The pH measurement originates in the electrode system. This system consists of a pH sensor, pH Half Cell, whose voltage varies proportionately to the hydrogen ion activity of the solution, and a reference electrode, Reference Half Cell, which provides a stable and constant reference voltage.

The pH electrode consists of a thin membrane of Hydrogen sensitive glass blown on the end of an inert glass tube. This tube is filled with an electrolyte, and the signal is carried through Ag/AgCl wire. This is a pH Half Cell. A similar system, but without using a Hydrogen sensitive glass, is used as a reference. A small filter (diaphragm) connects this tube to the external liquid. This system is called a Reference Half Cell.

The meter measures the difference between the pH Half Cell and the Reference Half Cell in Millivolts DC. This millivolt reading is read by the unit and displayed in either mV or pH units.

pH Half Cell
The electromotive force is based upon the following equation:				The slope factor N1 is called the Nernstian factor and is a characteristic of the glass membrane equal to:
Where:				Where:
Eobs = observed potential				R = gas constant (8.314 J/K Mol)
Ec = reference potential including other stable and fixed potentials				T = absolute temperature in °K
Nf = Nernstian slope factor				F = Faraday constant (9.648 x 104)
aH+ = hydrogen ion activity				n = valence factor (n = 1 in the case of hydrogen)

With R, F and n being constants, the observed potential is therefore dependent on the temperature of the sample. The relationship between the Nernstian factor and temperature is shown in the table below.

Hanna pH electrodes vary in shape and dimensions according to the needs of the market. Such variations are shown in the table below:

Theory and Practice

Similar to the manner in which acidic or alkaline solutions are quantified by pH measurements, solutions can also be graded as oxidizing or reducing based on measurements of ORP (sometimes called "REDOX") values.

Oxidation is a process during which a molecule or ion loses electrons. However, oxidation is always coupled together with reduction so that as one element gets oxidized, the other automatically is reduced.

Measurements

ORP measurements are based on the potential difference measured between the platinum or gold electrode and a reference electrode. The identical reference system utilized for the pH electrode (Ag/AgCl) is also used for redox measurements.

Redox electrodes are used to monitor many chemical processes particularly those involving reversible reactions. Common applications include:

Industrial Waste Water Treatment

The redox systems used in water treatment are the reduction of chromates and oxidation of cyanides. Waste hexavalent chromium is reduced to trivalent chromium by the addition of sodium bisulfate or sulphur dioxide. In the case of cyanide, chlorine or sodium hypochlorite is used to oxidize the cyanide, followed by the hydrolysis of cyanogen chloride to form cyanate.

This complementary oxidation-reduction process is known as a redox reaction system and the ORP value is a measure of the electron activity as compared to the hydrogen activity in the case of pH measurements.

Redox potentials are measured by an electrode normally made of an inert metal and is capable of absorbing or releasing electrons. The common metals used are platinum or gold.

When the redox electrode is immersed in a solution containing a reversible chemical reaction system, a migration of electrons is established between the electrode and the system. This electron flow can be construed as an exchange current density and is of paramount importance for accurate, fast and reproducible redox potential measurement.

Water Sterilization

ORP measurements are being increasingly used as an effective measure of the sanitizing activity in pool, spa and potable water. The kill time of E. Coli bacteria in water depends on the ORP value. ORP is a reliable indicator of bacteriological water quality. Water having an ORP value equal to or higher than 650 mV are well within accepted bacterial parameters for pool and spa water.