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Chapter: Clinical Anesthesiology: Perioperative & Critical Care Medicine: Acid-Base Management

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Hydrogen Ion Concentration & pH

In any aqueous solution, water molecules reversibly dissociate into hydrogen and hydroxide ions.

Hydrogen Ion Concentration & pH

 

In any aqueous solution, water molecules reversibly dissociate into hydrogen and hydroxide ions:


This process is described by the dissociation constant, KW:

 


The concentration of water is omitted from the denominator of this expression because it does not vary appreciably and is already included in the constant. Therefore, given [H+] or [OH], the concentration of the other ion can be readily calculated.

 

Example: If [H+]=108nEq/L, then [OH]=1014÷ 108= 106 nEq/L.

 

Arterial [H+] is normally 40 nEq/L, or 40 × 109 mol/L. Hydrogen ion concentration is more commonly expressed as pH, which is defined as the negative logarithm (base 10) of [H+] (Figure 50–1). Normal arterial pH is therefore –log (40 × 109) = 7.40. Hydrogen ion concentrations between 16 and 160 nEq/L (pH 6.8–7.8) are compatible with life.


 

Like most dissociation constants, KW is affected by changes in temperature. Thus, the electroneutral-ity point for water occurs at a pH of 7.0 at 25°C, but at about a pH of 6.8 at 37°C; temperature-related changes may be important during hypothermia .

 

Because physiological fluids are complex aqueous solutions, other factors that affect the dissociation of water into H + and OH are the SID, the Pco2, and ATOT.

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