An arbitrary standard scale is used as a means for matching of colour intensity.  A colour of 5 units means that the intensity of the colour of this sample is equal to the intensity of the colour of a sample of sulphuric acid 50% in water, containing 5 milligrams of platinum (as potassium chloroplatinate) per liter.

Treatment

Blending of acid is of limited effectiveness in improving the colour of the acid due to the large amount of colourless acid required to achieve an acceptable product quality.

Hydrogen peroxide (H₂O₂) is effective in removing colour from the acid and the rate at which hydrogen peroxide clears up the acid varies with the acid temperature, the quantity of SO₂ dissolved in the acid and the strength of H₂O₂.

Bench scale test should be performed to determine the amount of hydrogen peroxide to be added, the temperature and residence required to produce a product acid of acceptable quality.  

Based on the results of the testing, bleaching system can be designed based on conservative design parameters.

One acid producer found that to reduce the product acid colour to acceptable levels required between 0.5 to 1.4 kg of 50% H₂O₂ per metric tonne of 98% H₂SO₄ at an operating temperature of 80^ºC.  The residence time required for the reaction to take place was 2 hours.  A conservative design of a bleaching system based on these test results would be for double the dosing rate and a residence time of 4 to 5 hours.