Indirect Estimation of pH

Because of difficulty in measuring [H+] directly, blood pH was estimated by derivation from the Henderson-Hasselbach equation:

or where pKa is the equilibrium constant for the dissociation of carbonic acid, and a is the solubility coefficient for CO2. This method is inaccurate because the values of pKa and a vary with temperature.

pH Electrode

To overcome the problems of indirect estimation of pH, a glass electrode was developed to enable measurement of blood pH directly. This method is employed in most modern blood gas analysers. First described in 1933, the glass electrode has been extensively modified to produce what is now a stable, sensitive instrument capable of pH measurement using increasingly smaller volumes of blood. The electrode itself is non selective and will respond to more than one particular anion or cation. Specificity is achieved by enclosing the electrode in a membrane with ion selective permeability; in this way only H+ ions have access to the electrode.

CAUSES OF CHANGES IN END TIDAL CO2 CONCENTRATIONS

Change

Cause

Diagnosis

i ETCO2

alveolar ventilation X CO2 production Dead space

respiratory rate, tidal volume Hypothermia

Pulmonary embolus, shock, hypotension

Technical error

Calibration error, air contamination

ETCO2

X alveolar ventilation CO2 production inspired CO2

X respiratory rate, X tidal volume Sepsis, hyperpyrexia, thyrotoxicosis Rebreathing, CO2 added to inspired gases, administration of NaHCO3

Figure CM.35

Figure CM.35

The actual pH electrode consists of two component electrodes:

• Silver/silver chloride electrode (Ag:AgCl)

• Mercury/mercury chloride electrode (Hg:Hg2Cl2), calomel

Each component electrode consists of a metal conductor and electrolyte solution, the metal conducts electrons, the electrolyte solution conducts ions. A potential difference or electromotive force (EMF) is generated at the interface of the two electrodes (electrode potential). If the temperature remains constant, the only remaining variable is the pH difference between the inner buffer solution of the electrode, and the sample. A saturated solution of KCI provides a salt bridge that completes the circuit between the sample and second calomel electrode. To minimize diffusion of KCl into the sample, a porous plug is placed at the end of the measurement pathway (Figure CM.36).

As the pH electrode responds to the activity of H+ rather than concentration, the electrodes must be calibrated using standard solutions of known pH. The pH scale of the electrode depends on the pH of the standard solutions rather than the absolute concentration of H+. Modern blood gas analysers use two different buffers; the first (pH 6.841) has the same pH as the buffer inside the electrode and is taken as the arbitrary zero. The second buffer (pH 7.383), is used as a reference against which the blood pH is measured.

Figure CM.36 pH electrode

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