Determination of free chlorine

Our unit is automatic; I don’t know

According to the standards, interference should be eliminated, allowing measurement to be carried out using a spectrophotometer. Last edited by lifanwang on 2009-4-7 16:02.]

The poster is referring to the method of determining free residual chlorine using the DPD (N,N-diethylenediamine) spectrophotometric method, right? In the DPD spectrophotometric method, potassium iodide is added in different sequences, with the aim of determining combined residual chlorine, including monochloramine, dichloramine, trichloramine, and organic chlorinated compounds. Potassium iodide acts as a catalyst to enable DPD to react with combined residual chlorine to form a red compound for measurement. This post was last edited by xwtsq on 2009-1-16 18:17]

Water quality – Determination of free chlorine and total chlorine by the N,N-diethyl-1,4-phenylenediamine spectrophotometric method 1 Subject matter and scope: This standard is an equivalent adoption of the international standard ISO 7393/2–1985 \"Water quality – Determination of free chlorine and total chlorine – Part 2: Colorimetric method using N,N-diethyl-1,4-phenylenediamine, for routine control.\" This standard is applicable to the determination of free chlorine or total chlorine (expressed as Cl2) in the range of 0.0004–0.07 mmol/L (0.03–5 mg/L). When the sample concentration is high, dilution is required. Appendix A describes the procedures for determining monochloramine, dichloramine, and trichloramine in combined chlorine. 2 Definitions Table 1 Nouns and Their Components Noun Synonyms Components Free chlorine Free residual chlorine: Active free chlorine Potential free chlorine Elemental chlorine, hypochlorous acid Chlorite Total chlorine Total residual chlorine Elemental chlorine, hypochlorous acid, chlorites, chloramines 2.1 Free chlorine: Chlorine that exists in the form of hypochlorous acid, chlorite ions, and dissolved elemental chlorine. 2.2 Combined chlorine: Part of the total chlorine present in the form of chloramines and organic chloramines. 2.3 Total chlorine: Chlorine present in the form of \"free chlorine\", or \"combined chlorine\", or both. 2.4 Chloramines: Derivatives in which one, two, or three hydrogen atoms of ammonia are replaced by chlorine atoms (monochloramine NH2Cl, dichloramine NHCl2, trichloramine NCl3), as well as the chlorinated derivatives of organic nitrogen compounds, as determined by this method. 3 Principles 3.1 Determination of free chlorine At a pH of 6.2–6.5, free chlorine reacts directly with N,N-diethyl-1,4-phenylenediamine (DPD) to form a red compound, and its absorbance is measured using spectrophotometry. 3.2 Determination of total chlorine: A reaction is carried out in the presence of excess potassium iodide, and then its absorbance is measured as described in 3.1. 4 Reagents: All reagents used in the analysis are of analytical grade. 4.1 Water, water free of chlorinated reducing substances: Deionized water or distilled water is chlorinated to a level of about 0.14 m mol/L (10 mg/L), stored in sealed glass bottles for approximately 16 hours, then exposed to ultraviolet light or sunlight for several hours, or treated with activated carbon to remove chlorine. Check its quality by following the steps below. Add to two 250 mL Erlenmeyer flasks that do not require chlorine: a. To the first flask, add 100 mL of the water to be tested and about 1 g of potassium iodide (4.4), then mix well. After 1 minute, add 5.0 mL of buffer solution (4.2) and 5.0 mL of DPD solution (4.3) ； b. Second, 100 mL of water to be tested and 2 drops of sodium hypochlorite solution (4.8). After 2 minutes, add 5.0 mL of buffer solution and 5.0 mL of DPD solution (4.3). The first bottle should show no color, while the second bottle should turn pink. 4.2 Buffer solution, pH 6.5: Dissolve 24 g of anhydrous disodium hydrogen phosphate (Na2HPO4), or 60.5 g of disodium hydrogen phosphate monohydrate (Na2HPO4·12H2O) together with 46 g of potassium dihydrogen phosphate (KH2PO4) in water (4.1), sequentially. Add 100 mL of dihydrate disodium EDTA (C10H14N2O8Na2·2H2O) at a concentration of 8 g/L, or 0.8 g of solid. If necessary, add 0.020 g of mercury chloride to prevent mold growth and to avoid interference from trace iodides in the reagent with the detection of free chlorine. Dilute to 1000 mL and mix well. Note: Mercury salts are highly toxic and should be handled safely. 