Lab Quiz #9 - Electrochemistry

What is Ohm's Law?

E = I x R

What does E represent in Ohm's law?

electrical potential

What does I represent in Ohm's law?

current

What does R represent in Ohm's law?

resistance

Potentiometry

Measures electrical potential in volts

Conductivity

Combined ability of ions to carry a charge

Coulometry

Measures electrical charge per unit of time in coulombs

Amperometry

Measures current in amperes

Polarography

Measures current while controlling an increase in voltage

Voltammetry

Measures the current at an electrode using a specific voltage generated at another electrode

Osmometry

Measures dissolved particles in a solution using a series of variable resisters

What is a reference electrode?

an electrode that has a known, constant potential

What is a detection electrode?

an electrode where the patient potential is measured

What is the ISE membrane for Na⁺?

A Glass membrane made from lithium aluminum silicate or silicon dioxide contained ionophore, such as monesin

What is the ISE membrane for K⁺?

A Liquid membrane containing valinomycin

What is the ISE membrane for CO₂/HCO₃⁻?

An outer membrane that is gas permeable

&

an additional glass membrane made of silicon dioxide/lithium and calcium oxide glass that selects for H+

What is the ISE membrane for Cl⁻?

A Solid-state membrane containing silver chloride

What is the ISE membrane for Ca²⁺?

PVC, containing a neutral liquid carrier

How does the measurement of ionic substances occurs using ion-selective electrodes?

Directly or Indirectly

Direct measurement in ISE measures what?

measures electrolytes in undiluted samples

Indirect measurement in ISE measures what?

measures electrolytes by diluting the sample with diluent prior to analysis with the electrode

What are the limitations/interferences associated with measurement of electrolytes using the ion-selective electrode methodology?

False increases in potassium will be observed with hemolysis, fist clenching, or leakage from RBCs in older samples, or those that have been refrigerated.

False decreases due to protein build-up on the surface of the membrane

Indirect ISE systems cause false decreases in sodium and chloride due to dilutional effects caused by increased lipid and protein levels.

What is the type of potential that is applied in amperometry?

A fixed potential

What is the analyte that is measured using a Clarke electrode?

PO₂ (oxygen)

What is the method that determines the amount of O₂ (oxygen) present when using a Clarke electrode?

The Clark PO2 electrode consists of a gas-permeable membrane which allows dissolved oxygen to pass through.

This membrane also prevents other blood constituents from passing through that may interfere with the electrode.

Once the oxygen passes through the membrane, it mixes with a phosphate buffer solution and reacts with a polarized platinum cathode.

The cathode has an applied voltage which results in the reduction of oxygen.

Reduction of oxygen produces a flow of electrons (electrical current), and the magnitude of current is proportional to the amount of oxygen present in the sample.

What is the analyte that is often measured by coulometric titration?

Chloride (Cl⁻)

What is the method used in coulometric titration that determines the amount of analyte present?

This method uses a silver anode that releases silver ions at a constant rate due to constant electrical potential.

These ions react with the chloride ions, forming insoluble silver chloride.

When all of the chloride ions have reacted with the generated silver ions, there is a sharp increase in current conducted.

Since current relates to electrical charge per unit of time, the time of titration is proportional to the chloride activity in the sample.

This is the method that is commonly used for sweat chloride analysis (CF testing)

What is the rate at which potential is applied to a reaction in polarography?

applied by increasing at a constant rate by an external voltage source.

What is representative of the analyte present in polarography?

The current flow is measured at the indicator electrode (usually Ag/AgCl)

What are the two main steps in the reaction that result in final measurement of the analyte in voltammetry?

1. A negative potential is made and all the metal ions are attracted to the electrode.

2. A positive potential is made and the metal ions fall away.