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Beckman Model M pH Meter, 1937 [1] Beckman model 72 pH meter, 1960 781 pH/Ion Meter pH meter by Metrohm. A pH meter is a scientific instrument that measures the hydrogen-ion activity in water-based solutions, indicating its acidity or alkalinity expressed as pH. [2]
The bottom of a pH electrode balloons out into a round thin glass bulb. The pH electrode is best thought of as a tube within a tube. The inner tube contains an unchanging 1×10 −7 mol/L HCl solution. Also inside the inner tube is the cathode terminus of the reference probe.
The most common potentiometric electrode is by far the glass-membrane electrode used in a pH meter. A variant of potentiometry is chronopotentiometry which consists in using a constant current and measurement of potential as a function of time. It has been initiated by Weber. [7]
Quinones form a quinhydrone cocrystal by formation of hydrogen bonding between ρ-quinone and ρ-hydroquinone. [3] An equimolar mixture of ρ-quinones and ρ-hydroquinone in contact with an inert metallic electrode, such as antimony, forms what is known as a quinhydrone electrode.
Such an electrode has a "double reaction" mechanism - an enzyme reacts with a specific substance, and the product of this reaction (usually H + or OH −) is detected by a true ion-selective electrode, such as a pH-selective electrodes. All these reactions occur inside a special membrane, which covers the true ion-selective electrode.
In and of themselves, pH indicators are usually weak acids or weak bases. The general reaction scheme of acidic pH indicators in aqueous solutions can be formulated as: HInd (aq) + H 2 O (l) ⇌ H 3 O + (aq) + Ind − (aq) where, "HInd" is the acidic form and "Ind −" is the conjugate base of the indicator. Vice versa for basic pH indicators ...
Commercial reference electrodes consist of a glass or plastic tube electrode body. The electrode consists of a metallic silver wire (Ag (s)) coated with a thin layer of silver chloride (AgCl), either physically by dipping the wire in molten silver chloride, chemically by electroplating the wire in concentrated hydrochloric acid (HCl) [3] or electrochemically by oxidising the silver at an anode ...
The isohydric principle is the phenomenon whereby multiple acid/base pairs in solution will be in equilibrium with one another, tied together by their common reagent: the hydrogen ion and hence, the pH of solution. That is, when several buffers are present together in the same solution, they are all exposed to the same hydrogen ion activity.