According to the “Hygienic Standard for Drinking Water” (GB5749-2006), the residual chlorine content of the factory water is ≥ 0.3mg/L, and the residual chlorine concentration of the water at the end of the pipe network is ≥0.05mg/L, which is not harmful to the human body.
Although the residual chlorine of the latter still has the ability to disinfect, it is not enough to disinfect recontamination, and can be used as a signal to indicate recontamination. This is especially important when there is a dead end in a long dendrite. In the case of chlorination, the amount of chlorine should generally meet the amount of chlorine required to kill bacteria to achieve the specified disinfection index and oxidize organic matter, and the amount of residual chlorine required to inhibit the reproduction of residual pathogenic bacteria in the water. At the same time, too high dosage is easy to produce carcinogens chloroform, tetrachloroform, etc. Therefore, it is crucial to properly control the amount of chlorination added during the water treatment process.
1 What is residual chlorine
When the available chlorine is in contact with the water for a certain period of time, in addition to the consumption of a part of the chlorine with bacteria, microorganisms, organic matter, inorganic matter, etc. in the water, there is also a part of the chlorine, which is called residual chlorine.
There are three forms of residual chlorine:
1. Total residual chlorine: including free residual chlorine and chemical residual chlorine.
2. Free residual chlorine: including HOCl and OCl-, etc.
3. Chemical residual chlorine: including NH2Cl, NHCl2, NCl3 and other chloramine compounds.
The role of residual chlorine is to ensure continuous sterilization and also to prevent water from being recontaminated.
The main principle of sterilization of free residual chlorine is that the strong oxidizing HClO and ClO- invade the bacteria, reach the inside of the bacteria, and its oxidation destroys the bacterial enzyme system, so as to achieve the purpose of killing bacteria, and the sterilization effect of free residual chlorine is fast, according to the static experimental results, the free residual chlorine disinfection can kill more than 99% of bacteria within 5min; The oxidation ability of the synthetic residual chlorine is weak, and the sterilization speed is not as fast as the free residual chlorine, which can kill 60% of the bacteria within 5min, but the same sterilization effect can be achieved by increasing the concentration of the chemical residual chlorine or prolonging the sterilization time and ensuring a certain CT value. The residence time of the clear water pool of the water plant is generally more than 2 hours, and the sterilization effect can be guaranteed when the chemical residual chlorine is used for disinfection.
However, if the amount of residual chlorine exceeds the standard, it may aggravate the taste and odor of phenol and other organic substances in the water, and may also form chloroform and other organochlorine substances with “three causes”. Determination of residual chlorine content and presence in water is extremely important for drinking water disinfection and water hygiene safety. There are many methods for the determination of residual chlorine.
Residual chlorine in tap water is usually referred to as free residual chlorine.
2 Residual chlorine test method
There are several ways to test residual chlorine:
2.1 Portable DPD residual chlorine tester
DPD (N,N-diethyl p-phenylenediamine) colorimetric method is widely used for the determination of various types of chlorine, including swimming pools, drinking water, and wastewater treatment.
The DPD method is a colorimetric method in which the reaction between free chlorine and DPD reagent produces a colored compound at pH of 6.2~6.5. The intensity of the color is directly proportional to the concentration of chlorine in the water sample. Its concentration was determined spectrophotometrically.
1. Scope of application
(1). This method is suitable for the determination of free residual chlorine, total residual chlorine and chemical residual chlorine of domestic drinking water, source water, wastewater and seawater.
(2). The water sample is colored or turbid, and can be zeroed out to offset its influence.
(3). The maximum detection concentration of this method is 4.5mg/L available chlorine.
2. PrincipleWhen there are no iodide ions in the water sample, the free available chlorine immediately reacts with the DPD reagent to produce red, and the addition of iodine ions plays a catalytic role, so that the chlorine also reacts with the reagent to develop color. The absorbance was measured separately to obtain free chlorine and total chlorine, and total chlorine minus free chlorine to obtain chemical chlorine.
