pH Electrodes Designed and Manufactured by Hanna Single Junction vs. Double Junction Conventional electrodes are normally single junction. As depicted by the figure below, these electrodes have only a single junction, which serves to put the reference electrode system in contact with the sample. Under adverse conditions, such as high pressure, high temperature, highly acidic or alkaline solutions etc., the positive flow of the electrolyte through the junction is often reversed resulting in the ingress of sample solution into the reference compartment. If this is left unchecked, the reference electrode can become contaminated, leading to complete electrode failure. Another potential problem with single junction electrodes is the clogging of the junction due to AgCl precipitation. AgCl is less soluble in the sample than the reference electrolyte solution. Therefore, when the electrolyte solution makes contact with the sample, some AgCl will precipitate on the external face of the junction. The result is drifty readings obtained from the sensor. Hanna’s double junction system, as the name implies, has two junctions, only one of which is in contact with the sample as shown in the figure below. Under adverse conditions, the same tendency of sample ingress is possible. However, as the reference electrode system is separated physically from the intermediate electrolyte area, the contamination of the electrode is minimized. The likelihood of clogging of the junction is also reduced with a double junction electrode since the outer reference cell uses a fill solution that is “silver-free.” Since there is no silver present, no precipitate can form to clog the junction. Single junction electrodes use a fill solution such as the HI7071 that contains 3.5M KCl + AgCl, while double junction electrodes typically use HI7082 that contains 3.5M KCl. Silver-Free Electrolyte Inner Tube Housing the Membrane Sensing Wire Electrolyte Containing Silver Double Junction Electrode Single Junction Electrode Inner Ceramic Junction Outer Ceramic Junction Ag/AgCl Reference Wire Ceramic Junction Types of Junctions: Porous Ceramic Normally used in electrodes with glass bodies because ceramic with the correct expansion coefficient is easily welded to glass. Ceramic is available with different porosities and diameters. It may also be referred to as a diaphragm. Porous PTFE (Polytetrafluoroethylene) Porous PTFE is a hydrophobic material that is available with different porosities. Because of its chemical resistance, PTFE is widely used in industrial applications. Fiber Wick This type of junction is often used on plastic bodied electrodes with gel electrolytes. Open Junction This type of junction is often found in foodcare pH electrodes and is filled with a special gel which comes into direct contact with the solution to be measured. An advantage of an open junction is low contact resistance and low clogging potential. Cone Style This style junction is also renewable. As the sleeve or collar is moved, fresh fill solution cleans out the junction with fresh electrolyte. This has a higher flow rate than a ceramic type and is often specified for ISE measurements. Other types of junctions include: Capillary Junction This type of junction can be made with smooth or frosted glass. The advantage of a capillary junction is a fast flow rate and an open channel. It is typically used with thickened electrolytes. Open Platinum This style junction is made by partially sealing fine Pt wires through the stem glass, creating a leakage path. These have high flow rates. Fiberglass This style junction is very similar to a fiber wick. The junction is typically renewable and may have a high flow rate depending on strand number in the bundle. 2 pH 2.126 | www.hannainst.com electrodes
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