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Constant current coulometric titration is an analytical method based on electrolysis. Its principle involves adding an appropriate substance to the test solution and electrolyzing it with a constant current of a certain intensity. This electrolysis produces a reagent (called a titrant) at the working electrode (anode or cathode). This reagent reacts quantitatively with the analyte, and the reaction endpoint can be indicated by electrochemical methods. The content of the analyte can be calculated based on the amount of electricity consumed in electrolysis and Faraday's law of electrolysis. The mathematical expression of Faraday's law of electrolysis is:
W—mass of the electrode reactants (8);
I—electrolysis current (A);
t—electrolysis time (s);
96500—Faraday constant (C);
M—molar mass of the electrode reactants (8);
n—number of electrons transferred per gram molecule of reactant.
The working principle of a coulometric COD analyzer is shown in Figure 2-36. It consists of a coulometric titration cell, a circuit system, and an electromagnetic stirrer. The coulometric cell consists of a working electrode pair, an indicator electrode pair, and an electrolyte. The working electrode pair comprises a double platinum plate working cathode and a platinum wire auxiliary anode (placed in a glass tube filled with 3 mol/L H₂SO₄ with a liquid network at the bottom), used for electrolysis to generate the titrant. The indicator electrode (also in a glass tube with a liquid network at the bottom) indicates the coulometric titration endpoint through changes in its potential. The electrolyte is a mixture of 10.2 mol/L sulfuric acid, potassium dichromate, and ferric sulfate. The circuit system consists of an endpoint differential circuit, an electrolytic current conversion circuit, a frequency conversion integral circuit, and a digital display logic operation circuit. It controls the coulometric titration endpoint, converts and displays the electrolytic current, performs frequency conversion and integration of the electrolytic current, and performs logical operations according to the electrolysis laws to directly display the COD value of the water sample. The key points for determining the COD value of water samples using a coulometric COD analyzer are as follows: Add the same amount of potassium dichromate solution to both the blank solution (distilled water with sulfuric acid) and the sample solution (water sample with sulfuric acid), and reflux digest for 15 minutes each. After cooling, add an equal amount of ferric sulfate solution to each solution, and perform coulometric titration under stirring. Specifically, Fe²⁺ is reduced to Fe²⁺ (the titrant) at the working cathode to titrate (reduce) Cr²⁺O₂⁻. The result of the coulometric titration of Cr²⁺O₂⁻ in the blank solution represents the total oxidation amount of potassium dichromate added (as O₂); the result of the coulometric titration of Cr²⁺O₂⁻ in the sample solution represents the remaining oxidation amount of potassium dichromate (as O₂). Let the electrolysis time required for the former be 'o' and for the latter be ''. According to Faraday's law of electrolysis, we can obtain:
Where: 1r—weight of the analyte, i.e., the amount of oxygen equivalent to the amount of potassium dichromate consumed in the water sample;
I＝—electrolysis current;
M—molecular weight of oxygen (32);
n—number of electrons gained or lost by oxygen (4);
96500—Faraday constant.
Let the COD value of the water sample be c5 (mg/L); and the volume of the water sample be v (mL). Then, 1y·c2, substituting into the above formula and rearranging, we get:
This method is simple, rapid, and requires little reagent. It does not require standardization titration solutions and is particularly suitable for the control and analysis of industrial wastewater. When using 3 mL of '0.05 mol/L potassium dichromate solution for calibration, the lowest detectable concentration is 3 mL/L; the upper limit of detection is 100 mL/L. However, only by strictly controlling the digestion conditions to be consistent and paying attention to frequent electrode cleaning to prevent contamination can good reproducibility be obtained.
