Introduction to Salt Analysis

What Qualitative Salt Analysis Is

Analysis does not always mean breaking a substance into its ultimate constituents. Establishing the nature of a substance and the identity of its constituents is also analysis, and this branch is called qualitative analysis. For inorganic salts, qualitative analysis means the identification of the cations and anions present in a salt or a mixture of salts.

An inorganic salt is obtained by the complete or partial neutralisation of an acid with a base, or the reverse. In a salt, the part contributed by the acid is the anion and the part contributed by the base is the cation. In $\ce{CuSO4}$ and $\ce{NaCl}$, for instance, $\ce{Cu^2+}$ and $\ce{Na+}$ are cations while $\ce{SO4^2-}$ and $\ce{Cl-}$ are anions. Name both ions and you have named the salt.

The NEET-relevant aim, taken directly from Experiment 7.1, is to detect one cation and one anion in a given salt from a fixed list, with insoluble salts excluded. The reactions used are those that the senses can perceive — sight and smell — and they fall into three observable categories.

Analysis is carried out on different scales. In macro analysis 0.1–0.5 g of substance and about 20 mL of solution are used; in semimicro analysis about 0.05 g and 1 mL are sufficient; micro analysis needs far smaller amounts. School and NEET work is essentially macro/semimicro.

The Overall Workflow

Systematic analysis of an inorganic salt proceeds in a fixed order. The NCERT scheme breaks it into three principal stages, and the discipline of the sequence is itself part of the method.

Within these stages the physical examination feeds dry tests (heating, flame, borax bead, charcoal cavity); the anions are then tackled by treating the salt first with dilute acid, then with concentrated acid, then by independent precipitation tests; and finally the cations are sorted by systematic group precipitation. The flowchart below shows the spine of the whole exercise.

A point on the chemistry of the solution itself: the solubility of a salt and the pH of its aqueous solution carry information. If the solution is acidic or basic, the salt is being hydrolysed. A basic solution may point to a carbonate or sulphide; an acidic solution may indicate an acid salt or a salt of a weak base and a strong acid. In the latter case it is best to neutralise the solution with sodium carbonate before testing for anions.

Group Reagents and Selective Precipitation

Testing for thirteen-odd cations one at a time would be slow and error-prone. The systematic scheme avoids this by working in groups. A carefully chosen reagent, the group reagent, is added to the salt solution under controlled conditions so that it precipitates only a small set of cations and leaves the rest dissolved. This is selective precipitation.

The selectivity is not arbitrary. A precipitate forms only when the ionic product of a sparingly soluble salt exceeds its solubility product. By supplying a controlled concentration of the precipitating ion — chloride, sulphide, hydroxide, carbonate — the analyst pushes the ionic product of one group of cations above their $K_{sp}$ while keeping the others below theirs. The concentration of the precipitating ion is itself regulated by the common-ion effect, which is why, for example, $\ce{H2S}$ is passed in the presence of dilute $\ce{HCl}$ in Group II.

The Cation Analytical Groups I–VI

The cations in the NEET list are sorted into a zero group plus six numbered groups. Each group is defined entirely by its group reagent. The table below is the master key for the whole chapter; the dedicated sibling pages then expand each group's confirmatory tests.

The order is non-negotiable. Group I is precipitated first because lead chloride has the highest $K_{sp}$ among the chlorides that matter and dilute $\ce{HCl}$ removes only $\ce{Pb^2+}$. The filtrate then meets the next reagent, and so on down the table. The figure below shows how each reagent peels off one layer of cations.

The zero group is a special case. The ammonium ion forms no precipitate with any group reagent, so it is tested separately on the original salt by warming with sodium hydroxide: a smell of ammonia, white fumes with a glass rod dipped in $\ce{HCl}$, and a brown precipitate with Nessler's reagent confirm it.

The Anion Classification Overview

Anions are not arranged in a group ladder like cations. Instead they are classified by the reagent that betrays them. The NCERT scheme runs three steps in order, and an anion is assigned to whichever step first produces a positive result.

The chemistry of two anchor reactions illustrates the idea. A carbonate gives a brisk, odourless effervescence with dilute acid that turns lime water milky, while a chloride gives no reaction with dilute acid but releases pungent $\ce{HCl}$ fumes with concentrated acid.

The third class — sulphate, phosphate and borate — is examined only when Steps I and II are negative, because these anions do not evolve a characteristic gas and must be caught by direct precipitation. Sulphate gives a white barium sulphate insoluble in concentrated acids; phosphate gives the canary-yellow ammonium phosphomolybdate.

The Solubility-Product Basis

The NCERT Theory section names two principles of great use in the analysis: the solubility product and the common-ion effect. Everything in the group scheme reduces to these.

For a sparingly soluble salt $\ce{A_xB_y}$ the solubility-product expression is the equilibrium constant for its dissolution:

Precipitation begins the moment the ionic product — the same product computed with the actual ion concentrations — exceeds $K_{sp}$. The analyst therefore manipulates ion concentrations to flip individual cations across their solubility-product threshold. Adding a common ion (the dilute acid in Group II, the $\ce{NH4Cl}$ in Group III) suppresses the precipitating-ion concentration just enough to keep higher-$K_{sp}$ cations dissolved while the lower-$K_{sp}$ group drops out. This is the molecular logic behind every horizontal arrow in Figure 2.

Preliminary Examination First

Preliminary examination is not optional warm-up; it is the step that shapes everything after it. Performed in ten to fifteen minutes, it records the salt's appearance and physical properties — colour, smell, solubility, and the pH of its solution. These observations are clues, not proofs, but a good clue saves a wasted hour of wet testing.

The dry tests sit inside this stage: heating the dry salt, the blow-pipe test, flame tests, the borax bead test, the sodium carbonate bead test and the charcoal cavity test. Colour alone is suggestive — light green, yellow or brown salts hint at iron; blue at copper; bright green at nickel; pink at manganese or cobalt. Flame colours sharpen the picture: crimson red for strontium, apple green for barium, brick red for calcium, green with a blue centre for copper. None of these is conclusive on its own, which is exactly why confirmatory wet tests follow.

Preliminary tests should always be performed before starting the confirmatory tests for the ions. The gases evolved with dilute and concentrated $\ce{H2SO4}$ give good early indication of the acid radical present.

Safety and Systematic Discipline

Salt analysis handles concentrated acids and toxic gases, so the procedure carries a safety burden alongside its chemistry. The reagents are not benign: hydrogen sulphide and the halogen vapours are toxic by inhalation, bromine is corrosive, and the chromyl chloride test must be done with a minimal amount of substance to limit chromium pollution.

The deeper discipline is procedural. The scheme works only when the steps are followed in order: preliminary first, then anions, then cations; within anions, dilute acid before concentrated acid before independent tests; within cations, Group I before VI without skipping. The NCERT flowchart is explicitly drawn for detecting a single cation — modifications are needed for mixtures. Skip a step and a later confirmatory test loses its meaning, because each step assumes the earlier ones have already cleared their ions out of the way.