Thermo Fisher Scientific CyberScan pH 5000 User Manual
Page 136
Instruction Manual
CyberScan pH5500/ 5000
Temperature
4.01
6.86
9.18
0
10
20
25
30
40
50
4.003
3.998
4.002
4.008
4.015
4.035
4.060
6.984
6.923
6.881
6.865
6.853
6.838
6.833
9.464
9.332
9.225
9.180
9.139
9.068
9.011
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) 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 units in highly alkaline, high-sodium solutions.
The special FS-5 glass using in Eutech Instruments electrodes
exhibits must less sodium error in these solutions. < 0.05 pH units.
131