What is pH?

How a pH meter works?

Introduced by Søren Peter Lauritz Sørensen in 1909.

pH Definition

How pH works?

The pH measurement defines the quantity of hydrogen ions (H +) and hydroxide ions (OH-) present in an aqueous solution. In any set of water molecules, a small part is dissociated into ions:

The pH measures the degree of acidity or basicity of a solution, as a function of the concentration of H + ions.

Applicable to aqueous solutions.

What is a ph scale?

The pH value of pure water is 7 (at 25 ° C). The solutions can be separated into acids (generate H + ions) and basic (generate OH- ions), such as:

The pH scale varies from 0 to 14 units. Below 7 the solution is acidic (acid ph), above 7 the solution is basic (pH = 7 neutral ph solution).

How is ph measured?

The Industrial pH measurement is determined by measuring the voltage of an electrochemical cell. The following figure shows a simplified diagram of a pH probe:

ph Measurement Probe

The cell voltage is directly proportional to the pH of the test solution. In this figure, the measuring electrode and the reference electrode are separated. In most process sensors the two electrodes and the temperature sensor are combined in a single envelope.

The majority of pH electrodes available commercially are combination electrodes that have both glass H+ ion sensitive electrode and additional reference electrode conveniently placed in one housing.  In most cases combination electrodes are precise enough and much more convenient to use.

 

The Glass Electrode

The essential element of the measuring electrode is the pH-sensitive glass membrane. This glass can be considered as a ‘superfrozen’ electrolyte consisting of an irregular SiO2 structure and a number of other components moving in the gaps.

pH Glass electrode

These additional components allow to generate a balance of electrons through the glass.

When immersed in aqueous solutions all types of glasses used in pH measurement probes have the property of exchanging metal ions (La +++, Ba ++, Ca ++, …) with hydrogen ions H + present in the solution.

hydrated gel and ion behaviour at the glass electrode membrane

Because the hydrogen ions H + are surrounded by H2O molecules and are not free, a layer of silicic acid will form during the ion exchange.

As a result of this reaction a gel layer will be formed on the surface of the glass membrane (Hydrated gel in graph). The formation of this layer is essential for the proper functioning of the sensor. After about 2 days the situation reaches equilibrium and the gel layer stabilizes and reaches a thickness between 10 and 40 nanometers (depending on the fluid to be measured and the type of glass). At this point the ionic exchange of metal ions (La +++, Ba ++, Ca ++, …) with hydrogen H + ions is stopped and the voltage across the membrane is 0 volts.

 

Glass electrode Edge

To keep the gel-layer hydrated the sensor bulb must be kept moist constantly.

The glass electrode consists of a glass tube closed at the bottom with a glass membrane especially sensitive to H + ions. In the inner part of this membrane is a solution of potassium chloride KCl, a constant pH buffer, into which a silver Ag/AgCl wire is immersed.


If the concentration of hydrogen ions (H +) in the glass and the gel-layer is different from the concentration in the solution in which it is immersed, a transport of hydrogen ions occurs. This movement will affect the stability of the gel layer.

The value of the voltage will depend on the concentration of hydrogen ions (H +) present in the solution. As this potential can not be measured directly, it will be necessary to add a reference potential independent of the pH measurement to the circuit (reference electrode).

Variation with temperature

This potential not only depends on the concentration of H + ions, it also depends on the temperature of the medium (slope variation)

The potential depends on the temperature, for example it goes from 54.2 mV at 0 degrees Celsius to 74 mV at 100 degrees Celsius per unit pH.


The Reference Electrode

The reference electrode is composed of a Silver / Silver Chloride (Ag / AgCl) cable surrounded by an aqueous solution / Potassium Chloride (KCl) gel. The potential of the reference electrode is constant and only varies with temperature.

Reference Electrode

The salt bridge is a fundamental part of this type of electrode. It enables the electrical connection between the reference electrode and the liquid to be measured. Saline bridges are very porous, and these pores are filled with ions. The ions come from the solution of KCl and the solution that we are going to measure.

To measure the potential developed in the glass electrode it is necessary to have a second electrode in the so-called reference solution. This, in addition to closing electric circuit supplies a constant potential that serves as reference to measure the variable potential of the glass electrode (ph reading).

 

Electrical Circuit

The diagrams show a pH measuring circuit using a Yokogawa pH analyser. It consists of the pH glass electrode, reference electrode, solution ground and pH analyser built as dual amplifier system. The diagrams show the potentials which effect the final potential difference (Et) between the glass electrode and the reference electrode.

 

What can affect pH measurement?

 

References

What is Industrial pH Measurement?
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