Chemistry · Chemistry of the elements · Grade 8

Phosphorus and mineral fertilisers

What do the numbers “16-16-16” on a fertiliser bag mean, and how can a fertiliser be identified?

The situation

In spring fertiliser is brought to a cotton field: the sacks are marked with the numbers “34-0-0”, “12-52-0”, “0-0-60” and “16-16-16”. The raw material for phosphorus fertilisers comes from the phosphorite deposit in the Kyzylkum, potassium from Dehkanabad, and nitrogen from the air. The red strip on the side of a matchbox contains phosphorus too, but white phosphorus is so dangerous that it is kept under water. How do the forms of phosphorus differ, what do the numbers on the sack mean, and how can a fertiliser be identified?

Topic

Phosphorus and mineral fertilisers. From Kyzylkum phosphorite to ammophos: N, P and K for the cotton field — what the numbers on a fertiliser bag mean

Key ideas

Allotropy of phosphorus
NPK: nutrient content
Identifying a fertiliser

Formulas

4 P + 5 O_{2} \overset{}{\to} 2 P_{2} O_{5}

Phosphorus burns — white smoke: phosphorus(V) oxide.

P_{2} O_{5} + 3 H_{2} O \overset{}{\to} 2 H_{3} PO_{4}

Phosphoric acid is a tribasic acid of medium strength.

Ca_{3} (PO_{4})_{2} + 2 H_{2} SO_{4} \overset{}{\to} Ca (H_{2} PO_{4})_{2} + 2 CaSO_{4}

Ordinary superphosphate: the insoluble phosphate is converted into the water-soluble dihydrogenphosphate.

NH_{3} + H_{3} PO_{4} \overset{}{\to} NH_{4} H_{2} PO_{4}

Ammophos is a compound fertiliser containing nitrogen and phosphorus.

ω (N) = \frac{n A_{r} (N)}{M_{r}} \cdot 100 %

Mass fraction of a nutrient; the numbers on the sack are the mass fractions (%) of N, P₂O₅ and K₂O.

Theory

Phosphorus and its allotropy

Phosphorus (Z = 15, $3 s^{2} 3 p^{3}$ ) is an element of the nitrogen group, with valency III and V. In nature it occurs only in compounds: phosphorite and apatite (mainly Ca₃(PO₄)₂). White phosphorus consists of P₄ molecules (a tetrahedron, 60° angles); it is waxy, melts at 44.1 °C and is very toxic; slowly oxidising in air, it glows green in the dark (its name is Greek for “light-bearer”) and ignites spontaneously at ≈ 30–50 °C — so it is kept under water. Red phosphorus is a polymer, almost non-toxic, does not glow in the dark and does not ignite below ≈ 240 °C; it forms when white phosphorus is heated to ≈ 300 °C in the absence of air. The strip on the side of a matchbox contains red phosphorus and powdered glass, while the match head contains KClO₃ and sulfur. Black phosphorus is the most stable, layered (like graphite) and a semiconductor; in 1914 P. Bridgman obtained it by heating white phosphorus at a pressure of ≈ 12 000 atm. In industry phosphorus is made in an electric furnace: $Ca_{3} (PO_{4})_{2} + 3 SiO_{2} + 5 C \overset{}{\to} 3 CaSiO_{3} + 2 P + 5 CO$ .

Oxygen compounds of phosphorus

When phosphorus burns, white smoke — phosphorus(V) oxide — is formed: $4 P + 5 O_{2} \overset{}{\to} 2 P_{2} O_{5}$ (in fact, P₄O₁₀ molecules). P₂O₅ absorbs water extremely avidly — it is used for drying gases. With cold water it gives metaphosphoric acid HPO₃, and with hot water orthophosphoric acid: $P_{2} O_{5} + 3 H_{2} O \overset{}{\to} 2 H_{3} PO_{4}$ . H₃PO₄ is a tribasic acid of medium strength (pKₐ 2.15; 7.20; 12.35) and is not an oxidising agent. It gives three kinds of salt: dihydrogenphosphates (NaH₂PO₄, Ca(H₂PO₄)₂ — soluble in water), hydrogenphosphates (Na₂HPO₄, CaHPO₄ — the calcium one is sparingly soluble) and phosphates (Na₃PO₄, Ca₃(PO₄)₂ — the calcium one is insoluble). Plant roots mainly take up soluble phosphate. The qualitative test for the phosphate ion is a yellow precipitate with silver nitrate: $Na_{3} PO_{4} + 3 AgNO_{3} \overset{}{\to} Ag_{3} PO_{4} ↓ + 3 NaNO_{3}$ .

Mineral fertilisers

A plant takes N, P and K from the soil in the largest amounts; every year the elements removed with the harvest are returned with fertiliser. The three numbers on the sack are the mass fractions (%) of N, P₂O₅ and K₂O: “16-16-16” means 16 kg of N, 16 kg of P₂O₅ and 16 kg of K₂O in every 100 kg. Phosphorus is expressed as P₂O₅ and potassium as K₂O: P = 0.436·P₂O₅, K = 0.830·K₂O. Nitrogen fertilisers: ammonium nitrate NH₄NO₃ (34–34.4 % N; in the pure substance 35.0 %), urea CO(NH₂)₂ (46 % N), ammonium sulfate (21 % N). Phosphorus fertilisers: ordinary superphosphate Ca(H₂PO₄)₂ + 2CaSO₄ (≈ 20 % P₂O₅), double superphosphate (≈ 45 %), phosphorite flour. Potassium fertilisers: potassium chloride (≈ 60 % K₂O). Compound fertilisers: ammophos NH₄H₂PO₄ (10–12 % N, 46–52 % P₂O₅), nitroammophoska (NPK). In Uzbekistan: the Kyzylkum Phosphorite Complex (Navoi region) — phosphorite ore with ≈ 16–20 % P₂O₅, and ≥ 26–28 % in the washed and calcined concentrate; Ammofos-Maxam in Almalyk — ammophos and superphosphate; the Dehkanabad potash plant — potassium chloride; Navoiyazot, Fergana Azot and Maxam-Chirchiq — nitrogen fertilisers.

