Trends in Chemical Properties of Group 15 Elements and Their Compounds

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Senior 4 · Chemistry

Trends in Chemical Properties of Group 15 Elements and Their Compounds

5 lessons

Lesson 5 of 5

Tests for Phosphate and Nitrate Ions, and Uses of Group 15

25 min

Is it in the water?

Nitrates and phosphates are what make fertilisers work, but when they wash off fields into lakes and rivers they feed algae, which can cover the water and use up its oxygen when they die. Water laboratories therefore test for these two ions routinely, and a chemistry student needs to know the classic tests. This lesson describes them and explains the chemistry, then surveys how Group 15 elements and their compounds are used, from fertilisers and matches to medicines.

Definition

Eutrophication: the over-enrichment of a lake or river with nutrients, especially nitrates and phosphates, which causes rapid growth of algae; when the algae die and decay, bacteria use up the dissolved oxygen and fish die.

Devarda's alloy: an alloy of copper, aluminium and zinc used as a reducing agent in alkaline solution to reduce nitrate ions to ammonia.

Tests for nitrate ions, $\mathrm{NO_3^-}$

Almost all nitrates are soluble, so nitrate ions cannot be precipitated. The tests depend instead on reducing the nitrate ion.

Reduction to ammonia. Warm the solution with sodium hydroxide solution and Devarda's alloy (or aluminium foil). Ammonia is given off and turns damp red litmus paper blue: $\mathrm{3NO_3^-(aq) + 8Al(s) + 5OH^-(aq) + 18H_2O(l) \rightarrow 3NH_3(g) + 8[Al(OH)_4]^-(aq)}$ First warm the solution with sodium hydroxide alone: if ammonia is given off before the alloy is added, ammonium ions are present and must be boiled off first.
The brown ring test. Add freshly made iron(II) sulphate solution to the nitrate solution, then pour concentrated sulphuric acid slowly down the side of the tilted tube so that it forms a layer underneath. A brown ring forms where the layers meet (Figure 10.7). The acid and $\mathrm{Fe^{2+}}$ reduce some nitrate to nitrogen monoxide, which joins another $\mathrm{Fe^{2+}}$ ion as the brown complex $\mathrm{[Fe(H_2O)_5(NO)]^{2+}}$ : $\mathrm{NO_3^- + 3Fe^{2+} + 4H^+ \rightarrow NO + 3Fe^{3+} + 2H_2O} \qquad \mathrm{[Fe(H_2O)_6]^{2+} + NO \rightarrow [Fe(H_2O)_5(NO)]^{2+} + H_2O}$
Heating the solid. Most metal nitrates give brown $\mathrm{NO_2}$ and oxygen when heated (Units 6 and 7); sodium and potassium nitrates give only oxygen.

Figure 10.7: The brown ring test.

Tests for phosphate ions, $\mathrm{PO_4^{3-}}$

Ammonium molybdate. Acidify the solution with concentrated nitric acid, add ammonium molybdate solution and warm: a bright yellow precipitate of ammonium phosphomolybdate forms, $\mathrm{(NH_4)_3PO_4{\cdot}12MoO_3}$ . This is the most specific test.
Silver nitrate (neutral solution): a yellow precipitate of silver phosphate, $\mathrm{3Ag^+ + PO_4^{3-} \rightarrow Ag_3PO_4(s)}$ . Unlike yellow silver iodide, it dissolves in dilute nitric acid and in aqueous ammonia.
Magnesia mixture (magnesium chloride, ammonium chloride and aqueous ammonia): a white crystalline precipitate of $\mathrm{MgNH_4PO_4{\cdot}6H_2O}$ .
Barium chloride: a white precipitate of $\mathrm{Ba_3(PO_4)_2}$ that, unlike barium sulphate, dissolves in dilute nitric or hydrochloric acid.

Table 10.5: Summary of the tests

Ion	Reagent	Positive result
$\mathrm{NO_3^-}$	NaOH(aq) + Devarda's alloy, warm	ammonia: damp red litmus turns blue
$\mathrm{NO_3^-}$	$\mathrm{FeSO_4(aq)}$ , then concentrated $\mathrm{H_2SO_4}$ down the side	brown ring at the junction
$\mathrm{PO_4^{3-}}$	conc. $\mathrm{HNO_3}$ + ammonium molybdate, warm	bright yellow precipitate
$\mathrm{PO_4^{3-}}$	$\mathrm{AgNO_3(aq)}$	yellow precipitate, soluble in dilute $\mathrm{HNO_3}$ and in $\mathrm{NH_3(aq)}$

