2006A15 Briefly describe the measurement of pH in the blood sample using a pH electrode

Definition

·  ‘Potenz H⁺’

·  = -log₁₀([H⁺] x a)     (where a is the activity)

Importance

·  Macro: preservation of protein function (enzymes, ion channels, etc.)

·  Micro: trapping of metabolic intermediates in ICF and preservation of ICF volume

Diagramme

Components

·  Two half cells

o Ag/AgCl measuring electrode, buffer solution at ph 7

o Hg/HgCl reference electrode, saturated KCl solution

·  pH-sensitive glass membranes between each cell and the blood (contains Li⁺ and Cl^-)

·  Sample chamber

·  Voltmeter and amplifier

How it works

·  Addition of blood to sample chamber completes the circuit

·  H⁺ unable to pass through glass but changes polarity

·  ↓pH in KCl solution

·  ↔pH in buffer solution (by definition)

·  Potential difference generated, but no current

·  Calibrated with two phosphate solutions of known pH

Sources of error

·  ↓Temp -> ↓H₂O dissociation -> ↑pH (0.015 per 1°C – Rosenthal’s equation)

o FIX: temperature sensor

·  Damaged to glass, other