Overview

PEEK (PolyEtherEtherKetone) is a popular material for HPLC tubing and fittings because it offers excellent mechanical strength and broad solvent compatibility. However, while PEEK withstands a wide range of chemical environments, pH extremes—especially on the acidic end—can affect long‑term performance.

Understanding the recommended pH limits helps maintain the integrity and service life of PEEK components.

General Recommended pH Range

Manufacturer Guidance

• Many PEEK manufacturers specify a usable pH range of 2 – 14 for HPLC tubing and fittings.

This is based on short‑term and general laboratory use where the tubing is not exposed continuously to harsh conditions.

Caution With Low pH (0–6)

Why Extra Care Is Needed

• Long‑term exposure to pH 0–6 solutions is not advisable.
• PEEK is manufactured using phosphoric acid, and traces of this reagent may remain in the polymer granulate.
• Because phosphoric acid is reactive with the PEEK structure, highly acidic environments may:
  • Accelerate surface degradation
  • Increase brittleness over time
  • Reduce pressure‑handling stability

Implication for HPLC Users

• For acidic mobile phases, especially those with medium to high ionic strength, consider minimizing prolonged exposure of PEEK parts.

Recommended Practices for Long‑Term Use

For Acidic Environments

• Use a higher pH starting point whenever possible for long exposures.
• Consider switching to stainless steel or titanium components when very low pH is required for extended periods.

For Routine HPLC Use

• Short-term exposure within the pH 2–14 window is typically acceptable.
• Regular inspection for discoloration, swelling, or surface changes helps detect early degradation.

For High Ionic Strength Buffers

• Because ionic strength increases reactivity, increase caution when preparing acidic buffers in contact with PEEK.

Why PEEK Can Be Sensitive at Low pH

Material Background

• Traces of phosphoric acid in the polymer granulate indicate that PEEK can be susceptible to strong acids—even those present at moderate concentrations.
• Acidic attack can compromise the polymer chain structure, especially under elevated temperature or pressure.