How a pH Probe Works: A Comprehensive Guide

[Daniel2244]

Homework Statement

How does a pH probe work?

Homework Equations

The Attempt at a Solution

A combined pH probe is comprised of 2 electrodes a measuring and reference electrode. The measuring electrode is composed of glass with a silver alloy electrical wire submerged into a neutral KCl solution contained in a membrane made of metal salts. Like the measuring electrode the reference electrode has a metal wire submerged into a KCl solution contained in the same glass put sepereated from the rest and has a junction.
When dipped in a solution the h+ ions in the solution penetrate into the wall of the membrane, causing an ion exchange producing a voltage (potential). At the reference electode the outside solution mixes with the interal neutral solution (somtimes called reference fill) at the junction, if the pH of the outside solution if different fdrom the pH of the inside solution a potential is produced,.

(This is where I get confused why are both of the solutions pH7 in the combined electrode? Shouldn't only one be a unkown solution).

The voltage at the reference eletrode is constant and the voltage produced at the measuring electrode varies depeing on the solution.

 

[Borek]

(This is where I get confused why are both of the solutions pH7 in the combined electrode? Shouldn't only one be a unkown solution).

There are three solutions - external, inside, and inside the reference. Of these only one has unknown pH, exactly as you wrote it should be.

http://www.ph-meter.info/pH-electrode-single-double-junction

 

[Daniel2244]

There are three solutions - external, inside, and inside the reference. Of these only one has unknown pH, exactly as you wrote it should be.

http://www.ph-meter.info/pH-electrode-single-double-junction

Thanks