A positive brown ring test held up in a school laboratory: a thin dark brown band has formed at the boundary between the dense sulphuric acid layer at the bottom and the solution containing iron(II) sulphate and nitrate above it. Photo: Gan Kin Boon, CC BY-SA 3.0, via Wikimedia Commons

Uses of Group 15 elements and compounds

Table 10.6: Uses and the properties behind them

Element or compound	Use	Property that makes it suitable
Nitrogen gas	food packaging, inert atmospheres; liquid nitrogen for freezing	unreactive; boils at −196 °C
Ammonia	making fertilisers, nitric acid and nylon; household cleaners	basic, reacts with acids to give ammonium salts; source of nitrogen
Urea, ammonium nitrate, ammonium sulphate	nitrogen fertilisers	soluble sources of nitrogen; urea has 46.6% N
Ammonium nitrate (with fuel oil)	blasting in mines and quarries	decomposes explosively, releasing a large volume of gas
Nitric acid	fertilisers, explosives, dyes	strong acid and oxidising agent
Red phosphorus	striking surface of safety-match boxes	ignites by friction with the oxidising agent in the match head, but is stable in air
Phosphoric acid and phosphates	fertilisers (DAP, superphosphate); food acidulant; rust removal; toothpaste (calcium phosphate)	soluble source of phosphorus; non-oxidising, weak acid; hard, insoluble calcium salt
Arsenic	gallium arsenide semiconductors (LEDs, solar cells)	Ga–As forms a semiconductor that emits light
Antimony	hardening lead for battery grids; antimony(III) oxide as a flame retardant	hardens lead alloys; the oxide interferes with burning
Bismuth	stomach medicines (bismuth subsalicylate); low-melting alloys in fire sprinklers	low toxicity for a heavy metal; forms alloys that melt below 100 °C

Hazards. Ammonium nitrate is safe as a fertiliser when stored properly, but large stockpiles kept carelessly can explode: on 4 August 2020, about 2,750 tonnes of it stored in the port of Beirut exploded, devastating much of the city. Arsenic is highly toxic in its inorganic form, and the World Health Organization notes that arsenic-contaminated groundwater used for drinking and cooking is its greatest threat to health. Excess fertiliser washed into water causes eutrophication, so it should be applied in the right amount, at the right time.

Worked example 1: planning tests on an unlabelled fertiliser

A white fertiliser is thought to be either ammonium nitrate or diammonium phosphate. Plan tests to identify it, with expected results.

Solution. Both contain ammonium ions, so warming with NaOH(aq) gives ammonia from either; the anion decides.

Dissolve a little in water, add concentrated nitric acid and ammonium molybdate, and warm. A yellow precipitate shows phosphate (DAP).
On a fresh portion, do the brown ring test. A brown ring shows nitrate (ammonium nitrate).
For the Devarda's test, warm with NaOH(aq) alone until no more ammonia is given off (removing $\mathrm{NH_4^+}$ ), then add Devarda's alloy: more ammonia means nitrate.

Worked example 2: ammonia from a nitrate

A solution containing 0.850 g of sodium nitrate is warmed with excess sodium hydroxide and Devarda's alloy. All the nitrogen is converted to ammonia. Calculate the volume of ammonia at room temperature and pressure. $M(\mathrm{NaNO_3}) = 85.00\ \mathrm{g\,mol^{-1}}$ ; 24.0 dm³ mol⁻¹.

Solution. One nitrate gives one $\mathrm{NH_3}$ (nitrogen goes from +5 to −3, an eight-electron reduction). $n(\mathrm{NaNO_3}) = \frac{0.850}{85.00} = 0.0100\ \mathrm{mol} = n(\mathrm{NH_3}) \qquad V = 0.0100 \times 24.0 = 0.240\ \mathrm{dm^3} = 240\ \mathrm{cm^3}$

Warning

Common mistakes. (1) Trying to precipitate nitrate ions: nitrates are soluble; reduce them instead. (2) Doing the Devarda's test without first removing ammonium ions, which also give ammonia. (3) Shaking the brown ring tube: the ring forms only at the boundary. (4) Confusing yellow silver phosphate with yellow silver iodide: only the phosphate dissolves in dilute nitric acid.

Safety. The brown ring test uses concentrated sulphuric acid: wear goggles, add it slowly down the side of the tube, and never add water to concentrated acid. Hot sodium hydroxide solution is corrosive. Ammonium molybdate is harmful. If any reagent splashes on skin or into eyes, rinse with plenty of running water and get medical help.

