Electrochemical Cells:

Topics: Electrodes: Types of electrodes (reversible), reference electrodes, glass electrode & its application, Standard electrode potentials (E^o), Types of cells: chemical cells, reversible and irreversible cells, Principle of measuring E.M.F. of reversible cells, Potentiometer, standard cells, DG, DH in a reversible cells; Concentration cells, their types & application, liquid junction potential, Henderson equation, Nernst equation, H₂-scale potential, application of potentiometry.

Electrochemical Cells-Introduction: https://www.youtube.com/watch?v=V5TqMuHaDuY

Salt bridge: https://www.youtube.com/watch?v=1xKeFiyOemA&pbjreload=101

A salt bridge is a connection containing a weak electrolyte between the oxidation and reduction half-cells in a galvanic cell (e.g., voltaic cell, Daniell cell). Its purpose is to keep the electrochemical reaction from reaching equilibrium too quickly. If a cell is constructed without a salt bridge, one solution would quickly accumulate positive charge while the other would accumulate negative charge. This would halt the reaction and thus the  

In a galvanic (voltaic) cell, the anode is considered negative and the cathode is considered positive. This seems reasonable as the anode is the source of electrons and cathode is where the electrons flow. However, in an electrolytic cell, the anode is taken to be positive while the cathode is now negative.

Li Ion battery: https://www.youtube.com/watch?v=VxMM4g2Sk8U

Unit of EMF is volts. It is abbreviated E in the international metric system but also, popularly, as emf. Despite its name, electromotive force is not actually a force. It is commonly measured in units of volts, equivalent in the metre–kilogram–second system to one joule per coulomb of electric charge.

EMF: https://www.youtube.com/watch?v=cbSKkrzdXe4

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit.

The Henderson-Hasselbalch equation is the equation commonly used in chemistry and biology to determine the pH of a solution.

• This equation shows a relationship between the pH or pOH of the solution, the pK_a or pK_b, and the concentration of the chemical species involved.
• This equation was developed independently by the American biological chemist L. J. Henderson and the Swedish physiologist K. A. Hasselbalch to determine the pH of the bicarbonate buffer system in blood.
• This type of kinetic analysis has enabled us for nearly a century to relate theoretically the changes of the acidic intensity of dilute solutions to a quantity of acid or base added or subtracted.
• This equation can be considered as the backbone of acid-base physiology.
• This equation is commonly used to determine the amount of acid and conjugate base required to prepare a buffer of the desired pH.

The principal objectives of the Henderson Hasselbalch equation include the following:

1. To calculate the pH, pOH, [H3O+ ]_tot, [OH- ]_tot, [H3O+ ]_water, and [OH- ]_water in a solution containing a strong acid (base) given the initial concentration of the acid (base).
2. To describe how a buffer solution (either acidic or basic) can resist significant changes in pH when small amounts of either acid or base are added to the buffer solution.
3. To describe how either an acidic or basic buffer solution is prepared.
4. To describe a “buffer solution”.
5. To describe “buffer capacity”.
6. To determine whether an aqueous solution of salt will be acidic, basic, or neutral given values of Ka and Kb for conjugate acid-base pairs.
7. To describe how the relative strengths of the conjugate acids or bases can be evaluated using the values of Kb and Ka for the bases and acids, respectively.
8. To determine the protonation state of different biomolecule functional groups in a pH seven buffer.

9. Principle/Theory/Derivation of Henderson Hasselbalch Equation

According to the Bronsted-Lowry theory of acids and bases, an acid (HA) can donate a proton (H⁺) while a base (B) can accept a proton.

1. An acid after losing a proton forms a conjugate base (A^–), and the protonated base exists as conjugate acid (BH⁺).
2. The dissociation of acid is expressed in terms of the equilibrium equation as:

HA      ↔       H⁺  +  A^–

• This relationship can be described in terms of the equilibrium constant as:

 

 ...............................................................................................

A junction potential develops at the interface between two ionic solution if there difference in the concentration and mobility of the ions. 

Consider, for example, a porous membrane separating solutions of 0.1 M HCl and 0.01 M HCl, as shown here.

Because the concentration of HCl on the membrane’s left side is greater than that on the right side of the membrane, H⁺ and Cl^– diffuse in the direction of the arrows. 

The mobility of H⁺, however, is greater than that for Cl^–, as shown by the difference in the lengths of their respective arrows. 

Because of this difference in mobility, the solution on the right side of the membrane has an excess of H⁺ and a positive charge. 

Simultaneously, the solution on the membrane’s left side develops a negative charge because there is an excess concentration of Cl^–. 

This difference in potential across the membrane is called junction potential, which we represent as E_L.

................................----------------------------------------

Uses of Concentration Cells

• A pH meter is a specific type of concentration cell that uses the basic setup of a concentration cell to determine the pH, or the acidity/basicity, of a specific solution. 
• It is comprised of two electrodes and a voltmeter. One of the electrodes, the glass one has two components: a metal (commonly silver chloride) wire and a separate semi-porous glass part filled with a potassium chloride solution with a pH of 7 surrounding the AgCl. 
• The other electrode is called the reference electrode, which contains a potassium chloride solution surrounding a potassium chloride wire. 
• The purpose of this second electrode is to act as a comparison for the solution being tested. When the glass electrode comes into contact with a solution of different pH, an electric potential is created due to the reaction of the hydrogen ions with the metal ions. 
• This potential is then measured by the voltmeter, which is connected to the electrode. 
• The higher the voltage, the more hydrogen ions the solution contains, which means the solution is more acidic.