10.2 Physical & Chemical Drinking Water Testing: Turbidity, pH, Alkalinity, Hardness & Chlorine (DPD)

Physical & Chemical Drinking Water Testing: Turbidity, pH, Alkalinity, Hardness & Chlorine (DPD)

Drinking water treatment plants rely on precise bench-top and continuous on-line laboratory instruments to verify regulatory compliance under CDPHE Regulation 11 (Colorado Primary Drinking Water Regulations) and optimize chemical coagulation, softening, corrosion control, and disinfection.

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1. Turbidity Measurement via Nephelometry

Turbidity is an optical property of water causing light to be scattered and absorbed rather than transmitted in straight lines. In drinking water, suspended colloidal particles (clays, silts, organic matter, and microorganisms) shield pathogens from chemical disinfectants. Turbidity is quantified using a nephelometer and expressed in Nephelometric Turbidity Units (NTU).

                                [ Tungsten Lamp / LED (860 nm) ]
                                              |
                                              v (Incident Light Beam)
                                    +-------------------+
                                    |   Sample Cell     |
                                    |   (Indexed Glass) |
                                    +-------------------+
                                     /                 \
     (Transmitted Light Trap) <-----                    -----> [ 90-Degree Scattered Light Detector ]
                                                                                 |
                                                                                 v
                                                                      [ Nephelometer Digital Display ]
                                                                      [ (Output in NTU)              ]

Analytical Principles of Nephelometry

• 90-Degree Light Scatter: Standard Methods 2130B and EPA Method 180.1 specify that the primary photodetector must be positioned at a $90^\circ \pm 30^\circ$ angle relative to the incident light path. Scattered light intensity is directly proportional to particulate concentration.
• Light Source Specifications: Incandescent tungsten filament lamp operated at a color temperature of $2200–3000\text{ K}$, or an infrared LED light source ($860\text{ nm}$) complying with ISO 7027 (minimizes dissolved color interferences).

Calibration Standards & Sample Handling

1. Primary Standards: Formazin is the international primary reference standard. A 4000 NTU stock solution is synthesized by combining hydrazine sulfate ($\text{N}_2\text{H}_4\cdot\text{H}_2\text{SO}_4$) and hexamethylenetetramine ($(\text{CH}_2)_6\text{N}_4$) in ultrapure water, allowing 24 hours for polymer suspension maturation. AMCO Clear (styrene divinylbenzene copolymer beads) is also an EPA-approved primary standard that requires no mixing or dilution.
2. Secondary Standards: Sealed gel or silicone secondary standards are used strictly for daily calibration verification and drift checks. Secondary standards must never be used to calibrate or re-span a turbidimeter.
3. Optical Sample Cuvette Preparation: Cuvettes must be constructed of optical borosilicate glass. Clean cuvettes with non-abrasive laboratory detergent, rinse extensively with deionized water, and wipe with lint-free wipes (Kimwipes). Apply a single drop of high-purity silicone indexing oil to the exterior glass and buff with a velvet cloth to fill microscopic glass scratches that would otherwise refract incident light. Always align the cuvette index mark with the instrument indexing arrow.

Colorado Regulatory Turbidity Benchmarks

• Combined Filter Effluent (CFE): Must be $\le 0.30\text{ NTU}$ in at least 95% of monthly measurements, and must never exceed $1.0\text{ NTU}$ at any time.
• Individual Filter Effluent (IFE) Triggers: Continuous recording every 15 minutes. An IFE exceeding $0.50\text{ NTU}$ after 4 hours of continuous operation, or exceeding $1.0\text{ NTU}$ in two consecutive 15-minute readings, or exceeding $2.0\text{ NTU}$ in two consecutive readings within 15 minutes of startup triggers mandatory filter profiling or a comprehensive filter self-assessment.

