DPD spectrophotometry for the determination of residual chlorine

Please refer to the following method suggested by the original poster: Determination of residual chlorine (DPD method) I. Determination method: DPD spectrophotometry, using a HACH residual chlorine meter. II. Basis for the method: “Hygienic Standards for Drinking Water” (2001), as well as the instructions for use of the HACH residual chlorine meter. III. Measurement range: 1. Suitable for determining free residual chlorine and various forms of combined residual chlorine in drinking water that has been disinfected with chlorine, as well as in the water sources from which such water is obtained. 2. The minimum detectable mass is 0.1 µg; when a 10 mL water sample is used for analysis, the minimum detectable mass concentration is 0.01 mg/L. 3. High concentrations of monochloramine interfere with the determination of free residual chlorine; the reaction can be controlled using arsenite or thioacetamide to eliminate this interference. The interference from manganese oxide can be eliminated by using a water sample blank. The interference of chromates can be eliminated using thioacetamide. IV. Principle of measurement: DPD reacts rapidly with free residual chlorine in water to produce a red color. Catalyzed by iodides, monochloramine can also react with DPD to produce a color. When iodide is added before the DPD reagent, part of the trichloramine reacts to produce color along with the free residual chlorine; by changing the order in which the reagents are added, the concentration of trichloramine can be determined. V. Reagents 1. Potassium iodide crystals. 2. Potassium iodide solution (5 g/L): Weigh 0.50 g of potassium iodide (KI), dissolve it in freshly boiled and cooled pure water, then dilute to 100 mL. Store it in a brown bottle in the refrigerator; if the solution turns yellow, it should be discarded and prepared anew. 3. Phosphate buffer solution (pH: 6.5): Weigh 24 g of anhydrous disodium hydrogen phosphate (Na2HPO4), 46 g of anhydrous potassium dihydrogen phosphate (KH2PO4), 0.8 g of disodium ethylenediaminetetraacetate (Na2EDTA), and 0.02 g of mercury chloride (HgCl2). Dissolve sequentially in pure water and dilute to 1000 mL. Note: HgCl2 prevents mold growth and eliminates the interference caused by trace iodides in reagents on the determination of free residual chlorine. HgCl2 is highly toxic; do not ingest it or allow it to come into contact with the skin or fingers. 4. N,N-diethylenediamine (DPD) solution (1 g/L): Weigh 1.0 g of N,N-diethylenediamine hydrochloride, or 1.5 g of N,N-diethylenediamine sulfate, dissolve it in chlorine-free pure water containing 8 mL of sulfuric acid solution (13) and 0.2 g of Na2EDTA, then dilute to 1000 mL and store it in a brown bottle in a cool, dark place. Note: The DPD solution is unstable; it should not be prepared in large quantities at one time. If the color of the solution changes to darker or lighter during storage, it should be prepared again. 5. Potassium arsenite solution (5.0 g/L): Weigh 5.0 g of potassium arsenite (KAsO2), dissolve it in pure water, and dilute to 1000 mL. 6. Thioacetamide solution (2.5 g/L): Weigh 0.25 g of thioacetamide (CH2CSNH2) and dissolve it in 100 mL of pure water. Note: Thioacetamide is a suspected carcinogen; avoid contact with skin or inhalation. 7. No chlorine water required: Add a small amount of chlorine water or bleach solution to chlorine-free pure water to bring the total residual chlorine concentration in the water to about 0.5 mg/L. Heat to boiling to remove chlorine. Cool down and set aside for later use. Note: Potassium iodide can be added before use to test the total residual chlorine according to this procedure. 8. Chlorine standard stock solution [=1000 µg/mL]: Weigh 0.8910 g of high-purity potassium permanganate (KMnO4), dissolve it in pure water, and dilute to 1000 mL. 9. Chlorine standard working solution = 1 µg/mL]: Weigh 10.0 mL of the chlorine standard stock solution and dilute it to 100 mL with pure water. After mixing, take 1.00 mL and dilute it to 100 mL. VI. Instruments and Equipment 1. Spectrophotometer 2. Stoppered colorimetric tubes, 10 mL 3. HACH portable residual chlorine detector VII. Analysis Procedure 1. Preparation of standard curve: Transfer 0, 0.1, 0.5, 2.0, 4.0, and 8.0 mL of the chlorine standard solution into 6 10 mL stoppered colorimetric tubes, and fill them to the mark with dechlorinated water. Add 0.5 mL of phosphate buffer solution and 0.5 mL of DPD solution to each sample, mix well, measure the absorbance at a wavelength of 515 nm using a 1 cm cuvette with pure water as the reference, record the absorbance values, and prepare a standard curve. 2. Transfer 10 mL of the water sample into a 10 mL cuvette, add 0.5 mL of phosphate buffer solution and 0.5 mL of DPD solution, mix well, and immediately measure the absorbance at a wavelength of 515 nm using a 1 cm path length cuvette, with pure water as the reference. Record the reading as A; simultaneously measure the blank value of the sample and subtract it from the reading. 3. Add another small crystal of potassium iodide (about 0.1 mg) to the aforementioned test tube, mix well, then measure the absorbance and record the reading as B. Note: If the dichloramine content in the sample is too high, 0.1 mL of freshly prepared potassium iodide solution (1 g/L) can be added. 4. Add potassium iodide crystals (about 0.1 g) to the aforementioned test tube, mix well, and after 2 minutes, measure the absorbance and record the value as C. 5. Take two additional 10 mL colorimetric tubes. Place 10 mL of the water sample in one of the tubes, then add a small crystal of potassium iodide (about 0.1 mL) and mix well. In the second tube, add 0.5 mL of buffer solution and 0.5 mL of DPD solution; pour this mixture into the first tube and mix thoroughly. Measure the absorbance and record the reading as N. VIII. Calculation of free residual chlorine and various chloramines: Readings are calculated based on their presence. Water samples without trichloramine; Water samples containing trichloramine. A. Free residual chlorine; Free residual chlorine B – A. Monochloramine; Monochloramine C – B. Dichloramine; Dichloramine. 50% trichloramine: N ——— Free residual chlorine; 50% trichloramine: 2 (N – A) ——— Trichloramine: C – N ——— Dichloramine. The contents of free residual chlorine and various combined residual chlorines in the water sample are determined by referring to a standard curve using the readings from the table. m_ρ(Cl2) = ——— V. Where: ρ(Cl2) is the mass concentration of residual chlorine in the water sample, in mg/L ； m—the mass of residual chlorine obtained from the standard curve, in μg ； V—Volume of water sample, in mL. Precision and accuracy: Five laboratories used this standard to analyze samples with residual chlorine levels of 0.75 and 3.0 mg/L; the relative standard deviations were 2.5%~16.9% and 1%~8.5%, respectively. Using 0.05 mg/L for the spiking test, the average recovery rate was 97%–108%, with the spiked mass concentration ranging from 0.3 to 0.5 mg/L. The average recovery rate was 90%~103% ； When the spiked mass concentration was 1.0~3.0 mg/L, the average recovery rate was 94%~106%. 9. After adding 0.5 mL of phosphate buffer solution and 0.5 mL of DPD solution to the water sample and mixing well, it can also be measured using a HACH portable residual chlorine meter; refer to the operating instructions for the HACH portable residual chlorine meter.

This is great. Aren’t there chlorine residual electrodes available these days?