Thermo Fisher Scientific CyberScan pH 10/pH 100 - Manual (Page 58)

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14.1.3 pH and Temperature
The electromotive force generated in the glass electrode varies with the temperature of the
solution. As a result the response of the pH electrode is subjected to variations in temperature.
Automatic Temperature Compensation compensates for this variance.
The temperature dependable of solutions vary considerably. Some may show an increase in
pH while others a decrease for the same temperature variation. This is why when you are
measuring pH values at a particular temperature, even with a pH meter having ATC, you must
record the solution’s temperature along with the pH value, or the measurement may be
meaningless.
Temperature variation also cause variation in liquid junction potential, asymmetry potential,
and the pH value of the reference solution. To reduce these factors, it is important to ensure
that the temperature of the standard calibrating solution and the sample solution are the same.
14.2 Use of Standard pH Buffers
Standard pH buffer solutions are used to calibrate or standardize a pH meter before you
measure the pH of a sample. They serve as reference standards for the basis of comparison
between measurements.
The more common standard buffers are the pH 4.01, pH 7.00 and pH 10.01. The others
include pH 1.68 and pH 12.45.
For 1-point calibration, you only need a general pH value. Use a standard buffer of pH 7.00 or
a standard buffer whose pH value is close to that of the sample.
2-point calibration is used when you know that the sample is acidic or basic.
•
For acidic sample: use standard buffers of pH 7.00 and pH 4.01
•
For basic sample: use standard buffers of pH 7.00 and pH 10.01
3-point calibration is necessary when the sample’s pH is completely unknown. All pH 7.00, pH
4.01 and pH 10.01 standard buffers should be used.