Download 2001a(3) – Briefly describe measurement of pH in a blood sample

Measurement
2006a(15)/2001a(3): Briefly describe measurement of pH in a blood sample
using a pH electrode
General: pH = -log10[H+] x a
activity
where [H+] is hydrogen ion concentration, a is
Measurement using a pH electrode
- uses the principle that the electrical potential generated across a Hsensitive glass membrane is proportional to the pH difference across that
membrane
- consists of 2 x half cells: the glass electrode and the reference electrode
both connected via blood to make a complete circuit
o Glass electrode: Ag/AgCl core encased in glass. [H+] within glass
electrode kept constant by buffer solution. Glass is in contact with
blood
o Reference electrode: Ag/AgCl bathed in KCl (salt bridge) →
doesn’t participate in H-measureent. Semi-permeable membrane
separates it from blood
- Blood passes by both electrodes, H+ ions attracted to glass (negatively
charged). Buffer [H] fixed → creates a difference in [H+] across the glass
membrane.
- Difference across glass membrane measurable as a potential difference
b/n the electrodes (EMF).
o This allows the measurement of pH in blood as the pH of the
solution measured by the reference electrode is constant.
- Requirements:
o Standardisation of T°C (pH changes inversely with change T°C)
o Regular calibration (x2 buffers of known pH (PO4 buffers).
By Amanda Diaz