Anion Tests — Sulphate, Phosphate & Borate (Independent Group)

Why an "independent" anion group

In systematic salt analysis the anions are sorted by how they respond to acids. One set evolves a gas with dilute acid (carbonate, sulphite, sulphide, nitrite); another evolves a gas only with concentrated sulphuric acid (chloride, bromide, iodide, nitrate). Sulphate, phosphate and borate belong to neither. Their salts release no characteristic gas with dilute or concentrated acids, so a common gas-evolution scheme cannot flag them.

The NCERT Laboratory Manual therefore reserves Step-III for them: "If no positive test is obtained in Steps-I and II, then tests for the presence of sulphate and phosphate ions are performed." Because there is no shared gas to trap, each ion must be detected directly in solution using a reagent selective to it — barium chloride for sulphate, ammonium molybdate for phosphate, and the borate ester flame for borate. That is what makes the group "independent": every member carries its own confirmatory reagent.

A second consequence follows from the absence of a gas test. Detection rests on the solubility behaviour of a precipitate or on a flame colour, both of which can be imitated by other species. A white barium precipitate, a yellow molybdate precipitate and a coloured flame are each shared with a near neighbour, so the analyst leans on a precise diagnostic feature — insolubility in concentrated acid, a controlled heating step, or the formation of a specific volatile ester — to make the result unambiguous. The sections that follow treat each ion in the order the manual lists it, then close with the interference rules and a consolidated observation table.

Sulphate ion — barium chloride test

Take about 1 mL of the water extract (or sodium carbonate extract) of the salt, acidify it with dilute hydrochloric acid, and add barium chloride solution. A white precipitate of barium sulphate appears:

$$\ce{Na2SO4 + BaCl2 -> BaSO4 v + 2NaCl}$$

The white solid alone is not conclusive — barium sulphite and barium carbonate are also white. The decisive observation is what the precipitate does next: barium sulphate is insoluble in concentrated HCl or concentrated HNO₃, whereas the carbonate and sulphite precipitates dissolve with effervescence. That insolubility is the diagnostic property the NCERT manual records, and it is the single most examined fact in this section.

Sulphate — lead acetate confirmation

The manual gives a second, independent confirmation. Acidify the aqueous solution (or sodium carbonate extract) with acetic acid and add lead acetate solution. A white precipitate of lead sulphate confirms the sulphate ion:

$$\ce{Na2SO4 + (CH3COO)2Pb -> PbSO4 v + 2CH3COONa}$$

Lead sulphate is white and sparingly soluble; notably it dissolves in hot ammonium acetate solution, a property that overlaps with the Group I lead confirmation. For sulphate detection, the white $\ce{PbSO4}$ forming on adding lead acetate to an acetic-acid-acidified solution is the confirmatory observation. The acetic-acid medium is deliberate: a strongly acidic solution would keep lead sulphate dissolved, while a neutral or basic medium would precipitate lead hydroxide or basic lead carbonate and confuse the result. Acetic acid keeps the lead in solution as the acetate while still allowing the less soluble sulphate to crystallise out.

Why does the barium test take precedence over the lead test in routine work? Barium sulphate has an extremely low solubility product and forms even from dilute sulphate solutions, and its insolubility in concentrated acid is a property that almost nothing else shares — so it is both sensitive and selective. The lead acetate result is best treated as corroboration: two independent precipitations agreeing on sulphate is stronger evidence than either alone, which is exactly why the manual lists both under the same anion.

Phosphate ion — ammonium molybdate test

Acidify the sodium carbonate extract (or the salt solution in water) with concentrated nitric acid, add ammonium molybdate solution, and heat to boiling. A canary-yellow precipitate of ammonium phosphomolybdate forms — the signature confirmation for phosphate:

$$\ce{Na2HPO4 + 12(NH4)2MoO4 + 23HNO3 -> (NH4)3[P(Mo3O10)4] v + 2NaNO3 + 21NH4NO3 + 12H2O}$$

In this complex anion every oxygen of the original phosphate has been replaced by an $\ce{Mo3O10}$ group, building the bright yellow $\ce{(NH4)3[P(Mo3O10)4]}$. Warming is essential — at room temperature the colour may be a faint yellow tinge in solution rather than a clean precipitate, so the NCERT instruction to "heat to boiling" is part of the test.

