ORP/pH/Temperature Tester (HI98121) - Hanna Instruments

The HI98121 is a lightweight ergonomically designed tester. This tester is designed to float making it ideal for taking readings virtually anywhere.

By definition pH is the measurement of the voltage generated by an electrochemical cell at zero current. The potential generated is proportional to the thermodynamic activity of the ion being measured.

pH can further be described as the negative log of the concentration of hydrogen ions, pH=-log10[H+].  This is based on the fact that pure water, H2O, has an equal amount of hydrogen (H+) ions and hydroxide (OH-) ions.  In pure water there is an equal amount of positive ions and negative ions, making the solution neutral.  As a pH changes, there is a 10 times change in H+ concentration per pH unit.

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

When an oxidizing and/or reducing agent is dissolved into an aqueous solution, they may react with materials present and produce a voltage, or electromotive force (EMF), that is related to the ratio of oxidized to reduce species in the sample. An electron exchange can develop between this solution and an inert metal sensor immersed in the solution, and the voltage can be measured (when compared to a reference electrode) with a pH/mV meter.

This type of measurement is known as redox or ORP. The units of measurement are in mV. At a glance, an ORP electrode may look very similar to a pH electrode. Like a combination pH electrode, both the sensor and the reference are housed in a common body.

The scale of measurement may be positive (indicating oxidizing potential) or negative (indicating reducing). It should be noted that when zero mV is observed, it is really an oxidizing situation because the reference voltage (~200 mV for an Ag/AgCl with KCl electrolyte) is included in the observed mV value. In some cases the user may wish to offset the reading to remove the reference contribution. The mV is then said to be approaching the absolute mV scale that references a SHE (standard hydrogen electrode).

This type of calibration is called relative mV calibration. An ORP sensor must be chemically inert; it cannot be oxidized or reduced itself. It must also have the proper surface characteristics to promote rapid electron exchange, a property known as high exchange current density. Two noble metals have proven to work well for this purpose: pure platinum and pure gold are both used in the construction of ORP sensors.

The platinum sensor is often preferred because it is mechanically simpler and safer to produce. Platinum can be welded to glass and has the same thermal coefficient. Sensors made of gold cannot be welded to the glass and are often placed in plastic supports applied to the glass or plastic tube by means of tiny elastomeric bungs. The gold or platinum sensor signal is carried through the electrode body, and together with the reference signal is conducted to the measurement meter via a coaxial cable with BNC connector.

Features at-a-glance:

• Waterproof

• Automatic temperature compensation

• HOLD feature

• Simple to use

• Economical