Acid, Base & Salt

pH of Salts

Salts are formed by the neutralization reaction between acid and base consequently pH of salts should be equal to 7, i.e. salts should be neutral.

But, actually most of the salts have pH value less than or more than 7. pH of salts may be equal to 7 or more than seven or less than seven. This means salts may be neutral, acidic or basic.

Salts which are formed by the reaction of same strength of acid and base are neutral in nature and have pH value equal to 7. Such salts are called neutral salts.

Neutral salts i.e. salts having pH value equal to 7:

Salts produced after the reaction of a strong acid with a strong base, is neutral in character. For example: Sodium chloride, potassium chloride, sodium sulpahte, potassium sulphate, etc.

Examples of formation of neutral salt with pH value equal to 7

When sodium hydroxide reacts with hydrochloric acid, sodium chloride is formed. Since, sodium hydroxide is a strong base and hydrochloric acid is a strong acid, thus sodium chloride formed in this reaction is neutral in character. pH value of sodium chloride (common salt) is about equal to 7.

Similarly, sodium sulphate (a salt) formed by the reaction between sodium hydroxide and sulphuric acid, is neutral in chatater. Because sodium hydroxide is a strong base and sulphuric acid is a strong acid. pH value of sodium sulphate is nearly equal to 7, i.e. sodium sulphate is neutral in character.

Acidic salt i.e. salts having pH value less than 7:

(a) When a strong acid reacts with a weak or mild base, an acidic salt is formed. This happens because weak base could not neutralise the strong acid fully i.e. only partial neutralization takes palce. For example ammonium sulphate, ammonium chloride, etc.

When ammonium hydroxide reacts with hydrochloric acid, ammonium chloride is formed along with water.

In this reaction, ammonium hydroxide, which is a weaker base cannot able to neutralize hydrochloric acid, which is a strong acid fully. And some acidic character remains present in salt so formed. Thus, ammonium chloride is acidic in nature, and its pH value is less than 7.

(b) When ammonium hydroxide reacts with sulphuric acid, ammonium sulphate ((NH₄)₂SO₄) is formed.

In this reaction ammonium hydroxide is weaker base than sulphuric acid resulting ammonium hydroxide could not neutralizes sulphuric acid completely. Thus, salt ammonium hydroxide formed in this reaction is acidic in nature and its pH value is less than 7.

Sodium bicarbonate (NaHCO₃), Sodium hydrosulphide or sodium hydrogen sulphide (NaHS), Sodium bi sulphate (NaHSO₄), monosodium phosphate(NaH₂PO₄), disodium phosphate, etc. are examples of some acidic salts.

Basic salt i.e. salts having pH value more than 7:

Basic salts are formed when a strong base reacts with weaker acid. pH value of basic salt is more than 7. This happens because weaker acid could not neutralize strong base completely. And because of partial neutralization of base a basic salt is formed. For example: sodium carbonate (Na₂CO₃), calcium carbonate (CaCO₃), magnesium carbonate (MgCO₃), potassium carbonate (K₂(CO₃)₂), sodium acetate (NaCH₃COO), calcium acetate (Ca(CH₃COO)₂), etc.

Examples of formation of basic salts:

(a) When Carbonic acid (H₂CO₃) reacts with sodium hydroxide (NaOH), sodium carbonate is formed.

Since, carbonic acid is a weaker acid than sodium hydroxide (a base), thus sodium carbonate (salt) so formed in this reaction, is a basic salt. And pH value of sodium carbonate is greater than 7.

(b) When acetic acid reacts with sodium hydroxide, sodium acetate is formed.

Since, sodium hydroxide is stronger base than acetic acid, thus salt (sodium acetate) formed in this neutralization reaction is basic in nature and its pH value is more than 7. This happens because acetic acid does not neutralize sodium hydroxide completely.