Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 264

of the fi nal value. The response time is reported as t 95% or t 90% in seconds or minutes. In
many investigations, the response time of the overall measuring system is determined,
which infl uences the response time of the pH sensor. Generally speaking, metal/metal
oxide-based pH sensors present a faster response time than that of glass membrane elec-
trodes. However, when covered with a polymer membrane to eliminate or minimize
interference, the response time of oxide-based electrodes was adversely affected [41].

10.2.5 Reproducibility/accuracy
Accuracy is a measure of how close the result is to the true value; while reproducibility
or precision is a measure of how close a series of measurements on the same sample
are to each other. The accuracy and reproducibility of pH measurements can be highly
variable and are dependent on several factors: electrode stability (drift and hysteresis),
response slope/calibration curve, and accuracy of the pH meters. While some of these
factors are determined by the properties of electrodes, some measures can be taken to
improve measurement accuracy and reproducibility.
The concentration is proportional to the measured voltage and so any error in voltage
measurement will cause an error in the solution concentration. The measured voltage is
the cell voltage including different potentials generated at all-solid-solid, solid-liquid
and liquid-liquid interfaces of both sensing and reference electrodes. The potential of
the electrochemical cell, E cell , is mainly given by

E cell E pH E ref E j E xy (5)

where E pH is the half-cell potential of the pH electrode, E ref is the half-cell potential
of the reference electrode, E j is the liquid-junction potential, and E xy is the interfering
ions induced potential which affects the electrode's selectivity. Any variations in these
potentials will cause changes in the overall cell voltages. Frequent recalibration can
minimize potential drift, while using protection membranes can reduce the effect from
interfering ions. Liquid-junction potentials develop at the interface between two elec-
trolytes because of the differences in the migration rates of charged species across the
interface [42]. By using reference fi lling solutions with nearly equitransferrent electro-
lytes such as KCl, in which both ions have similar mobility when diffusing through the
liquid junction, E j can be minimized. E pH is more strongly dependent on temperature
in most cases than E ref and E j are. Calibrations and measurements should therefore be
carried out under temperature controlled conditions [38].
The error from pH measurement systems depends on the slope of the calibration
line. For a pH sensor with a slope of 59 mV/pH, an error of 1 mV in measuring the
electrode potential will cause a 0.017 pH change in the concentration. The lower the
slope, the higher the errors are on the sample measurements [39].
Several factors need to be considered to reduce the pH measurement error. First,
an electrode with a high response slope should be used. Second, it is important to
use a meter that is capable of measuring the millivolts or microvolts accurately and
precisely. With modern meter technology, this is not normally a limiting factor. The