Electrochemistry Lecture

Valinomycin enhances the selectivity of the electrode used to quantitate: A. sodium B. chloride C. potassium D. calcium

potassium

Commonly used reference electrodes:

1.standard hydrogen electrode special type of hydrogen electrode used for standardization of other reference electrodes 2.saturated calomel electrode (SCE) platinum electrode in contact with mercury, mercuric chloride, and KCl 3.silver-silver chloride electrode very reproducible

Coulometry

A coulometric titration makes use of an electrochemically generated titrant via constant current. The charge passing between two electrodes is directly proportional to oxidation or reduction of an electroactive substance at one of the electrodes. Measurement of current is related to charge as the amount of charge passed per unit of time (ampere = couloumb/s).

Electrolyte Test Panel

Main electrolytes are free ions (common panel) Na+ K+ Cl- HCO3- (pCO2 and pH often included) Others, including trace elements, are bound to proteins, like albumin Ca2+ Mg2+ Sample Types: serum/plasma, urine, body fluids, feces, GI fluid

Potentiometry

Measures an electrical potential difference between two electrodes (half-cells) in an electrochemical cell (salt bridge). An electrode is a half-cell that consists of a single metallic conductor in contact with an electrolyte solution; indicator electrode (measuring) is one half-cell (variable voltage) reference electrode is the second half-cell (constant voltage) Ion conductors (metal or electrolytes) consist of one or more phases (liquid or solid): that may be in direct contact with each other separated by permeable membrane only to specific ions The potential generated is proportional to the activity (concentration) of the ion being measured. Within a conductive phase, the potential is constant, as long as the current is zero.

Enzyme electrodes

Polymerizing a gelatinous membrane of enzyme directly over an ISE Enzyme catalyzes the production of ions from compounds Used for glucose and urea

Ion-selective electrodes measure electrical potential difference across a membrane using the principles of: A. Coulometry B. Conductivity C. Potentionmetry D. Amperometry

Potentiometry-build up of charge difference at the potential in the membrane conductivity=coulter: increased resistance colulometry: flouride measurement amperometry: O2

What you need for a potentiometric system:

Reference Electrode (Eref) --- Salt Bridge (Ej) --- Sample Solution --- Indicator Electrode (Eind) --- Digital Meter ---------------------------------------- Ecell = Eind - Eref + Ej

Ion Selective electrodes:

Simple, rapid, nondestructive, and able to measure a wide range of concentrations.

Sources of Error in ISE Measurements

Temperature variation between sample and analyzer Response to non-analyte pH dependent Different sample types Protein buildup on membranes

Summarization of ISEs

Used mainly for electrolyte measurements Na, K, Cl, and pH, and pCO2 can also measure Li, NH4+, Cai Direct and Indirect Methods Direct = non-diluted whole-blood or plasma sample (measurement performed on plasma water sample) Indirect = diluted with diluent (measures on a total plasma or serum sample)

How do we take advantage of potentiometry?

We measure the electrode potential and get a value of patient sample! Measure the potential difference between the indicator electrode and the reference electrode and calculate the concentration of ions in the solution using the Nernst equation. The voltage difference is proportional to the log of the ion activity.

Direct vs Indirect Methods:

Whole Blood Sample = Total Plasma and Cells Total Plasma = 93% water and 7% solids (like lipids and proteins)Electrolytes are present in water portion only Direct Method §Measures electrolyte content of plasma water only §Blood gas analyzers and POC electrolyte analyzers §Reported result is independent of the content of solids in a sample Indirect Method §Measures electrolyte content in volume of total plasma §Larger chemistry analyzers §Requires plasma separation §Due to the dilution step, this method measures the mean concentration in plasma (weighted avg. between the concentration in the electrolyte containing water part and the electrolyte free protein/lipid factor) §Reported result dependent on content of solids in the sample

What is solid-state electrodes used for?

anions, choride ISE

hydrogen ion concentration (pH) in blood is usually determined by means of which of the following electrodes? A. silver B. glass C. platinum D. platinum-lactate

glass

what is glass electrode most commenly used for?

to measure hydrogen ion activity

Example of ISEs with enzymes immobilized in their membranes for measurement of all of the following except: A. Urea B. Glucose C. Calcium

Calcium

What is the principle of potentionometry

A metal in a solution of its own ions will produce a potential related to the concentration of the ions in solution. Low concentration of ions in solution: metal ions will tend to pass into the solution, leaving a negative charge on the electrode. High concentration of metal ions in solution: fewer metal ions pass into the solution and the charge on the electrode will be smaller Net effect is that the charge on the electrode is proportional to the concentration (activity) of ions in solution. The indicator electrode must show high sensitivity to the activity of a given ion in solution (patient analyte), generating a potential that is proportional to the concentration of that single ionic species. Ion selective electrode (ISE); most common; heart of the system is the design of the semi-permeable membrane Redox electrode; potential the result of chemical equilibria involving transfer of e-; inert metal (platinum or gold); hydrogen electrode for pH; saturated calomel electrode; silver-silver chloride pCO2 gas sensing electrode; (led to development of three channel blood gas analyzer: pH, pCO2, pO2)

Conductometry

An electrochemical technique used to determine the quantity of an analyte present in a mixture by measuring its effect on the electrical conductivity of the mixture.

ISE = what are they?

Electrodes that selectively interact with a single ionic species. The potential produced at the membrane/sample solution interface is proportional to the logarithm of the ionic activity (concentration of free, unbound ion in solution).

Basic Electrochemical Cell

Galvanic/Voltaic or Electrolytic Two half cells are arranged together to form an electrochemical cell insert picture from slide 10

What are ISEs made of?

Glass Electrodes Polymer Membrane Electrodes Solid State Electrodes Gas Electrodes Enzyme Electrodes

Amperometry / Voltammetry

Measure current flowing through an electrochemical cell when a constant potential is applied.