Thermo Fisher Scientific CyberScan 6000 Series Meters v.2 - Manual (Page 89)

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Appendix: pH Theory
PH6000, PC6000, PH6500, PC6500 and PCD6500 meters
Technology Made Easy ...
pH Measurement
The pH value of a sample can be determined in several ways. These include the use of organic
dyes which change color 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 a 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 millivolt 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:
2.3RT
Emeas = E* – nF (pH)
In this equation, Emeas 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 differences
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 controlled, and
programmed to use electrode voltages, efficiency, and temperature to calculate an 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
prepackaged salts.
The following table lists three of the most widely used NIST buffers along with their pH values at
different temperatures.
Temperature (C) 4.01 6.86 9.18
0 4.003 6.984 9.464
10 3.998 6.923 9.332
20 4.002 6.881 9.225
25 4.008 6.865 9.180
30 4.015 6.853 9.139
40 4.035 6.838 9.068
50 4.060 6.833 9.011
( )