For a pH electrode, which is a valid option for the reference electrode tip?

For a pH electrode, which is a valid option for the reference electrode tip?

• A. Hg/HgCl (Calomel) or Ag/AgCl
• B. Ag/AgCl
• C. SCE (Saturated Calomel Electrode)
• D. Cu/CuSO4

Answer: (A)

The essential concept is that the reference electrode in a pH sensor must provide a stable, well-defined potential that remains essentially unchanged as the glass electrode responds to hydrogen ion activity. Calomel (Hg/HgCl) and silver/silver chloride (Ag/AgCl) reference electrodes meet this need because their potentials are set by a well-controlled redox couple in a chloride-containing filling solution. This makes their potential drift small and predictable, and their contact with the sample occurs through a salt bridge, helping keep the junction potential stable. Therefore, using either Hg/HgCl (calomel) or Ag/AgCl as the reference electrode tip gives a reliable, reproducible reference for pH measurement. Copper/copper sulfate is not suitable here because copper ions and the Cu/CuSO4 system can corrode or exchange ions, causing unstable potentials and drift that would distort the pH reading. Saturated calomel electrodes are also a form of calomel reference, so they fall under the same stable-reference category.

The essential concept is that the reference electrode in a pH sensor must provide a stable, well-defined potential that remains essentially unchanged as the glass electrode responds to hydrogen ion activity. Calomel (Hg/HgCl) and silver/silver chloride (Ag/AgCl) reference electrodes meet this need because their potentials are set by a well-controlled redox couple in a chloride-containing filling solution. This makes their potential drift small and predictable, and their contact with the sample occurs through a salt bridge, helping keep the junction potential stable. Therefore, using either Hg/HgCl (calomel) or Ag/AgCl as the reference electrode tip gives a reliable, reproducible reference for pH measurement. Copper/copper sulfate is not suitable here because copper ions and the Cu/CuSO4 system can corrode or exchange ions, causing unstable potentials and drift that would distort the pH reading. Saturated calomel electrodes are also a form of calomel reference, so they fall under the same stable-reference category.