pH METER DESIGN AND WORKING

• A pH meter provides a value as to how acidic or alkaline a liquid is. The basic principle of the pH meter is to measure the concentration of hydrogen ions. Acids dissolve in water forming positively charged hydrogen ions (H+). The greater this concentration of hydrogen ions, the stronger the acid is. Similarly alkali or bases dissolve in water forming negatively charged hydrogen ions (OH-). The stronger a base is the higher the concentration of negatively charged hydrogen ions there are. The amount of these hydrogen ions present solution is dissolved in some amount of water determines the pH. 

• A pH value of 7 indicates a neutral solution. Pure water should have a pH value of 7. Now pH values less than 7 indicate an acidic solution while a pH value greater than 7 will indicate an alkaline solution. A solution with pH value of 1 is highly acidic and a solution of pH value of 14 is highly alkaline. 

WHAT IS pH??

• A pH meter will be made up of a probe, which itself is made up of two electrodes. This probe passes electrical signals to a meter which displays the reading in pH units. The glass probe has two electrodes because one is a glass sensor electrode and the other is a reference electrode. Some pH meters do have two separate probes in which case one would be the sensor electrode and the other the reference point. 

• Both electrodes are hollow bulbs containing a potassium chloride solution with a silver chloride wire suspended into it. The glass sensing electrode has a bulb made up of a very special glass coated with silica and metal salts. This glass sensing electrode measures the pH as the concentration of hydrogen ions surrounding the tip of the thin walled glass bulb. The reference electrode has a bulb made up of a non-conductive glass or plastic. 

• When one metal is brought in contact with another, a voltage difference occurs due to their differences in electron mobility. Similar is the case with two liquids. A pH meter measures essentially the electro-chemical potential between a known liquid inside the glass electrode (membrane) and an unknown liquid outside. Because the thin glass bulb allows mainly the agile and small hydrogen ions to interact with the glass, the glass electrode measures the electro-chemical potential of hydrogen ions or the potential of hydrogen. To complete the electrical circuit, also a reference electrode is needed. 

BASIC PRINCIPLE OF A pH METER

GLASS BULB

A typical modern pH probe is a combination electrode, which combines both the glass and reference electrodes into one body. The combination electrode consists of the following parts :

• A sensing part of electrode, a bulb made from a specific glass• Internal electrode, usually silver chloride electrode or calomel electrode• Internal solution, usually a pH=7 buffered solution of 0.1 mol/L KCl for pH

electrodes • Reference electrode, usually the same type as 2• Reference internal solution, usually 0.1 mol/L KCl• Junction with studied solution, usually made from ceramics or capillary

with asbestos or quartz fiber.• Body of electrode, made from non-conductive glass or plastics.

The bottom of a pH electrode balloons out into a round thin glass bulb. The pH electrode is best thought of as a tube within a tube. The innermost tube (the inner tube) contains an unchanging 1×10−7 mol/L HCl solution. Also inside the inner tube is the cathode terminus of the reference probe. The anodic terminus wraps itself around the outside of the inner tube and ends with the same sort of reference probe as was on the inside of the inner tube. It is filled with a reference solution of 0.1 mol/L KCl and has contact with the solution on the outside of the pH probe by way of a porous plug that serves as a salt bridge.

HOW IS THE pH PROBE DESIGNED??

• A silver chloride electrode is a type of reference electrode, commonly used in electrochemical measurements. For example, it is usually the internal reference electrode in pH meters. The reaction is between the silver metal (Ag) and its salt — silver chloride (AgCl, also called silver(I) chloride).

The corresponding equations can be presented as follows:

• This reaction is characterized by fast electrode kinetics, meaning that a sufficiently high current can be passed through the electrode with the 100% efficiency of the redox reaction (dissolution of the metal or cathodic deposition of the silver-ions). The reaction has been proved to obey these equations in solutions of pH values between 0 and 13.5.

SILVER CHLORIDE ELECTRODE

The pH meter measures the potential difference and its changes across the glass membrane. The potential difference must be obtained between two points; one is the electrode contacting the internal solution. A second point is obtained by connecting to a reference electrode, immersed in the studied solution. Often, this reference electrode is built in the glass electrode (a combination electrode), in a concentric double barrel body of the device.

WORKING OF A pH METER

• Ag/AgCl | HCl | glass || probed solution | reference electrode)

• AgCl(s) | KCl(aq) || 1×10-7M H+ solution || glass membrane || Test Solution || ceramic junction || KCl(aq) | AgCl(s) | Ag(s)

• The potential difference relevant to pH measurement builds up across the outside glass/solution interface marked ||

• The bulb is sealed to a thicker glass or plastic tube, and filled, for example, with a solution of HCl (0.1 mol/dm3). In this solution is immersed a silver/silver chloride electrode with a lead to the outside through a permanent hermetic seal. The filling solution has constant Cl- concentration, which keeps the Ag/AgCl inner electrode at fixed potential.

• The pH sensing ability of the glass electrode stems from the ion exchange property of its glass membrane.

• Glass is mostly amorphous silicon dioxide, with embedded oxides of alkali metals. When the surface of glass is exposed to water, some Si–O- groups become protonated

• Si-O- + H3O+ ≡ Si-O-H+ + H2O (2)

• The exchange of hydronium (or written as proton, H+) between the solid membrane and the surrounding solution, and the equilibrium nature of this exchange, is the key principle of H3O+ sensing. As with any interface separating two phases between which ionic exchange equilibrium is established, the glass membrane/solution interface becomes the site of a potential difference

• Eglass electrode = E ‘ + RT/2.303F log a(H3O+)

Where E’ represents the sum of the constant offset potentials of the inner glass surface/solution and the two Ag/AgCl electrodes. At 30°C the potential of the glass membrane changes by about 60 mV for each one unit of pH