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2. Electrometric pH Measurement & Electrode Calibration

pH represents the negative logarithm of hydrogen ion activity ($\text{pH} = -\log_{10}[\text{H}^+]$). In water treatment, pH governs coagulant hydrolysis, chemical precipitation, chlorine speciation, and lead/copper pipe passivation.

The Combination pH Glass Electrode

Electrometric pH systems utilize a combination glass electrode comprising two electrochemical half-cells integrated into a single probe:

1. Measuring Half-Cell: Contains a pH-sensitive, hydrated lithium glass bulb enclosing an internal reference element ($\text{Ag/AgCl}$) in a buffer of constant pH (typically pH 7.00). An electrical potential develops across the glass membrane proportional to the difference in hydrogen ion activity between the internal solution and the external sample.
2. Reference Half-Cell: Contains a silver/silver chloride ($\text{Ag/AgCl}$) reference wire submerged in a saturated electrolyte solution ($3.0\text{ M } \text{KCl}$). The liquid junction (porous ceramic plug or Teflon wick) maintains continuous electrical contact with the sample.

The Nernst Equation & Temperature Compensation

The electrical potential ($E$) generated across the electrode membrane is governed by the Nernst Equation:

$E = E_0 - \left(\frac{2.303 \cdot R \cdot T}{n \cdot F}\right) \cdot \text{pH}$

At $25.0^\circ\text{C}$ (298.15 K), the theoretical Nernstian slope is $-59.16\text{ mV per pH unit}$. Because this voltage slope varies with absolute temperature ($T$), laboratory meters must incorporate an Automatic Temperature Compensation (ATC) probe to adjust the millivolt-to-pH conversion slope.

Theoretical Millivolt Output vs. pH at 25°C:
pH 4.01:  +177.48 mV
pH 7.00:     0.00 mV (Isoelectric Point)
pH 10.01: -177.48 mV

Calibration Protocols & Electrode Slope Check

• Multi-Point Calibration: Calibrate daily using at least two, preferably three, NIST-traceable standard buffer solutions (pH 4.01, 7.00, and 10.01) that bracket the anticipated sample pH.
• Electrode Slope Verification: Modern meters calculate the actual electrode slope percentage relative to theoretical Nernstian output:

$\text{Electrode Slope (\%)} = \left(\frac{\Delta V_{\text{measured}}}{\Delta V_{\text{theoretical}}}\right) \times 100$

An acceptable electrode slope must fall between 95% and 105% ($56.2\text{ to } 62.1\text{ mV/pH unit}$). A slope below 95% indicates electrode fouling, depleted electrolyte, or glass bulb dehydration, requiring electrode cleaning with $0.1\text{ M } \text{HCl}$ or electrode replacement.

• Electrode Storage: Store electrodes in $3.0\text{ M } \text{KCl}$ or pH 4.0 buffer. Never store electrodes in deionized/distilled water, as osmotic pressure leaches the internal electrolyte through the ceramic junction.

3. Alkalinity Titration & Chemical Speciation

Alkalinity is the quantitative capacity of aqueous media to neutralize strong acids, serving as the chemical buffer against pH depression during alum coagulation, chlorination, and nitrification. Alkalinity is expressed in $\text{mg/L as }\text{CaCO}_3$ and is primarily composed of three inorganic chemical species: hydroxide ($\text{OH}^-$), carbonate ($\text{CO}_3^{2-}$), and bicarbonate ($\text{HCO}_3^-$).

