Ferr Phos: How Ferric Phosphate Dihydrate Is Made
At a glance
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Chemical name |
Ferric phosphate; iron(III) phosphate dihydrate |
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Chemical formula |
FePO₄·2H₂O |
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Water solubility |
Practically insoluble |
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Natural mineral |
Strengite; phosphosiderite has the same composition in a different crystal structure |
Ferric phosphate is composed of iron in the ferric, Fe³⁺, state and phosphate. It occurs naturally as strengite, FePO₄·2H₂O, while phosphosiderite has the same overall chemical composition but a different crystal structure. These minerals are uncommon and are not generally the commercial sources of pharmaceutical-grade ferric phosphate.
Instead, manufacturers typically begin with abundant iron ores and phosphate rock, convert them into purified reactive intermediates, and then form ferric phosphate dihydrate under controlled conditions.
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Body connection: A few grams of iron support oxygen transport An average adult contains only about 3–4 grams of iron. Much of it is incorporated into hemoglobin inside red blood cells, where iron helps bind and transport oxygen. This small quantity supports oxygen delivery to trillions of cells every day. |
Making pharmaceutical-grade Ferr Phos
1. Beginning with iron ore
Iron commonly begins as hematite, Fe₂O₃, or magnetite, Fe₃O₄. The ore is mined, crushed, concentrated, and purified to remove silica, clay, and other naturally occurring minerals.
The purified iron oxide is then converted into a soluble ferric compound suitable for controlled chemical manufacture. The following equations are simplified examples of possible conversions.
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Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O |
Hydrochloric acid converts hematite into ferric chloride, FeCl₃.
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Fe₂O₃ + 3H₂SO₄ → Fe₂(SO₄)₃ + 3H₂O |
Sulphuric acid can produce ferric sulphate, Fe₂(SO₄)₃.
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Fe₂O₃ + 6HNO₃ → 2Fe(NO₃)₃ + 3H₂O |
Nitric acid can produce ferric nitrate, Fe(NO₃)₃. These soluble salts provide a purified and measurable source of Fe³⁺ ions.
2. Preparing the phosphate source
The phosphate component is obtained from purified phosphate materials derived from phosphate rock, as described in the Calc Phos article. Phosphoric acid is one possible purified phosphate source.
3. Precipitating ferric phosphate dihydrate
The purified ferric solution is combined with the phosphate source under controlled conditions. One simplified example uses ferric chloride and phosphoric acid:
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FeCl₃ + H₃PO₄ + 2H₂O → FePO₄·2H₂O↓ + 3HCl |
Ferric phosphate dihydrate forms as a sparingly soluble solid. During crystallization, two water molecules become incorporated into each FePO₄ unit and form part of the hydrated crystal lattice rather than remaining as ordinary surface moisture.
Manufacturers may use ferric nitrate, ferric sulphate, or another purified ferric compound instead of ferric chloride. The by-products differ, but the target ingredient remains FePO₄·2H₂O.
4. Final purification and testing
The precipitate is separated from the liquid, washed to remove soluble by-products and excess starting materials, and dried under conditions that preserve the specified hydration state. It may then be milled and classified.
Testing may include identity, iron and phosphate assay, moisture, hydration state, particle characteristics, residual anions such as chloride or sulphate, heavy metals, and other elemental impurities.