Fisher Scientific XL15 User Manual page 64

Temperature 4.01
0 4.003
10 3.998
20 4.002
25 4.008
30 4.015
40 4.035
50 4.060
pH Electrodes
The electrode system consists of two half cells: a pH indicating electrode, which is
primarily responsive to the acidity (the hydronium ion concentration-tion) of a solution,
and a reference electrode, which provides a constant voltage and completes the
electrical circuit.
Traditional pH indicating electrodes use a tip made of pH sensitive glass. Inside this
electrode is a buffer solution with a fixed pH and ionic strength. A silver wire coated with
silver chloride is immersed inside this internal solution, and establishes electrical contact
between the solution and the meter. The voltage associated with this wire and the
voltage associated with the pH of the internal solution and the inside wall of the pH
sensitive glass tip remain constant. Therefore, changes in voltage from this electrode
result from the voltage developed between the solution and the outside of the glass tip.
If the pH sensitive glass tip or membrane is to function properly, it must be hydrated.
A dry or dehydrated electrode membrane will not respond properly to changes in acidity.
pH electrodes are also effected by changes in temperature, and the presence of other
ions. Temperature effects can be countered by temperature compensation functions on
the pH meter. While temperature compensation will not allow you to predict what the pH
of the sample is at another temperature, it will permit you to accurately assess the pH
at whatever temperature you are presently working. pH sensitive membranes are also
sensitive, though to a lesser extent, to other ions than the hydronium ion. For example,
most pH glasses are somewhat sensitive to sodium ion as well. For some pH glasses,
this means an error as high as 0.5 pH units in highly alkaline, high-sodium solutions.
The special FS-5 glass used in Fisher's pH electrodes exhibits much less sodium error in
these solutions, <.05 pH units.
6.86 9.18
6.984 9.464
6.923 9.332
6.881 9.225
6.865 9.180
6.853 9.139
6.838 9.068
6.833 9.011
pH THEORY
Appendix
63