Analytical Titration Procedure

A $100\text{ mL}$ sample is titrated electrometrically (or with color indicators) using standard $0.0200\text{ N } \text{H}_2\text{SO}_4$:

1. Phenolphthalein Alkalinity ($P$): Titrate the sample to pH 8.3 (or until the pink color of phenolphthalein indicator turns colorless). Titration to pH 8.3 neutralizes all hydroxide ions and converts carbonate ions to bicarbonate: $\text{OH}^- + \text{H}^+ \longrightarrow \text{H}_2\text{O}$ $\text{CO}_3^{2-} + \text{H}^+ \longrightarrow \text{HCO}_3^-$
2. Total (Methyl Orange) Alkalinity ($T$): Continue titrating past pH 8.3 down to pH 4.5 (bromocresol green-methyl red indicator endpoint turning from blue-green to pinkish-gray). Titration from pH 8.3 to 4.5 converts all remaining bicarbonate ions to carbonic acid / dissolved carbon dioxide: $\text{HCO}_3^- + \text{H}^+ \longrightarrow \text{H}_2\text{CO}_3 \rightleftharpoons \text{CO}_2\uparrow + \text{H}_2\text{O}$

Alkalinity Calculation Formula

$\text{Alkalinity (mg/L as }\text{CaCO}_3) = \frac{A \times N \times 50{,}000}{\text{Sample Volume (mL)}}$

Where:

• $A$ = Volume of standard acid titrant used ($mL$)
• $N$ = Normality of sulfuric acid ($0.0200\text{ N}$)
• $50{,}000$ = Milligram equivalent weight factor for $\text{CaCO}_3$

For a standard $100\text{ mL}$ sample volume and $0.0200\text{ N}$ acid titrant, the equation simplifies to: $\text{Alkalinity} = A \times 10$.

Alkalinity Speciation Table (Standard Methods 2320B)

Worked Example 9.2.1: Determining Alkalinity Fractions

A $100\text{ mL}$ water sample has an initial pH of 9.2. Titration with $0.0200\text{ N } \text{H}_2\text{SO}_4$ requires $3.6\text{ mL}$ to reach the phenolphthalein endpoint (pH 8.3) and a cumulative total of $14.2\text{ mL}$ to reach the methyl red endpoint (pH 4.5). Calculate the Phenolphthalein Alkalinity ($P$), Total Alkalinity ($T$), and the individual concentrations of hydroxide, carbonate, and bicarbonate alkalinity in $\text{mg/L as }\text{CaCO}_3$.

Step 1: Calculate $P$ and $T$ alkalinity: $P = 3.6\text{ mL} \times 10 = 36.0\text{ mg/L as }\text{CaCO}_3$ $T = 14.2\text{ mL} \times 10 = 142.0\text{ mg/L as }\text{CaCO}_3$

Step 2: Compare $P$ and $T$: $\frac{1}{2}T = \frac{142.0}{2} = 71.0\text{ mg/L}$ Since $P (36.0) < \frac{1}{2}T (71.0)$, this falls into Case 2 ($P < \frac{1}{2}T$).

Step 3: Calculate species distribution:

• Hydroxide Alkalinity = $0\text{ mg/L as }\text{CaCO}_3$
• Carbonate Alkalinity = $2P = 2 \times 36.0 = 72.0\text{ mg/L as }\text{CaCO}_3$
• Bicarbonate Alkalinity = $T - 2P = 142.0 - 72.0 = 70.0\text{ mg/L as }\text{CaCO}_3$

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4. Total and Calcium Hardness via EDTA Titrimetry

Water Hardness is defined as the total concentration of polyvalent metallic cations in solution, predominantly Calcium ($\text{Ca}^{2+}$) and Magnesium ($\text{Mg}^{2+}$). Hardness causes scale formation in boilers and distribution piping while consuming soap.

