Determination of Chemical Oxygen Demand (COD) in Water - Potassium Dichromate Method

Chemical Oxygen Demand (COD) is one of the comprehensive indicators of organic pollution in water.

The COD in water is determined by the reflux method.

Experimental Principle

Under strong acidic conditions, an excess of K₂Cr₂O₇ standard solution is used to react with organic matter and inorganic reducing substances in the water. Then, using ferrophenanthroline (i.e., ferrous anhydride) as an indicator, the remaining K₂Cr₂O₇ solution is titrated back with (NH₄)₂Fe₂(SO₄)₂ standard solution. The titration endpoint is when the solution changes from light blue to red. The COD_cr (mg O₂/L) can be calculated based on the amount of (NH₄)₂Fe₂(SO₄)₂ standard solution used. C represents the organic matter and reducing inorganic substances in the water. The reaction formula is as follows:

3C + 2Cr₂O₇^2- + 16H⁺ = 2CO₂ + 4Cr³⁺ + 7H₂O

6Fe²⁺ + Cr₂O₇^2- + 14H⁺ = 6Fe³⁺ + 2Cr³⁺ + H₂O

Fe(C₁₂H₈N₂)₄³⁺ (light blue) → Fe(C₁₂H₈N₂)₃²⁺ (red)

Instruments and Reagents

1. Instruments

(1) Reflux apparatus. 250 mL or 500 mL ground-mouth triangular reflux condenser, electric furnace, and several glass beads (Figure 1).

(2) 50 mL acid burette.

2. Reagents

(1) Potassium dichromate standard solution (1/6 K₂Cr₂O₇ = 0.2500mol/L): Weigh 12.2579g of high-grade or analytically pure K₂Cr₂O₇ (dried at 120℃ for 2h, weighed after cooling in a desiccator) and dissolve it in water. Transfer it to a 1000mL volumetric flask and dilute it to the mark with distilled water. Shake well.

(2) Ammonium ferrous sulfate standard solution (NH₄Fe₂(SO₄)₂)·6H₂O x 0.25mol/L). Weigh 98.0g of analytically pure ammonium ferrous sulfate and dissolve it in distilled water. Slowly add 20mL of concentrated H₂SO₄ while stirring. After cooling, dilute it to 1L with distilled water and shake well. The concentration of this solution is about 0.25mol/L. Calibrate its accurate concentration before use.

(3) Test ferroin indicator: Weigh 1.485g of 1,2-diazophenanthroline or 1,2-diazophenanthroline (C₁₂H₈N₂·H₂O) and 0.695g of FeSO₄·7H₂O, dissolve in distilled water, dilute to 100mL, and store in a brown reagent bottle.

Figure 1 Reflux determination of COD by the K₂Cr₂O₇ method

(4) Ag₂SO₄-H₂SO₄ solution: Weigh 13.33 g of Ag₂SO₄ and add it to 1 L of concentrated H₂SO₄ (75 mL of this solution contains 1 g). Let it stand for 1-2 days, shaking occasionally to dissolve it.

(5) HgSO₄ crystals or powder.

(6) Organic-free distilled water. This can be obtained by further distilling distilled water containing a small amount of alkaline solution containing KMnO₄ (the water should remain red during the distillation process; otherwise, KMnO₄ should be added at any time).

Experimental Details

1. Standardization of Ammonium Ferrous Sulfate Solution

Accurately pipette 25.00 mL of 0.2500 mol/L potassium dichromate solution (1/6 K₂Cr₂O₇) into a 500 mL Erlenmeyer flask. Add distilled water to approximately 250 mL. Slowly add 20 mL of concentrated H₂SO₄ and mix thoroughly. After cooling, add 2 drops of ferrocyanine indicator (approximately 0.10 mL). Titrate with ammonium ferrous sulfate solution until the solution changes from orange-yellow to blue-green, then immediately turns brown-red. Record the amount of ammonium ferrous sulfate solution used (V, mL). Perform three replicates.

Calculation:

Where C_{(NH4)2Fe(SO4)2} is the concentration of the ammonium ferrous sulfate standard solution, in mol/L;

V_Standard is the volume of ammonium ferrous sulfate solution used during calibration, in mL.

2. Water Sample Determination - Reflux Method

(1) Take 50.00 ml of uniform water sample. Or take an appropriate amount of water sample and dilute it to 50.00 ml with distilled water, where the COD value is 50-900 mg O₂/L, and place it in a 500 ml ground-mouth reflux conical flask.

(2) Add a few glass beads and 1g HgSO₄, slowly add 5.0ml Ag₂SO₄-H₂SO₄ solution, and shake to mix until the Ag₂SO₄ dissolves.

(3) Accurately add 25.00 mL of K₂Cr₂O₇ standard solution (1/6 K₂Cr₂O₇ = 0.2500 mol/L), connect a ground-joint reflux condenser, and slowly add 70 mL of Ag₂SO₄-H₂SO₄ solution from the open end of the condenser. Heat and reflux for 2 h.

(4) After cooling, rinse the condenser wall with 25 mL of distilled water, remove the conical flask, and then dilute to 350 mL with distilled water (the total volume of the solution must not be less than 350 mL, otherwise the pH value will be too low and the end point will not be obvious).

(5) Add 2 drops of ferrous sulfate indicator and titrate with ammonium ferrous sulfate standard solution until the solution changes from yellow to blue-green and then immediately turns brown-red. This is the end point. Record the amount of (NH₄)₂Fe₂(SO₄)₂ standard solution used (V₁, mL). Make 2 parallel samples. (6) Simultaneously, use 50.00 mL of distilled water as a blank. The operation steps are the same as for the water sample. Record the amount of (NH₄)₂Fe₂(SO₄)₂ standard solution consumed (V₀, mL).

(7) Calculate:

• V₁ is the volume of (NH₄)₂Fe₂(SO₄)₂ standard solution consumed in the titration of the water sample, mL;
• V₀ is the volume of (NH₄)₂Fe₂(SO₄)₂ standard solution consumed in the blank test, mL;
• c is the concentration of the ammonium ferrous sulfate standard solution (NH₄)₂Fe₂(SO₄)₂, mol/L;
• 8 is the molar mass of oxygen, 1/2O, g/mol;
• V_Water is the volume of the water sample, mL.

If the CDD value of the water sample is <50 mg O₂/L, follow the above procedure, except for the use of potassium dichromate standard solution (1/6 K₂Cr₂O₇ = 0.0250 mol/L) for digestion and back titration with 0.0250 mol/L (NH₄)₂Fe₂(SO₄)₂ solution.

3. Experimental Results

Table 1 Chemical Oxygen Demand Results