The reactivity of phosphoric acid with various alkalis

Phosphoric acid (H₃PO₄) is a triprotic acid, meaning it can donate three protons (H⁺ ions) in aqueous solutions, making it an important chemical in both industrial and laboratory applications. When phosphoric acid reacts with alkalis, it forms salts, which are commonly referred to as phosphates. The reaction of phosphoric acid with alkalis is of significant interest in chemistry, particularly in the fields of fertilizer production, water treatment, and the synthesis of various phosphate compounds. The nature of these reactions depends on several factors, including the concentration of the acid, the type of alkali, and the conditions under which the reaction occurs.

 

1. Basic Chemical Reaction of Phosphoric Acid with Alkalis

Phosphoric acid reacts with alkalis (bases) to form salts and water, a process known as neutralization. The general equation for the neutralization of phosphoric acid with an alkali is:

H 3 PO 4 +Base→Salt+H2 O

However, the reaction is more complex with phosphoric acid due to its triprotic nature. Phosphoric acid can neutralize a base in stages, depending on how many hydrogen ions it donates during the reaction. This results in the formation of different phosphate salts, which are categorized into three main types:

 

Monobasic phosphate (H₂PO₄⁻), formed after one proton is neutralized.

 

Dibasic phosphate (HPO₄²⁻), formed after two protons are neutralized.

 

Tribasic phosphate (PO₄³⁻), formed after all three protons are neutralized.

 

The reactivity and the type of salt produced depend largely on the amount of alkali used and the stoichiometry of the reaction.

 

2. Reaction with Sodium Hydroxide (NaOH)

One of the most common alkalis that phosphoric acid reacts with is sodium hydroxide (NaOH), a strong base. The reactivity of phosphoric acid with sodium hydroxide can be broken down into three stages, corresponding to the neutralization of each of its three acidic protons:

 

First stage (monobasic phosphate formation):

H 3 PO4 +NaOH→NaH2 PO 4+H2 O

In this stage, one molecule of sodium hydroxide neutralizes one of the three acidic protons of phosphoric acid, forming sodium dihydrogen phosphate (NaH₂PO₄) and water.

 

Second stage (dibasic phosphate formation):

NaH 2 PO 4 +NaOH→Na 2 HPO4 +H 2 O

A second molecule of sodium hydroxide neutralizes another proton of phosphoric acid, forming disodium hydrogen phosphate (Na₂HPO₄).

 

Third stage (tribasic phosphate formation):

Na 2 HPO 4 +NaOH→Na3 PO4 +H2 O

The final stage occurs when another molecule of sodium hydroxide neutralizes the remaining proton of phosphoric acid, resulting in the formation of trisodium phosphate (Na₃PO₄).

 

Each of these reactions produces a different phosphate salt, depending on the amount of sodium hydroxide present. The pH of the solution and the amount of base used will dictate whether the product is a monobasic, dibasic, or tribasic phosphate.

 

3. Reaction with Potassium Hydroxide (KOH)

Potassium hydroxide (KOH), like sodium hydroxide, is a strong base and reacts with phosphoric acid in a similar manner. The reaction stages are analogous to those of sodium hydroxide, but the product is potassium salts rather than sodium salts.

 

First stage:

H 3 PO 4 +KOH→KH 2 PO 4 +H2 O

The reaction produces potassium dihydrogen phosphate (KH₂PO₄).

 

Second stage:

KH 2 PO 4 +KOH→K 2 HPO 4 +H2O

The result is potassium hydrogen phosphate (K₂HPO₄).

 

Third stage:

K2 HPO 4 +KOH→K 3PO 4 +H2O

The final product is potassium phosphate (K₃PO₄), a tribasic phosphate.

 

The key difference between the sodium and potassium salts of phosphoric acid lies in their solubility and their specific applications in various industries. Potassium phosphates, for example, are often used in agriculture to provide both potassium and phosphorus to plants, whereas sodium phosphates are more commonly used in detergents and water treatment.

 

4. Reaction with Calcium Hydroxide (Ca(OH)₂)

When phosphoric acid reacts with calcium hydroxide (lime), it forms calcium phosphates, which are of great importance in the production of fertilizers. The reaction proceeds in two main stages:

 

First stage:

2H 3 PO 4 +3Ca(OH) 2 →Ca3 (PO4 )2 +6H2O

In this reaction, phosphoric acid reacts with calcium hydroxide to form calcium phosphate (Ca₃(PO₄)₂), which is insoluble in water.

 

Second stage: Depending on the amount of calcium hydroxide used, the product may further react with excess lime to form other calcium phosphate compounds, such as dicalcium phosphate (CaHPO₄) or tricalcium phosphate.

 

This reaction is fundamental in the production of superphosphate fertilizers, where phosphoric acid is reacted with phosphate rock or lime to produce soluble phosphates that can be absorbed by plants.

 

5. Reaction with Ammonia (NH₃)

Ammonia, a weak base, reacts with phosphoric acid to form ammonium phosphates, which are key ingredients in fertilizers. The primary reactions are as follows:

 

Formation of monoammonium phosphate (MAP):

4H3PO4+NH3→NH4H2PO4

 

Monoammonium phosphate (MAP) is a highly soluble fertilizer that provides both nitrogen and phosphorus.

 

Formation of diammonium phosphate (DAP):

4H3 PO 4 +2NH3 →(NH4 )2 HPO 4

Diammonium phosphate (DAP) is another widely used fertilizer, containing a higher concentration of phosphorus and nitrogen than MAP.

 

These reactions highlight the importance of phosphoric acid in the production of phosphate-based fertilizers, which are essential for modern agriculture.

 

6. Conclusion

Phosphoric acid’s reactivity with alkalis is fundamental to the production of a wide variety of important phosphate compounds. The neutralization of phosphoric acid with different alkalis results in the formation of a range of phosphate salts, each with unique properties and applications. The type of alkali used, the concentration of acid, and the conditions under which the reaction takes place all influence the nature of the products. Whether it is used in agriculture for fertilizer production or in industrial processes, understanding the reactivity of phosphoric acid with alkalis is crucial for optimizing these processes and ensuring the efficient use of phosphorus resources.