3. Interference effect (1). The presence of alkalinity greater than 250mg/l or 150mg/l acidity in the water, such as CaCO3, etc., will inhibit all color development or the color will fade immediately. Neutralize this sample solution with 1 N of H2SO4 or 1 N of NaOH to pH 6-7. (2). Monochloramine will gradually cause an increase in free chlorine readings. In one minute of reading, every 3.0 mg/l monochloramine will increase the free chlorine reading by 0.1 mg/l. (3). Bromine, iodine, ozone and oxides of manganese and chromium will increase the reading of free chlorine. (4). In order to reduce the effect of Mn4+ and Cr6+, adjust the pH value to 6-7 as described in (1) above. Take 25ml of water sample, add 3 drops of 30g/lKI solution, mix and wait for one minute. Add 3 drops of 5g/l Na2AsO3 to mix (according to the “Water and Wastewater Standard Test Method?”) 15th edition can be used 0.25% thioacetamide solution instead of sodium arsenite, plus 0.5ml of 0.25% thioacetamide per 100ml of water sample). If chromium is present, it reacts with DPD in both assays, reading. This reading is then subtracted from the chlorine reading from the initial analysis.
2.2 Ortho toluidine colorimetric method (OT method)
1.Scope of application
(1). This method is suitable for the determination of total residual chlorine and free residual chlorine of domestic drinking water and its source water.
(2). The minimum detection concentration of this method is 0.01mg/l residual chlorine.
2.Principle
In an acidic solution with a pH value of less than 1.3, the residual chlorine reacts with o-toluidine to form a yellow quinone compound, which is quantified by visual method, and a permanent residual chlorine standard solution prepared by potassium dichromate-potassium chromate solution can also be used for visual colorimetry.
3. Interference affects
The determination of interference in the water containing suspended substances, which can be removed by centrifugation. The maximum allowable content of other interfering substances is as follows: high iron, 0.2mg/l; Tetravalent manganese, 0.01mg/l; Nitrite, 0.2 mg/l.
2.3 On-line electrochemical analysis of residual chlorine meter
2.3.1 Scope of application
This method is suitable for the determination of total residual chlorine, free chlorine, bromine chloride, bromine and iodine in drinking water, wastewater, cooling water and other waters.
2.3.2 Principle of galvanic batteries
In galvanic cells, changes in ion concentration can be detected by measuring the current in them.
The current in the bottle is proportional to the change in the concentration of chloride ions. The cathode in galvanic cells is metallic gold. When hypochlorous acid (or hypochlorous acid ions) is present in the solution, the cathode undergoes a chemical reaction, producing chloride ions. HOCl + 2e- ←→Cl- +OH- anode is metallic copper, and when the electrode reacts, the oxidation products remain on the anode. At this point, an abrasive machine (with the cleaning ball constantly stirring) works together to remove the oxidation products from the metal surface. The current in the galvanic cell is greatly affected by the pH change. At pH 4.0-4.5, the current in the bottle is very stable. Therefore, pH buffer is used to stabilize the current. If CO2 is used as a buffer, the pH value can be adjusted to 5.5-6.0. When KI is added, the total free iodine produced by the reaction is equivalent to the total residual chlorine participating in the reaction, so that the total residual chlorine is determined.
2.3.3 Interference Effects
(1). Changes in temperature and pH will affect the accuracy of the instrument. Therefore, a thermal resistor is used to compensate for the temperature change of the water sample and the pH value is added with a pH buffer to adjust the pH value to overcome the effects of these two factors.
(2). There is no indication that the presence of manganese, chromium, nitrite, etc. will have a disturbing effect.
merit
1. Allows real-time and continuous monitoring of chlorine content.
2. It has high sensitivity and is suitable for low chlorine concentration.
3. The interference of colored substances or turbidity is small.
4. It is suitable for automatic monitoring system in process control.
shortcoming
1. Usually more expensive than DPD colorimetry.
2. The electrode needs to be calibrated and maintained regularly.
3. It is susceptible to electrode fouling or drift over time.
4. Different methods are used for online measurement and laboratory measurement.
5. Ampere measurement provides sufficient accuracy near the set point.
2.4 Fluorescence method
It is an analytical method commonly used to determine the concentration of residual chlorine in water. It is based on the principle of fluorescence analysis, which measures the intensity of the fluorescence signal to determine the residual chlorine content in a water sample. The following are the details of the fluorescence test for residual chlorine:
2.4.1 Principle
In the fluorescence method, a fluorescent reagent is used to react with residual chlorine in a water sample to produce a fluorescent signal. When residual chlorine reacts with fluorescent reagents, it emits a specific fluorescence spectrum. By measuring the intensity of the fluorescent signal, the residual chlorine concentration in the water sample can be calculated.
