pH Measurement
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October 8, 2022Process Connections
October 9, 2022
pH is a measure of the acidity or alkalinity of a substance.
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?
Hydronium
| 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
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.