How To Measure Chlorine In Water
Chloride ion is a common anion in water. The too high content of chloride ions will cause bitterness and saltiness in drinking water, soil salinization, pipeline corrosion, difficulty in plant growth, and body health. Therefore, we must control the concentration of chloride ions in water. Total chlorine is an essential indicator of all kinds of water quality determination. So how to measure the total chlorine in the water?
There are three total chlorine analysis methods: rapid determination, spectrophotometry and titration.
Comparison of determination of total chlorine in water by rapid determination method, spectrophotometry and titration
The rapid determination method is simple and good, but the accuracy of the determination result is not as good as that of spectrophotometry and titration. It is suitable for preliminary analysis and the determination of samples on-site.
Although the accuracy of spectrophotometry is better, it needs to make the standard curve for measurement in a relatively complex process.
Step:
Suck 0ml, 1.00mL, 2.00ml, 3.00ml, 5.00ml, 10.00ml and 15.00ml potassium iodate standard solution with the concentration of 10.06mg/l into a 100ml volumetric flask accurately, and add an appropriate amount of water and 1.0ml1mol/l sulfuric acid solution, add 1ml1mol/l sodium hydroxide solution after 1min and then add water to fix the volume. Take a 250ml volumetric flask and orderly, add 15ml phosphate buffer solution and 5ml1 1g / ldpd solution. However, the standard solution in the previous volumetric flask into the conical flask respectively, measured the absorbance at 515nm with a 10mm cuvette and calculated the standard curve.
It involves unequal volume addition (potassium iodate standard solution) and fixed volume addition of various reagents (sulfuric acid solution, sodium hydroxide solution, phosphate buffer solution, DPD solution, etc.).
The titration accuracy is good, but we must use micro burette in the process, which requires a high technical level of laboratory personnel. With the development of technology, the digital display titrator and electronic titrator can realize extraction and liquid addition, hand rotation/hand pressing to control the titration speed and direct display on the screen, which can solve traditional problems, such as slow liquid filling, difficult speed control and disordered reading. It is very suitable for on-site analysis and determination.
How to measure chloride ions in water on site?
For the determination of chloride ions in water, there are a variety of test methods for different industries. For example, ion chromatography is commonly used to determine trace chloride ions in-unit water vapour for the power industry, during the determination of trace chloride ions by spectrophotometry or electrode method, the determination of chloride ion content in coal mine water by molar titration method etc.
However, these methods have disadvantages, such as high equipment, long time and highly toxic chemicals, which are not suitable for rapid on-site detection. We can use the determination method of adding reagent powder combined with a spectrophotometer for rapid on-site detection.
The principle:
According to the principle of silver chloride turbidimetry, adding solid powder instead of a traditional solution avoids high operation requirements and short storage time of reagent. Combined with a portable spectrophotometer, it can realize the rapid detection of chloride ions in the field or sampling site.
Step:
Instruments and reagents
Spectrophotometer.
Reagents: boric acid, polyethylene glycol (2000), silver nitrate and amino sulfonic acid are all analytical pure and distilled water.
Chloride ion standard solution: 100 mg / L.
Preparation of test powder
Test powder I: sulfamic acid;
Test powder II: weigh 2g boric acid, 2g polyethylene glycol (2000) and 0.12g silver nitrate, mix and grind.
Sample determination
Put 10 ml of the sample into the test tube, add 0.5 g of powder I, shake and dissolve, add 0.1 g of powder II, shake and dissolve rapidly, place it in the dark for 20 minutes, use a 1 cm cuvette, and measure the absorbance value at 650 nm of wavelength.
Selection of test conditions
Wavelength measurement
The turbidity measurement is done according to the test method. The absorption spectrum of 10 mg / L chloride ion standard solution conforms to the common law of turbidity photometric detection. With the increase of wavelength, the turbidity absorbance value gradually decreases. Since the interference of chloride ion content in water and the background colour of water samples, we can choose the determination wavelength of chloride ion as 650 nm.
Dosage test of powder I
Powder I is a solid, strong acid amino sulfonic acid, which makes the water sample strong acidic to avoid precipitation of other anions and silver ions.
Add 0.1 g, 0.3 g, 0.5 g, 0.7 g, 0.9 g and 1.1 g of amino sulfonic acid to 10 mg / L chloride ion standard solution orderly, and determine the absorbance value of the solution after the reaction.
The results showed that the amount of sulfamic acid was stable between 0.3 and 0.9 g.
Dosage test of powder II
Powder II contains precipitant silver nitrate, stabilizer boric acid and polyethylene glycol (2000), in which polyethylene glycol is evenly wrapped on the surface of silver nitrate to make it react with chloride ions slowly and evenly in the sample to ensure the reproducibility of the results.
