Chlorine Residuals Measurement Terry Engelhardt Application Development Manager – Drinking Water Hach Company 1 Reaction with Water • Forms hydrochloric (HCl) and hypochlorous (HOCl) acids: + - Cl 2 + H 2O HOCl + H + Cl • Reaction is reversible. Above pH 4, reaction is to the right • HOCl dissociates to the hydrogen ion and hypochlorite ion (OCl -) varying with temperature and pH HOCl H + + OCl - 2 1 HOCl vs. OCl - 3 Free Available Chlorine • Chlorine existing in water as hypochlorous acid (HOCl) or the hypochlorite ion (OCl -) is defined as free available chlorine 4 2 Hypochlorite Salts • Salts used for chlorination include – Lithium hypochlorite LiOCl + - LiOCl + H 20 Li + HOCl + OH – Sodium hypochlorite NaOCl + - NaOCl + H 20 Na + HOCl + OH – Calcium hypochlorite Ca(OCl)2 2+ - Ca(OCl)2 + 2H20 Ca + 2HOCl + 2OH 5 Combined Chlorine - Chloramination • Chlorine (HOCl and OCl -) reacts with ammonia to form chloramines, commonly referred to as ‘combined chlorine’ • The predominate species are monochloramine and dichloramine. A small fraction is trichloramine or nitrogen trichloride 6 3 Breakpoint Curve Cl 2:N < 5:1 Cl 2:N > 5:1 Cl 2:N > 9:1 Free Residual Breakpoint Dichloramine predominates Total Chlorine Residual Chlorine Total Monochloramine predominates A BC Chlorine Added Breakpoint Curve Considerations • Shape of the curve is dependent upon – amount of ammonia and other chlorine demand substances in the water – temperature – pH – contact time • Most effective disinfection, least taste and odor occurs with free residual chlorine • Free chlorine may lead to formation of DBP 8 4 Chloramination • Chloramination: Purposeful use of chlorine and ammonia to form monochloramine. – Minimizes formation of chlorinated organics – Ammonia to chlorine Ratio is controlled to favor formation of monochloramine, typically 5:1 Cl 2:N • Total residual chlorine test: All free and combined chlorine species 9 Chloramine Formation • Monochloramine - NH 2Cl NH 3 + HOCl NH 2Cl + H 2O • Dichloramine - NHCl 2 NH 2Cl + HOCl NHCl 2 + H 2O • Tricholoramine (Nitrogen Trichloride) - NCl 3 NHCl 2 + HOCl NCl 3 + H 2O • Chloramines are not as effective disinfectants as free chlorine 10 5 Definition of Unreacted Ammonia • Ammonia in solution as – NH 3 Free ammonia gas dissolved in water or; - – NH 4 The ammonium ion 11 Breakpoint curve for chlorination and chloramination Cl 2:N <5:1 Cl 2:N >5:1 Cl 2:N >9:1 Total Ammonia Dichloramine Formation Free Ammonia Monochloramine Free Residual Formation Chlorine Total Residual Chlorine TotalResidual Total and Free Ammonia and Free Total Chlorine Added 12 6 Calculate Ratio as Cl 2:N! Atoms/ Ammonia, Atomic molecule NH 3 mass Nitrogen 14 X 1 = 14 Hydrogen 1 X 3 = 3 Molecular = 17 Weight (Mass) 3 x 100 Percent Hydrogen = 17.6 17 If the feed rate is calculated on NH 3 as NH 3 instead of as N, the feed rate is off 17.6%! 13 Effect of pH on Chloramine Species 100 0 • Distribution of 90 10 80 20 chloramine % % Monochloramine 70 30 species is 60 40 effected by: 50 50 40 60 – pH % % Dichloramine 30 70 – Ammonia 20 80 concentration 10 90 (see breakpoint curve) 0 100 5 6 7 8 9 pH 14 7 Comparison of Methods Method Range Detection %RSD Use Skill mg/l Level * DPD colorimetric 0-5 0.005 1-2 F & T 1 Ultra low-range 0-0.500 0.002 5-6 T 2 DPD colorimetric DPD titration 0-3 0.018 2-7 F & T 2 Iodometric Up to Total 1 NR 2 4% Oxidants Amperometric Up to 10 0.015 1-2 F & T 3 Titration - Forward -Back 0.006-1 0.006 15 T 3 Electrode Total 0-1 0.05 10 2 Oxidants Monochlor-F W 0-4.5 Mono- 0.09 2 1 WW 0-10 chloramine Skill Level: 1= Minimal training; 3 = Experienced *Under ideal laboratory conditions. Practical limit for all methods is really about 0.02 15 DPD-Chlorine Reaction Products H H H H H H H H N+ N+ N+ Cl 2 + N+ N+ N+ Et Et Et Et Et Et H AMINE WÜRSTER DYE IMINE (colorless) (magenta colored) (colorless) 16 8 DPD Würster Dye Absorbance Curve 530 nm Maximum sensitivity 512 nm 553 nm 510-515 nm 0.2500 0.1500 Absorbance 0.0500 400.00 440.00 480.00 520.00 560.00 600.00 Wavelength, nanometers Colorimetric Methods – Lab or Field Use Chlorine – DPD Chloramination – MonoChlor F 18 9 Measuring Free and Total Residual • Free residual • Total residual measurement measurement – Add sample to sample – Add sample to sample cell cell – Blank – Blank – Add reagents – Add reagents – Read within 1 minute – Wait 3 minutes – Read within 3-5 minutes 19 Test Kits Compara Test -tors Strips Chlorine X X MonoChloramine NA NA Avoid use of color comparators for regulatory reporting due to subjective errors 20 10 Common Interferences • Other oxidants: ClO 2, • Buffer capacity O3, Br 2, H 2O2, I2, • Sample color KMnO 4 • Mn +3 to Mn +7 • Disinfection by- • Cr +7 products, I.e. chlorite • Organic N-Cl (organic and chlorate chloramines in • Particulate wastewater) contamination - turbidity 21 Compensating for Manganese Interference • Split sample. Analyze • Subtract result of first portion as usual second portion from • Second Portion: first portion – Adjust pH w/1N