For optimal pH measurements, the correct electrode must first be selected.
The most important sample criteria to be considered are: chemical composition, homogeneity, temperature, pH range and container size (length and width restrictions). The choice becomes particularly important for non-aqueous, low conductivity, protein-rich and viscous samples where general purpose glass electrodes are subject to various sources of error.
The response time and accuracy of an electrode is dependent on a number of factors. Measurements at extreme pH values and temperatures, or low conductivity may take longer than those of aqueous solutions at room temperature with a neutral pH.
The significance of the different types of samples is explained below by taking the different electrode characteristics as a starting point. Again, mainly combined pH electrodes are discussed in this chapter.
Figure 14. Electrode with ceramic junction.
a) Ceramic junctions
The opening that the reference part of a pH electrode contains to maintain
the contact with the sample can have several different forms. These
forms have evolved through time because of the different demands put
on the electrodes when measuring diverse samples. The ‘standard’ junction
is the simplest one and is known as a ceramic junction. It consists
of a porous piece of ceramic which is pushed through the glass shaft of
the electrode. This porous ceramic material then allows the electrolyte to
slowly flow out of the electrode, but stops it from streaming out freely.
This kind of junction is very suitable for standard measurements in aqueous
solutions; the METTLER TOLEDO InLab®Routine Pro is an example
of such an electrode. A schematic drawing of the principle of this junction
is shown below in figure 14.