Editing PH meter (section)

===pH electrode and reference electrode design===
Details of the fabrication and resulting microstructure of the glass membrane of the pH electrode are maintained as [[trade secrets]] by the manufacturers.<ref name="Galster">{{cite book|last1=Galster|first1=Helmuth|title=pH Measurement: Fundamentals, Methods, Applications, Instrumentation|date=1991|publisher=VCH Publishers, Inc.|location=Weinheim|isbn=978-3-527-28237-1}}</ref>{{rp|125}} However, certain aspects of design are published. Glass is a solid electrolyte, for which alkali-metal ions can carry current. The pH-sensitive glass membrane is generally spherical to simplify the manufacture of a uniform membrane. These membranes are up to 0.4 millimeters in thickness, thicker than original designs, so as to render the probes durable. The glass has [[silicate]] [[chemical function]]ality on its surface, which provides binding sites for alkali-metal ions and hydrogen ions from the solutions. This provides an ion-exchange capacity in the range of 10<sup>−6</sup> to 10<sup>−8</sup>&nbsp;mol/cm<sup>2</sup>. Selectivity for hydrogen ions (H<sup>+</sup>) arises from a balance of ionic charge, volume requirements versus other ions, and the coordination number of other ions. Electrode manufacturers have developed compositions that suitably balance these factors, most notably lithium glass.<ref name="Galster" />{{rp|113–139}}

The [[silver chloride electrode]] is most commonly used as a [[reference electrode]] in pH meters, although some designs use the [[saturated calomel electrode]]. The silver chloride electrode is simple to manufacture and provides high [[reproducibility]]. The reference electrode usually consists of a platinum wire that has contact with a silver/silver chloride mixture, which is immersed in a potassium chloride solution. There is a ceramic plug, which serves as a contact to the test solution, providing low resistance while preventing mixing of the two solutions.<ref name="Galster" />{{rp|76–91}}

With these electrode designs, the voltmeter is detecting potential differences of ±1400 millivolts.<ref name="Bye">{{cite journal|title=Potentiometric pH Meter|journal=Journal of Scientific Instruments|date=1962|volume=39|issue=6|page=323|doi=10.1088/0950-7671/39/6/442|last1=Ltd|first1=W G Pye and Co}}</ref> The electrodes are further designed to rapidly equilibrate with test solutions to facilitate [[ease of use]]. The equilibration times are typically less than one second, although equilibration times increase as the electrodes age.<ref name="Galster" />{{rp|164}}