Electrode Measurements Flashcards

measurement of the electrical potential difference between two electrodes in an
electrochemical cell, when no current is allowed to flow.

in which chemical concentrations oscillate between high and low values.

When two dissimilar electrolyte solutions are placed in contact, a voltage difference develops at the interface.

Direct

• Inherent inaccuracy

Liquid-liquid junction at 4% among 14 measurements by direct potentiometry

1. different indicator electrodes
2. varying liquid junction potential

Relative

• By titration
• relatively precise
• permit an end point to be identified with little uncertainty

• Caused by unequal mobility of different ions at liquid-liquid interfaces.
• Unknown junction potentials can result in a limitation of accuracy of potentiometric measurements.

typically used for ISE [Ion selective electrode] analysis where the leached electrolyte must not interfere with the ISE being used.

They are also particularly useful for pH analysis of products that must not have KCl or a strong electrolyte contaminating the sample

is an analytical technique used to determine the activity of ions in aqueous solution by measuring the electrical potential.

1. It is relatively inexpensive and easy to operate.
2. It has wide concentration measurement range.
3. As it measure the activity, instead of concentration, it is particularly useful in biological/medical application.
4. It is a real-time measurement, which means it can monitor the change of activity of ion with time.
5. It can determine both positively and negatively charged ions.

selectively binds one ion—no redox chemistry

It is a surface on which redox reaction takes place.

• pH glass surfaces from a hydrate gel in water where metal diffuses out and ^H+ in.
• an ion exchange equilibrium occurs
• the porous plug serves as the salt bridge.

• Standards{Buffer accuracy}
• Junction potential {analyte composition different that electrode}
• Junction potential drift{ precipitate in the plug recalibrate pH meter }
• Sodium error{ H⁺ is low Na⁺ is high}
• Acid error{glass is saturated with H⁺}
• Equilibrium time{ didn't wait for pH to equilibrate}
• Hydration of Glass{ electrode too dry}
• Temperature{pH meter not at same temp as sample}
• Cleaning{probe has hydrophobic liquid}

incorporating both the glass and reference electrodes in one body

They must be calibrated before use. It should be calibrated every 2 h in sustained use. Ideally, calibration standards should bracket the pH of the unknown.

Don’t leave it out of water (or in a nonaqueous solvent) longer than necessary.

Advantages:

• unaffected by color turbidity
• short response time
• Non-contaminating

Disadvantages:

• electrodes can be fouled by organic solutes
• precision ≤ 1%
• certain ions can interfere

• Inorganic crystal act as the membrane
• ion of interest moves through the crystal lattice by going to each vacant site.

• uses a hydrophobic membrane that contains a liquid organic complexing agent that reacts selectively with the analyte
• Potential difference builds up between inside and outside of the membrane depending on the analyte.

a conventional electrode surrounded by a membrane that isolates (or generates) the analyte to which the electrode responds.

• Electrode have membrane of multiple type

There are four main types of ion-selective membrane used in ion-selective electrodes (ISEs): glass, solid state, liquid based, and compound electrode.

• Quantity: potential voltage
• Unit: voltage
• Analyte: H⁺ and PCO₂

• Quantity: Electric charge
• Unit: Coulombs
• Analyte: Cl^-

• Quantity: Current
• Unit: Ampere
• Analyte:PO₂

• Quantity: Current
• Unit: Volt
• Analyte: Vitamin C