Hardness Classifications (USGS / EPA)

• Soft: $0–60\text{ mg/L as }\text{CaCO}_3$ ($0–3.5\text{ gpg}$)
• Moderately Hard: $61–120\text{ mg/L as }\text{CaCO}_3$ ($3.5–7.0\text{ gpg}$)
• Hard: $121–180\text{ mg/L as }\text{CaCO}_3$ ($7.0–10.5\text{ gpg}$)
• Very Hard: $> 180\text{ mg/L as }\text{CaCO}_3$ ($> 10.5\text{ gpg}$)
• Conversion Factor: $1\text{ grain per gallon (gpg)} = 17.12\text{ mg/L as }\text{CaCO}_3$

The EDTA Complexometric Titration Method

Ethylenediaminetetraacetic acid (EDTA) chelates divalent metal ions to form stable, soluble chelate complexes:

1. Total Hardness: Buffer a $50.0\text{ mL}$ sample to pH 10.0 $\pm 0.1$ using an ammonium chloride/ammonium hydroxide buffer ($\text{NH}_4\text{Cl}/\text{NH}_4\text{OH}$). Add Eriochrome Black T (EBT) or Calmagite indicator. In the presence of $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$, the indicator forms a wine-red complex. Titrate with standard $0.0100\text{ M } (0.0200\text{ N})$ EDTA. The EDTA displaces the indicator, turning the solution to a sharp, distinct sky blue at the endpoint.
2. Calcium Hardness: Buffer a separate sample to pH 12.0 to 13.0 by adding $1.0\text{ N } \text{NaOH}$. At this elevated pH, magnesium precipitates quantitatively as solid magnesium hydroxide ($\text{Mg(OH)}_2\downarrow$). Add Hydroxy Naphthol Blue (or Murexide) indicator and titrate with EDTA from pink/red to pure royal blue.
3. Magnesium Hardness Determination: Calculated mathematically by subtraction: $\text{Magnesium Hardness (mg/L as }\text{CaCO}_3) = \text{Total Hardness} - \text{Calcium Hardness}$

$\text{Total Hardness (mg/L as }\text{CaCO}_3) = \frac{\text{mL EDTA Titrant} \times 1{,}000}{\text{Sample Volume (mL)}}$ (When utilizing $0.0100\text{ M}$ EDTA standard titrant where $1.00\text{ mL EDTA} = 1.00\text{ mg }\text{CaCO}_3$.)

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5. Chlorine Residual Analysis: DPD Colorimetric & FAS Titration

Maintaining a legal disinfectant residual is mandatory across distribution systems ($0.20\text{ mg/L}$ free chlorine minimum under Colorado Reg 11).

DPD Colorimetric Method (Standard Methods 4500-Cl G)

• Free Available Chlorine (HOCl + OCl⁻): $N,N\text{-diethyl-}p\text{-phenylenediamine}$ (DPD) reagent buffered to pH 6.2–6.5 is added to the sample. Free chlorine instantly oxidizes DPD to form a magenta/pink Würster dye. Photometric absorbance is read at $515\text{ nm}$ within 1 minute.
• Combined Chlorine (Chloramines): Excess potassium iodide ($\text{KI}$) crystals or DPD-3 reagent is introduced. Chloramines oxidize iodide ($\text{I}^-$) to iodine ($\text{I}_2$), which in turn oxidizes additional DPD to produce total magenta color representing Total Chlorine Residual.
• Combined Residual Calculation: $\text{Combined Chlorine} = \text{Total Chlorine} - \text{Free Chlorine}$.

Analytical Interferences & Matrix Precautions

• Oxidized Manganese ($\text{MnO}_2$): Produces a false-positive free chlorine reading. Corrected by running a sodium arsenite ($\text{NaAsO}_2$) or thioacetamide blank.
• High Chlorine Bleaching: High chlorine concentrations ($> 4.0–5.0\text{ mg/L}$) oxidize DPD past the colored dye to a colorless imine compound, yielding a false-low reading. If a transient flash of pink vanishes instantly, dilute the sample with chlorine-demand-free water and re-test.
• DPD FAS Titrimetric Method (SM 4500-Cl F): Standard Ferrous Ammonium Sulfate ($0.00282\text{ N } \text{FAS}$) titrates the pink DPD dye to a colorless endpoint ($1.00\text{ mL FAS} = 0.100\text{ mg Cl}_2$), eliminating turbidity and colorimeter optical bias.