DPD spectrophotometry is the standard method for detecting free residual chlorine and total residual chlorine in China’s national standard “Water Quality Vocabulary and Analytical Methods” GB11898-89, jointly developed by the American Public Health Association, the American Water Works Association and the Water Pollution Control Federation. In the edited “Standard Test Methods for Water and Wastewater”, the DPD method has been developed since the 15th edition and is recommended as the standard method for testing chlorine dioxide.
Advantages of DPD method
It can separate chlorine dioxide from various other forms of chlorine (including free residual chlorine, total residual chlorine and chlorite, etc.), making it easier to perform colorimetric tests. This method is not as accurate as amperometric titration, but the results are sufficient for most general purposes.
principle
Under the conditions of pH 6.2~6.5, ClO2 first reacts with DPD to form a red compound, but the amount appears only reaches one-fifth of its total available chlorine content (equivalent to reducing ClO2 to chlorite ions). If a water sample is acidified in the presence of iodide, chlorite and chlorate also react, and when neutralized by the addition of bicarbonate, the resulting color corresponds to the total available chlorine content of ClO2 . The interference of free chlorine can be inhibited by adding glycine. The basis is that glycine can immediately convert free chlorine into chlorinated aminoacetic acid, but has no effect on ClO2.
Potassium iodate standard stock solution, 1.006g/L: Weigh 1.003g potassium iodate (KIO3, dried at 120~140°C for 2 hours), dissolve in high-purity water, and transfer to 1000ml volume.
Dilute the measuring flask to the mark and mix.
Potassium iodate standard solution, 10.06mg/L: Take 10.0ml of the stock solution (4.1) in a 1000ml volumetric flask, add about 1g of potassium iodide (4.5), add water to dilute to the mark, and mix. Prepare on day of use in brown bottle. 1.00ml of this standard solution contains 10.06μg KIO3, which is equivalent to 1.00mg/L available chlorine.
Phosphate buffer: Dissolve 24g anhydrous disodium hydrogen phosphate and 46g anhydrous potassium dihydrogen phosphate in distilled water, and then mix into 100ml of distilled water with 800mg EDTA disodium salt dissolved. Dilute with distilled water to 1L, optionally add 20mg mercuric chloride or 2 drops of toluene to prevent mold growth. Adding 20 mg of mercuric chloride can eliminate the interference of trace amounts of iodide that may remain when measuring free chlorine. (Note: Mercury chloride is toxic, handle with caution and avoid ingestion)
N,N-diethyl-p-phenylenediamine (DPD) Indicator: Dissolve 1.5g DPD sulfate pentahydrate or 1.1g anhydrous DPD sulfate in chlorine-free distilled water containing 8ml1+3 sulfuric acid and 200mg EDTA disodium salt , dilute to 1 liter, store in a brown ground glass bottle, and store in a dark place. When the indicator fades, it needs to be reconstituted. Regularly check the absorbance value of blank samples,
If the absorbance value of the blank at 515nm exceeds 0.002/cm, the reconstitution needs to be abandoned.
Potassium iodide (KI crystal)
Sodium arsenite solution: Dissolve 5.0g NaAsO2 in distilled water and dilute to 1 liter. Note: NaAsO2 is toxic, avoid ingestion!
Thioacetamide solution: Dissolve 125 mg of thioacetamide in 100 ml of distilled water.
Glycine solution: Dissolve 20g glycine in chlorine-free water and dilute to 100ml. Store frozen. Need to be reconstituted when turbidity occurs.
Sulfuric acid solution (about 1mol/L): Dissolve 5.4ml concentrated H2SO4 into 100ml distilled water.
Sodium hydroxide solution (about 2mol/L): Weigh 8g NaOH and dissolve it in 100ml pure water.