4.5 N,N-Diethyl-1,4-phenylenediamine sulfate (DPD) solution, 1.1 g/L: Mix 250 mL of water (4.1), 2 mL of sulfuric acid (ρ = 1.84 g/mL), and 25 mL of an 8 g/L disodium ethylenediaminetetraacetate dihydrate solution (or 0.2 g of solid). Dissolve 1.1 g of anhydrous DPD sulfate (or 1.5 g of the pentahydrate), or 1 g of DPD oxalate in this mixture, dilute to 1000 mL, and mix well. The test solution is stored in a brown bottle in the refrigerator. If the solution changes color after one month, it should be reprepared. 4.4 Potassium iodide, crystals. 4.5 Sulfuric acid, approximately 1 mol/L: To 800 mL of water (4.1), 54 mL of sulfuric acid (ρ = 1.84 g/mL) is carefully added while stirring continuously; after cooling to room temperature, the mixture is diluted to 1000 mL and mixed well. 4.6 Sodium hydroxide, solution, approximately 2 mol/L: Weigh 80 g of granular sodium hydroxide and add it to a beaker containing 800 mL of water (4.1). Stir until it is completely dissolved, then cool to room temperature and dilute to 1000 mL, mixing well. 4.7 Sodium hypochlorite, solution (trade name, Antifolin), containing about 0.18/L of Cl2: prepared by diluting a concentrated solution. 4.8 Potassium iodate standard stock solution, 1.006 g/L: Weigh 1.006 g of potassium iodate (KIO3), which has been dried at 120–140°C for 2 hours, dissolve it in water (4.1), transfer the solution to a 1000 mL volumetric flask, dilute to the mark, and mix well. 4.9 Potassium iodate standard working solution, 10.06 mg/L: Take 10.0 mL of the stock solution (4.8) and transfer it to a 1000 mL volumetric flask; add about 1 g of potassium iodide (4.4), then dilute with water (4.1) to the mark and mix well. Prepare it on the day of use and store it in a brown bottle. 1.00 mL of this standard solution contains 10.06 μg of KIO3 (equivalent to 0.141 μmol of Cl2): 4.10 g/L of sodium arsenite (NaAsO2) solution, or 2.5 g/L of thiourea (CH3CSNH2) solution. 5 Instruments 5.1 Common laboratory instruments and the following instruments. 5.2 Micropipette, with a total volume of 5 mL and precision to 0.02 mL. 5.3 100mL volumetric flask. 5.4 Spectrophotometer (using a 510nm wavelength), with 10mm square cuvettes. Preparation of chlorine-free glassware: Soak the glassware in sodium hypochlorite solution (4.7) for 1 hour, then rinse thoroughly with water (4.1). 6 Measurement Steps 6.1 Sample After sampling, measurement should be carried out immediately, with strong light, shaking, and heat being avoided at all times. 6.2 Test samples: Take two samples of 100 mL each as test samples (Vo). If the total chlorine concentration exceeds 70 μmol/L (5 mg/L), take a smaller volume of sample (V1) and dilute it to 100 mL with water (4.1). 6.3 Preparation of the standard curve: To a series of 100 mL volumetric flasks, add 0.0, 0.30, 0.50, 1.00, 5.00, 10.0, 20.0, 30.0, 40.0, and 50.0 mL of the potassium iodate standard solution (4.9), respectively; to each flask, add 1.0 mL of sulfuric acid (4.5). After 1 minute, add 1.0 mL of sodium hydroxide solution (4.6) to each, and dilute to the mark. The chlorine concentrations C(C12) in each bottle were 0.00, 0.423, 0.705, 1.41, 7.05, 14.1, 28.2, 42.3, 56.4, and 70.5 μmol/L (i.e., 0.00, 0.03, 0.05, 0.10, 0.50, 1.00, 2.00, 3.00, 4.00, and 5.00 mg/L). Prepare a sufficient number of 250 mL Erlenmeyer flasks; add 5.0 mL of buffer solution (4.2) and 5.0 mL of DPD solution (4.3) to each. Within 1 minute, pour the standard solutions that have just been diluted to the mark into these flasks (without rinsing), mix well, and measure the absorbance of each standard solution at 510 nm using a 10 mm cuvette (the measurement time should not exceed 2 minutes). Draw a standard curve, or determine the regression equation x = by + a. Check one point on the standard curve every day. Note: ① For the various standard color solutions prepared separately, it is necessary to avoid leaving the buffer solution mixed with the DPD reagent for too long, as this may cause a false red color to appear. ②If the monochromatism of the spectrophotometer used is poor, it will result in a narrower concentration range when measuring standard color solutions, allowing measurement only within the range of C(Cl2) 0.0007–0.021 mmol/L (0.05–1.5 mg/L). Therefore, the concentration range of the prepared standard color solutions should be suitable for the performance of the spectrophotometer. 