Phosphate ion (PO₄^3-) testing in Qualitative Analysis | Phosphoric acids, compounds

There are tests to identify phosphate ion in qualitative analysis. Some metal phosphates are soluble in water and some are not. Physical characteristics such as precipitating, colour changes, vapor emissions and etc. are observed when PO₄^3- ion is tested. The main source of phosphorous is apatite. Additionally, Phosphoric acid (H₃PO₄) is a weak acid. In many occasions phosphate ion give coloured precipitates. Phosphate ions are very important in fertilizing industry. In this tutorial, we will study how to identify phosphate ion from other anions.

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Content

Structure of phosphate ion (PO₄^3-)

According to the lewis structure of phosphate ion, there is only one double between center phosphorus atom and one oxygen atom. As well, there are three more three single bonds.

In this tutorial we first discuss about identifying phosphate ion (PO₄^3-). Next we study some compounds and reactions of phosphorus and acids of phosphorus.

Identify phosphate ion - Qualitative analysis/

Test for phosphate ion - Concentrated nitric acid and ammonium molybdate

Add concentrated nitric acid (HNO₃) to phosphate ion solution. Next, add excess ammonium molybdate (NH₄)₂MoO₄. You can see a yellow precipitate forms in the solution. That yellow precipitate will dissolve in ammonia solution.

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Test for phosphate ion - Silver nitrate and phosphate ion solution

Silver nitrate and phosphate ion solution react and form silver phosphate ( Ag₃PO₄ ) which is a yellow precipitate. That precipitate dissolve in dilute nitric acid (HNO₃) or in ammonia solutions.

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Test for phosphate ion with Barium chloride

Barium chloride (BaCl₂) aqueous solution and phosphate ion aqueous solution react and give Barium phosphate (Ba₃(PO₄)₂), a white precipitate which dissolve in dilute hydrochloric acid (HCl).

Barium chloride (BaCl₂) and Sodium biphosphate (Na₂HPO₄) reaction

Barium chloride reacts with Sodium biphosphate solution to give white precipitate of BaHPO₄. which is soluble in acids (except H₂SO₄).

In the presence of alkali or ammonium hydroxide, HPO₄^3- ion is converted to phosphate ion (PO₄^3-).

Testing of phosphate ion with Ferric chloride

Ferric chloride (FeCl₃) and PO₄^3- react and give ferric phosphate (FePO₄) which is a yellow precipitate. This yellow precipitate will dissolve in dilute nitric acid. But do not dissolve in acetic (CH₃COOH) acid.

Testing of phosphate ion With Magnesia mixture

Magnesia is a mixture of of Magnesium chloride (MgCl₂). ammonium hydroxide (NH₄OH) and ammonium chloride (NH₄Cl). It gives a white crystalline precipitate of MgNH₄PO₄.

CrPO₄ is a greenish precipitate.

Fertilizer Production and phosphate ion

Naturally occurring calcium phosphate(Ca₃(PO₄)₂) is the primary resource utilized in producing fertilizer. But Ca₃(PO₄)₂ is insoluble in water. Because PO₄^3- ion is basic, it reacts with an acid to produce H₂PO₄^- and it gives Ca(H₂PO₄)₂ is more soluble than Ca₃(PO₄)₂. Sulfuric acid is used as the acid.

Now, we discuss about some phosphorus compounds and their reactions/

Oxyacids of phosphorus

H₃PO₄, H₃PO₃, H₃PO₂, H₄P₂O₇ are oxyacids of phosphorus.

Phosphoric(V) acid (H₃PO₄)

Phosphoric acid is one of the chemicals produced in enormous(very large quantity) quantities, and it is used in many industrial processes. H₃PO₄ is not a oxidizing acid and a weak acid.

There are three salts of Na as NaH₂PO₄, Na₂HPO₄, Na₃PO₄,

Phosphoric acid preparing

Phosphoric(III) acid (H₃PO₃)

H₃PO₃ is a weak acid and a dibasic acid. Oxidation number of P in H₃PO₃ is +3.

Phosphoric(I) acid (H₃PO₂)

H₃PO₂ is a weak acid and a monobasic acid. Oxidation number of P in H₃PO₃ is +3. H₃PO₂ is an oxidizing acid.

Occurrence of phosphate ion

Phosphate minerals include calcium phosphate (Ca₃(PO₄)₂) , apatite(Ca₅(PO₄₂)₃OH), fluoroapatite (Ca₅(PO₄)₃F) and chloroapatite (Ca₅(PO₄)₃Cl)

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Questions

What are the yellow precipitates of phosphate ion?

silver phosphate - Ag₃PO₄

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