How Do Double Junction Electrodes Work?
As mentioned above, the combination electrode is separated from the solution being tested by the junction that the electrolyte flows out of. In adverse conditions where the solutions being tested can be strongly acidic or alkaline, or be under high pressure or high temperatures, the positive flow of the electrolyte solution through the junction can be reversed. Once reversed the sample being tested can flow into the reference compartment contaminating the reference electrode, ultimately destroying the electrode.
In applications where the electrolyte flow reversal may occur, a double junction is used. To protect the reference from contamination, a second junction is built into the electrode. The second junction, or double junction electrode as it is known, has an additional chamber between the solution being tested and the reference electrode. Before contamination from the test solution can get to the reference electrode it must first diffuse not only through the original junction but also through the second. The additional chamber works as a buffer, slowing changes in the electrolyte solution of the reference electrode. The advantage of double junction electrodes is that they last longer and work well under adverse conditions.
How Do Calomel Electrodes Work?
General purpose electrodes constructed with the silver wire and silver chloride and potassium chloride (KCl) electrolytes work well with most aqueous solutions that do not react with silver. When working with solutions with heavy metals, proteins, tris buffers, organics, and low ion solutions all of which do react with silver, the life of a general-purpose electrode is shortened.
The Calomel combination electrode is constructed with the reference electrode being made of mercury and mercuric chloride and having KCl electrolyte. It is less reactive with the above-mentioned solutions and gives a longer service life to the electrode.
The drawback to the Calomel electrode is the mercury and mercurous compounds it contains. They are considered hazardous materials and require specific disposal practices.
Improvements in technology in the construction of the Double Junction electrodes have eliminated the need for Calomel electrodes.
How Do Ion Selective Field Effect Transistors (ISFET) Work?
Ion Selective Field Effect Transistors (ISFET), or solid state electrodes, rely on a silicon chip which, when placed in contact with the test solution, detects and measures the variable voltage potential between its surface and underlying semiconductor material. This variable potential is proportional to the hydrogen ion concentration in the sample, and is used to determine the pH value.
ISFET electrodes are durable and easy to maintain. The probes are made of stainless steel and because there is no glass they are ideal for the food industry. They are virtually unbreakable.
Other advantages of the ISFET electrode include; a rugged pH sensing area that can be cleaned by a toothbrush; they can be stored dry; have a quick response time and can be used for testing extreme levels of either acids or alkaline on the pH scale.
The disadvantages of the ISFET are that the are two to three times more expensive than traditional electrodes; they do not offer the same stability and accuracy as glass electrodes; they have a known drifting problem and the majority only work with pH meters adapted to ISFET technology.
How Do Ion Selective Electrodes Work?
Ion Selective Electrodes (ISE) are used to measure the specific ion concentration in a solution usually in a real time basis. The electrodes consist of a sensor that converts the activity of an ion that is dissolved in a solution into an electrical potential. Whereas the traditional pH electrode measures the potential of dissolved hydrogen ions; ISE can measure the potential of the ions many substances; some common examples are Ammonia, Cadmium, Calcium, Bromide, Fluoride, Copper and Cyanide dissolved in a solution.
ISE can only be used with pH meters that display reading in a millivolt scale as well as a pH scale. Measurements are converted to Parts per Million (ppm).
Rather than the glass bulb the sensing part of the electrode is made as an ion-specific membrane along with an ion specific reference electrode. When using an ISE ionic strength adjuster, an ion-specific calibration solution and replacement electrolyte solution are required.
ISE are used in water treatment plants, manufacturing facilities and laboratories where real time measurements of specific ions are required for purification results, quality control and solution analysis.
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