1. What is chloride ion?
In a chemical reaction, chlorine atoms gain electrons, so that the chlorine atoms participating in the reaction are charged. The charged chlorine atoms are called chloride ions. It is the most common anion in water. Excessively high chloride ion content will cause bitter and salty drinking water, soil salinization, pipeline corrosion, plant growth difficulties, and endanger human health, so the accuracy of the chloride ion content measurement is very important.
2. Source of chloride ion?
The sources of chloride in surface water are mainly divided into natural sources and man-made sources.
Natural sources: First, the water flows through the chloride-bearing formation, causing the salt deposits and other chlorine-containing sediments to dissolve in the water. The second is that the rivers or rivers close to the ocean are affected by the wind blown by the tidal water and seawater, which causes the chloride content in the water to increase.
⑵Artificial sources: Mainly include industrial wastewater discharged from industries such as mining, petrochemicals, food, metallurgy, tanning (tannery), chemical pharmaceuticals, papermaking, textiles, paints, pigments, and machinery manufacturing, as well as domestic sewage generated by human life , Of which industrial emissions are the main source.
3. Common chloride ion limit standards
(1) Stainless steel pipeline: The mandatory requirement of the chloride ion content of the stainless steel pipeline test water is less than 50ppm (TSG D0001-2009) and the specification of 25ppm (GB 50235-2010) is recommended. When the chloride ion content is greater than 25 ppm, stainless steel will undergo stress corrosion, pitting corrosion, and intergranular corrosion.
⑵ Urea circulating water: The control range of chloride ions is determined by the chloride ion content in the supplemental water and the circulation multiple of water. If the supplemental water chloride ion is 10 ppm and the circulating water is concentrated 3 times, then the circulating water chloride ion should be less than 30 ppm. If the circulating water chloride ion is greater than 30 ppm, the concentration of circulating water is greater than 3 times, and the system may scale. The circulating water chloride ion is less than 30ppm, which means that the circulating water concentration factor is less than 3 times, resulting in waste of water resources.
(3) Boiler water: There is no requirement for chloride ion in GB/T1576-2008 "Industrial Boiler Water Quality" standard, the general chlorine control of boiler water is less than 400mg/L, and the feed water degree is less than 30mg/L. The chloride ion in the boiler water should be no more than 20 times that of the feed water. Low chlorine can reduce the discharge volume accordingly.
The chloride ion index is usually used to measure the accumulation of electrolyte concentration in boiler water (Cl- does not react in the steam and does not deposit). For example, the water freshly added to the boiler at first contained Cl-10 ppm, and after a period of operation, it was found that the Cl- content became 100 ppm. From this, it can be shown that the electrolyte chemical composition of boiler water has increased tenfold. In order to avoid the accumulation of electrolyte, fresh water should be added continuously to maintain and control the chloride ion index in the boiler. Indirectly control the content of dissolved solids, the medicine treatment in the pot is <5000mg/L, and the chemical treatment outside the pot is <500mg/L.
⑷Drinking water: CJ3020-1993 drinking water source water quality standards do not require the content of chloride ions, but only stipulates that the chloride content of chloride should be <250mg/L. After testing, the chloride ion in tap water is usually around 50mg/L, and the surface water is 50-100mg/L. There will be thousands of chloride ions in the area near the sea.
⑸ COD detection: In the COD detection, when the content of chloride ion is higher than 1500mg/L, the measurement result will be higher, resulting in inaccurate data. Quantitative dilution should be done first to reduce the content of chloride ion to below 1500mg/L, and then determine .
4. Inspection method
Chlorine ion detection kit 20-400mg/L 50 times/box
Testing method: Same as nitric acid titration method (GB5750-85), using potassium chromate as indicator, titrating chloride with silver nitrate.
[Instructions for use]
1. Use a pipette to accurately pipette 10ml of water. .
2. Add the water sample to the clean Erlenmeyer flask.
3. Add 1 package of chloride ion (1) reagent to the Erlenmeyer flask and shake it to dissolve. At this time, the solution is bright yellow.
4. While fully shaking the conical flask, add the chloride ion (2) reagent vertically, and add 1 drop every 3 seconds until the solution just turns orange or the precipitate changes to a flesh color, and record the number of drops of consumed reagent (N).
5. Calculation of results: chloride ion content = NX20 (mg/I)
1. Please clean the Erlenmeyer flask with deionized water before testing.
2. In order to ensure the accuracy of the test results, first discard the first titration solution and perform sample titration.
3. If the solution or precipitate is orange-red or diamond-red after the titration, it means that the titration has been excessive.
Instructions and precautions for the use of chloride ions.
1. PH value. Should be controlled at 5-8.
2. Both bromide and iodide can cause the same reaction and are included in the result with a mass equivalent to chloride.
3. Sulfide, sulfite, thiosulfate and oxygen consumption exceeding 15mg/I can interfere with the determination of this method. Sulphite and other disturbances can be taken out with hydrogen peroxide, and water samples with high oxygen consumption can be treated with potassium permanganate or ashing after evaporation to dryness.
⑵ Chloride test paper 500-3000mg/L 100 times/box Colorimetry
1. Remove a test strip and quickly close the lid
2. Immerse part of the reaction zone into the test solution for 1 second, remove and throw off excess water drops
3. After being placed for 60 seconds, it is compared with the color card, and the color close to the color block is the density value.
5. How to remove chloride ion?
⑴ distillation method. This is the easiest and safest method to remove chloride ions in water. Direct distillation is used to greatly reduce chloride ions in water.
⑵ Add silver pernitrate. It reacts with chloride ions to form a silver chloride precipitate. After standing, it can obtain chlorine-standard water. Disadvantages: At the same time, new ions are introduced __NO3- (nitric acid ions), whether the content of NO3- exceeds the standard needs to be calculated. A. Compared with the standard. If NO3-does not exceed the standard, this method can be used to remove chloride internal ions. In the industry, the cost is too high, the application is not widespread, and it is limited to laboratory use.
(3) Ion exchange method, using double bed or mixed bed to remove chloride ions, can be recycled, low cost, but anion exchange resin is easily saturated and needs regeneration.
The removal rate of reverse osmosis (RO) membrane method is higher than that of electrodialysis, which is convenient to operate, but the investment is large, and the membrane is easily clogged. It is not suitable for the treatment of wastewater with high conductivity and high ion concentration.