17. pH measurement

Question 1

Define pH. >

Answer 1

pH stands for ‘power of hydrogen’.

> It is a measure of the hydrogen ion
activity in an aqueous solution.

> pH =

negative log
to the base 10
of the
hydrogen ion concentration [H+]:

pH = − log10 [H+]

There are a few rules of thumb worth remembering:

> Pure water is neutral and has a pH of 7.

> For each 1 unit change in pH there is a 10-fold change in [H+].

> For each 0.3 change in pH there
is a 50% change in [H+].

E .g.:

pH 6 = [H+] 1000 nmol/L or 10–6 mol/L

pH 7 = [H+] 100 nmol/L or 10–7 mol/L

pH 7.4 = [H+] 40 nmol/L or 10–7.4 mol/L

pH 8 = [H+] 10 nmol/L or 10–8 mol/L

pH 9 = [H+] 1 nmol/L or 10–9 mol/L.

Question 2

How is [H+] measured?

Method -

O2

Answer 2

Arterial blood gas analysers measure

[H+]

and

PCO2 using

potentiometric electrodes

(i.e. voltage-producing electrodes)

and PO2 via an
amperometric technique
(i.e. current-producing electrodes).

[H+] is measured using a

pH (glass) electrode,
which is an ion-sensitive
electrode whose operation depends
upon the ion-sensitive glass at its tip.

It is an example of a potentiometric 
electrode in that a potential difference
develops across the ion-sensitive glass, 
the potential difference being
dependent upon the 
difference in [H+] across the glass.

Question 3

Draw a simple schematic pH

electrode.

Answer 3

Draw a simple schematic pH electrode.

> Reference electrode:

mercury/mercury chloride electrode

within a potassium chloride solution.

This solution is saturated and acts
as a salt bridge to complete

the circuit between the
sample and the electrode.

The KCl solution is prevented from diffusing
into the sample by a porous
membrane that is permeable
to only H+ ions.

> pH electrode:

silver/silver chloride electrode
within a buffer solution of
hydrochloric acid.

The tip of the electrode is
composed of pH-sensitive
glass.

> The temperature of the system is
maintained at 37 °C because
dissociation of acids and bases increases with increasing temperature.

> [H+] is held constant around the
pH electrodes by the buffer solution and
so any potential difference 
across this electrode is due 
to [H+] within the blood sample.

> Potential difference between the
two electrodes is measured and
converted to a direct reading of pH or [H+].

> Potential output is linear (60 mV per unit pH).

Question 4

How is the pH electrode

calibrated?

Answer 4

The pH scale is not an absolute scale; 
it is relative to a set of standard
solutions whose pH has 
been established by 
international agreement.
The system is calibrated using 
two of these standard buffer solutions of
known pH.

Question 5

What are the sources of error

in this measuring system?

Answer 5

The following conditions may result in erroneous [H+] results:

> Calibration errors

> Drift of the measuring system

> Membrane damage resulting
in electrode contamination

> Temperature –
hypothermia increases CO2
solubility resulting in reduced
PaCO2 and increased pH

> Sampling errors

> Effect of over-heparinisation –
acidic heparin lowers pH.

Delays in analysis of arterial blood gas –
cellular metabolism continues:

> PCO2 rises about 0.009 kPa per minute

> pH falls approximately 0.0006 units per minute

> PO2 falls approximately 0.13–0.39 kPa per minute.

Question 6

What other method can be

used to measure pH?

Answer 6

A pH indicator is a 
substance that will change colour 
at a particular pH value
(e.g. litmus paper turns red in acidic 
and blue in alkaline conditions).