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pH Measurement

pH is a measure of the acidity or alkalinity of a substance.

A low pH number means acidic while a high pH number means more alkaline. pH7 is considered ‘neutral’.

Both acidic and alkaline chemicals can be quite corrosive, so material compatibility needs consideration.

So what makes things Acidic or Alkaline and what does acidity (or alkalinity) actually mean?


The formula for water is H2O, but in fact it’s more complex than that. Water auto-ionises, it is continually adding and removing a Hydrogen ion, such that at any given time, a cup of water will contain both H2O and H3O molecules.
Water-Molecules H3O is called Hydronium. The more Hydronium, the more acidic the water. In its natural state there will be a fixed number of H2O vs. H3O molecules. The pH number is equal to the negative logarithm of the number of Hydronium ions.
E.g. in a sample of water at 25 °C there will be 1 X 10 -7 Mole of Hydronium.
The negative log of 1 X 10 -7 is 7, which has been standardised as the pH of pure water at room temperature.

Defining an Acid or Alkali

There are several definitions for Acids but the most common definition is the ‘Arrhenius Acid Definition’ that states:
“An acid is a substance that when added to water, increases the amount of Hydronium”.

Acids contain hydrogen ions (H+) and have a pH less than 7.
Hydrogen 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)
ph7 Bulbs 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.
ph8 BUlb-drawing
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.
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