I have made a test to investigated: I measured the potential in mV of a standard +220mV solution using my pHmeter which gives a mV value with the pH. The result was +9mV. But this was the result for the reference used in the electrode (Ag AgCl with KCl 3M), comparing it with Hydrogen standard (Which is 209mV for 25 Celsius Degree) the result is 218mV.
I couldn't understand how I could have found a value so accurated, since a glass electrode is seletive ideally only to H+, hence it is used measure pH. Maybe it is a coincidence.
Lucas Silva
The absolute value of the electrode potential cannot be obtained. Therefore, we agreed to measure the electrode potential relative to a standard hydrogen electrode. The pH electrode is calibrated with respect to the hydrogen electrode.
Hi Lucas,
The pH electrode is consideted as an ion-selective electrode (for hydrogen ions), while ORP electrode is a non-selective electrode because the latter measures the concentration of the oxidized form of all the compounds found in the medium related to their reduced form according to Nernest equation.
Because the thin glass bulb of pH electrode allows mainly the small hydrogen ions to interact with the glass, the glass electrode measures the electro-chemical potential of hydrogen ions or the potential of hydrogen only. The ORP electrode is characterized by the presence of a ring of a noble metal such as platinum or gold on the head of measuring electrode that permits all the oxidants and reductants found in the medium to exchange electrons on the surface of this ring. So, ORP electrode is non selective electrode. The value in millivolts obtained by pH electrode that you found in your measurement is related to electrical potential of hydrogen only and excludes the electrical potential of other ions found in the medium.
Coincidence. The pH of your standard solution is close to 7. Add a few drops of acidic or basic solution. You will obtain completely different results.
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