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In a strongly acidic solution, potassium dichromate is used to oxidize reducing substances in the water sample. Excess potassium dichromate is used as an indicator, and the solution is back-titrated with ferrous ammonium sulfate standard solution. The amount of oxygen consumed by the reducing substances in the water sample is calculated based on the amount of potassium dichromate used. The reaction equation is as follows:
The determination process is shown in Figure 2-35.
20 mL of water sample (original or diluted) in an Erlenmeyer flask
↓← H8SO₄·0.48 (to eliminate interference)
Mix well
← 0.25 mol/L (1/6 K₂Cr₂O₄) 100 mL
↓← A few zeolite particles
Mix well, connect to reflux apparatus
↓← Add 30 mL of A82SO₄-H₂SO₄ solution (catalyst) through the top of the condenser
Mix well
↓
Reflux and heat for 2 h
↓
Cool
↓← Add 80 mL of water to the reaction solution through the top of the condenser
Remove the Erlenmeyer flask
↓← Add 3 μL of ferrin indicator
Titrate with 0.1 mol/L Fe(SO₄)₂ standard solution; the endpoint changes from blue-green to reddish-brown.
Figure 2-35 CoDcr determination process
Potassium dichromate is a strong oxidizing agent, capable of oxidizing most organic compounds, but pyridine is not oxidized, and aromatic organic compounds are not easily oxidized; volatile straight-chain aliphatic compounds, benzene, etc., exist in the vapor phase; it cannot come into contact with liquid oxidizing agents, and oxidation is not significant. Chloride ions can be oxidized by potassium dichromate and react with silver sulfate to form a precipitate; a suitable amount of mercuric sulfate can be added to conjugate it.
The determination result is calculated according to the following formula:
Where: V<sub>o</sub> — Volume of ferrous ammonium sulfate standard solution consumed in the titration of blank (mL)
V<sub>l</sub> — Volume of ferrous ammonium sulfate standard solution consumed in the titration of water sample (mL);
V — Volume of water sample (mL);
c — Concentration of ferrous ammonium sulfate standard solution (mol)
g — Molar mass of oxygen (1/20) (g/mol). A 0.25 mol potassium dichromate solution can be used to determine COD values greater than 50 mg/L; a 0.025 mol potassium dichromate solution can be used to determine COD values of 5-50 mg/L, but the accuracy is poor.
