Regarding the issue of residual chlorine

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Determination of residual chlorine: This method is applicable to the determination of 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, indicating 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 these two solutions and add distilled water until the total volume reaches 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 the mixture to 1000 mL with chlorine-free distilled water. 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) Ferrous ammonium 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 aforementioned 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, mol/l ； V—Volume of standard solution consumed by the sample, ml ； This is the analysis method for residual chlorine in water. Recently, we installed a new incinerator, which has a type of alkali scrubber that absorbs hydrogen chloride and hydrogen fluoride from the flue gases; it is necessary to determine the residual chlorine level inside this scrubber. The sample is alkaline and contains iron ions. The sample must be treated first by adding 32% alkali to eliminate the effect of iron ions, and then the procedures should be carried out accordingly. Does this method require the sample to be neutral, or is an alkaline pH acceptable? Is it necessary to adjust the pH?