A reliable mercury free chemical oxygen demand (COD) method

Abstract

A simple, but reliable mercury free method to analyse chemical oxygen demand is introduced. The method is based on international standard procedures but avoids the highly toxic mercury. In the procedure the oxidation temperature is brought down from 148 °C to 120 °C. At this temperature the chloride interference is reduced to a large extent, whereas the oxidation of most compounds is still complete. Chloride interference is further reduced by addition of silver ions and practically absent at a molar ratio Ag⁺/Cl⁻ over 1.7. Compounds that are difficult to oxidise even at 148 °C or only in the presence of silver ions are investigated at 120 °C. A slight reduction in recovery of 20–35% is observed for compounds like acetic acid, ethanol or lauryl sulfate in the absence of chloride. In the presence of 3000 mg L⁻¹chloride only the reduction for acetic acid is more pronounced due to the low availability of silver ions. The other compounds studied showed no further decrease in recovery. The interference of bromide is about 50% less at 120 °C than at 148 °C. The interference of ammonium in the presence of chloride is not confirmed in this method. The recovery of the method at high chloride and low COD concentration, i.e., at 3000 mg Cl⁻ L⁻¹ and 25 mg COD L⁻¹, is acceptable (122%) whereas at 2000 mg Cl⁻ L⁻¹ and 25 mg COD L⁻¹ it is better (110%). Precision is good; Relative standard deviations are 5.6% respectively 2.6%. The results of 99 wastewater samples over a wide range of chloride concentrations are similar compared to analyses based on ISO 15705 (cuvette) or ISO 6060. With the addition of 20 mL silver sulfate–sulfuric acid solution, the chloride in samples up to 3000 mg L⁻¹chloride is largely precipitated. More volume silver sulfate–sulfuric acid solution or a higher concentration of silver sulfate increases the range of samples that can effectively be analysed. The believed favourable effect of addition of Cr³⁺ to samples has not been confirmed. In the Netherlands 95% of about 100 000 wastewater samples that are analysed for COD each year have chloride concentrations below 3000 mg L⁻¹ and could be analysed without the use of mercury. This may save as much as 10 kg mercury per annum. It will however require the additional use of about 0.5 kg of silver.