There are many detection methods for total chlorine and free chlorine in water, commonly used are spectrophotometry, titration and electrode method. Among them, the operation steps of spectrometry and titration have been introduced to you. Today we will talk about the steps of electrode method to determine total chlorine and free chlorine in water quality. Detection principle The working electrode set contains two working electrodes, which are used to detect free chlorine and total chlorine respectively. The free chlorine electrode is coated with tetramethylbenzidine. After a fixed voltage is applied, the free chlorine undergoes a reduction reaction on the surface of the working electrode to obtain electrons and generate current; the total chlorine working electrode is coated with potassium iodide to promote the reduction reaction of combined chlorine. Gain electrons and generate an electric current. When measuring, insert the working electrode group into the water sample to be tested, apply a fixed working voltage to the working electrode through the voltage stabilizer, and record the resulting current-power-on time relationship in real time. The intensity of the generated current is proportional to the concentration of free chlorine and total chlorine in the water sample. Chlorine dioxide, a common interference in the detection process, will undergo a reduction reaction similar to chlorine on the surface of the working electrode, which will interfere with the detection of chlorine. However, chlorine dioxide itself is volatile and can be removed by blowing air into the water for 5 minutes. The acceptable pH range for water sample detection is 4-9, and the detection will be affected if the pH value is too high or too low. Reagents and equipment used in the test 1. Sodium hydroxide solution: 2.0mol/L Weigh 80g of sodium hydroxide, dissolve in 500ml of laboratory grade 1 pure water, transfer the solution to a 1000ml volumetric flask after cooling, add pure water to the mark, mix uniform. 2. Laboratory primary pure water 3. Portable electrochemical residual chlorine analyzer
includes free chlorine working electrode, total chlorine working electrode, reference electrode and counter electrode, etc. The water quality testing instrument should meet the following working conditions: ambient temperature, 0°C-50°C; ambient humidity, ≤90%. The instrument should have temperature compensation function. Accuracy performance: free chlorine: ≤±0.05mg/L, 10±0.05mg/L (20°C); ≤±2mg/L, 20±2mg/L (10°C). Total chlorine: ≤±0.5mg/L, 10±0.5mg/L L (20°C); ≤±10mg/L, 200±10mg/L (10°C). Precautions for water sample collection Free chlorine and total chlorine are unstable, and water samples should be measured on site as much as possible. If the water sample cannot be measured on site, add the 1% NaOH solution of the sampling volume into the brown glass bottle in advance, collect the water sample to fill the sampling bottle, and seal it immediately to prevent the water sample from being lost and exposed to the air. If the water sample is acidic, the amount of NaOH solution should be increased to ensure the pH of the water sample>12. In addition, the turbidity and color of the water sample have no effect on the measurement and can be directly measured. The temperature of the water sample should be between 2°C and 35°C. Water samples were stored at 4°C protected from light and measured within 5 days. Test steps Before the test, connect the working electrode group and the measuring instrument correctly, and check the performance of the instrument. Determination of the blank water sample, the measured value should be less than the detection limit of the method. Adjust the pH value of the water sample to 4-9, rinse the measuring cup with the water sample, and then measure the water sample to the mark. Select free chlorine and total chlorine measurement items on the instrument, insert the working electrode group of the water quality detector into the water sample, and read after it is stable. After the measurement is completed, discard the water sample and clean the measuring cup, turn off the power, and end the measurement. The concentration of free chlorine and total chlorine can be read directly from the display of the instrument (calculated as Cl2). If the water sample is diluted before determination, the concentration of free chlorine and total chlorine should be directly read out and multiplied by the dilution factor. The concentration of combined chlorine in the water sample can be calculated according to the formula C3=C2 (total chlorine concentration)-C1 (free chlorine).
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