Standard Solutions for COD Determination in Water Quality Testing

Chemical Oxygen Demand (COD) is one of the core indicators for assessing organic pollution in water bodies. It is defined as the amount of oxidant consumed when a water sample is treated with potassium dichromate as the oxidizing agent under strong acid and heating conditions, expressed as the mass concentration of oxygen (mg/L). 

The accuracy of COD determination directly affects the reliability of water quality assessment and environmental management. In this context, standard solutions serve as critical reference materials that play an irreplaceable role in the quality assurance system.

Definition and Nature of Standard Solutions

A standard solution for COD determination—also referred to as a reference material—is a solution prepared by simulating the matrix of natural fresh water, using specialized processes under prescribed environmental conditions, with a known chemical oxygen demand concentration. The core characteristics of a standard solution are the certainty and traceability of its concentration value.

Based on their intended use, standard solutions are categorized into two types: stock standard solutions and working standard solutions. Stock standard solutions are prepared at relatively high concentrations and are typically preserved as mother liquors for long-term storage; working standard solutions are obtained by stepwise dilution of the stock solution and are used for routine instrument calibration and quality control.

Primary Reference Material: Potassium Hydrogen Phthalate

The most widely adopted reference material for COD determination is potassium hydrogen phthalate (KHP, C₈H₅KO₄). This substance is favored for its high purity, stable properties, and theoretically calculable COD value, and it is universally prescribed by both domestic and international standard methods.

According to the national standard Water Quality—Determination of Chemical Oxygen Demand—Dichromate Method (HJ 828-2017), the preparation procedure for KHP standard solution is as follows: accurately weigh 0.4251 g of primary standard or guaranteed-grade potassium hydrogen phthalate that has been dried at 105°C for 2 hours and cooled to room temperature。

dissolve it in a small amount of distilled water; transfer the solution to a 1000 mL volumetric flask; dilute to the mark with water and mix thoroughly. The resulting solution has a theoretical COD value of 500 mg/L. If the same 0.4251 g of KHP is dissolved in a 500 mL volumetric flask, a standard solution with a COD value of 1000 mg/L is obtained. The theoretical COD equivalent of potassium hydrogen phthalate is 1.167 g of oxygen per gram of KHP.