Phosphate — magnesia mixture test

A second confirmation uses magnesia mixture — magnesium chloride with ammonium chloride and ammonium hydroxide. Added to a phosphate solution (and on scratching the inner wall of the tube), it gives a white crystalline precipitate of magnesium ammonium phosphate:

$$\ce{Mg^2+ + NH4+ + PO4^3- -> Mg(NH4)PO4 v}$$

This is the same compound encountered in the magnesium cation confirmation (Group VI), where disodium hydrogen phosphate is added to a magnesium solution to precipitate $\ce{Mg(NH4)PO4}$. Here the roles reverse: the unknown supplies phosphate and the reagent supplies magnesium, but the white crystalline solid is identical. As the NCERT note warns, the precipitate sometimes appears slowly, so warm the solution and scratch the tube walls to hasten crystallisation — the mechanical scratch provides nucleation sites that let the supersaturated solution deposit crystals.

The ammonium chloride and ammonium hydroxide in the magnesia mixture are not optional extras. The ammonium hydroxide keeps the medium mildly alkaline so that the triple salt can form, while the ammonium chloride suppresses the precipitation of magnesium hydroxide by the common-ion effect — without it, $\ce{Mg(OH)2}$ would precipitate in the alkaline medium and the result would be ambiguous. A clean white crystalline solid, rather than a gelatinous one, is the hallmark of the genuine magnesium ammonium phosphate.

Borate ion — ethyl borate flame test

Borate is the only member of this group confirmed by a flame rather than a precipitate. Place a little of the salt in a china dish, add ethanol (or methanol) and a few drops of concentrated sulphuric acid, and ignite the vapour. The acid esterifies the borate; the volatile ester carries boron into the flame, which burns with a characteristic green-edged flame:

$$\ce{H3BO3 + 3C2H5OH ->[H2SO4] B(OC2H5)3 + 3H2O}$$

The product $\ce{B(OC2H5)3}$ is triethyl borate (methanol gives trimethyl borate, $\ce{B(OCH3)3}$). It is volatile and combustible, and its combustion is what produces the green colour at the edge of the flame. The concentrated sulphuric acid is essential both to drive the esterification equilibrium by removing water and to liberate boric acid from the borate salt. If a metaborate or tetraborate salt is taken, the acid first sets free boric acid, which then esterifies — so the test works equally for orthoborate $\ce{BO3^3-}$, metaborate $\ce{BO2-}$ and tetraborate $\ce{B4O7^2-}$ such as borax.

Two practical cautions accompany the test. First, the green colour shows at the edge of the flame as the ester vapour burns; do not wait for the whole flame to turn green. Second, copper salts also impart a green flame, so the borate flame test should not be the sole evidence when copper may be present — the green of $\ce{B(OC2H5)3}$ is a momentary green fringe on the burning alcohol vapour, whereas a copper flame is a sustained green-blue. The transient, ester-driven nature of the borate flame is its signature.

Borate — turmeric paper test

A supporting confirmation uses turmeric paper. Dip a strip of turmeric paper in a borate solution acidified with hydrochloric acid and let it dry. The yellow turmeric turns red (reddish-brown) as boric acid reacts with the curcumin pigment. When this reddened paper is then moistened with an alkali such as dilute sodium hydroxide or ammonia solution, the colour shifts to green-black. The two-stage colour change — red on acid drying, then green on alkali — is the diagnostic sequence for borate.

The reason the test needs both stages is that an ordinary acid will not turn turmeric red on its own; it is specifically the boric acid (and a small group of related species) that produces the rosy colour with curcumin. The subsequent reversal to a darker green-black on alkali distinguishes the boron-curcumin colour from any incidental browning of the paper. In an exam setting the safest phrasing is to quote the full red-then-green sequence rather than either colour alone.

Interference removal & sequencing

Because these anions are tested in solution, dissolved interferents must be cleared first. Coloured cations and heavy metals can mask a faint canary-yellow or obscure a precipitate, so analysts use the sodium carbonate extract: boiling the salt with sodium carbonate converts interfering metal ions to insoluble carbonates and leaves the anions in solution as their soluble sodium salts. The clear extract is then acidified to the appropriate acid before each reagent is added.

Sulphate itself is an interferent in cation work. It forms sparingly soluble precipitates with Group V cations — barium, strontium and calcium — so where these cations must later be detected, the sulphate is converted to carbonate via the sodium carbonate extract so the two halves of the analysis do not collide. Acidity also matters: sulphate is taken to dilute HCl or acetic acid, phosphate to concentrated nitric acid, and borate is treated with concentrated sulphuric acid in the flame test. Using the wrong acid is a common cause of a failed or misleading result.

Observation → inference table