I corrected for the LJP and got around 20-25 mV, but when I look at other papers this seems high.
I corrected for the LJP in a patch clamp rig by leaving external solution in bath and using a silver chloride pellet as ground. I first added external solution into a recording pipette in contact with a freshly chloridized silver wire, put the pipette into the bath and offset to 0. I waited about 1-3 minutes to make sure that the voltage was stable at 0 +-0.3mV. I then switched the pipette solt. to my K+ based internal recording solution and put it into the bath, this resulted in ~23 mV junction potential. I then switched the internal solt. back to be the same as the bath solt. again and saw that it was close to 0 again, +- ~0.4mV. I repeated this process a second time and it was relatively consistent. In between the switches I would lightly wipe off the chloridized silver recording wire...
I then repeated the process above using a Cs+ based solution, with the same (10mM) concentration of Cl-. The junction potential was ~21 mV.
Hi Phiip,
Erwin Neher wrote an excellent short article on the practicalities of junction potentials including how to measure them. It is in Methods in Enzymology, 207, 1992, pg123-130. It also contains a table of junction potentials for stadard solutions. It will help you iron out your method for measuring the junction potential for your solutions. The article will also provide you with the needed insight to correctly apply the recorded junction potential....do you add it or subtract it!
The following link might be helpful, too. Try using a high (3M KCl) as your reference to avoid the junction potential changes at the AgCl electrode (see comments in link).
http://junctionpotential.blogspot.com/2010/10/how-do-i-measure-liquid-junction.html
Thank you Michael and Alexander for the citation and link. I referred to the Nat Blairs very nice blog and Erwin Neher's thorough paper. I gathered that if the bath solution in contact with the pellet did not change between the control and test pipette solution, that the measurement would still be accurate, and that only the junction potential change if switching bath solutions would be an issue, due to changing junction potentials at the pellet.
If I only switch the pipette solution and not the bath, shouldn't the junction potential at the pellet remain basically the same?
Or am I mistaken here...
If the pellet is in contact with the bath and the bath solution is not changed then all is fine. The problem is when you chage the bath and in particular change chloride. What also needs to be considered is the junction potential between the pipette and the bath when you place the electrode in the bath and null out the current before actually patching. When you make the seal, the junction potential that you nulled out is then removed from the circuit and needs to be accounted for when determining the real applied voltage. This can be quite considerable when using different chloride solutions in the bath and in the pipette. The Neher reference and Alexander's link should ensure that you are as accurate as possible. Best of luck with the experiments.
Even though this answer comes several years too late, it might still help others, guided here by a search engine...
The obvious problem with your protocol is that you did change the composition of a solution touching a chloride electrode. Just not the bath electrode. The measured potential difference is the sum of two potentials changing at once: the LJP and the chloride potential at your pipette electrode - presumably a chlorides silver wire.
The solution in contact with both electrodes must not change when you measure the LJP.
- Badges
- Science topic
- Similar topics
- Mathematical Sciences
- Graphs