Standard Examination Methods for Drinking Water - Disinfectants Parameters
1 Free Residual Chlorine
1.1 N, N-diethyl p-phenylenediamine (DPD) spectrophotometry
1.1.1 Scope
This standard specifies determination of free residual chlorine in drinking water and its source water by N,N-diethyl p-phenylenediamine (DPD) spectrophotometry.
This method is applicable to the determination of free chlorine residual in chlorinated drinking water and its source water and all kinds of compound residual chlorine.
Minimum detectable mass in this method is 0.1μg; the minimum detectable mass concentration is 0.01 mg/L if 500mL of water sample is taken for determination.
Due to interference of high concentration monochloro amine in determination of free residual chlorine, the arsenite or sulfo acetamide may be used for control response to remove the interference. The interference from manganese oxide may be removed by water sample blank and that from chromate may be removed by thio acetamide.
1.1.2 Principle
DPD reacts with free residual chlorine in water quickly to produce red composition. Catalysed by iodide, monochloramine reacts with DPD to produce red composition. When iodide is added before DPD reagent, a part of trichloramine reacts with free residual chlorine to develop red and the concentration of trichloramine may be obtained by changing the order to add reagents. The permanent standard series may be prepared by potassium permanganate solution by this method.
1.1.3 Reagents
1.1.3.1 Potassium iodide crystal.
1.1.3.2 Potassium iodide solution (5g/L): weigh 0.50 g of potassium iodide (KI), dissolve it in pure water which is newly-boiled and cooled and dilute it to 100mL, store it in a brown bottle, preserve in a refrigerator and re-prepare it if the solution gets yellow.
1.1.3.3 Phosphate buffer solution (pH 6.5): weigh 24g of anhydrous sodium hydrogen phosphate (Na2HPO4), 46g of anhydrous potassium dihydrogen phosphate (KH2PO4), 0.8g of ethylenediamine tetraacetic acid disodium (Na2-EDTA) and 0.02g of mercury bichloride (HgCl2). Dissolve them into pure water successively and dilute to 1000mL.
Note: HgCl2 can be used to prevent fungus growth and eliminate interference of slight trace of iodide in determination of free residual chlorine. The HgCl2 is virulent and never enters into mouth and contacts with skin and finger in use.
1.1.3.4 N,N-diethyl-p-phenylenediamine (DPD) solution (1g/L): weigh 1.0g of hydrochloric acid N,N-diethyl-p-phenylenediamine [H2N·C6H4·N(C2H5)2·2HCl] or 1.5g of sulfuric acid N,N-diethyl-p-phenylenediamine [H2N·C6H4·N(CaH5)2·H2SO4·5H2O], dissolve it into a chlorine-free pure water containing 8mL of sulfuric acid solution (1+3) and 0.2g of Na2-EDTA, and dilute to 1000mL. Store into a brown bottle and preserve in cool dark place.
Note: DPD solution is instable, it is not suitable to prepare too much in one time. If it gets darker or faded, it shall be re-prepared.
1.1.3.5 Potassium arsenite solution (5.0 g/L): weigh 5.0g of potassium arsenite (KASO2), dissolve it in pure water and dilute to 1000mL.
1.1.3.6 Thioacetamide solution (2.5 g/L): weigh 0.25g of thio acetamide (CH2CSNH2) and dissolve it in 100mL of pure water.
Note: Thio acetamide is a possible human carcinogen and never contacts with skin or is inhaled.
1.1.3.7 Chlorine water: add a small amount of chlorine water or high test bleaching powder solution into a chlorine free pure water so that the total residual chlorine concentration in water is about 0.5 mg/L. Heat and boil to remove chlorine. Use it when being cooled.
Note: Prior to use, add potassium iodide to inspect total residual chlorine by this standard.
1.1.3.8 Chlorine standard stock solution [ρ(Cl2)=1000 μg/mL]: weigh 0.8910g of guaranteed reagent potassium permanganate (KMnO4). Dissolve it with pure water and dilute to 1000mL.
Note: Preparation of standard solution with ammoniated water is complicated and instable. The test shows that the potassium permanganate amount is similar with the red produced by DPD and indicated residual chlorine in the standard solution.
1.1.3.9 Chlorine standard working solution [ρ(Cl2)=1 μg/mL]: suck 10.0mL of chlorine standard stock solution (1.1.3.8), dilute it with pure water until 100mL. Shake well, take 1.00 mL and then re-dilute until 100mL.
1.1.4 Apparatus
1.1.4.1 Spectrophotometer.
1.1.4.2 Colorimetric cylinder with stopper, 10mL.
1.1.5 Analytical procedure
1.1.5.1 Drawing of standard curve: suck 0, 0.1, 0.5, 2.0, 4.0 and 8.0 mL of chlorine standard working solution (1.1.3.9) and put them into 6 10mL colorimetric cylinder with stopper respectively and dilute to the scale with chlorine water (1.1.3.7). Add 0.5 mL of phosphate buffer solution (1.1.3.3) and 0.5 mL of DPD solution (1.1.3.4) into the six cylinders each, shake well, at 515 nm wave length and 1cm cuvette, take purity water as reference, determine absorbance and draw the standard curves.
1.1.5.2 Suck 10mL of water sample into a 10mL colorimetric tube, add 0.5 mL of phosphate buffer solution (1.1.3.3) and 0.5 mL of DPD solution (1.1.3.4), shake well, at 515 nm wave length and 1cm cuvette 1cm cuvette, take purity water as reference and immediately measure absorbance and record the reading as A, measure blank sample, deduct from the reading.
Note: If the content of chloramine is too high in the sample, the water sample may be processed by arsenite or thio acetamide.
Foreword I
1 Free Residual Chlorine
2 Available Chlorine in Chlorine Disinfectant
3 Chloramine
4 Chlorine Dioxide
5 Ozone
6 Chlorate
