Hanna Instruments offers a wide variety of ORP electrodes that are designed for many different applications. The type of material used for the sensing pin, type of glass used for body material, type of junction, type of reference and electrolyte used are just some of the design considerations.
The HI3618D uses a platinum pin, glass body, single ceramic junction and is refillable with 3.5M KCl + AgCl.
Platinum Sensing Pin
The HI3618D ORP sensor is made with platinum. 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. The platinum sensor signal is carried through the electrode body, and together with the reference signal is conducted to the measurement meter.
Glass Body
The glass body is ideal for laboratory use. The glass is resistant to many harsh chemicals and is easily cleaned. The glass body also allows for a fast transfer of heat to the internal reference electrolyte. The mV generated by the reference cell is temperature-dependent. The faster the electrode reaches equilibrium, the steadier the reference potential.
Single Ceramic Junction
The outer junction of an electrode, also known as a salt bridge, is a necessary component of the electrical circuit. The movement of ions must flow through the junction for a steady reading. The outer reference has a single ceramic frit. The ceramic is a porous material that is easily fused with the glass body and has a similar expansion coefficient. A single ceramic junction has a flow rate of 15-20 μL/hour. Other junction types are available with higher flow rates and made with different materials.
Refillable
The HI3618B is a refillable probe. Since it is a single junction ORP electrode the fill solution is the HI7071 3.5M KCl + AgCl. If using a refillable ORP electrode, the fill cap should be removed prior to measurement. Removing the cap creates positive head pressure in the reference cell allowing for a higher flow rate of electrolyte through the outer junction. A higher flow rate will result in a faster and more stable reading.
DIN Connector
The HI3618D uses a DIN connector. This type of connector is generally proprietary to the meters they are supplied with and may not be interchangeable. Other types of connectors include BNC, screw type, T-type, and 3.5mm to name a few.