Sample sulfuric acid Size 5 ml 10 ml 25 ml – Add drops of 30 g/l Adjust to Adjust to Adjust to H SO , 1N potassium iodide; 2 4 pH 6-7 pH 6-7 pH 6-7 wait one minute Potassium – Add drops of 5 g/l Iodide, 2 drops 2 drops 3 drops sodium arsenite 30 g/l – Add DPD and Sodium Arsenite, 2 drops 2 drops 3 drops complete test 5 g/l 22 11 Chemistry of Amperometric Titration • For total chlorine determinations, KI is oxidized by chlorine and chloramines, at pH 4, to form tri-iodide: − + − Cl 2 + 3KI → I3 + 3K + 2Cl • Then the two half reactions are : − − − I3 + 2e → 3I • Stoichiometry is thus 2:2 (titrant : sample) + − PhAsO + 4H 2O → PhAsO (OH )2 + 2H 3O + 2e Forward Titration • Amperometry – Electrochemical technique in which a small electrical voltage is applied across two electrodes – Chemical reactions caused by titrant addition cause a change in current, which is measured and recorded by the instrument 12 Forward Titration • Amperometry – Results are obtained by calculating the current change as a function of the amount of titrant added Forward Titration • Amperometry – A potential is applied across the electrodes prior to the titration. – Buffer is added to the sample and KI is added to total chlorine samples Potential Applied 13 Forward Titration • Amperometry – Current can flow as long as there is a substance that can be reduced at the cathode (+) and oxidized at the anode (-). + - Forward Titration • Amperometry (for free residual chlorine) – Chlorine is titrated with PAO titrant. The chlorine is reduced at the cathode. The PAO is oxidized at the anode. Chlorine reduced + - PAO Oxidized (or iodine for total chlorine determination) 14 Forward Titration • Amperometry – The more chlorine (or oxidant) in solution, the greater the amount of current flow. Current Forward Titration • Amperometry – As the PAO titrant is added, the PAO reduces the chlorine, and the chlorine concentration decreases. 15 Forward Titration • Amperometry – As the chlorine concentration decreases, the amount of current also decreases. Current Forward Titration • Amperometry – When all of the chlorine has been reduced by the PAO, the amount of current falls to near zero. Current 16 Forward Titration • Amperometry – The chlorine concentration is calculated based on the amount of PAO added to reduce the measured current to zero. Typical Amperometric Titration System Titrant delivery system Dual Platinum or Silver/Platinum Electrode 1.123 Microampere meter Magnetic Stirrer 34 17 Forward Titration • Titration curves and calculations On-line Chlorine Measurement Amperometric Probe – Free or Total Chlorine Monochlor F – Monochloramine, Colorimetric Free and Total DPD – Free or Ammonia Total Chlorine 36 18 Online Chlorine Monitoring – Major Technologies Colorimetric : Amperometric: measuring intensity of color developed measuring electrical current by reaction of chlorine with indicator generated in a circuitry by reaction of (chemical compound, e.g. DPD). The chlorine with electrodes . The larger deeper color, the higher chlorine current value, the higher chlorine concentration. concentration. Main Differentiators: Main Differentiators: • Independent of major sample • No chemical reagents required parameters (pH, flow, temperature), • Fast response to analyte • Established calibration curve concentration changes Really? 37 Online Chlorine Monitoring Comparison Colorimetric Amperometric Pros Pros •Accuracy - no calibration •Fast response •Unattended operation •Reagentless technology (up to 30 days) •No waste stream? •Predictable and simple maintenance •Results independent of changes in sample pH, Really? temperature, conductivity, sample pressure 38 19 Online Chlorine Monitoring Comparison Colorimetric Amperometric Cons Cons •Reagents and waste •Greater influence from stream management sample pH, temperature, flow, pressure, Cl 2 concentration, etc. 39 Keys to Application Success Steps to choosing your chlorine analyzer: 1. Look at the instrument's major performance specifications to make your initial decision. • Chlorine concentration range • Sample pH range 2. Next, consider each technology's key differentiators to determine which is preferred for your application. • Colorimetric • Amperometric 3. Finally, consider the treatment process details - key to application success to make sure that your preferred instrument is right for your application. 40 20 CL17 Chlorine Analyzer Pocket Colorimeter Use a portable colorimeter to verify operation of on- line chlorine analyzers. Do not use color comparitors 41 Monitoring Hypochlorite and Aqua Ammonia Bulk Solutions • Know what you’re buying • Know the concentration being used • Digital Titration or drop count (Bleach only) • 5-15% 42 21 Contact Information N Illinois S Illinois Paul Gauger Brad Baldwin Hach Company Hach Company 800-227-4224 X2060 800-227-4224 X2327 [email protected] [email protected] Terry Engelhardt Hach Company Application Development Manager – Drinking Water 800-227-4224 X2327 [email protected] 43 22.