pH - Appendix pH pH measurements are usually made with a pH...

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Appendix – pH pH measurements are usually made with a pH meter. Correct use is necessary for accurate results. Knowledge about the theory of the meter response will minimize errors in techniques. pH meter responses relate to potentiometric measurements of galvanic cells. Two half cells are necessary. One, called the indicator electrode , is set up to respond to the concentration (more precisely, the activity) of a certain ion in solution. In pH measurements, this is the hydrogen ion. The other electrode is a reference electrode, one for which the potential is known and remains constant during the measurement. In some potentiometric cells, oxidation and reduction actually occur at the anode and cathode, respectively (see Experiment 5). However, a potential can also develop by the movement of any charged species (not necessarily electrons). Membrane electrodes utilize this principle. In the early 1900s, it was reported that certain thin glass membranes allowed the passage of certain ions, the characteristic makeup of the glass determining which ions. Today, electrodes selective to more than two dozen different ions have been constructed. In pH measurements, the indicating electrode is made up with a thin bulb of hydrogen ion-selective glass separating an internal standard solution from the external solution to be measured. The migration of hydrogen ions depends on the difference in hydrogen ion activity between these two solutions. The internal solution is usually dilute hydrochloric acid
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pH - Appendix pH pH measurements are usually made with a pH...

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