It’s hard to tell the difference between free chlorine and residual chlorine

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The following are the analysis methods for residual chlorine and free chlorine: Determination of residual chlorine. This method is suitable for determining the residual chlorine content in water samples. Principle: In a phosphate-buffered solution, free chlorine reacts with the DPD reagent to form a red complex; this red complex is then titrated using an ammonium ferrous sulfate standard solution until the red color disappears, which marks the end of the titration. Instruments and reagents: a) Phosphate buffer solution: Weigh 24 g of anhydrous disodium hydrogen phosphate (or 60.5 g of disodium hydrogen phosphate·12H2O) and 46 g of anhydrous potassium dihydrogen phosphate, dissolve them in distilled water; separately, dissolve 800 mg of disodium ethylenediaminetetraacetate in 100 ml of distilled water. Combine the two solutions and add distilled water to bring the total volume to 1000 mL. b) DPD solution (N,N-diethyl-1,4-phenylenediamine sulfate) indicator solution: Weigh 1 g of DPD oxalate or 1.5 g of the pentahydrate sulfate, or 1.1 g of the anhydrous sulfate, and dissolve it in 8 mL of sulfuric acid (1:3) and 20 mg of Na2-EDTA in chlorine-free distilled water; then dilute with chlorine-free distilled water to a total volume of 1000 mL. Store in a brown glass bottle with a glass stopper, in a dark place. If the solution fades, discard it immediately. Regularly check the solution blank; if its absorbance at 515 nm is greater than 0.002/cm, discard it immediately (1000 ml – 20 days). c) Ammonium ferrous sulfate standard solution: 0.0025 mol/L ； Analysis steps: a) Using a pipette, transfer 5.00 ml of phosphate buffer solution and 5.00 ml of DPD solution into a 250 mL conical flask. b) Add 100 mL of water sample to the above solution and mix well. c) If the solution is colorless, the residual chlorine is zero ； d) If the solution turns red, it indicates the presence of residual chlorine; titrate with an 0.0025 mol/L ammonium ferrous sulfate solution until the red color disappears as the endpoint. Result calculation: In the formula: C — concentration of the standard solution of ammonium ferrous sulfate, in mol/l ； V—Volume of standard solution consumed by the sample, ml ； Analysis of free chlorine content. This method is suitable for the determination of free chlorine content in saline solutions. Principle: In an acidic medium, free chlorine reacts with potassium iodide to produce iodine. Starch is used as an indicator, and sodium thiosulfate solution is added drop by drop until the blue color disappears, indicating the endpoint. The reactions are as follows: C12 + 2KI → I2 + 2KCl; I2 + 2Na2S2O3 → 2NaI + Na2S4O6. Instruments and reagents: 1) General laboratory instruments ； 2) Starch indicator: 10g/l ； 3) Potassium iodide solution: 100 g/l ； 4) Acetic acid solution: 10% ； 5) Sodium thiosulfate standard solution: C(Na2S2O3)=0.02 mol/L. Analysis procedure: Measure 100 ml of the sample and place it in a 250 ml iodometric flask. Add 10 ml of potassium iodide solution and 10 ml of acetic acid solution. Seal the flask with a water seal and leave it in the dark for 2–3 minutes. Then titrate with sodium thiosulfate standard solution until a light yellow color is reached. Add 1 ml of starch indicator, and continue titrating with the standard solution until the blue color disappears, at which point the reaction is complete. Result calculation: In the formula, C – the molar concentration of the sodium thiosulfate standard solution, in mol/L ； V – Volume of sodium thiosulfate solution consumed in titration, ml ； 35.45――grams per mole of chlorine, g. What is the difference between free chlorine and residual chlorine? Are both measured as chloride ions?