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IĆ8 800 Series Operator's Manual
Hydrogen Ion Activity or pH
The notation of pH expresses the hydrogen ion activity in a solution as the negative
logarithm of the hydrogen ion concentration. The hydrogen ion is actually the
determinant of the acidity of blood or plasma. Normal cellular metabolism requires
an exacting environment where hydrogen ion concentration must be maintained
within narrow limits. Hydrogen ion activity reflects the acid-base balance within
blood. Acids are substances that donate hydrogen ions; bases are substances that
remove hydrogen ions from solution. The lungs, kidneys and blood bases all work
to maintain the acid-base status within the strict limits for normal cell functioning.
Expressed in concentration units, hydrogen ion concentrations are very small
numbers that are cumbersome to use. (For example the common “neutral” pH of
7.00 is 0.0000001 mol/L.) In 1909 Sorenson
13
converted the numbers
mathematically to simplify their use and described the notation pH
pH = –log
10
cH
+
where (H
+
) is the molar concentration of hydrogen ions.
Using this formula, a hydrogen ion concentration of 1 x 10
–7
mol/L has a pH value
of 7. Because pH is the negative logarithm, it’s value is inversely proportional to
the actual hydrogen ion concentration in a sample. Therefore, as the hydrogen ion
concentration decreases, the pH value increases and visa versa.
The normal pH range of human blood is 7.35 – 7.45.
The Henderson-Hasselbalch equation describes how pH expresses the interaction
of acid and base in blood.
pH + pKĂ ) logĂ
base
acid
where K is the dissociation constant, which describes the ability to release
hydrogen ions.
Since K, and thus pK, is a constant, this equation can be used to demonstrate that
pH is proportional to the acid-base concentrations
in blood.
pH T
base
acid
Therefore, if base increases without a corresponding increase in acid, the pH rises,
and if acid increases without a corresponding increase in base, the pH decreases.