The mechanism: two simultaneous attacks

A standard pH electrode contains two electrochemical subsystems: the glass membrane (pH-sensitive) and the reference electrode (a fixed-potential anchor). In clean aqueous solutions, H₂S affects neither. In real O&G process streams, H₂S primarily attacks the reference electrode — dissolved sulphide enters through the liquid junction and poisons the Ag/AgCl element. The glass membrane is generally resistant to H₂S at process-relevant concentrations.

1. Reference poisoning — the Ag/AgCl pathway

The most common pH reference design uses a silver wire coated with silver chloride (Ag/AgCl), immersed in a concentrated KCl filling solution. The half-reaction that maintains the stable reference potential is:

AgCl(s) + e⁻ ⇌ Ag(s) + Cl⁻     E° = +0.222 V (vs. SHE at 25°C)

When dissolved H₂S enters the filling solution through the liquid junction, it reacts with the silver wire directly:

2 Ag(s) + H₂S → Ag₂S(s) + 2H⁺ + 2e⁻

Silver sulphide (Ag₂S) has a solubility product of approximately 6 × 10⁻⁵⁰ — it is essentially insoluble. Once formed, it precipitates as a black coating on the silver wire, altering the effective electrode area and, critically, shifting the reference potential. The half-reaction is no longer Ag/AgCl but a mixed potential incorporating the Ag/Ag₂S system (E° = −0.691 V vs. SHE), a shift of up to 0.9 V in the theoretical limit; in practice, mixed potentials typically produce shifts of 50–400 mV depending on the extent of poisoning.

Critical The Ag₂S poisoning is irreversible. Cleaning or recalibrating the electrode does not restore its original reference potential. Once sulphide has access to the silver wire, the electrode must be replaced.

2. Liquid junction blockage — the KCl pathway

Even before H₂S reaches the silver wire, it causes a secondary problem at the liquid junction. The junction connects the KCl filling solution to the process stream. In H₂S-rich environments:

A partially blocked junction increases the junction resistance, slows electrode response, and introduces a residual liquid junction potential (RLJP) that cannot be corrected by simple two-point calibration.

Key point Both failure modes — reference poisoning and junction blockage — produce pH readings that appear numerically reasonable. The transmitter shows a stable value. No fault is logged. But the reading is wrong by up to 1.5 pH units.

How large is the error?

The magnitude of the error depends on H₂S concentration, temperature, process ionic strength, and how far the poisoning has progressed. From field data across sour water stripper and SAGD produced water applications:

H₂S (mg/L) Exposure time (hours) Typical pH error Visual indicator
< 1 Any ± 0.05 None
1 – 10 < 24 ± 0.1 – 0.3 Light discolouration of junction
1 – 10 24 – 168 ± 0.3 – 0.7 Black tinge on Ag wire (if visible)
> 10 < 8 ± 0.5 – 1.5 Often none until junction is inspected
> 50 > 2 Total loss of reference stability Readings drift continuously

These ranges assume a standard Ag/AgCl double-junction reference with ceramic porous pin junction. Actual errors in your process will vary.

Why the glass membrane is usually not the problem

Engineers often suspect the glass membrane first — it is the visible sensing element. In H₂S environments, the glass membrane is typically not the primary failure mode. Most commercial pH glass formulations are chemically resistant to H₂S at concentrations below several thousand mg/L. The Ag/AgCl reference fails orders of magnitude faster.

This is why replacing the glass bulb (or buying an entirely new combination electrode and using the same KCl filling solution) does not fix the problem: the new glass membrane is calibrated against a still-poisoned reference.

Diagnostic If your electrode gives inconsistent calibration slopes (e.g., 52 mV/pH one week, 47 mV/pH the next) in H₂S service, the reference is failing — not the glass. Slope decay is a reference symptom, not a membrane symptom.

Four engineering solutions, in order of preference

1. Double-junction reference with non-silver inner chamber

The most effective approach eliminates silver from the reference path. Designs using alternative reference elements that avoid silver — such as Tl/TlCl (Thalamid® by Metrohm), Ross-type double junction, or iodide/iodine references — maintain stable potential in H₂S without sulphide poisoning. The inner filling solution uses a non-reacting electrolyte (e.g., 3 M KNO₃), with the outer KCl chamber acting as a sacrificial barrier.

This is the preferred option for continuous in-line monitoring in sour water streams above 10 mg/L H₂S. Electrode cost is 3–5× standard, but maintenance frequency drops by the same factor.

2. High-flow sleeve junction

A PTFE sleeve junction with high KCl outflow (typically 0.5–2 mL/day) physically prevents process ingress. The continuous outward KCl flow keeps H₂S from reaching the silver wire. This requires pressurised filling solutions (or a pressurised reference reservoir) and regular filling solution top-up.

Practical for applications where the electrode is accessible for weekly maintenance. Not suitable for remote or unattended installations.

3. Solid polymer electrolyte (SPE) pH sensors

ISFET and solid-state polymer electrolyte sensors eliminate liquid junctions and Ag/AgCl references entirely. They have no junction to block and no silver to poison. Their limitation in H₂S environments is different: the SPE membrane can be attacked by dissolved sulphide at pH < 4 where H₂S is the dominant species (pKa₁ = 6.98 at 25°C).

For pH 5–12 sour water applications, solid-state sensors are an excellent choice. Below pH 5 in high H₂S, evaluate membrane compatibility with the specific polymer type.

4. Sample conditioning and dilution

In some applications — particularly lab or at-line measurement of process samples — conditioning the sample before electrode contact is more economical than specialised electrodes. Stripping dissolved H₂S by acidification (drop sample pH to < 2 briefly, then neutralise to measurement pH) or by nitrogen sparging removes sulphide before electrode contact.

This adds complexity and introduces its own sources of uncertainty, but allows standard electrodes to be used reliably.

What to specify in your next RFQ

When sourcing pH electrodes for H₂S service, require the following from vendors:

Note on sourcing Schrödinger fabricates glass-membrane and ISE electrodes. Solid-state specialty references for H₂S service are sourced from established manufacturers without brand preference — the goal is matching the right reference design to your process, not selling a specific product line.

Documentation and QA implications

If your process operates under ISO 17025, ASTM D1293, or an internal QMS that requires pH measurement traceability, a poisoned reference is not just a calibration problem — it is a data integrity problem. Any pH records collected with a compromised electrode are suspect.

Recommended practice for H₂S environments: calibrate before and after each measurement session (or each shift for continuous analysers), keep the calibration records, and establish acceptance criteria for slope and offset. A slope below 54 mV/pH at 25°C in a new electrode is a hard stop — recalibrate with a known-good electrode before accepting data.

Summary