PH and the Henderson Hasselbach equation

H+ ions are difficult to measure as there are literally billions of them. We use pH instead, which simply put is the negative logarithm of the H+ concentration in moles.

Buffers are weak acids. When a weak acid dissociates:

where A is the acid and AH is the conjugate base, the product of [A] and [H+] divided by [AH] remains constant. Put in equation form:

Ka is called the dissociation constant. pKa is like pH - it is the negative logarithm of Ka. The Henderson-Hasselbach equation puts the pH and the dissociation equations together and describes the relationship between pH and the molal concentrations of the dissociated and undissociated form of a dissolved substance. The Henderson-Hasselbach equation is:

A simplified version is:

[HCO3] dissociated pH

PaCO2 undissociated

H + concentration is sometimes used instead of pH. A simple conversion between pH 7 2 and 7 5 is that [H+] _ 80 - the two digits after the decimal point. So if the pH is 735, [H+] is 80 -35 _ 45 nmol/l. From the law of mass action:

PaCO2

181 x PaCO2 in kPa (24 x PaCO2 in mmHg)

hco3

[H+] is related to the PaCO2/HCO3 ratio. This is clinically relevant when checking the consistency of arterial blood gas data.

Table 3.1 pH and equivalent [H+]

pH

[H+] nmol/l

7-6

26

7-5

32

7-4

40

7-3

50

7-2

63

7-1

80

7-0

100

6-9

125

6-8

160

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