6.4 Determination of free chlorine: Transfer the sample (without rinsing) to a 250 mL Erlenmeyer flask containing 5.0 mL of buffer (4.2) and 5.0 mL of DPD solution (4.3), and mix well (see note 6.3). The absorbance is measured to obtain a concentration of C1. In the case of water samples that are slightly acidic, slightly alkaline, or contain high levels of salts, the amount of buffer (4.2) should be increased to bring the pH of the sample to 6.2–6.5. To obtain accurate results, it is very important to control the pH. At pH 6.2–6.5, the resulting red color accurately reflects the free chlorine concentration. If the pH is too low, monochloramine in the total chlorine can cause coloration during the measurement of free chlorine; on the other hand, if the pH is too high, dissolved oxygen can lead to coloration. 6.5 Determination of total chlorine: Transfer the second sample (without rinsing) to a 250 mL conical flask containing 5.0 mL of buffer (4.2) and 5.0 mL of DPD solution (4.3), add about 1 g of potassium iodide (4.4), and mix well (see note 6.3). Pour the color-developing solution into the cuvette ； After 2 minutes, the absorbance is measured to obtain the concentration C2. In the case of water samples that are slightly acidic, slightly alkaline, or contain high levels of salts, the amount of buffer (4.2) should be increased to bring the pH of the water sample to 6.2–6.5. 7 Correction for the interference of manganese oxide and hexavalent chromium: Supplementary measurements were carried out by pre-adding sodium arsenite or thioacetamide solution (4.10) to the samples, in order to eliminate all oxides other than manganese oxide and hexavalent chromium, thereby determining the effect of these substances. Take 100 mL of the sample into a 250 mL conical flask, add 1 mL of sodium arsenite (or thioacetamide) solution (4.10), and mix well. Add another 5.0 mL of buffer (4.2) and 5.0 mL of DPD solution (4.3). Then, the absorbance is measured, and the concentration C3 is recorded, which corresponds to the interference from manganese oxide and hexavalent chromium. 8 Representation of Results 8.1 Calculation Methods 8.1.1 Calculation of Free Chlorine The free chlorine concentration C(Cl2), expressed in millimoles per liter, is calculated using Equation (1): Where: C1 —— the free chlorine concentration obtained in measurement (6.4), in m mol/L ； C3 —— The concentration of manganese chloride and hexavalent chromium equivalent to chlorine obtained in determination (7), in mmol/L ； Note: If there is no manganese oxide and hexavalent chromium, C3 = OmL. Vo——maximum volume of the sample (6.2) (Vo=100 mL) ； V1 —— Volume of the specimen in the test material (6.2), in mL. 8.1.2 Calculation of total chlorine The total chlorine concentration C(Cl2), expressed in millimoles per liter, is calculated using Equation (2): Where ； C2 —— The total chlorine concentration obtained in determination (6.5), in m mol/L. 8.2 To convert the concentration in terms of amount of substance to a mass concentration, the chlorine (Cl2) concentration expressed in millimoles per liter is multiplied by 70.91 to obtain the value in milligrams per liter. 8.3 Reproducibility The Anhui Provincial Environmental Monitoring Center organized eight laboratories to verify this method. The intra-laboratory precision for free chlorine is shown in the table below: Table 2 Results of free chlorine determination in laboratories 1) Sample name, Number of samples, Range of free chlorine concentration (Cl2, mg/L), Mean value of Cl2 (mg/L), Standard deviation of Cl2 (mg/L), Relative standard deviation (%) Distilled water containing free chlorine: 10 samples; range: 0.14–0.16 mg/L; mean value: 0.74–0.80 mg/L; standard deviation: 1.33–1.39 mg/L; relative standard deviation: 0.15%; 0.77%; 1.35%; values for standard deviation: 0.0038, 0.0046, 0.0054; 2.5; 0.6; 0.4 Drinking water: 9 samples; range: 0.36–0.70 mg/L; mean value: 0.51 mg/L; standard deviation: 0.0054; relative standard deviation: 1.1 Hospital wastewater: 4 samples; range: 0.52–0.64 mg/L; mean value: 0.57 mg/L; standard deviation: 0.0069; relative standard deviation: 1.2 Papermaking wastewater: 6 samples; range: 0.52–0.87 mg/L; mean value: 0.70 mg/L; standard deviation: 0.0059; relative standard deviation: 0.8 Textile and dyeing wastewater: 4 samples; range: 0.17–0.28 mg/L; mean values: 7, 9 mg/L; ranges for standard deviation: 0.38–0.60, 0.72–1.17; relative standard deviations: 0.23%, 0.50%, 0.94%. Note: 1) The measurement results from the 8 laboratories (6 replicate measurements per sample) demonstrate the reproducibility of this method. 