 1: ${C O}_{3}^{2 -}, {N O}_{3}^{-}, {S O}_{3}^{2 -}, S^{2 -}$

Group 2: $C l^{-}, B r^{-}, I^{-}, {C H}_{3} C O O^{-}, {N O}_{3}^{-}, C_{2} O_{4}^{2 -}$
Group 3: $: {P O_4^{3 -}}_$ , ${N O}_{3}^{-}, {S O}_{4}^{2 -}$

Preliminary test for the anions

Testing chemicals, observation and inference of various ions are given in the table below.

Group Number	Testing reagent	Observation	Likely presence of
1	Add dil.Sulphuric acid to the salt/solution. Warm if necessary	a) Brisk effervescence of a colourless gas, that turns lime water milky b) Vinegar odour c) Rotten egg smell	Carbonate Acetate Sulphide
2	Add Conc.Sulphuric acid to the salt/solution. Warm if necessary	a) A colourless gas that gives white fumes with a glass rod wetted with ammonium hydroxide solution b) Brown gas c) Violet vapours d) Vinegar smell e) Light brown gas on strong heating only	Chloride Bromide Iodide Acetate Nitrate
3	Add dil.HCl. Heat and then add burial chloride solution	White precipitate insoluble in Conc. HCl	Sulphate
3	Add Conc. Nitric acid. Heat. Add ammonium molybdate solution	Yellow precipitate	Phosphate

Confirmatory test for Anions:

Anion	Test	Observation
Carbonate	Add magnesium sulphate to an aqueous salt solution	White precipitate
Chloride	a) Heat the substance with Conc. sulphuric acid and a little manganese dioxide b) Chromyl chloride test: Heat the substance with Conc. Sulphuric acid and potassium dichromate Pass the gas through dil. NaOH Add lead acetate solution c) Silver nitrate test: Dissolve the salt in dil. Nitric acid and add silver nitrate	Greenish yellow gas with pungent odour Reddish brown vapours Solution turns yellow in colour Yellow precipitate forms Curdy white precipitate is formed
Bromide	Add dil HCl and silver nitrate	Yellow precipitate partial soluble in ammonium hydroxide solution
Nitrate	a) Heat the salt with conc. Sulphuric acid and copper turnings b) Add the salt to a ferrous sulphate solution. Shake. Add conc. Sulphuric acid slowly along the sides	Reddish brown gas and a greenish solution A brown ring at the juncture of two layers
Acetate	a) heat the salt with dil. sulphuric acid. Add alcohol(ethyl alcohol) and heat b) To a separate clear ferric chloride solution of the salt- i) add dil.HCl ii) add water and boil	Fruity smell Reddish colour is decolourised Reddish brown precipitate is formed
Sulphate	a) Dissolve the salt in dil. HCl and add barium chloride b) Add dil acetic acid to the salt and then add lead acetate solution	White precipitate insoluble in Conc. HCl White precipitate soluble in hot ammonium hydroxide
Phosphate	Add magnesia mixture (Ammonium chloride + magnesium Chloride + excess ammonium hydroxide)	White precipitate

Analysis of Cations:

Preliminary test for Cations:

Colour and appearance of the substance

A solubility test can be conducted to detect the presence of cations since the cations always release colour when mixed with another compound.

If the given salt is colourless, proceed toward the flame test. It is very effective in identifying the presence of more than one cation. To perform the flame test, take a pinch of the given salt and add a little quantity of hydrochloric acid. This releases colour on heating.

Cations present in some salts can be easily identified based on their appearance and odour. For example, calcium carbonate salt is chalk-like in appearance and the salts which smell like camphor contain naphthalene.

Test	Observation	Presence of
Colour of the aqueous solution	a) Blue b) green c) Yellowish Brown d) Pink	Curic ions Ferrous ions Ferric ions Manganese ions
Solubility in sodium carbonate solution	Soluble and no precipitate formation	Ammonium ions
Solubility in dil HCl	Slightly yellowish precipitate	Lead ions
Flame test: salt is mixed with Conc. HCl to a paste. A small amount of paste is introduced into a bunsen flame with a glass rod. Note the colour of the flame	a) Blue flame b) Brick red flame c) Crimson red flame d) Green flame	Copper ions Calcium ions Strontium ions Barium ions
Ash test: Salt is heated with Conc. Nitric and and cobalt nitrate solution. A piece of filter paper dipped in the solution is burned to ashes. Note the colour of the ash	a) Green b) Blue tint c) Pale Pink	Zinc ions Aluminium ions Magnesium ions

List of some Cations and their groups for analysis:

Cations are classified into six groups based on the similarity of reaction with test reagents.

Group	Cations	Group Reagent
0	NH⁴⁺	NaOH
1	Pb²⁺	dil HCl
2	Cu²⁺	dil HCl, H₂S
3	Fe²⁺, Fe³⁺, Al³⁺	NH₄OH, NH₄Cl
4	Co²⁺, Mn²⁺, Ni²⁺,Zn²⁺	H₂S in presence of NH₄OH, NH₄Cl
5	Ba2+, Sr²⁺, Ca²⁺	NH4OH, NH4Cl, (NH₄)2CO₃
6	Mg²⁺	NH₄OH, NH₄Cl, (NH₄)3PO₄

Preliminary chemical tests of Cations:

The analysis works on the solubility product of the cation that is in the solution. If the cation achieves the concentration that it needs to precipitate, precipitation happens. After the precipitation, we can easily conduct further tests to detect the species with better accuracy.

