15.7–15.8 Making Fertilisers in the Lab and in Industry

A free GCSE Chemistry lesson for AQA (8462) from OBA School: nPK fertilisers, how ammonium salts and phosphates are made, and how lab and industrial production compare. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain why fertilisers containing nitrogen, phosphorus and potassium (NPK) are used
• Write equations for making ammonium nitrate and ammonium sulfate from ammonia
• Name the products formed when phosphate rock is treated with nitric, sulfuric and phosphoric acid
• Describe how to make ammonium sulfate in the laboratory by titration and crystallisation
• Compare the laboratory preparation of a fertiliser with its industrial production, and calculate percentage of nitrogen and (Higher tier) titration concentrations

Key words

• Fertiliser: A substance added to soil to give plants the elements they need, increasing crop yields.
• NPK fertiliser: A formulation (a mixture made for a particular use) of salts containing nitrogen, phosphorus and potassium in suitable percentages.
• Phosphate rock: A mined rock containing calcium phosphate. It is insoluble, so it cannot be used directly as a fertiliser.
• Superphosphate: A soluble phosphate fertiliser made by treating phosphate rock with sulfuric acid (single) or phosphoric acid (triple).
• Integrated process: Several linked industrial processes on one site, where the product of one is the raw material for another.

Why farmers use NPK fertilisers

Plants take mineral ions from the soil. Harvesting crops removes these elements, so farmers add fertilisers to keep the soil fertile and to increase crop yields. The three main elements needed are nitrogen (N), phosphorus (P) and potassium (K). The fertiliser must be soluble in water so that roots can absorb the ions.

• Nitrogen (N); Needed for making proteins; healthy leaf and stem growth; Examples of compounds that supply it ammonium nitrate, ammonium sulfate, ammonium phosphate, potassium nitrate
• Phosphorus (P); Needed for healthy root growth; respiration; Examples of compounds that supply it ammonium phosphate, single and triple superphosphate
• Potassium (K); Needed for flowers, fruit and resistance to disease; Examples of compounds that supply it potassium chloride, potassium sulfate, potassium nitrate

Bags of NPK fertiliser show three numbers, such as 15 : 9 : 20. They give the proportions of nitrogen, phosphorus and potassium, always in the order N, P, K, so a farmer can match the fertiliser to the crop. No single compound contains all three elements in useful amounts, so an NPK fertiliser is a formulation: a mixture of several salts. Plants also need tiny amounts of other elements, such as magnesium and zinc.

Worked example: percentage of nitrogen in a fertiliser

Ammonium nitrate, NH₄NO₃. Ar: N = 14, H = 1, O = 16
Mr = 14 + (4 × 1) + 14 + (3 × 16) = 80. Mass of nitrogen = 2 × 14 = 28
Percentage of N = 28 ÷ 80 × 100 = 35%
Ammonium sulfate, (NH₄)₂SO₄: Mr = 132, so % N = 28 ÷ 132 × 100 = 21.2%

Answer: ammonium nitrate supplies more nitrogen per kilogram (35% compared with 21.2%)

Making the compounds

Ammonia from the Haber process is an alkali. It is neutralised by acids to make ammonium salts. Some of the ammonia is also oxidised to make the nitric acid.

NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq) (ammonium nitrate)

2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq) (ammonium sulfate)

3NH₃(aq) + H₃PO₄(aq) → (NH₄)₃PO₄(aq) (ammonium phosphate)

Potassium chloride and potassium sulfate are mined and purified. Phosphate rock is also mined, but it is insoluble, so it is treated with an acid to make soluble salts.

• nitric acid; Products phosphoric acid, H₃PO₄, and calcium nitrate, Ca(NO₃)₂; the phosphoric acid is then neutralised with ammonia to make ammonium phosphate, (NH₄)₃PO₄
• sulfuric acid; Products single superphosphate: a mixture of calcium phosphate, Ca₃(PO₄)₂, and calcium sulfate, CaSO₄
• phosphoric acid; Products triple superphosphate: calcium phosphate, Ca₃(PO₄)₂

Making ammonium sulfate in the laboratory

In the lab, a fertiliser salt is made in a small batch by neutralising an alkali with an acid. A titration finds exactly how much acid is needed, so that the final solution contains only the salt and water. The salt is then crystallised from the solution.

