pH Measurement

There are several definitions for Acids but the most common definition is the ‘Arrhenius Acid Definition’ that states:

Acids contain hydrogen ions (H⁺) and have a pH less than 7.

 

Under the Arrhenius definition, “An alkali is a substance that when added to water, increases the number of Hydroxide (OH) ions”. In another definition, called the “Broensted-Lowry acid definition”, acids are known as “Proton Donors” as the Hydrogen ion is a proton and bases are known as “Electron Donors”.

 

Strong Acid vs. Weak Acid

A strong acid, like Hydrochloric acid (HCL), will completely break up (disassociate) into H⁺ and Cl^- ions. While weak acids will only partially disassociate.
 

Measuring pH

pH is most commonly measured with a glass bulb possessing properties such as a semi permeable membrane.
This bulb contains an electrode made of silver and an Alkaline Gel of Potassium Chloride (KCl)

	Because the glass is permeable, and because the gel inside is strongly alkali, there is a ‘Concentration Gradient’ between the substance the probe is in, and the gel inside. This causes H+ ions to move through the glass. The Hydrogen moving through the glass creates a very small electric charge which we can measure in millivolts (mV). The amount of this charge will indicate the pH of the material under test in comparison with the gel inside the glass.
	As you might imagine, this special semi-permeable glass takes some caring for and this may explain why so many people have had failures of their pH probes. When one purchases a pH probe, the manufacturer will give care instructions, but typically these are things like keeping the bulb moist, not wiping it in a way to cause static and not allowing the build-up of protein.
	Honeywell approached the problem of delicate glass bulbs, by building special pH probes based on a transistor.  These DuraFET sensors typically have lower accuracy but are much more robust than traditional glass probes.

Feel free to contact the technical team if you have any questions on pH.