2.4.2 Steps
Prepare the fluorescent reagent and add it to the water sample to be tested. Excite the fluorescent reagent to make it fluorescent. Measure the intensity of the fluorescence signal using a fluorescence instrument. Based on the standard curve or a standard solution of known concentration, the residual chlorine concentration in the water sample is calculated.
2.4.3 Advantages
High sensitivity: The fluorescence method can also accurately measure residual chlorine at low concentrations. Can be monitored online: suitable for real-time monitoring.
2.4.4 Limitations
Higher equipment costs: Fluorescence instruments require specialized equipment. Requires professional operation and maintenance: The operation of the fluorescence method requires a certain amount of technical knowledge. In conclusion, the fluorescence method is an accurate and sensitive method for the determination of residual chlorine at low concentrations in water. In practice, it is necessary to select the appropriate measurement method for the specific situation
2.5 Ultraviolet absorption method
Ultraviolet absorption (UV-Vis absorption) is an analytical method commonly used to determine the concentration of residual chlorine in water. It is based on the principle of absorption of ultraviolet light, and the residual chlorine content is determined by measuring the degree of absorption of specific wavelengths of ultraviolet light in a water sample. Here are the details of the UV absorption test for residual chlorine:
2.5.1 Principle In the ultraviolet absorption method, ultraviolet light is used to irradiate a water sample, and the residual chlorine absorbs a specific wavelength of ultraviolet light. The chlorine atoms in the residual chlorine molecule absorb ultraviolet light, producing electronic transitions that reduce the intensity of ultraviolet light. By measuring the intensity of the absorbed light, the residual chlorine concentration in the water sample can be calculated.
2.5.2 Steps
Prepare the UV absorption instrument and select the appropriate wavelength. The water sample is placed in an absorbent tank and ultraviolet light is irradiated with the water sample. Measure the intensity of the absorbed light. Based on the standard curve or a standard solution of known concentration, the residual chlorine concentration in the water sample is calculated.
2.5.3 Advantages
High sensitivity: UV absorbance can accurately measure residual chlorine at low concentrations. No reagents required: No reagents need to be added compared to other methods. 2.5.4 Limitations Higher equipment costs: UV absorption instruments require specialized equipment. Affected by other substances in the water sample: Other organic matter and impurities may interfere with the absorption of ultraviolet light. In conclusion, UV absorption is an accurate and sensitive method for the determination of residual chlorine at low concentrations in water. In practice, it is necessary to select the appropriate measurement method for the specific situation.
2.6 Gas chromatography
When it comes to the determination of residual chlorine concentration in water, gas chromatography (GC) is a commonly used analytical method. While gas chromatography is primarily used for the separation and quantification of volatile organic compounds, it can also be used to determine residual chlorine in water under certain conditions. Here are the details about the gas chromatography test for residual chlorine:
2.6.1 Principle
Gas chromatography is based on the partitioning behavior of molecules in the gas phase, by separating and detecting compounds in a sample. In the determination of residual chlorine, the residual chlorine in the water sample is evaporated into the gas phase and then separated by a gas chromatography column. Gas chromatography-mass spectrometry (GC-MS) is commonly used for the detection of residual chlorine to improve accuracy and sensitivity.
2.6.2 Steps
Preparing a water sample often requires pre-treatment, such as extraction or concentration. A gas chromatograph is used to inject the sample into the column and separate out the different compounds. Residual chlorine in the sample is detected by a detector, and a mass detector (MS) is often used to confirm the identity of the compound. 2.6.3 Advantages
High resolution: Gas chromatography can separate complex samples. High sensitivity: GC-MS can detect low concentrations of residual chlorine.
2.6.4 Limitations
Higher equipment costs: Gas chromatographs and mass spectrometers require specialized equipment. Requires professional operation and maintenance: Operators need to have relevant technical knowledge.