Tip

Nitrate: reduce it (NH₃ or a brown ring). Phosphate: precipitate it (yellow with molybdate or silver). For uses, link each to a property: inert $\mathrm{N_2}$ , soluble N and P for fertilisers, red P for friction ignition, GaAs for light, Bi for low toxicity.

Sergei's Chemistry: Brown ring test for nitrate ions (2.5 min). Watch how the concentrated sulphuric acid is run down the side of the tube to form a layer, and where the brown ring of the iron–nitrosyl complex appears.

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Practice

Explain why nitrate ions cannot be identified by a precipitation test.
Describe the brown ring test, including how the acid is added and what is seen.
In the Devarda's test, what change in oxidation number does nitrogen undergo? Why must ammonium ions be removed first?
Describe two tests for phosphate ions and give the observations.
How would you distinguish silver phosphate from silver iodide, both yellow precipitates?
For each use, name the Group 15 element or compound and the property behind the use: (a) striking strip of matchboxes; (b) packaging crisps; (c) fire-sprinkler plugs; (d) blue and red LEDs.
A solution contains 1.70 g of sodium nitrate. What volume of ammonia, at room temperature and pressure, would the Devarda's test release if conversion were complete? $M(\mathrm{NaNO_3}) = 85.00$ .
Evaluate the claim: "Because fertilisers increase crop yields, farmers should apply as much as they can afford."

Show answersHide answers

Almost all nitrates are soluble in water, so no reagent gives a precipitate with nitrate ions.
Add fresh iron(II) sulphate solution to the sample; tilt the tube and pour concentrated sulphuric acid slowly down the side so it forms a lower layer; a brown ring of $\mathrm{[Fe(H_2O)_5(NO)]^{2+}}$ appears at the junction.
From +5 (in $\mathrm{NO_3^-}$ ) to −3 (in $\mathrm{NH_3}$ ). Ammonium ions also give ammonia with hot NaOH, so they would give a false positive.
Concentrated $\mathrm{HNO_3}$ + ammonium molybdate, warm: bright yellow precipitate. Silver nitrate (neutral): yellow precipitate that dissolves in dilute nitric acid. (Or magnesia mixture: white crystalline precipitate.)
Add dilute nitric acid (or aqueous ammonia): silver phosphate dissolves, silver iodide does not.
(a) red phosphorus: ignites by friction but is stable in air; (b) nitrogen: unreactive, keeps out oxygen; (c) bismuth alloys: melt at low temperature; (d) gallium arsenide (arsenic): a light-emitting semiconductor.
$n = \frac{1.70}{85.00} = 0.0200$ mol; $V = 0.0200 \times 24.0 = 0.480\ \mathrm{dm^3}$ .
Not true. Beyond the amount the crop can use, extra fertiliser adds cost without extra yield; the surplus nitrate and phosphate wash into rivers and lakes and cause eutrophication, and nitrate can contaminate drinking water. The right amount at the right time, matched to the soil, is better for both the farmer and the environment.

Quick check

Why is the test for nitrate ions based on reducing them, not on precipitating them?

Key points

Nitrate ions are reduced, not precipitated: NaOH + Devarda's alloy gives ammonia (after removing $\mathrm{NH_4^+}$ ); $\mathrm{FeSO_4}$ + concentrated $\mathrm{H_2SO_4}$ gives a brown ring of $\mathrm{[Fe(H_2O)_5(NO)]^{2+}}$ .
Phosphate ions give a bright yellow precipitate with ammonium molybdate in nitric acid, and yellow $\mathrm{Ag_3PO_4}$ (soluble in dilute $\mathrm{HNO_3}$ ) with silver nitrate.
Uses: $\mathrm{N_2}$ (inert atmosphere), $\mathrm{NH_3}$ , $\mathrm{HNO_3}$ and their salts (fertilisers, explosives), red P (matches), phosphates (fertilisers, food, toothpaste), GaAs (LEDs), Sb (alloys, flame retardants), Bi (medicines, low-melting alloys).
Hazards: ammonium nitrate stockpiles can explode; inorganic arsenic is highly toxic; excess fertiliser causes eutrophication.

1Is it in the water?

2Tests for nitrate ions, NO3− \mathrm{NO_3^-} NO3−​

3Tests for phosphate ions, PO43− \mathrm{PO_4^{3-}} PO43−​

4Uses of Group 15 elements and compounds

Is it in the water?

Tests for nitrate ions, $\mathrm{NO_3^-}$

Tests for phosphate ions, $\mathrm{PO_4^{3-}}$

Uses of Group 15 elements and compounds