Table 3: Optimisation of the mass of applied Na2S2O8 required for decolourisation.

Test solution no. Na2S2O8 applied (g) pH (before heating) Observation
of treated water 		Comments
1 10.01(3) 9.4 Clear and colourless solution.
No sludge formation.		 Oxidant applied was in large excess.
2* 5.00(2) 10.2 Clear and colourless solution.
No sludge formation.		 Oxidant applied was in excess of about 25%.
3 4.01(6) 10.9 Clear and colourless solution.
No sludge formation.		 “Optimal” concentration was established with this data point.
4 3.04(4) 11.6 Transparent, very pale orange.
No sludge formation.		 Inadequate oxidant.
5 2.01(0) 12.1 Transparent and bright orange.
No sludge formation.		 Inadequate oxidant.
6 1.03(7) 12.3 Transparent and bright orange.
No sludge formation.		 Inadequate oxidant.

Note:*The 25% excess was calculated by (5.002 – 4.016)/4.016 = 24.5% ≈ 25%.