Add 0.02 g, 0.04 g, 0.06 g, 0.08 g, 0.10 g, 0.12 g, 0.15 g and 0.20 g of powder II into 10 mg / L chloride ion standard solution, and determine the absorbance value after reaction solution.
The results:
The results show that when the amount of powder II is more than 0.08 g, it tends to be the largest and stable, and 0.10 g in this test.
Color development time
After adding powder II into 10 mg / L chloride ion standard solution, measure its absorbance value within 5 ~ 60 min. the change of absorbance value with time is shown below.
According to the curve, we know that the absorbance value tends to be stable after adding powder II for 20 min, so the colour development time is determined as 20 min.
Interference test
Determine 10 mg / L chloride ion standard solution according to the test method.
Add possible interfering anions, and the relative error is within 6%. The allowable amount of coexisting ions is 100 times of SO2-4, NO3 -, NO2 -, F -, 1 time of sio2-3, PO3-4 and 0.1 times of S2 -, which do not interfere with the determination.
Standard curve and detection limit
measure the absorbance values of 0.0 mg / L, 2.0 mg / L, 5 mg / L, 10 mg / L, 15 mg / L and 20 mg / L of chloride ion standard solution at the wavelength of 650 nm by spectrophotometer. The standard curve is drawn with the mass concentration of chloride ion as the X and the absorbance value as the Y.
The results show that the mass concentration of chloride ions is in the range of 0 ~ 20 mg / L, which is an excellent linear relationship with the absorbance value. The linear regression equation is y = 0.0227 X, and the correlation coefficient is 0.9995.
Accuracy test
The raw and coal mine water was measured in parallel 6 times, and the results are below.
According to the above, the relative standard deviation (RSD) of the determination results of raw water and coal mine water is less than 5.16%.
Determination of chloride ion content in industrial circulating cooling water and boiler water
The molar and potentiometric titration methods are suitable for determining chloride ion content in natural water, circulating cooling water, softened water and boiler water.
The determination range of the molar method is 3mg / L ~ 150mg / L. when it is more than 150mg / L, and we can appropriately reduce the sampling volume and determine after dilution; The determination range of potentiometric titration is 5mg/l~1000mg/l; The coprecipitation enrichment spectrophotometry is suitable for the determination of chloride ion content in demineralized water and boiler feed water, and the determination range is 10 μ g/L~100 μ g/L。
Principle of the molar method
Potassium chromate as an indicator, titrate with silver nitrate standard titration solution in the pH 5.0 ~ 9.5. Silver nitrate reacts with the chloride to produce silver chloride white precipitation. Excess silver nitrate reacts with potassium chromate indicator to produce brick red silver chromate precipitation, indicating the endpoint.
The reaction formula :
Potentiometric titration
The principle
The composite silver electrode as the measuring electrode or the silver/silver chloride electrode as the reference electrode and the silver electrode as the indicator electrode, immerse the composite silver electrode or the indicator electrode and the reference electrode into the measured solution, titrate with the silver nitrate standard titration solution to the potential jump point, and then calculate the chloride ion content through the volume of the silver nitrate standard titration solution consumed by the jump point.
Reagent
Nitric acid solution: 1 + 300.
Sulfuric acid solution: C (1 / 2 H2SO4) is about 0.1mol/l.
Silver nitrate standard titration solution: C (AgNO3) is about 0.05mol/l.
Methyl orange indicator solution: 1g / L.
Instruments and equipment
Automatic potentiometric titrator: equipped with silver/silver chloride reference electrode, silver electrode or composite silver electrode. Blender.
Step:
Put an appropriate volume of water sample into the measuring cup by pipette. For water samples with pH > 7 or water samples containing scale inhibitor, it should adjust the water sample to pH 3 ~ 4 . Place the measuring cup on the titration table, insert the electrode, turn on the potentiometric titrator and stirrer, titrate with silver nitrate standard titration solution, and stop the titration when the complete jump curve appears.
Calculation
Chloride ion content is determined by mass concentration ρ 2, (mg / L)
ρ 2=（V1cM/V ）*10^3
Where:
V1 — the volume of silver nitrate standard titration solution consumed when the sample is titrated to the potential jump point, unit: ml;
C — the accurate value of actual concentration of silver nitrate standard titration solution, unit: mol / L;
M — value of molar mass of chlorine, in grams per mole (g / mol) (M = 35.45);
V — the value of the sample volume in millilitres (ML).
Difference
The same operator uses the same instrument and test method, and the absolute difference in the similar measurement results in a short time should meet the requirements.
