Several METHODS of measuring pH have been developed and upgraded. Currently, the glass electrode method is most often used in various fields.

Indicator method:

• The indicator method uses litmus papers or reagents such as methyl orange and phenolphthalein solutions and is old and simple way of measuring pH.
• This method has a long history and is easy to USE; however, there are various errors and inaccuracies.

Hydrogen electrode method:

• A platinum electrode, on which hydrogen gas is adequately adsorbed, is called a hydrogen electrode. Placing this electrode in a test solution generates a potential corresponding to the hydrogen ion concentration (to be precise, the hydrogen ion activity) of the solution, and thus the pH of the test solution can be DETERMINED.
• This hydrogen electrode method is to directly measure the hydrogen ion activity and is used as one of the standard pH measurement methods today. However, it is not used as an industrial standard because it requires hydrogen gas that has an explosion risk, its operation is troublesome, and furthermore, it may involve a sodium ion error and other errors.

Quinhydrone electrode method:

This method is to determine the pH from the potential difference between the two electrodes: a platinum electrode and a reference electrode. A small amount of quinhydrone is added to a test solution and after vigorous stirring; two electrodes are immersed in the solution to determine the potential difference. This method utilizes the fact that dissolving quinhydrone in a solution changes the ratio of benzoquinone to hydroquinone depending on the pH of the solution, thereby changing the potential difference. It is simple, but it can apply only to a test solution with pH 8 or less and cannot be used when a test solution contains oxidizing or reducing SUBSTANCES. Because of these application limitations, now this method is almost out of use.

Antimony electrode method:

In the antimony electrode method, an antimony rod with a polished tip is immersed into a test solution together with the reference electrode to obtain pH based on the potential difference between the antimony rod and reference electrode. This method is no longer used often with the exception of certain APPLICATIONS (where accuracy is not required when measuring a solution containing fluorine) because the reading varies depending on the condition of the polished electrode and reproducibility is poor.

Glass electrode method:

If two solutions with different pH exist separately on two sides of a thin glass membrane, a potential difference develops between the two sides, which are proportional to the difference in pH of the two solutions. This method of measuring pH is often used due to advantages such as the potential quickly reaches equilibrium, good reproducibility, and little effects of an oxidizing or reducing substance. “Methods of pH Measurement” also specifies that the applicable scope is limited to the “method of measurement by a pH meter using a glass electrode.”

Several methods of measuring pH have been developed and upgraded. Currently, the glass electrode method is most often used in various fields.

Indicator method:

Hydrogen electrode method:

Quinhydrone electrode method:

This method is to determine the pH from the potential difference between the two electrodes: a platinum electrode and a reference electrode. A small amount of quinhydrone is added to a test solution and after vigorous stirring; two electrodes are immersed in the solution to determine the potential difference. This method utilizes the fact that dissolving quinhydrone in a solution changes the ratio of benzoquinone to hydroquinone depending on the pH of the solution, thereby changing the potential difference. It is simple, but it can apply only to a test solution with pH 8 or less and cannot be used when a test solution contains oxidizing or reducing substances. Because of these application limitations, now this method is almost out of use.

Antimony electrode method:

In the antimony electrode method, an antimony rod with a polished tip is immersed into a test solution together with the reference electrode to obtain pH based on the potential difference between the antimony rod and reference electrode. This method is no longer used often with the exception of certain applications (where accuracy is not required when measuring a solution containing fluorine) because the reading varies depending on the condition of the polished electrode and reproducibility is poor.

Glass electrode method:

If two solutions with different pH exist separately on two sides of a thin glass membrane, a potential difference develops between the two sides, which are proportional to the difference in pH of the two solutions. This method of measuring pH is often used due to advantages such as the potential quickly reaches equilibrium, good reproducibility, and little effects of an oxidizing or reducing substance. “Methods of pH Measurement” also specifies that the applicable scope is limited to the “method of measurement by a pH meter using a glass electrode.”