Acid Base

Question 1

Describe the methods of measurement of pH in blood

Syllabus

Answer 1

pH electrode (aka glass electrode):

• Consists of:
  
  • 2 electrodes:
    
    • Measuring electrode (Silver/silver chloride) bathed in a buffer solution
    • Reference electrode (Calomel – mercury/mercury chloride) bathed in potassium chloride solution with no buffering properties
  • pH-sensitive glass which acts as a semi-permeable membrane that only hydrogen ions can cross (H+ diffuses from the blood sample across the glass and into the solution)
  • A voltmeter
• The process:
  
  • Both electrodes with corresponding solutions are separated from the blood sample by the glass
  • H+ ions diffuse from the blood sample to the solutions which bathe the electrodes
  • The buffer of the measuring electrode ensures that the pH of the solution doesn’t change - this causes H+ ions to continue to diffuse across the glass
  • Diffusion of H+ ions into KCl bathing the reference electrode (with no buffering properties) will result in a changing pH until it equilibrates to the pH of the blood sample
  • The potential difference between the measuring electrode buffer solution and the reference electrode solution can be measured by a voltmeter
  • The voltage is a reflection of the H+ concentration, and pH can be calculated using pH = -log10[H+]
• Limitations:
  
  • Must be kept at constant temperature (hypothermia increases CO2 solubility)
  • Electrodes must be kept clean and protein free