Specific Objectives
By the end of this Lecture the student is expected to:
• Define the pH and pOH concepts.
• Calculate the pH and pOH concepts.
• Prepare deferent
concentration of buffer solution.
• Operate and calibrate the pH meter.
L-10
Hydrocarbons derivatives
(functional groups)
Carboxylic acid Phys./chem.
properties
Self-Ionization of Water
• Water molecules collide with one another to cause the self-ionization reaction
represented by this equation:
2H2O H3O+ + OH-
• It is a reversible reaction so the equation is usually written with the arrows going in both directions:
2H2O H3O+ + OH-
• The reaction does not form very much H3O+ or OH-.
• In one liter of water there are about 55 moles of water molecules, but only 1.0 x 10-7 moles of H3O+ and OH- are formed (at room temperature).
• So the concentrations of H3O+ and OH- in pure water are 1.0 x 10-7 M. It is
the 7 in the exponent or power of this
number that gives neutral water a pH of 7.
Definition of pH
• pH can be viewed as an abbreviation for power of hydrogen or more
completely, power of the concentration of hydrogen ion .
• The mathematical definition of pH says that the pH is equal to the negative log of the hydrogen ion concentration,
or pH = -log [H+].
• Using the Brønsted-Lowry approach that would be pH = -log [H3O+].
Hydronium/Hydroxide Balance
• When an acid dissolves in water,
additional H3O+ is formed, increasing the concentration of H3O+.
• For example, the concentration of
H3O+ might be increased from 10-7M up to 10-5 M. That is 100 times more
concentrated. Note that the pH, -the
number behind the negative sign in the exponent-, changes from 7 to 5.
• This is why acidic solutions have pH values lower than 7.
• The acidity or basicity of a solution is related to the relative concentrations of H3O+ and OH-.
• If the concentration of H3O+ is more than the concentration of OH-, the solution is
acidic.
• If the concentration of OH- is more than
the concentration of H3O+, then the solution is basic.
• If the concentrations of H3O+ and OH- are equal to one another, the solution is neutral.
• There is also an internal relationship
between the concentrations of H3O+ and OH-.
• They are not independent of one another. As one goes up, the other goes down.
• They cannot both go up because the higher concentrations of H3O+ and OH- would react with one another to make water molecules.
• That is a consequence of the reversibility of the self-ionization reaction of water.
(2H2O H3O+ + OH-)
[H3O+] [OH-]
1.0 x 10-7 M 2.0 x 10-7 M 0.5 x 10-7 M 1.0 x 10-6 M 1.0 x 10-5 M
1.0 x 10-7 M 0.5 x 10-7 M 2.0 x 10-7 M 1.0 x 10-8 M 1.0 x 10-9 M
• Using the self-ionization of pure water as our starting point. The concentrations of both H3O+ and OH- are 1.0 x 10-7 M.
• If the concentration of H3O+ is doubled, the concentration of OH- will be halved.
• If the concentration of H3O+ is halved,
the concentration of OH- willl be doubled.
• The relationship between the concentration of H3O+ and the concentration of OH- shows a very consistent pattern.
• That pattern can be expressed
mathematically by saying that the product of the concentration of H3O+ times the
concentration of OH- remains a constant value of 1.0 x 10-14. That value has both a name and a symbol.
• It is variously called the ion product
constant for water, the ionization constant of water , or simply the water constant. The
symbol is Kw.
[H3O+] x [OH-] = 1.0 x 10-14 [H3O+] x [OH-] = Kw
• This equation for the ionization
constant of water can be used in a number of calculations…
CalculatiAng Hydronium and Hydroxide Concentrations:
Problem:
Given that [H3O+] = 4.5 x 10-5 M, calculate [OH-].
[H3O+]·[OH-] = Kw, [OH-] = Kw/[H3O+].
[OH-] = (1.0 x 10-14) ÷ (4.5 x 10-5) [OH-] = 2.2 x 10-10 M
As an example, to calculate [OH-] given that [H3O+] = 4.5 x 10-5 M. Starting with the water constant equation
[H3O+]·[OH-] = Kw, we can figure that [OH-] = Kw/[H3O+].
Then, substitute the known values for Kw and [H3O+] to get that [OH-] is equal to 1.0 x 10-14 divided by 4.5 x 10-5. That comes out to be 2.2 x 10-10 M for the concentration of
hydroxide ion.
Calculating pH and pOH
• pOH is simply the power of hydroxide ion concentration and is figured the same way as pH but using the concentration of
hydroxide ion instead.
