Chlorides and residual chlorine: chlorides are found in almost all water and waste water. The content of chloride in fresh water is low, which is about mg / L, and the source is mainly brought in when the water flows through the stratum containing chloride; The content of chloride in seawater, salt lake and some groundwater can reach to ten grams / liter; There are also a lot of chloride ions in domestic and industrial wastewater.
Chlorination is a reliable method to disinfect drinking water, kill pathogenic bacteria in water and prevent the spread of water medium diseases. Because of its strong ability of sterilization and algae killing, convenient operation and low price, it is still the main disinfection method of drinking water in the world. In order to ensure the sterilization effect of the water supply network terminal, a certain amount of residual chlorine must be used in the pipe network, but the high concentration of chloride will also damage the health of human body. The drinking water sanitation standard gb5749-85 stipulates that: after 30 minutes of contact with water, it shall not be less than 0.3mg/l, and the end water of the pipe network shall not be less than 0.05mg/l. Meanwhile, chlorination of drinking water will produce trihalomethane, haloacetic acid and other disinfection by-products, which leads to the risk of three causes (carcinogenesis, teratogenesis and mutation) in drinking water.
The determination of disinfection by-products such as haloalkanes will be introduced in the section of the determination of organic compounds, and the determination of residual chlorine in water will be emphasized. Residual chlorine refers to the amount of chlorine left in water after being sterilized with chlorine for a certain time. The chlorine added less the residual chlorine is the chlorine demand of the water. The role of residual chlorine in drinking water is to characterize the disinfection effect and prevent the drinking water from being polluted again. There are three forms of residual chlorine: total residual chlorine, chemical residual chlorine and free residual chlorine. The determination methods of residual chlorine are as follows: the iodine method is used to determine the total residual chlorine and free residual chlorine (gb11898-89) by the total residual chlorine (gbj48-83), N, n-diethyl-p-phenylenediamine (DPD) - ammonium ferrous sulfate titration (gb11897-89).
(1) Iodimetry
Under the acidic condition, the residual chlorine and potassium iodide release the iodine from the single substance, making the water brown yellow, and using standard sodium thiosulfate
Titrate the solution to light yellow, add starch indicator and continue titration until blue disappears. The reaction formula is as follows
The content of residual chlorine is calculated according to the amount of sodium thiosulfate consumed, and the calculation method is as follows:
Total residual chlorine (Cl2, mg/l) =
Where: C -- standard solution concentration, mol/l;
——Volume of consumption standard solution, ml;
V - water sample volume, ml;
35.45 - molar mass of chlorine, g/mol
This method is suitable for the determination of water samples with total residual chlorine (calculated by Cl2, mg/l) content greater than 1mg/l.
（2） N, n-diethyl-p-phenylenediamine (DPD) - ammonium ferrous sulfate titration
When iodine free ions exist in water, the free residual chlorine reacts with DPD to form red compounds,
Titrate with ammonium ferrous sulfate solution until red disappears, and record the titration volume as V1 (ML). The content of free chlorine is:
Where: C -- concentration of free chlorine, mg / L;
C1 - concentration of standard solution of ammonium ferrous sulfate, mg/l;
V0 - volume of water sample, ml.
If 1g of potassium iodide is added after DPD is added, at this time, both NH2Cl, nhcl2 and NCl3 react with DPD to form red compounds. The results are total chlorine content at the same time as the determination of free chlorine. The results can be calculated by the following formula:
Where: C2 - volume of standard solution of ammonium ferrous sulfate consumed by titration after iodization, ml.
If the conical bottle containing buffer solution and DPD test solution is added to the sample, a small amount of potassium iodide is added, the reaction will be limited to the free chlorine and the chloroamine in the chlorine; Before the sample is added with the buffer value and DPD, a small amount of potassium iodide is added. At this time, the reaction occurs between the free chlorine, the one chloroamine and the nitrogen trichloride in the chlorine; Dichloroamine does not react in both cases. Therefore, the concentrations of monochloramine, dichloroamine and nitrogen trichloride in the chemical chlorine can be calculated according to different reaction conditions.
This method is suitable for the determination of free chlorine or total chlorine (calculated by Cl2) from 0.0004-0.07mmol / L (0.03-5mg / L). This method is not available for determination when the following oxides are present in water samples: bromine, iodine, bromine, iodamine, ozone, hydrogen peroxide, chromate, manganese oxide, nitrite, copper ion and iron ion. The interference of copper ion < 8mg / L and Fe < 20mg / l can be masked by EDTA disodium.
（3） N, n-ethyl-p-phenylenediamine (DPD) colorimetry
The principle of this method is basically the same as method 2, but it is not quantitative by titration with ammonium ferrous sulfate solution, but the red compound formed by DPD and chlorine is compared in spectrophotometer. The determination wavelength is 515nm, and quantitative analysis is carried out according to the standard curve.
This method is suitable for the determination of free chlorine or total chlorine (calculated by Cl2) from 0.0004-0.07mmol / L (0.03-5mg / L). This method is applicable in the absence of the following oxidants: bromine, iodine, bromine, iodamine, ozone, hydrogen peroxide, chromate, manganese oxide, nitrite, copper ion and iron ion. The interference of copper ion < 8mg / L and Fe < 20mg / l can be masked by EDTA disodium.
