ISFET Sensors Versus Glass Bulb pH Electrodes
The method by which ISFET sensors and glass bulb pH electrodes achieve a response is significantly different. In buffers and most samples, the pH value based on both technologies will be the same. However, differences due to method may occur in applications like milk and cheese. As these are reproducible readings, the bias can be set in the pH meters to come to the same readings as the glass electrodes.
The reason for different outcomes is primarily the following. Glass bulb electrodes use a permeable glass membrane involving an ion exchange process. The pH value measured by glass electrodes is based on the hydrogen ion (H+) concentration. Instead, ISFET sensors use an instantaneous surface reaction. The pH value measured by ISFET electrodes is based on the H+ activity as described below.
What Is pH?
The pH is a widely used way of expressing the acidity or alkalinity of aqueous solutions. In essence it is a means of indicating the hydrogen ion activity (aH+) in products, which in turn is the direct availability of free H+. The commonly used pH scale is from 0 to 14: a pH value of 7 is neutral, values below 7 are acidic and those above 7 are alkaline or basic.
Hydrogen Ion Activity Versus H+ Concentration
Mathematically, the pH is expressed as pH = -10log (aH+), where aH+ is the hydrogen ion activity. Hydrogen ions are made available (i.e. free) by the ability to dissociate from their parent compound. Strong acids like sulfuric acid, H2SO4, and weak acids like boric acid, H3BO3, both contain H+, but the H+ in strong acids dissociates in the presence of water to become free H+. In weak acids, when added to water, very little of the hydrogen is liberated as free H+. Instead it remains relatively inert in the undissociated molecules of the acid. Thus, other than expressed as concentration or strength, the pH expressed as hydrogen ion activity is a more precise way to know about the quality or effectiveness of water based products.
Principle of the ISFET Technology
The principle of the ISFET technology is that when the H+ activity changes, the current driven voltage will change and a different pH value is measured accordingly. To complete the electrical circuit, the ISFET sensor (also called the indicator electrode) needs a reference electrode. For any change in the analyte concentration, the potential of the reference electrode will remain virtually constant and so the change of potential will only be caused by the indicator electrode.
Wellinq has a broad experience in the development and manufacture for the circuitry and reference electrodes and owns tiny, stable designs compatible with medical and food safety standards.
Webshop for ISFET pH products
Various pH probes and meters can be purchased in the webshop at the website of Wellinq Sensor Solutions. The probes all are based on the ISFET technology, but differ in dimension as each application requires each own restrictions. More information on applications can be found at the `pH meter line` page of the Wellinq Sensor Solutions website.
