Potentiometry Parts 3 and 4

Question 1

What does a high potential indicate?

Answer 1

More charges

Question 2

What is a liquid junction potential (Eⱼ)?

Answer 2

Potential difference that develops across an ion-permeable boundary between different electrolyte solutions

Question 3

Give an example situation where the creation of separation of charge is observed

Answer 3

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-)

Question 4

Liquid junction potentials limit the accuracy of potential measurements

How do you minimize liquid junction potentials (few mV)?

Answer 4

By using concentrated electrolytes where the cation and anion have the same mobility

Question 5

What is a boundary potential?

Answer 5

The difference between the two membrane potentials. Its mathematical equation is Eb = E1 - E2

Question 6

When is Eb = 0?

Answer 6

When the ph of the two solutions are equal

Question 7

When is Eb < 0

Answer 7

When pH of solution 2 < pH of solution 1

Question 8

When is Eb > 0?

Answer 8

When pH of solution 2 > pH of solution 1

Question 9

In which direction will low pH go in a chemical equilibrium/reaction?

Answer 9

To the neutral compound/species

Question 10

In which direction will high pH go in a chemical equilibrium/reaction?

Answer 10

To the negatively charged compound/species

Question 11

What are the two electrodes used in potentiometric measurement?

Answer 11

Indicator electrode and reference electrode

Question 12

What does ISE (ion-selective electrode) require during a potentiometric measurement?

Answer 12

External reference electrode, internal reference electrode, ion-sensitive membrane

Question 13

What does Combination glass electrode require during a potentiometric measurement?

Answer 13

Internal Ag/AgCl reference electrode, junction (to maintain electrical contact)

Question 14

How do you determine boundary potential (Eb)?

Answer 14

Determined by the H₃O⁺ activity on either side of the membrane

Question 15

What is the alkaline error?

Answer 15

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.

Question 16

What is the alkaline error?

Answer 16

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.

Question 17

Why is the [H₃O⁺] very low at high pH?

Answer 17

Na⁺ (and some other monovalent metals) can associate with the glass membrane, which generates a change in potential (false positive result)

Question 18

How can you avoid/fix alkaline error?

Answer 18

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

Question 19

How can you determine the selectivity coefficient (k)?

Answer 19

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

Question 20

What does TISAB (Total Ionic Strength Adjustment Buffer) do?

Answer 20

Adjusts ionic strength (increases the total ionic strength to a high level)

Question 21

If we want the ionic strength of two samples to be similar, what do we add?

Answer 21

TISAB (Total Ionic Strength Adjustment Buffer)

Question 22

Explain the two possible sources of error in pH measurements with glass electrodes

Answer 22

1. Chemical Errors: junction potentials are different between the standard and the sample, may be caused by alkaline error or acid error
2. 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

Question 23

What are the three types of Ion-Selective Electrodes?

Answer 23

Liquid membranes, glass, solid-state

Question 24

What is the purpose of having Ion-selective electrodes?

Answer 24

To selectively bind to certain metal ions as the internal and external solutions will have different binding, creating a boundary potential

Question 25

What happens in an alkaline error?

Answer 25

Na⁺ will affect the [H]

Question 26

What is a liquid membrane ISE?

Answer 26

Its function is analogous to glass electrodes, except the glass membrane is replaces with an ionophore in a hydrophobic matrix

Question 27

What is an ionophore?

Answer 27

A hydrophobic organic molecule that serves as an ion exchanger. It is a receptor or ligand that binds the ion of interest

Question 28

How does a membrane potential develop?

Answer 28

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

Question 29

What is a solid-state ISE?

Answer 29

They are inorganic crystals that can respond to certain metal ions. Mobile monovalent ions conduct electricity through crystal vacancies (created by doping)