Prepare an aqueous solution of the salt and use a part of it as an original salt for group analysis.

Group	Test	Observation	Presence of
1	Add dil HCl to the salt solution	White precipitate	Lead ions
2	Pass hydrogen gas through the above clear solution	Black precipitate	Copper ions
3	Add solid ammonium chloride and excess ammonium hydroxide to the salt solution	White gelatinous solution	Aluminium ions
4	Pass hydrogen sulphide gas through the above clear solution	White precipitate	Zinc ions
5	Add solid ammonium chloride and excess ammonium hydroxide to the salt solution. Add ammonium carbonate solution	White precipitate	Barium or calcium or strontium ions
6	Add solid ammonium chloride and excess ammonium hydroxide to the salt solution. Add sodium hydrogen phosphate solution	White precipitate	Magnesium ions

Confirmatory test for Cations:

Ions	Test	Observation
Ammonium	a) Heat the solution with dil.NaOH b) Add dil NaOH to the salt solution. Then add Nessler’s reagent	Smell of ammonia that gives white fumes with a rod wetted with the HCl solution. Brown precipitate
Lead	a) Add Potassium iodide solution to the salt solution b) Add potassium chromate to the salt solution	Yellow precipitate Yellow precipitate
Copper	a) To the salt solution, add acetic acid and potassium ferrocyanide solution b) To the salt solution add ammonium hydroxide c) To the salt solution add acetic acid and KI	Chocolate coloured precipitate Blue coloured solution Yellow precipitate
Aluminium	a) Add excess of NaOH solution to the salt solution b) acidify the salt solution with dil. HCl. Add 2 drops of blue litmus solution and then add ammonium hydroxide through the sides slowly	White precipitate formed dissolves in excess NaOH Blue precipitate floats on the surface.
Zinc	Acidify the salt solution with dil. HCl and add potassium ferrocyanide solution	Bluish precipitate
Barium	Add potassium chromate solution to the salt solution	Yellow precipitate
Calcium	To the salt solution add ammonium oxalate	White precipitate
Strontium	To the salt solution add ammonium sulphate solution	White precipitate
Magnesium	To the salt solution add ammonium phosphate solution	White precipitate

In addition to the previously described method, sodium carbonate is also used to distinguish between some common metal ions. Reaction with sodium carbonate leads to precipitation of the ions in the form of their respective carbonates. This method is useful in differentiating between copper, iron and calcium or zinc and lead. The method involves adding sodium carbonate to the salt of the metal to form carbonates. If the precipitate is blue, it indicates the presence of copper. A dirty green colour signifies the presence of iron ions. Calcium, zinc and lead give white precipitates. This method works because the carbonates formed by the reaction are not soluble. They precipitate easily with a certain colour and the carbonate may be used for further tests.

Practice Problems

Q1. What are the different types of salts?

Answer: Salts are formed by the neutralisation reaction of acid and base, the nature of salts is decided by the strength of acid and base.

Types of salts:

Neutral salts: NaCl, generally formed by the combination of strong acid and strong base.

HCl+NaOHNaCl+H₂O

Acidic salts: NH4Cl, generally formed by a combination of strong acid and weak base.

HCl+NH₄OHNH₄Cl+H₂O

Basic salts: CH3COONa, generally formed by a combination of strong base and weak acid.

CH₃COOH+NaOHCH₃COONa+H₂O

Q2. What are acid and base radicals?

Ans. Salts are composed of acid and base radicals.

E.g- NaCl, Acid radical: Cl-& base radical Na⁺

Q3. Which factors decide the solubility of salt in water?

Answer: generally for any solid salt, majorly three factors decide their solubility and those factors are

Lattice energy
Hydration energy of cations
Hydration energy of anions

Q4. Why PbCl2 is insoluble in cold water but soluble in hot water?

Answer: Generally, the dissolution of salt in an aqueous medium increases the entropy and exothermic process in nature. But for a few salts like PbCl2 dissolution is endothermic in nature.

From Gibbs Helmholtz law; G=H-TS

S= positive

H= positive

T= temperature in K, always positive

G= change in free energy

Case 1: at low T, |TS|<|H|

So, G become positive, process is non spontenious . So, insoluble in cold water.

Case 2: at high T, |TS|>|H|

So, G become negative, process is spontenious . So, soluble in hot water.

Frequently Asked Questions

Q1. What role does salt analysis play?
Answer: A salt analysis is a series of steps that aid in the identification of salt and its constituent ions. Under laboratory conditions, the given salt sample is subjected to a series of preliminary and confirmatory tests that aid in distinguishing and separating the ions.

Q2. What is the purpose of grouping salts?
Answer: Salts are classified into six groups based on the differences in the solubilities of their chlorides, sulphide hydroxides, and carbonates at various pH values.

Q3. What are the three kinds of salts?
Answer: Neutral, acidic, and basic salts are the three types of salts. Neutral salts are salts that, when mixed and dissolved with water, produce a neutral solution. Strong acid and a strong base react to form neutral salt. For instance, sodium chloride and sodium sulphate.

Q4. What are the applications of salts?
Answer: Salt has long been used to flavour and preserves foods. It has also been used in the tanning, dyeing, and bleaching processes, as well as the manufacture of pottery, soap, and chlorine. It is currently widely used in the chemical industry.