The first pH electrodes had a glass bulb or bubble that was filled with strong electrolyte and had a Ag/AgCl (silver/silver chloride) half-cell inside, with Ag wire as a contact.
The workings of the original pH electrodes and modern electrodes have not changed much, however there have been new designs as technology improved such as combination electrodes, double junction electrodes, gel-filled electrodes, Calomel junctions, solid state electrodes, Ion Selective Electrodes and epoxy body electrodes.
The majority of today's pH electrode types are known as combination electrodes. Combination electrodes have the glass Hydrogen Ion (H+) sensitive electrode and an additional reference electrode all in one housing.
How Does the pH Electrode or Combination Electrode Work?
The combination or pH electrode measures the difference in potentials between the two sides in the glass electrode. To measure the potentials it must be a closed circuit. The circuit is closed through the internal solutions of the electrode and the external solution that is being measured and the pH meter.
As the electrode is immersed in the test solution the glass bulb senses the hydrogen ions as a millivolts (mV) due to the positive charge of the hydrogen ions. The electrolyte or internal solution picks up the mV signal from the glass bulb. That signal is then passed to the internal electrode. The Ag/AgCl wire then passes that signal to the electrode cable that leads to the meter.
The reference electrode containing electrolyte or filling solution generates a constant mV, which is transferred to the Ag/AgCl wire. The wire then passes the signal, which can be considered a "control" being measured to the electrode's cable.
The circuit is closed by a minute amount of internal solution from the reference electrode flowing through a porous membrane made of a ceramic wick. This membrane or junction as it is called is located the electrode body.
The pH meter measures the difference between the internal electrode and the reference electrode in millivolts DC. This mV reading is then read by the meter and is displayed in pH units.
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.
Commonly Asked Questions
Q. What is the difference between a Glass body and Epoxy-body Electrode?
A. Epoxy-body Electrodes have an impact resistant body that suits them for field work or rough handling. The bulbs are made of glass so some caution must still be followed when handling. The plastic body also restricts the electrodes from being used with high temperature solutions and solvents.
Q. What is the difference between Refillable (liquid filled) and Gel-filled electrode?
A. In a refillable electrode a small amount of electrolyte solution leaks through the junction into the test solution it can be replenished or refilled through a fill port on the body of the electrode. This helps in maintaining accuracy as well as extending the life of the electrode.
In gel-filled, the electrolyte is just that, jelled; it slows the leaking of the electrolyte but does not prevent back flow through the junction which leads to contamination. Gel-filled electrodes are easier to maintain but have a shorter service life.
Q. What is the expected life of an electrode?
A. In general pH electrodes will last from one to two years. There are many factors involved that determine the life expectancy. If the wrong electrode is selected for an application, its life will be shortened. When testing aggressive acids or alkalines the working life will be shortened. Finally, if the electrode is not maintained and stored properly, the service life of the electrode may be shortened.