Analytical Instrumentation: pH, Conductivity, Gas & Composition

Measuring what a process stream is made of. Learn pH (glass electrode, Nernst equation, buffer calibration, temperature compensation), ORP, conductivity, dissolved oxygen, turbidity, gas detectors (catalytic, electrochemical, infrared), oxygen analyzers (zirconia, paramagnetic), gas chromatography, spectroscopy (NDIR, UV-Vis, NIR, Raman), sampling systems and continuous emissions monitoring (CEMS).

Temperature, pressure, flow and level tell you how a process is running; analyzers tell you whether the product is right and whether emissions are legal. They are the most complex and maintenance-intensive instruments in a plant.

Most analyzers need a sample system that takes a representative portion of the stream, conditions it (filter, cool, reduce pressure) and returns or vents it — often the main source of analyzer problems.

An analogy

If process instruments are the plant's thermometer and blood-pressure cuff, analyzers are its blood test — slower, more expensive, but they reveal what is really going on inside.

Liquid analysis

The water-quality workhorses:

• pH — glass electrode + reference; 0–14 scale; calibrate with buffers
• ORP — oxidation-reduction potential (platinum electrode), disinfection control
• Conductivity — total ionic content; contacting or toroidal (inductive) sensors
• Dissolved oxygen — optical (luminescence) or amperometric (Clark cell)
• Turbidity — light scattering (NTU), drinking-water compliance
• Chlorine, ammonia, TOC, silica — specialised analysers

Gas detection and analysis

Safety gas detectors: catalytic bead (combustibles in %LEL), infrared (hydrocarbons, CO₂, fail-safe), electrochemical (toxic: H₂S, CO), photoionisation (VOCs). Process gas analysers: zirconia O₂ probes on boilers (combustion trim), paramagnetic O₂, thermal conductivity (H₂), NDIR (CO/CO₂), and gas chromatographs that separate components in a column and measure each (natural-gas BTU, refinery streams).

Sampling systems

A good sample system is fast (short transport lag — use a fast loop), representative (no condensation or reaction), and maintainable. Components: probe, isolation valve, filter, pressure reducer, cooler/heater, flow indicator, stream selector, calibration gas injection. The analyzer shelter (house) protects sensitive equipment.

Spectroscopy, CEMS and validation

Spectroscopic analyzers (UV-Vis, NIR, FTIR, Raman, tunable diode laser) measure in-situ with fibre probes; NIR with chemometric models predicts properties like octane or moisture. Continuous emission monitoring systems (CEMS) measure SO₂, NOₓ, CO, O₂, particulates for regulators with daily calibration drift checks. Analyzer validation uses reference samples and statistical quality control (ASTM D3764/D6299).

Formulas

• Nernst (pH electrode): E = E₀ − S·(pH − 7), S = 2.303·RT/F
• Slope vs temperature: S = 0.1984·(T[°C] + 273.15) mV/pH
• Conductivity: κ = G · K_cell (G = 1/R measured)
• Beer–Lambert (absorption): A = ε·l·c = −log₁₀(I/I₀)
• Zirconia O₂ sensor: E = (RT/4F)·ln(P_O2,ref / P_O2,sample)

Worked example: pH from millivolts

Given: Ideal electrode, 25 °C, reading −177.5 mV

Find: pH.

• pH = 7 + 177.5/59.16

Answer: pH 10.0

Worked example: Slope at 50 °C

Given: S = 0.1984 × (T + 273.15)

Find: Slope at 50 °C.

• 0.1984 × 323.15

Answer: 64.1 mV/pH

Worked example: Gas detector alarm

Given: Methane LEL = 5.0% vol; alarm at 20% LEL

Find: Concentration at alarm.

• 0.2 × 5.0%

Answer: 1.0% vol methane

Two-point pH calibration and temperature compensation

R, F = 8.314, 96485
def slope(Tc):                              # ideal Nernst slope, mV/pH
    return 2.303 * R * (Tc + 273.15) / F * 1000

# Two-point calibration in pH 7.00 and pH 4.01 buffers at 25 C
E7, E4 = -3.0, 166.0                        # measured mV
S_meas = (E4 - E7) / (7.00 - 4.01)
eff = S_meas / slope(25) * 100
print(f"ideal slope {slope(25):.2f} mV/pH, measured {S_meas:.2f} mV/pH ({eff:.1f}% efficiency)")

# Sample reads -180 mV at 50 C
Tc, E = 50.0, -180.0
S50 = S_meas * (Tc + 273.15) / 298.15      # scale slope with absolute temperature
pH = 7.00 - (E - E7) / S50
print(f"slope at 50 C {S50:.2f} mV/pH -> sample pH {pH:.2f}")
# ideal slope 59.17 mV/pH, measured 56.52 mV/pH (95.5% efficiency)
# slope at 50 C 61.26 mV/pH -> sample pH 9.89

A new electrode typically shows 95–102% slope efficiency; below ~90% it should be replaced. Automatic temperature compensation (ATC) corrects the electrode slope — it does not correct the real pH change of the solution with temperature.

Common mistakes

• Wrong: Letting pH electrodes dry out. — Right: Store wet in KCl solution.
• Wrong: Long, slow sample lines. — Right: Use fast loops; transport lag ruins control.
• Wrong: Catalytic detectors in low-O₂ or poisoned atmospheres. — Right: Use IR detectors.

Applications

• Water treatment: pH, chlorine, turbidity for compliance.
• Boilers: O₂ trim, conductivity for blowdown, silica.
• Refineries: GCs and NIR for product quality.
• Plant safety: Fixed gas detection with shutdown logic.

Key takeaways

• pH: Nernst 59.16 mV/pH at 25 °C, two-buffer calibration.
• Gas detection: catalytic, IR, electrochemical.
• GCs and spectroscopy for composition.
• Sample systems make or break analyzers.

Going further

• Chemometrics: PLS models convert NIR spectra into properties.
• TDLAS: Laser absorption for fast in-situ O₂, H₂O, NH₃.
• Analyzer management systems: Automatic validation and reliability tracking.