The following two reactions show the dibasic nature of sulphuric acid.

Sulphuric Acid, H2SO4, is a dibasic acid in nature  as it contains 2 hydrogen atoms which ionise in aqueous solution to become Hydrogen Ions, H(+).
H2SO4 ↔ H+ + HSO4-HSO4- ↔ H+ + SO4

2-The reaction for the overall dissociation can be represented as
H2SO4 ↔ 2H+ + SO4 2- H2SO4 reacts with Na2CO3 to form sodium sulphate, water with the emission of CO2 gas.

The two acidic hydrogen ions are replaced by two sodium ions. Similar reaction occurs with potassium carbonate.  Na2CO3 + H2SO4 -------->  Na2SO4  +   H2O + CO2 

Sodium  sulphuric  Sodium sulphate  water  carbondioxide  carbonate  acid 
K2CO3 + H2SO4 -------->  K2SO4  +  H2O + CO2 
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