This method involves dipping traditional litmus paper in the solution and comparing the standard color corresponding to a certain pH with the color of the indicator paper. This is the simplest method, but it has a high capacity for error due to temperature, salt concentration and organic compounds within the test liquid.
The hydrogen electrode method is considered to be very reliable and other results are only considered reliable when they align with the results of the hydrogen electrode method. A hydrogen electrode is made by adding platinum black to a platinum wire or plate. When immersed in the test solution, an electric charge is applied to the solution and platinum black with hydrogen gas. This method, however, is not ideal for frequent use due to the expense and the danger of dealing with hydrogen.
The quinhydron-electrode method consists of dipping a polished antimony rod into the solution as well as a reference electrode and measuring the difference in potential between them.
The antimony-electrode method is now only used in situations that can allow a small room for error such as industrial purposes. This method requires that the test solution contain fluorine. This method consists of dipping the tip of a polished antimony rod into a test solution and comparing the potential with that of a reference electrode, similar to the quinhydron-electrode method.
The glass-electrode method is very widely used for pH measurement because it has high potential to be reproduced, can be used for various solutions and is rarely affected by reducing and oxidizing agents. This method consists of pH measurement being applied to plating, etching of metal services and production of batteries.