Chlorine dioxide was first produced in 1814 by reacting sulphuric acid with potassium chlorate.
Today, most methods involve using either a chlorate (ClO3-), or chlorite (ClO2-) salt, with an acid,
sometimes in the presence of a Chlorine Donor.
Methods for chlorine dioxide production:

5NaClO2 + 4HCl → 4ClO2 + 5NaCl + 2H2O
2NaClO2 + HOCl → 2ClO2 + 5NaCl + NaOH
2NaClO3 + H2O2 + H2SO4 → 2ClO2 + 2Na2SO4
2NaClO2 + Cl2 → 2ClO2 + 2NaCl
5NaClO2 + 4NaHSO4 → 4ClO2 + NaCl + 4Na2SO4 + 2H2O
2NaClO2 + Na2SO8 → 2ClO2 + 2Na2SO4