pH Laboratory Systems

pH measurement is based on the Nernst equation.

E = E₀ − (0.0591 × pH) at 25°C

Slope must be ~59 mV per pH unit at 25°C.

Calibration Example

Buffer 7 → 0 mV Buffer 4 → +177 mV Slope = 59 mV/pH → acceptable

Conductivity Systems

Conductivity depends on ionic mobility.

κ = G × K

Where:
κ = Conductivity
G = Conductance
K = Cell constant

Troubleshooting

• Drift → Dirty electrode
• Slow response → Aging membrane
• Unstable conductivity → Air bubbles

Laboratory Hub

pH – Full Nernst Equation (Engineering Form)

E = E₀ − (2.303 RT / nF) × pH

Where:
R = Gas constant
T = Absolute temperature (K)
n = number of electrons (1 for H⁺)
F = Faraday constant

At 25°C, slope = 59.16 mV per pH unit.

pH Slope Validation Calculation

Measured slope = 57 mV Theoretical slope = 59.16 mV

Slope % = (57 / 59.16) × 100 Slope % = 96.3%

Acceptable range under laboratory control: 95–105%.

Offset Check (Zero Point)

At pH 7: Expected potential ≈ 0 mV (±30 mV)

Deviation beyond ±30 mV indicates:

• Reference junction clogging
• Electrolyte depletion
• Aging reference system

pH Troubleshooting Flow

• No slope → Glass membrane crack
• Slow response → Dehydrated bulb
• Noise → Ground loop issue

Conductivity – Worked Engineering Example

Measured resistance = 2000 Ω Conductance G = 1 / 2000 = 0.0005 S Cell constant K = 1.0 cm⁻¹

κ = G × K κ = 0.0005 × 1.0 κ = 500 µS/cm

Temperature Compensation

κ₂₅ = κT / [1 + α(T − 25)]

Where:
α ≈ 0.02 /°C for many aqueous systems

Example:
κ at 35°C = 600 µS/cm
κ₂₅ = 600 / [1 + 0.02(35 − 25)]
κ₂₅ = 600 / 1.2 = 500 µS/cm

Cell Constant Verification

K = κ_standard / G_measured

This must be verified periodically using certified KCl solution.

ISO 17025 Laboratory Control Points

• Daily buffer verification (pH 4, 7, 10)
• Conductivity standard verification
• Temperature probe validation
• Electrode maintenance log
• Uncertainty estimation documentation