9 Interferences Two types of interference are worth noting. 9.1 Interference from other chlorinated compounds Some chlorine dioxide or chlorite may be detected as total chlorine. This interference can be corrected by measuring chlorine dioxide in the water. 9.2 Interference with DPD oxidation by compounds other than chlorides is not caused solely by chlorides. Depending on the concentration and chemical oxidation potential, this reaction can be induced by other oxidants. Therefore, this law applies in the absence of the following oxidants: bromine, iodine, bromamine, iodoamine, ozone, hydrogen peroxide, chromates, manganese oxide, nitrites, copper ions, and iron ions. The interference from copper ions at <8 mg/L and iron ions at <20 mg/L can be masked by disodium EDTA in reagents 4.2 and 4.3. Appendix A: Separate determination of chlorine in the forms of monochloramine, dichloramine, and nitrogen trichloride (Supplementary document) A1 Scope of application: This appendix specifies the method for distinguishing chlorine in the forms of monochloramine, dichloramine, and nitrogen trichloride. The applicable concentration range for this method is the same as that for free chlorine and total chlorine (see Chapter 1). A2 Principle: After measuring free chlorine and total chlorine, two additional samples are tested: a. When the third sample is added to the conical flask containing the buffer and DPD solution, a small amount of potassium iodide is added, and the reaction occurs only with monochloramine among free chlorine and combined chlorine. b. The fourth sample had a small amount of potassium iodide added first, before the buffering agent and DPD were added. At this point, half of the free chlorine and half of the monochloramine and nitrogen trichloride in the combined chlorine undergo reaction. Dichloramine in combined chlorine does not react under either of the above two conditions. Calculate the concentrations of monochloramine, dichloramine, and nitrogen trichloride in combined chlorine separately. A3 Reagents: The reagents listed in Chapter 4 and the following reagents. A3.1 Potassium iodide solution, 5 g/L. Prepare on the day of use and store in a brown bottle. For A4 instruments, see Chapter 5. A5 Determination Procedure A5.1 Sample See 6.1. A5.2 Test specimens: The same two specimens as those used in 6.2 are measured. A5.3 Preparation of the standard curve see 6.3. A5.4 Determination of monochloramine in free chlorine and combined chlorine: Add 5.0 mL of buffer (4.2), 5.0 mL of DPD solution (4.3), the third sample, and 2 drops (approximately 0.1 mL) of potassium iodide solution (A3) or a very small crystal of potassium iodide (approximately 0.5 mg) into a 250-mL conical flask sequentially and quickly. Immediately measure the absorbance of the solution under the same conditions as described in A5.3, and record the concentration C4. A5.5 Determination of free chlorine, combined chlorine, monoammonium chloride, and half of nitrogen trichloride: Add the fourth sample to a 250 mL beaker, along with 2 drops (about 0.1 mL) of potassium iodide solution (A3) or a very small piece of potassium iodide crystal (about 0.5 mg), and mix well ； Pour the solution in the beaker into a 250 mL Erlenmeyer flask into which 5.0 mL of buffer (4.2) and 5.0 mL of DPD solution were just added within 1 minute. Quickly pour the solution into the cuvette, measure the absorbance, and record the concentration C5. Representation of A6 results A6.1 Calculation method A6.1.1 Calculation of monochloramine The concentration X1(Cl2) of monochloramine in combined chlorine, expressed in millimoles per liter, is calculated using equation (A1): Where: C4 —— the chlorine concentration obtained in measurement (A5.4), in m mol/L. A6.1.2 Calculation of dichloramine The concentration of dichloramine, X2(Cl2), in combined chlorine expressed in millimoles per liter, is calculated using Equation (A2): Where: C5 —— the chlorine concentration obtained in measurement (A5.5), in m mol/L. A6.1.3 Calculation of nitrogen trichloride: The concentration of chlorine trichloride, X3(Cl2), in terms of millimoles per liter, is calculated using equation (A3). A6.2 Conversion from molar concentration to mass concentration: The concentration of chlorine (Cl2) expressed in millimoles per liter is multiplied by 70.91 to convert it to milligrams per liter.

