How is pH Measured in Potentiometry?

Ph measurement is widely used to measure the acidity of any kind of fluid. There are two ways of measuring pH; the first is the colourimetric method using a colour indicator like solutions or papers, and the second is through potentiometry. Potentiometry, also known as potentiometric titration, is a technique to characterise and measure the potential of an analyte. There are four types of potentiometric titrations used in analytical chemistry. It is the acid-base titration where the role of the pH metre comes into play.

Potentiometry

Potentiometry or potentiometric titration is a technique used to determine the potential of an analyte. The amount of analyte is determined by adding titrants of known concentration till the endpoint. Potentiometric titrations can be divided into four categories based on the type of reaction taking place.

• Acid-Base titration: This type of titration is used in potentiometry to calculate the concentration of an acid/ base using another base/acid with a known concentration respectively. An acid and a base neutralise each other.
• Complexometric titration: It is also known as chelatometry, where the formation of a coloured complex marks the endpoint of the titration. It is used to determine the mixture of metal ions in a given solution.
• Precipitation titration: Here, the reaction between an analyte and a titrant leads to the formation of a precipitate. When the precipitate formation stops even after the addition of titrant, the titration is considered to reach its endpoint.
• Redox titration: Here, the analyte and titrant simultaneously perform the oxidation and reduction reactions.

Acid-Base Titration

Acid-base titrations are neutralisation reactions involving strong, weak acids or bases. This type of titration can be performed in two ways, using a colour indicator and a potentiometer. 

Potentiometric pH Metres

Potentiometric pH metres, commonly known as pH metres, are instruments used to measure the pH of a solution by measuring the H+ ion activity. The pH metre is known to measure the difference in the potential between two electrodes; a pH electrode and a reference electrode. The principle used by the pH metre is that the electrodes record the potential of a solution and display the results in the form of pH based on H+ ion concentration. It consists of an amplifier, two electrodes, and a display for showing the results.

History

S.P.L. Sorensen was the first to define the concept of pH in 1909. However, the pH meter came into play in 1934 because of Arnold Orville Beckman, who initially called it an acidimeter. In 1935, the pH meter started to be used for medical purposes. By 1970, a portable pH meter was launched by Jenco Electronics of Taiwan.

Design of a pH metre

The pH metre consists of two electrodes which are rod-like in structure. The electrodes are formed from glass membranes in a pH metre. The lower part of the electrode has a bulb-like structure which is the sensor for ionic changes in the solution. This bulb is highly selective to H+ ion concentration, and potential is generated around this bulb. The difference in potential between the electrodes is detected by the amplifier and is converted to pH units and displayed on the screen.

Uses of Potentiometric pH

There is a wide range of applications of potentiometric pH measurement in laboratories, chemical industries, pharmaceutical industries, and clinical laboratories. Many universities and colleges use this technique to purify or identify the pH of a chemical formed. It is also used for measuring the pH of the soil, quality of water, and more.

Factors Involved in Potentiometric Measurement of pH

Several factors affect measurement through a pH meter, such as

• Temperature: The measurement of pH, voltage signals, and pH values are highly dependent on temperature.
• Pressure: Under pressure, the glass of the pH metre might break; hence pressure also plays an important role.
• Moisture: Due to lack of liquid content, the electrode’s glass membrane can be damaged; hence, the electrode is always dipped in a liquid solution.
• Electrical default: It can cause signal loss, leading to incorrect readings.
• Mechanical default: Any mechanical damage to the electrode’s glass membrane can lead to faulty outcomes and results.

Conclusion

The pH is a value that tells us about the acidity or basicity of any particular substance. The pH measurement is important as it helps measure the concentration of H+ ions present in a solution. Every system in the world is maintained at a specific pH, and various chemical reactions around us take place only at a specific pH. For example, the digestion of food in our body is carried out only at acidic pH. Similarly, the soil of specific ph is only fertile for cultivation. Hence, the measurement of pH is very important. pH can be measured through colour indicators, pH strips, or pH metres. pH metres are more accurate for measuring pH as they track the H+ ions in the solution and determine the pH of the solution.