What acid is commonly used to preserve COD and nitrate-nitrite samples?

Sulfuric acid (H2SO4) is commonly used to preserve COD (Chemical Oxygen Demand) and nitrate-nitrite samples because it effectively suppresses microbial activity and prevents the degradation of organic material in the samples. By lowering the pH of the samples, sulfuric acid helps to stabilize the concentration of nitrate and nitrite compounds, ensuring that they remain in their original forms until laboratory analysis can be performed. This ensures that the measurements for parameters like COD, which varies with time due to biological activity, remain accurate and representative of the true conditions of the wastewater being tested.