4.1 Chlorine Chemistry, Forms & Breakpoint Chlorination

4.1 Chlorine Chemistry, Forms & Breakpoint Chlorination

Disinfection is the targeted destruction or inactivation of disease-causing pathogenic organisms—including bacteria, viruses, and protozoan cysts—to prevent waterborne disease transmission. Unlike sterilization, which destroys all living microorganisms, disinfection reduces pathogen counts to safe public health thresholds. In drinking water (potable compliance) and wastewater treatment (protecting receiving waters), chlorine remains the predominant chemical disinfectant due to proven efficacy, ease of measurement, and protective residual capability.

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1. Commercial Forms of Chlorine & Physical Properties

Water and wastewater utilities apply chlorine in three primary commercial configurations:

Because chlorine gas is 2.5 times heavier than air, leaking vapors accumulate in trenches and basements, mandating floor-level exhaust and top fresh air louvers. Sodium hypochlorite requires climate-controlled storage (< 20°C) in UV-inhibited tanks to prevent rapid strength loss. Calcium hypochlorite must be strictly segregated from petroleum products to avoid spontaneous combustion, and requires decanting or filtering to keep calcium carbonate scale from plugging feed equipment.

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2. Chlorine Reactions in Water & The HOCl / OCl- Equilibrium

When chlorine contacts water, it undergoes instantaneous hydrolysis followed by pH-dependent dissociation.

Hydrolysis Reactions

Chlorine gas hydrolyzes in milliseconds to yield hypochlorous acid ($HOCl$), hydrogen ions ($H^+$), and chloride ions ($Cl^-$): $Cl_2 + H_2O \rightleftharpoons HOCl + H^+ + Cl^-$

This reaction produces hydrochloric acid, consuming natural alkalinity. In contrast, sodium hypochlorite dissolves to form hypochlorous acid and hydroxide alkalinity, mildly elevating pH: $NaOCl + H_2O \rightleftharpoons HOCl + Na^+ + OH^-$

Acid Dissociation & Germicidal Potency

Hypochlorous acid ($HOCl$) dissociates reversibly into hydrogen ions ($H^+$) and hypochlorite ions ($OCl^-$): $HOCl \rightleftharpoons H^+ + OCl^-$

The dissociation constant ($pK_a$) is approximately 7.5 at 25°C. Water pH dictates the operational distribution:

• pH 6.5: ~90% $HOCl$ and 10% $OCl^-$
• pH 7.5: ~50% $HOCl$ and 50% $OCl^-$
• pH 8.5: ~10% $HOCl$ and 90% $OCl^-$

Hypochlorous acid ($HOCl$) is 80 to 100 times more potent as a germicide than the hypochlorite ion ($OCl^-$). Bacterial cell walls and viral capsids carry a negative electrical charge. Because $HOCl$ is an electrically neutral, uncharged molecule, it encounters no electrostatic barrier and rapidly diffuses through the fatty lipid bilayer of the microbial cell membrane, oxidizing vital sulfhydryl ($-SH$) bonds in respiratory enzymes. Conversely, the negatively charged hypochlorite ion ($OCl^-$) is repelled by the cell's electrostatic field, severely retarding penetration. Consequently, disinfection kinetics decline drastically at pH values above 7.5 to 8.0, requiring higher doses or longer contact times.

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3. Free Available Chlorine vs. Combined Available Chlorine

Chlorine residual exists in two primary states:

1. Free Available Chlorine: The sum of unreacted hypochlorous acid and hypochlorite ions ($[HOCl] + [OCl^-]$). Fast-acting, potent oxidant that dissipates rapidly in distribution mains.
2. Combined Available Chlorine: Reaction products of chlorine with ammonia-nitrogen ($NH_3\text{-}N$) or organic nitrogen to form chloramines:
   • Monochloramine ($NH_2Cl$): $NH_3 + HOCl \rightleftharpoons NH_2Cl + H_2O$. Favored at pH 7.5–8.5 and chlorine-to-ammonia weight ratios up to 5:1. Stable, non-irritating secondary disinfectant.
   • Dichloramine ($NHCl_2$): $NH_2Cl + HOCl \rightleftharpoons NHCl_2 + H_2O$. Favored at acidic pH (5.0–6.5). Unstable, strong "swimming pool" odor, causes eye irritation.
   • Trichloramine ($NCl_3$): $NHCl_2 + HOCl \rightleftharpoons NCl_3 + H_2O$. Forms at pH < 4.5 or high chlorine excess (> 7.6:1). Pungent, volatile lachrymator causing severe eye tearing.

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4. The Breakpoint Chlorination Curve

When chlorine is continuously added to water containing reducing minerals and ammonia, the residual follows a four-zone curve:

1. Zone 1: Initial Chlorine Demand (Inorganics Oxidation): Chlorine immediately oxidizes reducing compounds including ferrous iron ($Fe^{2+}$), manganous manganese ($Mn^{2+}$), hydrogen sulfide ($H_2S$), nitrite ($NO_2^-$), and readily oxidizable organic matter. No residual forms; measured chlorine residual remains 0.0 mg/L.
2. Zone 2: Formation of Chloramines & Chloro-Organics: After satisfying immediate demand, chlorine reacts with ammonia to form chloramines (primarily monochloramine). Total chlorine residual rises steadily to a peak at approximately a 5:1 $Cl_2:NH_3\text{-}N$ weight ratio.
3. Zone 3: Destruction of Chloramines: Adding chlorine past the 5:1 peak oxidizes chloramines into inert nitrogen gas ($N_2$), nitrous oxide ($N_2O$), and hydrochloric acid: $2NH_2Cl + HOCl \rightarrow N_2\uparrow + 3HCl + H_2O$ Total residual declines to a minimum known as the breakpoint, occurring at a $Cl_2:NH_3\text{-}N$ ratio between 7.6:1 and 10:1. Offensive chlorinous odors and taste complaints peak within this zone.
4. Zone 4: Free Available Chlorine Residual: Once all ammonia is completely oxidized at the breakpoint, all further added chlorine remains as free available chlorine ($HOCl$ and $OCl^-$). The curve rises along a 1:1 slope (each 1.0 mg/L chlorine added produces a 1.0 mg/L increase in free residual).

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5. DPD Colorimetric Testing & Field Troubleshooting

Residual analysis relies on the DPD (N,N-diethyl-p-phenylenediamine) colorimetric method (Standard Methods 4500-Cl G) measured spectrophotometrically at 515 nm:

$\text{Combined Chlorine Residual} = \text{Total Chlorine Residual} - \text{Free Chlorine Residual}$

Analytical Interferences & Remedies

• Monochloramine Bleed: In samples containing high chloramines, monochloramine slowly reacts with DPD #1 if reading is delayed beyond 60 seconds, yielding a false-positive free chlorine reading. Read free chlorine strictly within 60 seconds.
• Oxidized Manganese Interference: Oxidized manganese ($Mn^{4+}$, e.g., permanganate) directly oxidizes DPD into the pink semiquinoid dye, creating false-positive readings. Treat a duplicate sample with sodium arsenite ($NaAsO_2$) to destroy chlorine, measure the resulting manganese absorbance, and subtract it from the raw reading.
• Color Bleaching: Chlorine concentrations exceeding 10 mg/L over-oxidize DPD into a colorless imine compound, causing a false zero or low reading. Dilute high-chlorine samples with deionized demand-free water before analysis.