The ion concentration is measured as the potential difference between an ion selective electrode and a reference electrode which are soaked into the sample solution and connected with ah ion meter or a meter with an ion measurement function. The fucntion of each electrode is shown in the table below. The method of measuring ion concentration using two electrodes in this way is called the ion electrode method.
| Electrode type | Electrode Function |
| Ion selective electrode | An electromotive force is generated through the ion selective memebrane by the difference of the ion concentration between in the internal solution and in the sample soluiton. |
| Reference electrode | This maintains a stable potential no matter what kind of sample solution or standard solution is present. |
The following figure shows the structure of a combination type ion selective electrode where two electrodes are integrated.
State of liquid junction of reference electrode
The reference electrode generates the potential that is used as the reference against the potential measured with the ion selective electrode. That potential needs to be stable with very little variation no matter what kind of sample solution or standard solution is present.
The internal solution in the reference electrodeis discharged to the sample solution or standard solution as a mechanism of generating such a stable potential. The stable potential with almost no change, V≅0, can be generated by the diffusion of the outflow of the internal solution into the sample solution or standard solution.
If the internal solution in the reference electrode is not discharged, the ion concentration cannot be measured accurately because the reference potential may chnage depending on the sample solution or the standard solution.
It happens if the amount of the internal solution in the reference electrode is insufficient or the rubber stopper is not opened which does not make the internal solution exposed to the atmosphere.
