The pH of a substance indicates how many hydrogen ions (H+) it forms in a certain measured volume of water. A pH meter shows the value as to how acidity or alkalinity a liquid is. The basic principle of the pH meter is measuring the density of hydrogen ions. Positively charged hydrogen ions (H+) is formed when acids dissolve in water. A stronger acid is measured by the greater dense accumulation of hydrogen ions. Bases or alkaline substances dissolve in water forming negatively charged hydrogen ions (OH-). A high concentration of negatively charged hydrogen ions forms a stronger base. A number of hydrogen ions present in a solution dissolved in some amount of water determine the pH. pH value of solutions ranges from 1 to 14.Solutions with pH value 1 is highly acidic with solutions with pH value 14 are highly basic. A pH meter is used to determine the pH of different solutions in pharmaceuticals.
A pH meter is made of a pH probe which passes the electrical signals to the meter and the meter shows the pH value of the solution. The glass probe contains two electrodes, a reference electrode and a sensor electrode which are in the form glass tubes. One contains saturated potassium chloride solution and other contains pH 7 buffer. The sensor electrode bulb interpolates permeable or porous glass membrane with metal salts and silica coatings. A silver wire coated with silver chloride is submerged in a pH buffer in the bulb. Another silver wire with silver chloride coating is immersed in the saturated potassium chloride solution.
pH Meter Operation:
When the probe is immersed in a liquid solution for measurement, hydrogen (H+) ions accumulate around the bulb replacing the metal ions from the bulb. An exchange of ions occurs which generates an electric flow that is captured by the silver wire. pH meter computes the voltage of the electric flow by changing it into pH value by comparing the generated voltage with the reference electrode. A high voltage is realized when there is a higher accumulation or density of hydrogen ions due to increased acidity of the solution. Reading in a meter decreases due to increased voltage.
Likewise, hydrogen ions decrease when there is an increase in alkalinity. In other words, an increase in hydroxyl ions concentration decreases the voltage and increases the pH value in the meter.
All-inclusive, working principle of the pH meter and pH sensor depends upon the exchange of ions from sample solution to pH 7 buffer (inner solution) of glass electrode through the glass membrane. Continuous use of the glass integument decreases the porosity of the glass thus decreases the performance and efficiency of the probe.
Making Accurate pH Measurements:
For accurate measurements, pH meters have to be properly calibrated by dipping it into buffers which are test solution of known pH and adjusting the meter accordingly. You can alternatively correct the measurement yourself by calibrating the pH meter and making measurements at broadly the same measurements.