Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 265

potential measuring devices regularly used generally have an accuracy of 0.1mV or
higher (0.01 mV).
Third, the electrode should be calibrated in the pH range close to the intended appli-
cations. Although the calibration curve may show a straight line over several decades
of concentration with an average slope, it is unlikely that this will be exactly the same
across the whole range. For example, when multi-points (fi ve points) calibrations are
made, there may be a variation of several millivolts between the individual slopes calcu-
lated from two adjacent points at different concentration ranges or between the individual
slopes and the overall slope. Therefore, for the most accurate results, it is recommended
that the electrode slope be determined by using two-point calibration (also called brack-
eting) using two standard buffer solutions. These two buffer solutions are often selected
that closely cover the expected range of the samples. This is especially important for
some in-vivo measurements, where the pH variation is within a very narrow range.
In some in-vivo applications, one-point calibration is used. Although a one-point
calibration is insuffi cient to determine both the slope and one-point pH value, by
assuming that the slope of prior calibration is unchanged, the electrode's performance
in vivo can be assessed during operation. For example, in measuring esophageal acid
exposure, since pH 4 is accepted as a cut-off value, one-point calibration was carried
out during in-vivo ambulatory pH monitoring in which patients swallowed a juice with
a predetermined pH close to 4 [33].
The desired results for a practical measurement vary from application to applica-
tion. In many pH measurements, accuracy within 0.1 pH unit is more than suffi cient
while 0.01 pH is required in some other more accurate tests. Reaching an accuracy of
less than 0.01 pH will be a challenge for most measurement systems.
10.2.6 Selectivity
Selectivity is one of the most important characteristics of an electrode, as it often
determines whether a reliable measurement in the sample is possible or not. In prac-
tice, most pH sensitive membranes will also respond slightly to some interfering ions.
As can be seen in Eq. (5), the potential of such a membrane is governed mainly by the
activity of the hydrogen ion and also by the concentration of other interfering ions. To
improve selectivity, advanced membrane compositions or protection membranes with
size-exclusion or ion-exchange properties are often utilized.
It is reported that the greatest interference that affects selectivity of an oxide-based
pH electrode is from redox couples. For practical applications, some oxide-based sen-
sors are covered with a thin layer of a size-exclusive protection membrane such as
Nafi on or polyphenol [43]. However, the improvement in selectivity is at the expense
of the response slopes and, sometimes, the response times.
10.2.7 Stability and reliability
Poor operational stability due to drift has largely limited the long-term or implanta-
ble application of pH sensors. Some oxide-based electrodes present very high initial