Methods for Analyzing Organic Matter in Water Bodies - Permanganate Index

The chemical oxygen demand (COD) measured using potassium permanganate solution as an oxidant was previously known as the manganese method COD. In my country's new environmental water quality standards, this value has been renamed the permanganate index, while only the value measured by the acidic potassium dichromate method is referred to as COD. The International Organization for Standardization (ISO) recommends that the potassium permanganate method be limited to surface water, drinking water, and domestic sewage.

Depending on the medium of the measuring solution, it is divided into the acidic potassium permanganate method and the alkaline potassium permanganate method. Because the oxidizing power of potassium permanganate is slightly weaker under alkaline conditions than under acidic conditions, it cannot oxidize chloride ions in water under alkaline conditions, and therefore is often used to measure water samples with high chloride ion concentrations.

The acidic potassium permanganate method is suitable for water samples with a chloride ion content not exceeding 300 mg/L. When the permanganate index exceeds 5 mg/L, a smaller water sample should be taken and diluted before measurement. The measurement process is shown in Figure 2-37. Take 100 mL of water sample (original or diluted) into an Erlenmeyer flask

↓←(1+3)H₂SO₄‘5mL ‘

Mix well

↓←o. olmoI 10. omL of potassium permanganate standard solution (+KMnO₄)

Boil in a water bath for 30 min

↓←o. olo omt 10. omL of sodium oxalate standard solution (Nasc2O‘)

Decolorize ‘

↓←o. 01 om01 omTick back with potassium pergerate standard solution

Endpoint: slightly red

Figure 2-37 Determination process of pergerate index

The determination result is calculated according to the following formula:

1. 1. Water Sample Without Dilution

Permanganate Index

Where: Vl—volume of potassium permanganate standard solution consumed in titrating the water sample (mL);

K—correction coefficient (the number of milliliters of sodium oxalate standard solution equivalent to each milliliter of potassium permanganate standard solution);

M—concentration of sodium oxalate standard solution (1/2Na2C2Od) (ntO1);

g—molar mass of oxygen (1/20) (g/mol);

100—volume of water sample taken (mL).

2. Water Sample With Dilution

Permanganate Index

Where: 2V₀—volume of potassium permanganate standard solution consumed in the blank test (mL);

V_z—volume of water sample taken (mL);

f—ratio of dilution water in the diluted water sample (e.g., 10.0 mL water sample diluted to 100 mL, Ng/ = 0.90)

Other items are the same as the calculation formula for water sample without dilution. Chemical oxygen demand (CODcr) and permanganate index are measured using different oxidants under their respective oxidation conditions, making it difficult to find a clear correlation. Generally speaking, the oxidation rate of the potassium dichromate method can reach 90%, while that of the potassium permanganate method is around 50%. Neither method achieves complete oxidation, and therefore both are only relative reference data.