1. Use a pipette to put 25.0 cm³ of dilute ammonia solution into a conical flask. Add a few drops of methyl orange (yellow in alkali). 2. Add dilute sulfuric acid from a burette, swirling, until the indicator just turns red. Record the volume of acid used. 3. Repeat with the same volumes but no indicator, so the product is pure. 4. Pour the solution into an evaporating basin and heat it over a water bath until about half the water has gone and crystals start to form. 5. Leave to cool so crystals form. Filter, then dry the crystals between filter papers.

Safety: Wear eye protection: dilute acids and ammonia solution are irritants. Ammonia gas has a strong smell, so work in a well-ventilated room.

Worked example: concentration of the ammonia solution (Higher tier)

21.60 cm³ of 0.150 mol/dm³ sulfuric acid neutralises 25.0 cm³ of ammonia solution. 2NH₃ + H₂SO₄ → (NH₄)₂SO₄
Moles of H₂SO₄ = 0.150 × 21.60 ÷ 1000 = 0.003 24 mol
Ratio NH₃ : H₂SO₄ = 2 : 1, so moles of NH₃ = 2 × 0.003 24 = 0.006 48 mol
Concentration = 0.006 48 ÷ (25.0 ÷ 1000) = 0.2592 mol/dm³

Answer: 0.259 mol/dm³ (3 s.f.)

Lab compared with industry

• Scale; In the laboratory a few grams; In industry thousands of tonnes a day
• Type of process; In the laboratory batch: one batch at a time; In industry continuous: reactants in and product out all the time
• Raw materials; In the laboratory dilute ammonia solution and acid bought from a supplier; In industry made on site from raw materials: ammonia from the Haber process, sulfuric acid from sulfur, nitric acid from ammonia
• Concentration; In the laboratory dilute solutions; In industry concentrated reactants
• Energy; In the laboratory heat supplied by a Bunsen burner to evaporate the water; In industry the neutralisation is exothermic; its heat is used to evaporate water, saving energy
• Number of stages; In the laboratory few: titrate, evaporate, crystallise; In industry many linked stages in an integrated plant
• Equipment; In the laboratory glass apparatus: burette, pipette, conical flask, evaporating basin; In industry large reaction towers and pipes made of stainless steel to resist corrosion, heat and pressure
• Safety; In the laboratory dilute solutions, small amounts, eye protection; In industry concentrated, corrosive chemicals in huge amounts; fertiliser plants must be designed and monitored carefully, as there have been fatal explosions
• Form of product; In the laboratory crystals, filtered and dried; In industry hot slurry dried with air into round granules (pellets) of equal size, which spread evenly on fields and do not clog spreaders

Exam tip: If asked to “compare”, give a point for each method in the same sentence, e.g. “In the lab the reactants are dilute, but in industry they are concentrated.”

Common mistakes

• Saying phosphate rock can be spread straight onto fields. It is insoluble, so plants cannot absorb it; it must be treated with acid.
• Writing NH₃ + H₂SO₄ → NH₄SO₄. The ammonium ion is NH₄⁺ and sulfate is SO₄²−, so the formula is (NH₄)₂SO₄ and 2NH₃ is needed.
• Forgetting to repeat the titration without indicator. The indicator would contaminate the crystals.
• Heating the solution until it is dry. Evaporate some of the water, then leave it to crystallise.
• Using the mass of one N atom when a formula has two: NH₄NO₃ contains two nitrogen atoms.

Revision questions

Try these, then check your answers below each one.

Question 1. Calculate the percentage by mass of nitrogen in ammonium nitrate, NH₄NO₃. (Ar: N = 14, H = 1, O = 16)

Answer: 35 %

Working: Mr = 80. % N = 28 ÷ 80 × 100 = 35%.

Question 2. Name the salt made when ammonia reacts with sulfuric acid.

Answer: ammonium sulfate

Working: Ammonia + sulfuric acid → ammonium sulfate.

Question 3. Why can phosphate rock not be used directly as a fertiliser? (Choose from: It is insoluble in water, It contains no phosphorus, It is too acidic, It is a liquid.)

Answer: It is insoluble in water

Working: Phosphate rock is insoluble, so plants cannot absorb it. It is treated with acid to make soluble salts.

Related lessons

• Previous lesson: 15.5–15.6 The Haber Process and Its Economics
• In this chapter: 15.1 Rusting
• In this chapter: 15.2 Useful Alloys
• In this chapter: 15.3 The Properties of Polymers

Continue in the full course

This lesson is section 15.7–15.8 (AGCCH1507) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.