Question 6 - Reference Electrodes Flashcards
(18 cards)
What is the SCE?
Saturated calomel electrode.
How would you draw a diagram of the SCE?
Box with a mini box inside with hole at the bottom. Inside box has two halves. Pourous wick at the bottom. Fill hole. Wire out of the top.
What solutions are in the SCE?
Inside box, top = Hg(l), bottom = saturated KCl(aq), Hg(l), Hg2Cl2(s), KCl(s). KCl crystals in porous wick. Saturated KCl solution im outside box.
Why is the SCE useful as an electrode?
Prescence of crystals implies a saturated solution.
Saturated solution has a specific concentration, governed by solubility product.
This ensures a stable repeatable reduction as the final term in the Nernst equation is fixed.
How is the SCE calibrated against the SHE?
Linking the two together and setting the reduction potential for SHE to 0.
Why is SCE used in the lab instead of SHE?
Pressurised hydrogen gas is dangerous. Difficult, expensive and dangerous to keep SHE at conditions.
What is the SSCE?
Silver Silver Chloride Electrode.
How would a diagram of the SSCE be drawn?
Glass tube with porous plug, silver wire coated in AgCl, saturated solution of KCl.
Why is the SSCE used instead of the SHE?
SHE fiddly, expensive and dangerous, SSCE safe and cheap.
How is the SSCE used as a reference electrode?
SSCE has a reduction potential of 0.21V when measured against SHE. As long as solution remains saturated Ecell = difference between measured voltage and 0.21V.
What is the reduction potential of the SHE?
0
What is the reduction potential of the SSCE?
0.21V
How is the SSCE used in combination with a thin membrane glass bulb and a buffer solution to measure pH?
Calibrated by inserting a solution with known pH.
When placed in solution of unknown pH there is a potential formed across the glass membrane which alters the voltage being measured.
What is the voltage altered by in a pH probe when the potential is formed?
Alters voltage being measured by 0.0592 x pH value.
How is the diagram of the SSCE being used as a pH probe drawn?
SSCE in pH 7 buffer in glass bulb alongside SSCE in KCl with bulb protruding through the plug.
How is a diagram of the SHE drawn?
Hydrogen gas at 1bar in equilibrium with molar H+ connected to an unknown electrode via voltmeter and salt bridge. Pt wire connected to Pt electrode.
How are electrode potentials determined in practice (calibration of reference electrodes)?
Use the SHE to calibrate other reference electrodes and use them to find standard potentials of other couples.
Why do we use the SSCE or SCE? (not about conditions)
Their cell potentials are reliably reproducible.
