Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 263

hydrogen ions should be used in the Nernst equation. However, in dilute solutions, activ-
ity of H almost equals its molar concentration. In in-vivo and clinical applications, the
molar concentration is used rather than the activity of H .
Using a series of calibration solutions, the response curve or calibration curve of a
pH electrode can be experimentally determined by plotting the cell voltage vs the pH
of the calibration solution. The linear range of the calibration curve is applied to deter-
mine the pH in any unknown solution. The slope of the calibration curve within the
linear range is used to determine the response slope or electrode sensitivity in mV/pH.
This response slope is an important diagnostic characteristic of the electrode; generally,
the slope gets lower as the electrode gets old or contaminated [39].
For a pH sensor with a Nerstian response, the slope, calculated from 2.303 RT/F , is
59.16 mV/pH at 298 K. A useful slope range can be regarded as 50-70 mV/pH. Super-
Nerstian slopes are mostly reported from electrodes based on metal/metal oxide sensing
materials. It has been reported that some electrodes present two linear response ranges.
For example, IrOx microelectrodes prepared by Wipf et al. [40] have shown two linear
slopes in the range of pH 2-6 and 6-12.
It is clear from the Nernst equation that the temperature of the solution affects the
response slope (2.303 RT/F ) of the calibration curve. The electrode voltage changes
linearly in relationship to changes in temperature at a given pH; therefore, the pH of
any solution is a function of its temperature. For example, the electrode response slope
increases from 59.2 mV/pH at 25ºC to 61.5 mV/pH at a body temperature of 37ºC. For
modern pH sensing systems, a temperature probe is normally combined with the pH
electrode. The pH meter with an automatic temperature compensation (ATC) function
automatically corrects the pH value based on the temperature of the solution detected
with the temperature probe.
10.2.3 Sensitivity
The sensitivity of a pH electrode is determined by the linear response slope of the pH
electrode as defi ned by the Nernst equation. Typically, the electrode calibration curve
exhibits a linear response range between a pH of 2 and 9. At very high and very low
pH, there are deviations from linearity. The high detection end (high [H ], low pH)
of most pH sensors is limited by the so-called acid error . The electrode reads higher
than the actual pH in very acidic solutions. The mechanism of such an error is not
well understood. The lower detection limit (low [H ], high pH) is often governed by
the selectivity of the sensor. At high pH, alkaline interfering ions such Na or K are
about 8 to 9 orders of magnitude higher than H in the solution. Electrodes respond
slightly to Na or K , giving a lower reading than the actual pH. This deviation from
the actual pH is often referred to as alkaline error .
10.2.4 Response time
Response time is defi ned as the time at which the pH concentration in a solution is
changed on contact with a pH sensor and a reference electrode has reached 95% (or 90%)