Biogenic elements

The elements that are part of living organisms and necessary for life are the biogenic elements. Macroelements: C, H, O, N (together ≈ 96 % of body mass), P, S, Ca, K, Mg, Na, Cl. Microelements: Fe, Zn, Cu, Mn, Co, Mo, B, I and others. Nitrogen is in proteins and nucleic acids; phosphorus in DNA, ATP and bones (calcium phosphate); potassium regulates the water balance and cell pressure in plants; magnesium is in chlorophyll; iron in haemoglobin. If a plant lacks nitrogen, the lower leaves turn yellow; if it lacks phosphorus, growth slows and the leaves take on a brown-purple tint; if it lacks potassium, the leaf edges dry out and “scorch”. For the elements in the human body, see the experiment “Chemistry in our body”.

Identifying fertilisers (lab work 10, practical 5)

1. Appearance (colour, granules or crystals) and solubility in water: superphosphate dissolves only partly — the gypsum stays in the precipitate — while the others dissolve well. 2. Heating with alkali: the smell of ammonia, damp red litmus turns blue — an ammonium salt. 3. BaCl₂ (in hydrochloric acid): a white precipitate of BaSO₄ — a sulfate. 4. AgNO₃: a white curdy precipitate of AgCl — a chloride; a yellow precipitate of Ag₃PO₄ — a phosphate. 5. Flame: purple through cobalt glass — potassium. 6. On a glowing coal: if it flashes and burns — a nitrate (ammonium nitrate); if it melts and gives off the smell of ammonia — urea. Safety: do not taste fertilisers or touch them with your hands; smell ammonia by wafting the vapour towards you; AgNO₃ stains the skin black; white phosphorus is not handled in school — it is only a sample shown by the teacher.

University: a fertiliser plant

Phosphoric acid and superphosphate. Kyzylkum phosphorite (fluorapatite) is decomposed by sulfuric acid: extraction phosphoric acid — $Ca_{5} (PO_{4})_{3} F + 5 H_{2} SO_{4} + 10 H_{2} O \overset{}{\to} 3 H_{3} PO_{4} + 5 CaSO_{4} \cdot 2 H_{2} O + HF$ (the waste is phosphogypsum); ordinary superphosphate — $2 Ca_{5} (PO_{4})_{3} F + 7 H_{2} SO_{4} + 3 H_{2} O \overset{}{\to} 3 Ca (H_{2} PO_{4})_{2} \cdot H_{2} O + 7 CaSO_{4} + 2 HF$ : 1 g of P₂O₅ needs 1.61 g of H₂SO₄ (100 %) (carbonate impurities consume acid too). Double superphosphate: $Ca_{3} (PO_{4})_{2} + 4 H_{3} PO_{4} \overset{}{\to} 3 Ca (H_{2} PO_{4})_{2}$ . Halurgy. Sylvinite (the raw material of the Dehkanabad potash plant) is a mixture of KCl and NaCl. In a solution saturated with both salts at 25 °C there are ≈ 11 % KCl and ≈ 20 % NaCl, while at 100 °C there is about 2 times as much KCl and slightly less NaCl (approximate). If sylvinite is treated with hot solution, KCl dissolves and NaCl remains; when the filtrate is cooled, KCl crystallises, and the solution is heated and used again. Ammonium nitrate and urea. $NH_{3} + HNO_{3} \overset{}{\to} NH_{4} NO_{3}$ — the heat of neutralisation is used to evaporate the solution, which is then granulated. Urea: $2 NH_{3} + CO_{2} \overset{}{\to} CO (NH_{2})_{2} + H_{2} O$ — via ammonium carbamate, at ≈ 170–200 °C and 130–200 atm; 50–65 % is converted in a single pass. NPK calculation: the required P₂O₅ → ammophos, K₂O → KCl, the remaining N → urea. Model: the ignition temperatures of the allotropes are taken as 40 and 240 °C; the pH of phosphate is for an ideal solution; solubilities are approximate and times are shortened.

Did you know?

Phosphorus was discovered in 1669 by the Hamburg alchemist Hennig Brand while he was evaporating urine in search of the “philosopher's stone”. Because it glowed green in the dark, it was named phosphoros — Greek for “light-bearer”.
Urea was synthesised in 1828 by the German chemist Friedrich Wöhler from inorganic ammonium cyanate — one of the first artificial syntheses of a substance found in a living organism. Today urea is the most widely used nitrogen fertiliser in the world.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Non-metals and halogens — Grade 8
Chlorine and hydrochloric acid — Grade 8
The oxygen group and sulfur — Grade 8
Sulfuric acid and the contact process — Grade 8
Nitrogen, ammonia and nitric acid — Grade 8
Carbon and its compounds — Grade 9
Silicon and silicates — Grade 9
Making cast iron and steel — Grade 9
Metals and alloys — Grade 9
Alkali metals and soda — Grade 9