Thermo Fisher Scientific CyberScan pH 5000 - Manual (Page 135)

Instruction Manual CyberScan pH5500/ 5000
pH Measurement
The pH value of a sample can be determined in several ways. These
include the use of organic dyes which change colour in certain pH
ranges. The dyes can be added directly to the solution or
impregnated onto paper which may be dipped into the solution. At
best, these “colorimetric” methods yield approximate pH values, often
with an accuracy of plus or minus 0.5 pH units.
The preferred and most accurate way to measure pH value is the
potentiometric measurement, using a pH electrode, a reference
electrode, and a pH meter. This method is based on the fact that
certain electrodes, immersed in solution, produce a millivolts potential
(i.e. voltage) that is related to the hydronium ion concentration or pH
of a solution in a precise way.
The relationship between the electrode’s voltage and the solution pH
is defined by the Nernst equation:
E* - (pH)
2.3RT
nF
( )
E =
meas
In this equation, E
meas
is the voltage output of the electrodes, E* is the
total of all other voltages in the system including the reference
voltage, R is the Gas Law constant, T is the temperature in °Kelvin, n
is the charge on the hydronium ion (1+), and F is the Faraday
constant.
pH Meter
The pH meter is a sensitive voltmeter capable of accurately
measuring small voltage difference between the pH electrode and the
reference electrode. This voltage difference is amplified, and shown
as a pH reading. Almost all modern meters are microprocessor
based, and programmed to use electrode voltages, efficiency, and
temperature to calculate and accurate pH for the sample. Since the
voltage output from different electrodes will vary, it is essential to
calibrate a pH meter. The ability to calibrate or to standardize a pH
meter permits the meter to match the pH reading on the meter with
the known pH value of a buffer solution. For best accuracy, it is
recommended that at least two buffer solutions be used to calibrate a
pH meter. Buffer solutions are available as ready-to-use solutions,
concentrated solutions, capsules, or pre-packaged salts. The
following table lists three of the most widely used NIST buffers along
with their pH values at different temperature.
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