-2
$\begingroup$

If the current is kept to zero then how do electrons move from one electrode to the other electrode? How would the reduction happen if electrons don't move from one electrode to the other electrode?

$\endgroup$
3
  • $\begingroup$ As per my knowledge, nothing like potentiometric cell exists. Can you please give details on what cell are you talking about in terms of the electrode reactions? $\endgroup$ – Nisarg Bhavsar Apr 8 at 6:58
  • $\begingroup$ @NisargBhavsar A potentiometric cell is the arrangement e.g. for potentiometric titrations, using a probe inert electrode ( usually Pt, perhaps glassy carbon too ) and the reference electrode. the probe electrode mirrors potential belonging to the present redox systems. $\endgroup$ – Poutnik Apr 8 at 7:21
  • $\begingroup$ @Poutnik Ok I got it, the electrode used in the potentiometric titration. Than the current is actually not zero but a very small quantity, just like in the case of Voltmeter. I still can't believe how I didn't see the obvious relation between potentiometric cell and potentiometric titration. $\endgroup$ – Nisarg Bhavsar Apr 8 at 8:18
3
$\begingroup$

Zero in science is not taken as rigorously as in mathematics. It is seldom exactly zero, but rather any value smaller than some significant or noticeable threshold.

Scientific zero is in data processing context any value with its difference to mathematical zero being statistically not significant.

In the question context, the zero current means a current small enough not to noticeably disturbing electrode equilibrium.

It is comparable to a voltmeter measuring voltage on particular points of electronic circuits, that has ideally infinite resistance and draws zero current from the circuit. In reality, its resistance is finite and it does draws some current, that is considered negligible, taking as zero for circuit servicing purposes.

$\endgroup$
1
  • $\begingroup$ Yes i finally understand how the current is kept zero and voltage is measured. It's actually the primary circuit of the potentiometer having a battery of known voltage and an adjustable resistance wire which apply the voltage across the terminals of secondary circuit having unknown voltage so that no current flows. In this way, we measure that voltage applied by source battery which is actually of unknown battery. $\endgroup$ – Pakeeza Iram Jun 3 at 7:19
1
$\begingroup$

No current needs to flow from an open circuit half cell. Well, no external current needs to flow. An equilibrium is established between the solution and the electrode. This equilibrium involves many transitions, back and forth, but is in equilibrium. There is a net current of zero.

Now, how do you measure a voltage without getting a current? You back it up with an equal and opposite voltage so as to establish a current of zero. Now, I admit that zero is pretty difficult to hit, exactly, but you can come so close that whether the current in or out of the half cell is + or - an electron or two or three, it doesn't change the measured voltage significantly.

A Wheatstone Bridge is the way to get zero current; normally, the galvanometer is very sensitive and resistances to be measured are adjusted for zero current. https://en.wikipedia.org/wiki/Wheatstone_bridge

enter image description here

To measure the voltage of a half cell without drawing current, the half cell is placed in series with the galvanometer and the resistances are adjusted for zero current. Then half cell is disconnected and its voltage is read off the actually unbalanced bridge, which can provide enough current for a non-infinite input resistance voltmeter.

$\endgroup$
3
  • 2
    $\begingroup$ I am glad you have brought this fine piece of the classics. I was aware of the W. bridge, called also potentiometric bridge, but I do not think it is useful for potentiometry or potentiometric cells. The high impedance modern voltmeters based on MOS FET transistors or Operation amplifiers draw lower current that is the residual current of the balanced bridge. They can have easily input impedance $\pu{e12 \Omega}$ $\endgroup$ – Poutnik Apr 8 at 15:10
  • $\begingroup$ The classics are still in use: "The basic principle of the experimental determination of electrolytic conductance is that of the Wheatstone’s bridge." qsstudy.com/chemistry/experimental-determination-conductance $\endgroup$ – James Gaidis Apr 9 at 2:08
  • $\begingroup$ Sure, why not ? But it is rather a different case. $\endgroup$ – Poutnik Apr 9 at 5:38

Not the answer you're looking for? Browse other questions tagged or ask your own question.