METTLER TOLEDO (2017) How pH probes work
METTLER TOLEDO (2017) How pH probes work

This short animated video briefly explains what pH is and how a pH sensor works. pH is a numerical representation of the hydrogen ion concentration of a liquid. The pH scale runs from 1 to 14, where 7 is neutral, below 7 is acidic (the liquid contains an excess of hydronium ions) and above 7 is alkaline (the liquid contains an excess of hydroxyl ions). Wine, with a pH of 3-4, is therefore acidic; while soapy water, with a pH of 10, is alkaline. pH sensors have a number of elements that are required for them to measure the pH of a solution. The most important part of a pH sensor is the end which is in contact with the liquid. The glass tip is a type of membrane on which a gel layer forms when the sensor is in an aqueous solution. A similar gel layer also forms on the inside of the membrane glass, since the electrode is filled with an aqueous electrolyte solution (a buffer). The hydrogen ions in and around the gel layer can either diffuse into or out of this layer, depending on the pH value and therefore the hydrogen ion concentration of the liquid. This causes a potential to build up on the outside of the glass. The sensor’s inner buffer has a constant pH value; therefore, the potential on the inner surface of the membrane remains the same during the measurement. The pH electrode potential, and hence the pH, is therefore the difference between the inner and outer charge of the membrane. Also vital to the function of a pH sensor is the reference electrode. This provides a defined stable reference potential for the pH sensor potential to be measured against. To be able to do this, the reference electrode needs to be made of a glass which is not sensitive to the H+ ions in the solution. It must also be open to the sample environment into which it is dipped. To achieve this, an opening or junction is made in the shaft of the reference electrode through which the inner solution or reference electrolyte is in contact with the sample

 

 

METTLER TOLEDO (2017) How pH probes work
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