Calibration (working) curve
To a series of 50 colorimetric tubes, add 0.0, 0.25, 0.50, 1.50, 2.50, 3.75, 5.00, 10.00ml of potassium iodate standard solution, respectively, add about 1g of potassium iodide and 0.5ml of sulfuric acid solution, mix and let stand for 2 minutes , then add 0.5ml sodium hydroxide solution and dilute to the mark. The concentrations in each bottle are respectively equivalent to 0.00, 0.05, 0.10, 0.30, 0.50, 0.75, 1.00, and 2.00 mg/L of available chlorine. Add 2.5ml of phosphate buffer and 2.5ml of DPD indicator solution, mix well, and immediately (within 2 minutes) measure the absorbance at 515nm using a 1-inch cuvette. Draw a standard curve and find the regression equation.
Determination steps
Chlorine dioxide: Add 1ml of glycine solution to 50ml of water sample and mix, then add 2.5ml of phosphate buffer and 2.5ml of DPD indicator solution, mix well, and measure the absorbance immediately (within 2 minutes) (reading is G).
Chlorine dioxide and free available chlorine: Take another 50ml water sample, add 2.5ml phosphate buffer and 2.5ml DPD indicator solution, mix well, and measure the absorbance immediately (within 2 minutes) (the reading is A).
7.3 Chlorine dioxide, free available chlorine and combined available chlorine: Take another 50ml of water sample, add about 1g of potassium iodide, add 2.5ml of phosphate buffer and 2.5ml of DPD indicator solution, mix well, and measure the absorbance immediately (within 2 minutes) (Reading is C).
Total available chlorine including free chlorine dioxide, chlorite, free residual chlorine and combined residual chlorine: After obtaining the reading C, add 0.5ml sulfuric acid solution to the water sample in the same colorimetric bottle, and mix After standing still for 2 minutes, add 0.5 ml sodium hydroxide solution, mix and measure the absorbance immediately (the reading is D).
ClO2=1.9G (calculated as ClO2)
Free available chlorine=A-G
Combined available chlorine = C-A
Total available chlorine=D
Chlorite=D-(C+4G)
Effects of Manganese: The most important interfering substance encountered in drinking water is manganese oxide. After adding phosphate buffer (4.3), add 0.5~1.0ml sodium arsenite solution (4.6), and then add DPD indicator to measure the absorbance. Subtract this reading from the reading A to eliminate
Remove interference from manganese oxide.
Influence of temperature: Among all current analytical methods that can distinguish ClO2, free chlorine and combined chlorine, including amperometric titration, continuous iodometric method, etc., temperature will affect the accuracy of the distinction. When the temperature is higher, combined chlorine (chloramine) will be prompted to participate in the reaction in advance, resulting in higher results of ClO2, especially free chlorine. The first method of control is to control the temperature. At around 20°C, you can also add DPD to the water sample and mix it, and then immediately add 0.5ml thioacetamide solution (4.7) to stop the combined residual chlorine (chloramine) from DPD. Reaction.
Influence of colorimetric time: On the one hand, the red color produced by ClO2 and DPD indicator is unstable. The darker the color, the faster it fades. On the other hand, as the phosphate buffer solution and DPD indicator are mixed over time, they themselves will also fade. Produces a false red color, and experience has shown that this time-dependent color instability is a major cause of reduced data precision. Therefore, speeding up each operating step while controlling the standardization of the time used in each step is crucial to improving precision. According to experience: the color development at a concentration below 0.5 mg/L can be stable for about 10 to 20 minutes, the color development at a concentration of about 2.0 mg/L can only be stable for about 3 to 5 minutes, and the color development at a concentration above 5.0 mg/L will be stable for less than 1 minute.
The LH-P3CLO currently provided by Lianhua is a portable residual chlorine meter that complies with the DPD photometric method.
The analyzer has already set the wavelength and curve. You only need to add reagents and perform colorimetry to quickly get the results of residual chlorine, total residual chlorine and chlorine dioxide in the water. It also supports battery power supply and indoor power supply, making it easy to use whether outdoors or in the laboratory.
Post time: May-24-2024
