pH Meter Electrodes

Wine Analysis Home

The Glass and Reference Electrodes of a pH Meter

Two electrodes are involved in the measurement of pH, the glass electrode and the reference electrode.

The Glass Electrode.

The glass electrode can also be referred to as the sensing, measuring, indicating or pH electrode.
The glass electrode is a half cell consisting of –
- an internal silver/silver chloride (Ag/AgCl) electrode
- an internal, pH neutral (pH 7) solution of concentrated potassium chloride (KCl)
- a sealed glass outer body with a special pH sensitive glass bulb joined at the end.
  
  This special glass bulb is able to develop an electric potential corresponding to the solutions’ pH. (see explanation).
- The special glass bulb is capable of absorbing water from both the inside and outside of bulb facilitating the flow of electrical charges at the inner and outer surfaces.
- At these hydrated, gel like glass surface layers hydrogen ions and lithium ions (Li⁺) are interchangeable within the glass matrix..
- With a greater then neutral H⁺
- It is the build up of hydrogen charges
- The reference electrode of a pH meter generates a fixed electrical potential
- It is the difference in electrical potential between the reference electrode and the glass electrode that generates an electric current corresponds and is used to determine the pH of a solution.
- The circuit for this electric current is completed via the sample being measured.
- The reference electrode has a number of parts to it but suffice to say that the electric current flows through two internal solutions.
  
  One of these solutions is a concentrated solution of potassium chloride (1 MKCl) or a supersaturated solution of KCl, where a few crystals of KCl remain undissolved.
  
  This solution can be topped up through a fill hole usually located at the top of the electrode.
- The electric circuit, measuring the current flow between the reference and glass electrodes due to the difference in electrical potential is completed by the sample being tested.
- The reference electode completes the electrical circuit through the solution being tested with the glass electrode either through a porous plug (e.g. sintered glass frit) or a wet junction. The frit or wet junction must be kept clean and moist at all time and not allowed to dry out.

Temperature Probe

The Temperature Probe is able to automatically read and adjust for temperature differences of solutions being measured through a temperature probe incorporated or separate from the electrodes.

This glass able to develops an electric potential corresponding to a solutions’ pH. (see explanation).