Potentiometry Parts 3 and 4 Flashcards
What does a high potential indicate?
More charges
What is a liquid junction potential (Eⱼ)?
Potential difference that develops across an ion-permeable boundary between different electrolyte solutions
Give an example situation where the creation of separation of charge is observed
In a concentration gradient, Na+ moves at a faster rate than Cl- through the porous membrane. Because of this, one side has more positive charge (Na+) while the other is more negative (Cl-)
Liquid junction potentials limit the accuracy of potential measurements
How do you minimize liquid junction potentials (few mV)?
By using concentrated electrolytes where the cation and anion have the same mobility
What is a boundary potential?
The difference between the two membrane potentials. Its mathematical equation is Eb = E1 - E2
When is Eb = 0?
When the ph of the two solutions are equal
When is Eb < 0
When pH of solution 2 < pH of solution 1
When is Eb > 0?
When pH of solution 2 > pH of solution 1
In which direction will low pH go in a chemical equilibrium/reaction?
To the neutral compound/species
In which direction will high pH go in a chemical equilibrium/reaction?
To the negatively charged compound/species
What are the two electrodes used in potentiometric measurement?
Indicator electrode and reference electrode
What does ISE (ion-selective electrode) require during a potentiometric measurement?
External reference electrode, internal reference electrode, ion-sensitive membrane
What does Combination glass electrode require during a potentiometric measurement?
Internal Ag/AgCl reference electrode, junction (to maintain electrical contact)
How do you determine boundary potential (Eb)?
Determined by the H₃O⁺ activity on either side of the membrane
What is the alkaline error?
In a basic solution in the presence of alkali ions, the potential of the glass electrode differs from the hydrogen function to an extent which is the higher, the higher the pH and the alkali concentration is.
What is the alkaline error?
In a basic solution in the presence of alkali ions, the potential of the glass electrode differs from the hydrogen function to an extent which is the higher, the higher the pH and the alkali concentration is.
Why is the [H₃O⁺] very low at high pH?
Na⁺ (and some other monovalent metals) can associate with the glass membrane, which generates a change in potential (false positive result)
How can you avoid/fix alkaline error?
Measure a pH between 0 and -1 units lower than the true value (positive bias) and use modern glass membranes as they are relatively immune to alkaline error up to pH ∼ 12
How can you determine the selectivity coefficient (k)?
1: separate solution method (measure the analyte and interferent ion(s) separately) and 2: fixed interference method (prepare a calibration curve with different activities of analyte and a fixed activity of interference
What does TISAB (Total Ionic Strength Adjustment Buffer) do?
Adjusts ionic strength (increases the total ionic strength to a high level)
If we want the ionic strength of two samples to be similar, what do we add?
TISAB (Total Ionic Strength Adjustment Buffer)
Explain the two possible sources of error in pH measurements with glass electrodes
- Chemical Errors: junction potentials are different between the standard and the sample, may be caused by alkaline error or acid error
- User Errors: standard solutions used for calibration are inaccurate, calibration and sample temperature are not matched, glass membrane is not clean, glass is not fully hydrated, insufficient time to reach equilibrium
What are the three types of Ion-Selective Electrodes?
Liquid membranes, glass, solid-state
What is the purpose of having Ion-selective electrodes?
To selectively bind to certain metal ions as the internal and external solutions will have different binding, creating a boundary potential
What happens in an alkaline error?
Na⁺ will affect the [H]
What is a liquid membrane ISE?
Its function is analogous to glass electrodes, except the glass membrane is replaces with an ionophore in a hydrophobic matrix
What is an ionophore?
A hydrophobic organic molecule that serves as an ion exchanger. It is a receptor or ligand that binds the ion of interest
How does a membrane potential develop?
When the activity of the analyte ion is different between the two sides of the membrane, that is, the interfacial ion-ionophore binding equilibria have different positions
What is a solid-state ISE?
They are inorganic crystals that can respond to certain metal ions. Mobile monovalent ions conduct electricity through crystal vacancies (created by doping)
