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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. |
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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”. |
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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. |
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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. |
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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. |