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


(NOTE: currently used device is the solid-state ion-sensitive field-effect transistor)



·      pH

·      Electrode: diagramme, components, how it works, sources of error




·  ‘Potenz H+

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


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

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


pH electrode:



·  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



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