Difference between Phosphorous acid (H₃PO₃) and phosphoric acid (H₃PO₄)

Phosphorous acid (H₃PO₃) and phosphoric acid (H₃PO₄) are two oxygen-containing acids of phosphorus. The differences between them are mainly reflected in their chemical structure, oxidation state, properties and uses:

1. Chemical structure and composition.

-Phosphorous acid (H₃PO₃)

-Structure: actually a dibasic acid (only 2 ionizable H), with the molecular formula HPO (OH)2.

-Phosphorus is in the oxidation state of +3, and contains 1 P-H bond (not ionized) and 2 P-OH bonds (ionized).

-Phosphoric acid (H₃PO₄)

-Structure: tricarboxylic acid (3 ionizable H), with the molecular formula PO (OH) ₃.

-Phosphorus is in the +5 oxidation state and contains 3 P-OH bonds (both ionizable).

2. Acidic differences.

-phosphorous acid:

-pKa₁ ≈ 1.26, pKa₂ ≈ 6.70 (only 2 ionizable H).

-Stronger than phosphoric acid (due to the electron-withdrawing effect of the P-H bond).

-ortho-phosphoric acid:

Pka₁ ≈ 2.15, pKa₂ ≈ 7.20, and pKa₃ ≈ 12.35 (3 ionizable H).

-Less acidic but more stable.

3. Chemical properties

-phosphorous acid:

-Reductivity: easily oxidized to phosphoric acid (e.g., reaction with halogen, H₂O₂).

-Thermal decomposition: heating isomerizes to phosphoric acid and phosphine (PH₃).

-ortho-phosphoric acid:

-Oxidizing: almost no reducing, can be dehydrated to form pyrophosphate or polyphosphate at high temperature.

-Stability: stable at room temperature, no volatility.

4. Physical properties

-Phosphorous acid: colorless crystal, hygroscopic, soluble in water.

-Phosphoric acid: colorless solid or viscous liquid (commonly sold as 85% aqueous solution), high boiling point.

5. Uses

-phosphorous acid:

-Used as a reducing agent, pesticide intermediate (such as herbicide).

-Used as a stabilizer in plastics industry.

-Ortho-phosphoric acid:

-Preparation of fertilizers (such as ammonium phosphate), food additives (acidifiers).

-Metal surface treatment, pharmaceutical and detergent raw materials.

Attention

The molecular formula of phosphorous acid is often mistakenly written as H₃PO₃, but its true structure has only two acidic H.

-Phosphoric acid is a more common industrial feedstock, while phosphorous acid is more commonly used in specific chemical synthesis.

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