In the spectrophotometric method using DPD (N,N-diethylenediamine) to determine free residual chlorine, what standard samples are used to prepare the calibration curve? What is the laboratory method? Thank you

In the current **standard testing methods for drinking water**, the standard curve used for determining free chlorine is prepared using a potassium permanganate standard solution instead of a standard solution made from chlorinated water. This is because preparing standard solutions with chlorinated water involves complicated procedures and results in unstable solutions; whereas standard solutions prepared with potassium permanganate have a color similar to that of the red color produced by DPD and residual chlorine, and their color remains stable, allowing for the creation of a \"permanent\" standard series. For specific methods, see http://bbs.hcbbs.com/viewthread.php?tid=324554&highlight=%C9%FA%BB%EE%D2%FB%D3%C3%CB%AE%B1%EA%D7%BC%BC%EC%D1%E9%B7%BD%B7%A8: GB/T 5750.7, 10, 11, 12, 13-2006 Test Methods for Standards of Drinking Water. This post was last edited by xwtsq on 2009-1-16 17:27]

Do we need a spectrophotometer for free chlorine? We use titration

Iodide ions can be reduced by free chlorine to produce elemental iodine; this is what causes the color change, allowing for a reading to be taken

It should be that iodide ions can be oxidized by free chlorine, not reduced.