What should be done if the chloride ion concentration in the effluent is too high and affects the COD determination?

　　During COD analysis,chloride ions in water samples are readily oxidized by the oxidizing agent,and their high concentrations can lead to elevated measurement results.Determining COD in wastewater with high chloride and low organic carbon content remains a significant challenge.

　　In practical monitoring,it has been observed that many types of wastewater—such as chemical industry effluent,monosodium glutamate production wastewater,and seafood-processing wastewater—contain very high chloride(Cl⁻)concentrations,necessitating the suppression of Cl⁻prior to COD determination.

　　1,Interference of CI-on COD determination

　　In GB 11914-89“Determination of Chemical Oxygen Demand in Water—Potassium Dichromate Method,”it is explicitly stated that when the chloride ion(Cl⁻)concentration in a water sample is excessively high,a masking agent such as mercuric sulfate must be added to suppress its interference.The primary influencing factors can be summarized as follows:

　　1,Cl⁻is oxidized by the oxidizing agent,thereby consuming the oxidizing agent and leading to an overestimated analytical result.The specific reaction equation is:

　　6Cl⁻+Cr₂O₇²⁻+14H⁺→3Cl₂+2Cr³⁺+7H₂O

　　2 In the reaction system,the addition of a silver salt as a catalyst leads to the formation of an AgCl precipitate upon reaction with Cl⁻,which deactivates the catalyst and adversely affects the measurement results.

　　2 Methods for Eliminating Chloride Interference in COD Determination

　　Diluting water samples is one of the simplest and most effective approaches;the national standard also recommends diluting samples with chloride concentrations exceeding 1,000 mg/L.However,for high-chloride,low-COD water samples,excessively high dilution factors can compromise analytical accuracy.Currently,numerous methods are available to eliminate chloride interference,including the mercuric salt method,the silver salt precipitation method,the standard curve correction method,the chlorine gas correction method,the closed-vessel digestion method,the low-concentration oxidant method,the KI–KMnO4 oxidation method,and the bismuth absorbent–chloride removal method,among others.

　　Mercury salt method

　　The mercuric salt method,also known as the mercuric sulfate complexation method,is the standard national procedure for masking chloride ions.Specifically,HgSO4 is used as a masking agent for Cl⁻,with an optimal mass ratio of HgSO4 to Cl⁻of 10:1.

　　This method is highly effective when the chloride ion concentration is below 200 mg/L;however,at higher chloride concentrations,the measured results remain biased high,and the error increases with increasing chloride levels.

　　Because HgSO4 is highly toxic,mercury salts in the waste effluent are difficult to treat,and they can cause secondary environmental pollution,many countries now discourage the use of this method for eliminating chloride ion interference,favoring instead the development of non‑toxic,pollution‑free analytical approaches.

　　Error in Determining COD of Wastewater by the National Standard Dichromate Method Due to Cl⁻Interference

　　Silver salt method

　　The silver salt precipitation method involves adding AgNO3 to form an AgCl precipitate,thereby removing Cl⁻interference;it is suitable for water samples with a Cl⁻concentration exceeding 10,000 mg/L.

　　This method typically takes one of two forms:In the first,AgNO3 is added during pretreatment,and the supernatant is analyzed for COD.This approach requires that the amount of AgNO3 be carefully controlled—just enough to precipitate all Cl⁻without excess.The second method uses AgNO3 and KCr(SO4)2 as masking agents for Cl⁻;the latter serves to suppress the oxidation of trace amounts of Cl⁻during the digestion process.

　　The silver salt precipitation method employs costly silver salts,thereby increasing the analytical cost;hence,the recovery and reuse of silver are highly necessary.Another drawback is that,during the formation of AgCl precipitate,co-precipitation and flocculation can lead to the loss of a portion of the organic matter in the water sample,resulting in underestimation of the analytical results.

　　Standard Curve Calibration Method

　　Procedure for the standard curve correction method:First,prepare a series of sodium chloride standards with varying chloride concentrations and determine their COD values to construct a COD–Cl⁻standard curve.Next,take two aliquots of the same water sample:in one aliquot,measure the COD without masking the chloride ions and record this as COD_total;in the other aliquot,determine the chloride ion concentration and use the standard curve to obtain the corresponding COD value,which is recorded as COD·Cl⁻.The difference between COD_total and COD·Cl⁻represents the true COD value of the sample.

　　The standard curve calibration method does not use mercury or silver salts,making it environmentally friendly and cost-effective;it is the preferred approach in laboratories.

　　Chlorine Correction Method

　　During the digestion process,a reflux‑absorption apparatus is employed to collect the evolved Cl₂,which is then absorbed by an NaOH solution.The solution is titrated with a standard Na₂S₂O₃solution,and the amount of Na₂S₂O₃consumed is converted into the equivalent oxygen demand,yielding the corrected value for Cl⁻.

　　The actual COD value of the wastewater is the difference between the apparent COD value and the correction value for chloride ions.This method is suitable for determining the COD in high-chloride wastewater with a chloride ion concentration below 20,000 mg/L and a COD greater than 30 mg/L.However,the method requires meticulous experimental procedures;otherwise,it may introduce additional errors.

　　Sealed digestion method

　　The underlying principle is that COD is determined by digestion in a sealed vessel;when chloride ions(Cl⁻)in the water are oxidized to chlorine gas(Cl₂)and equilibrium between the gas and liquid phases is established,further oxidation of Cl⁻ceases.By employing an appropriate masking agent,the COD of the sample can be accurately measured.

　　Compared with the standard method,this approach yields results with higher accuracy and precision.Data indicate that the sealed digestion method provides greater accuracy for COD analysis of high-chloride wastewater;when the COD ranges from 100 mg/L to 1000 mg/L and the chloride ion concentration is below 10,000 mg/L,the relative error of this method does not exceed 4.2%.Although sealed digestion is rapid,it carries certain risks;therefore,experimental safety must be strictly ensured.

　　KI-KMnO4 Oxidation method

　　Under alkaline conditions,substances in the wastewater are oxidized with KMnO4;the excess KMnO4 is then reduced with KI,followed by titration with a standard Na2S2O3 solution.The amount of Na2S2O3 consumed is converted into an equivalent oxygen demand,yielding a COD value.

　　However,the results obtained using this method differ from those of the K2Cr2O7 oxidation method;the two methods yield values that are related by a ratio,K.Therefore,once this ratio K is known,the COD value measured by the KI–KMnO4 oxidation method can be converted accordingly.This approach is suitable for wastewater with chloride concentrations ranging from several tens of thousands to several hundred thousand milligrams per liter,but it requires the determination of the K value,making the procedure rather cumbersome.

　　Bismuth absorbent method for chlorine removal

　　In the water sample,Cl⁻is released as HCl gas in an acidic medium,absorbed and removed by a bismuth absorbent,after which the COD value is determined.

　　The accuracy and precision of this method are not significantly different from those of the standard method;however,the digestion procedure differs,employing either microwave digestion or oven digestion.

　　The methods for eliminating Cl⁻interference described above all have limitations in practical applications,and each requires further refinement.At present,when the Cl⁻concentration in the water sample is≤1000 mg/L,we prioritize the national standard method;when the Cl⁻concentration exceeds 1000 mg/L and the COD level is relatively low,the standard curve method may be considered.