1、Current Technology ofChlorine Analysisfor Water and WastewaterTechnical Information Series Booklet No.17By Danial L. HarpLit. no. 7019L21.5 Printed in U.S.A.Hach Company, 2002. All rights are reserved.In memory of Clifford C. Hach(1919 1990)inventor, mentor, leader and, foremost,dedicated chemistCur
2、rent Technology of Chlorine Analysis for Water and WastewaterTable of Contents Page1. Overview of Chlorine Chemistry in Water Treatment 12. Analytical Methods for Chlorine and Chloramines 2a. DPD Colorimetric Method 2b. DPD Titration Method 5c. Iodometric Titration Method6d. Amperometric Titration M
3、ethods 6e. Other Analytical Methods: 8Orthotolidine8Syringaldazine (FACTS) 9Electrode 93. Method Interferences and Sources of Errors 11a. Sampling Considerations 11b. Interferences Common to All Chlorine Methods 11Other Disinfectants11Manganese Compounds12Organic Chloramines 12Bromides in Chlorinate
4、d Waters 12c. Errors Common to Total Chlorine Determinations 12d. Interferences in the DPD Methods13Calibration Non-Linearity 13Precautions in Using Permanganate as an Equivalent Standard 13Monochloramine Interference in the Free Chlorine Test 15Stability of the Colored Reaction Product 16Compensati
5、on for Sample Color and Turbidity 16e. Interferences in the Amperometric Titration Methods 17Deposition on Electrode Surfaces 17Manganese Interference 17Nitrite Interference 17Choice of Reductant19Effect of Iodine Demand on End Point Determinations 19Order of Reagent Addition204. Method Comparisons
6、and Performance Evaluations 21a. Field Kit or Laboratory Comparisons 21b. Performance Evaluations of Residual Chlorine Process Analyzers 235. Selection of the Appropriate Testing System 24a. Field Testing 24b. Laboratory Testing 25c. On-line Automated Testing266. Conclusions 27References 29Acknowled
7、gements 301. Overview of Chlorine Chemistry inWater TreatmentChlorination of public water supplies has been practicedfor almost 100 years in the United States. Although thepros and cons of disinfection with chlorine have beenextensively debated, it remains the most widely usedchemical for disinfecti
8、on of water in the U.S.Comprehensive information explaining chlorinechemistry in water treatment is available in severalexcellent references describing chlorination anddisinfection practices. (See Ref. 1.1 - 1.4). An overviewemphasizing general chemistry of chlorine disinfectionwill be presented her
9、e.Chlorine usually is added to water as the gaseous formor as sodium or calcium hypochlorite. Chlorine gas rapidly hydrolyzes to hypochlorous acid according to the following equation:Cl2+ H2O HOCl + H+ ClSimilarly, aqueous solutions of sodium or calciumhypochlorite will hydrolyze according to:Ca(OCl
10、)2+ 2H2O Ca2+ 2HOCl + 2OHNaOCl + H2O Na+ HOCl + OHThe two chemical species formed by chlorine in water,hypochlorous acid (HOCl) and hypochlorite ion (OCl),are commonly referred to as “free available” chlorine.Hypochlorous acid is a weak acid and will disassociateaccording to:HOCl H+ OClIn waters wit
11、h pH between 6.5 and 8.5, the reaction isincomplete and both species (HOCl and OCl) will bepresent. Hypochlorous acid is the more germicidal ofthe two.A relatively strong oxidizing agent, chlorine can reactwith a wide variety of compounds. Of particular importance in disinfection is the chlorine rea
12、ction withnitrogenous compoundssuch as ammonia, nitrites andamino acids.Ammonia, commonly present in natural waters, will reactwith hypochlorous acid or hypochlorite ion to formmonochloramine, dichloramine and trichloramine,depending on several factors such as pH andtemperature. Typical reactions fo
13、llow:NH3+ HOCl NH2Cl (monochloramine)+ H2ONH2Cl + HOCl NHCl2(dichloramine)+ H2ONHCl2+ HOCl NCl3(trichloramine) + H2OKnown as “break-point” reactions, they are important inwater disinfection. The chloramines are potent biocidesbut not as effective as hypochlorous acid or thehypochlorite ion.Chlorinat
14、ion of water to the extent that all ammonia isconverted to either trichloramine or oxidized to nitrogenor other gases is referred to as “break-point chlorination.”Figure 1.1 shows a typical break-point chlorinationcurve. Prior to the break point,“combined” chlorine(monochloramine plus dichloramine)
15、predominates.In disinfection systems in which chloramination ispracticed, the goal is to remain at the peak of the curveprior to the break point. If the amount of unreactedammonia is minimized, monochloramine will be thepredominant chloramine.After the break point, free chlorine (hypochlorous acidpl
16、us hypochlorite) is the dominant disinfectant. Typically,the free chlorine residual is adjusted to maintain aminimum level of 0.2 mg/L Cl2throughout thedistribution system.The importance of break-point chlorination lies in thecontrol of taste and odor and increased germicidalefficiency. The killing
17、power of chlorine on the right sideof the break point is 25 times higher than that of the leftside (Ref. 1.1). Hence, the presence of a free chlorineresidual is an indicator of adequate disinfection. Theshape of the break-point curve is very dependent oncontact time, water temperature, concentration
18、s ofammonia and chlorine, and pH.1Figure 1.1: Typical Break-point Chlorination CurveThe use of monochloramine as an alternate disinfectantfor drinking water has received attention lately due toconcern about the possible formation of chlorinated by-products when using free chlorine disinfection.Consi
19、derable debate continues about the merits ofchloramination disinfection. The reader is referred toWhites handbook (Ref. 1.1) for an animated discussion ofthe pros and cons of chloramination practices in drinkingwater treatment.In chloramination disinfection, monochloramine isformed from the reaction
20、 of anhydrous ammonia andhypochlorous acid. In general, ammonia is added firstto avoid formation of chlorinated organic compounds,which can exhibit objectionable taste and odors. Hachoffers a method specific for inorganic monochloraminedisinfectant in the presence of organic chloramines(Ref 1.2).Thr
21、oughout the U.S. Today, wastewater effluents arechlorinated to kill pathogens and then dechlorinatedbefore discharge. This common practice resulted fromseveral comprehensive studies (Ref. 1.5) which quantifiedthe toxicity of chlorinated effluents on aquatic life. Theamount of total residual chlorine
22、 in the final effluent isregulated by a National Pollutant Discharge EliminationSystem (NPDES) permit. Typical permit limits for totalresidual chlorine (TRC) in the final effluent range from0.002 to 0.050 milligram per liter (mg/L). To thechlorination-dechlorination practitioner, this leveltranslate
23、s to zero mg/L TRC.Dechlorination by sulfur dioxide (SO2) is the mostcommon process to meet zero TRC effluent limits.Sodium bisulfite and sodium metabisulfite also have beenused for chemical dechlorination. In the dechlorinationprocess using SO2, sulfurous acid is formed first:SO2+ H2O H2SO3Sulfurou
24、s acid then reacts with the various chlorineresidual species:H2SO3+ HOCl HCl + H2SO4H2SO3+ NH2Cl + H2O NH4Cl + H2SO42H2SO3+ NHCl2+ 2H2O NH4Cl + HCl + 2H2SO43H2SO3+ NCl3+ 3H2O NH4Cl + 2HCl + 3H2SO4It is common practice to overdose the sulfur dioxide tomaintain a level up to 5 mg/L SO2in the effluent.
25、 Thisensures the reduction of all chlorine residual species.2. Analytical Methods for Chlorineand Chloramines2a. DPD Colorimetric MethodThe DPD (N, N-diethyl-p-phenylenediamine) method forresidual chlorine was first introduced by Palin in 1957(Ref. 2.1). Over the years it has become the most widelyu
26、sed method for determining free and total chlorine inwater and wastewater. Hach Company introduced its firstchlorine test kit based on the DPD chemistry in 1973.The chemical basis for the DPD chlorine reaction isdepicted in Figure 2.1. The DPD amine is oxidized bychlorine to two oxidation products.
27、At a near neutral pH,the primary oxidation product is a semi-quinoid cationiccompound known as a Wrster dye. This relatively stablefree radical species accounts for the magenta color in theDPD colorimetric test. DPD can be further oxidized to arelatively unstable, colorless imine compound. WhenDPD r
28、eacts with small amounts of chlorine at a nearneutral pH, the Wrster dye is the principal oxidationproduct. At higher oxidant levels, the formation of theunstable colorless imine is favored resulting inapparent “fading” of the colored solution.The DPD Wrster dye color has been measuredphotometricall
29、y at wavelengths ranging from 490 to 555nanometers (nm). The absorption spectrum (Figure 2.2)indicates a doublet peak with maxima at 512 and 553 nm.For maximum sensitivity, absorption measurements can be made between 510 and 515 nm. Hach Company hasselected 530 nm as the measuring wavelength for mos
30、t of its DPD systems. This “saddle”between the peaksminimizes any variation in wavelength accuracy betweeninstruments and extends the working range of the teston some instruments.2Figure 2.1: DPD-Chlorine Reaction ProductsMonochloramine and dichloramine are slow to reactdirectly with DPD at a near n
31、eutral pH. To quantify thesespecies, the test is performed under slightly acidicconditions in the presence of iodide ion. The iodidereacts with the chloramines to form iodine as thetriiodide ion (I3):NH2Cl + 3I+ H2O + H+ NH4OH + Cl+ I3NHCl2+ 3I+ H2O + 2H+ NH4OH + 2Cl+ I3The triiodide, in turn, react
32、s with DPD, forming theWrster oxidation product. There is very little confirmedevidence that trichloramine species can be quantifiedwhen using iodide with DPD (Ref. 2.2).In practice, only a trace of iodide is required at pH 6.2- 6. 5 to resolve monochloramine. Standard Methods forthe Examination of
33、Water and Wastewater (Ref. 2.3)stipulates the addition of approximately 0.1 mg ofpotassium iodide to a 10-mL sample to determinemonochloramine. By adding excess potassium iodide (anadditional 0.1 gram or more per 10-mL sample),dichloramine is included. It is not entirely clear at whatlevel of iodide
34、 the dichloramine fraction begins tointrude into the monochloramine results.Two “standard” DPD colorimetric methods generally arerecognized in the international community. These arethe Standard Methods 4500-Cl G and InternationalOrganization for Standardization (ISO) Method 7393/2(Ref. 2.4). The ISO
35、 method has been adopted by most ofthe members of the European Union. Germanys DINStandard 38 408 G4 for free and total chlorine ismodeled after ISO 7393/2. Table 2.1 shows the maindifferences between Standard Methods 4500-Cl G andISO 7393/2.Both Standard Methods and ISO procedures call forliquid DP
36、D reagents prepared from DPD sulfate or DPDoxalate salts. Liquid DPD reagents, inherently unstable, aresubject to oxidation from either atmospheric oxygen ordissolved oxygen present in the preparation water. It hasbeen shown that the oxidation of DPD by oxygen ispHdependent (Ref. 2.5). The liquid DP
37、D formulationsattempt to retard oxidation by lowering the pH of theindicator reagent.The liquid formulations also incorporate disodiumethylenediamine tetraacetate (Na2EDTA) in order to“retard deterioration due to oxidation and, in the testitself, provide suppression of dissolved oxygen errors byprev
38、enting trace metal catalysis”(Ref 2.6). The practice of adding Na2EDTA to the DPD indicator reagent isquestionable because of the low solubility of EDTA indilute acid solutions.Standard Methods and ISO procedures both usephosphate buffers to adjust the sample pH to between6.2 and 6.5. The slightly a
39、cidic pH is preferred toquantitatively resolve the chloramine species and tominimize interferences. Phosphate buffers, however, donot work in hard or brackish waters. Calcium andmagnesium ions in the sample will precipitate thephosphate and destroy the buffering capacity (Ref. 2.7).Because aqueous p
40、hosphate solutions are excellentgrowth media for biological growth, highly toxicmercuric chloride is added to preserve the reagent.3Figure 2.2: Absorption Spectrum - DPD Wrster CompoundHach Company DPD powder formulations overcome thedisadvantages of using liquid reagents. The DPDindicator and buffe
41、r are combined in powder form,minimizing degradation by oxidation and microbialaction. Because Hachs DPD powder indicator does notexist in an ionized state, it is not subject to air oxidationas is the liquid DPD reagent. Hachs combined DPDreagents also incorporate EDTA to prevent metal-catalyzed oxi
42、dation.Hachs buffer component makes use of a carboxylate-phosphate system which works extremely well in highhardness and brackish water samples. Up to 1000 mg/LCaCO3hardness can be tolerated with either the free ortotal chlorine powder formulations. Mercuric salts are notused in any of Hach Company
43、DPD formulations.Hach Companys DPD powder reagents are quite stablewhen protected from moisture, light and temperatureextremes. Excellent reagent stability is achieved bysealing the reagent in unit-dose foil pouches. AccuVacDPD reagent ampuls are air-evacuated and hence areprotected from oxidation a
44、nd moisture. It isrecommended that all DPD reagents, both liquids andpowders, be stored between 10 to 25 C (50 to 77 F)for greatest stability.Hach Company has produced a stable liquid DPDreagent. The DPD Indicator Solution for Ultra Low Range(ULR) Chlorine, Cat. No. 24932, is sealed in a unit-doseam
45、pule under argon gas. The use for this reagent is intrace determinations of total chlorine in water andwastewaters. Liquid reagents are preferred for trace levelsof chlorine less than 20 micrograms per liter (g/L).Powdered reagents typically leave a very smallundissolved residue when added to the wa
46、ter sample.Although the resulting turbidity is not evident visually, itmay be sufficient to interfere in trace colorimetricmeasurements. Shelf studies indicate the ULR-DPDreagent exhibits no loss in sensitivity to chlorine over aone-year period (Ref 2.8).For trace determinations of chlorine, purity
47、of the bufferand iodide components are critical. Organic bufferimpurities can exhibit a “chlorine demand” when addedto a sample containing trace amounts of chlorine. Asstated previously, phosphate buffers generally are uselessin samples containing hardness. Liquid phosphatebuffers can contain insolu
48、ble impurities ormicrobiological growth which may cause turbidity whenadded to the sample. Iodide often contains iodine oriodate impurities which react directly with the DPDindicator. Exposure to oxygen and light will graduallyoxidize iodide to triiodide even in the solid state.A stable liquid buffe
49、r/iodide reagent developed by Hachis suitable for trace chlorine analysis. The ULR ChlorineBuffer, Cat. No. 24931, is specially treated to remove anychlorine demand from its components. Iodide in thereagent is controlled to minimize oxidation impurities.The ULR Chlorine Buffer is packaged under argon in alight-protected, unit-dose ampule.Another important consideration for trace analyticalmeasurements is the “reagent blank.”This is the amountof interference due to the addition of the reagents. In theDPD colorimetric test fo