•When [H3O+] is 10-7 M, the pH is 7. Also the [OH-] is 10-7 M and the pOH is 7. Note that the pH and pOH add up to 14.
pH + pOH = 14
pOH [OH-]
[H3O+] pH
1.0 x 10-7 M 7 1.0 x 10-7 M
7
1.0 x 10-8 M 8 1.0 x 10-6 M
6
1.0 x 10-9 M 9 1.0 x 10-5 M
5
1.0 x 10-6 M 6 1.0 x 10-8 M
8
1.0 x 10-5 M 5 1.0 x 10-9 M
9
• An organic acid is an organic compound with acidic properties.
• The most common organic acids are :
• carboxylic acids, whose acidity is
associated with their carboxyl group – COOH.
• Sulfonic acids, containing the group –
SO
2OH, are relatively stronger acids.
• The relative stability of the conjugate base of the acid determines its acidity .
• Other groups can also confer
acidity , usually weakly: –OH, –SH, the enol group, and the phenol
group.
• A few common examples include:
Acetic acid
Formic acid
Lactic acid
Citric acid
Oxalic acid
Uric acid
Characteristics:
• weak acids .
• do not dissociate completely in water.
• Lower-molecular-weight organic acids such as formic and lactic acids are
miscible in water, but higher-molecular- weight organic acids, such as benzoic acid, are insoluble in molecular (neutral) form.
• Most organic acids are very soluble in organic solvents.
Applications:
• Simple organic acids like formic or acetic acids are used for oil and gas well stimulation treatments.
• The conjugate bases of organic acids such as citrate and lactate are often used in biologically-compatible buffer solutions.
• Citric and oxalic acids are used as rust removal.
Application in food
Organic acids are used in food
preservation because of their effects on bacteria.
• Lactic acid and its salts sodium lactate
and potassium lactate are widely used as antimicrobials in food products, in
particular, meat and poultry such as ham and sausages.
Basicity & basic organic
compounds
• An organic base is an organic
compound which acts as a base.
• Organic bases are usually, but not always, proton acceptors. They
usually contain nitrogen atoms, which can easily be protonated.
• Amines and nitrogen-containing
heterocyclic compounds are organic bases. Examples include:
• Pyridine, methyl amine, histidine
• Two of the factors which influence the strength of a base are:
• The ease with which the lone pair picks up a hydrogen ion,
• The stability of the ions being
formed
Basicity of Amines
• Amines are slightly basic. This because they have a lone pair of electrons to donate to a proton.
• This same feature makes them nucleophiles.
RNH2 + H OH RNH3 + OH
• Amines are stronger bases than alcohols, ethers, or water
• Amines establish an equilibrium with water in which the amine becomes protonated and hydroxide is
produced
• Alkyl amines are usually stronger bases than ammonia.
• Increasing the number of alkyl
groups decreases solvation of ion, so 2 and 3 amines are similar to 1
amines in basicity
• Most simple alkylammmonium ions have pK
a's of 10 to 11
• Arylamines and heterocyclic aromatic amines are
considerably less basic than
alkylamines (conjugate acid
pK
a5 or less
• Given the basicities of amines, we can
determine which form of an amine exists in body fluids, say blood
– in a normal, healthy person, the pH of blood is approximately 7.40, which is slightly basic
– if an aliphatic amine is dissolved in blood, it is present predominantly as its protonated
(conjugated acid) form
HO HO
NH2
D opamine
HO HO
NH3+
Con jugate acid of d op amine (the major form p res ent
in b lood p lasma)
pH measurements
can be measured by using either :
•pH indicators : (like phenolphtaleine) - in deferent type of titration.
•pH strips : are very useful when all you need is 0.2- 0.5 pH unit accuracy.
•potentiometric method. When you
need higher precision, pH meter is the
only way .
pH meter :
Device used for
potentiometric pH measurements.
•Methods you measure potential difference
between known reference electrode and the
measuring pH electrode.
•Potential of the pH
electrode depends on the
activities of hydronium ions.
Potentiometric measurements pH meter measurements
The pH meter calculates a value by measuring the voltage differences between the pH electrode and the reference electrode
• Voltage differences described by Nernst equation, thus once the
potential has been measured you can calculate the activity
Nernst equation
•The general Nernst Equation:
E = Eo -(RT/nF)lnQ
Eo = standard electrochemical cell potential (voltage)
R = ideal gas constant T = temperature
n = moles of electrons
F = Faraday constant = 96,485 C mol-1
Q = mass-action expression (approximated by the equilibrium expression
pH meter device consists of
•Potentiometer.
• Glass or calomel electrode
• Reference electrode.
•Temperature compensation devise
•Measures potential difference
•Meter indicates pH value or volts,
accuracy of ± 0.01 pH
Types of pH meter electrodes
pH meter consists of a special measuring probe (electrode)
connected to an electronic meter that measures the pH reading. two
types of electrodes1- Glass electrode.
2- Calomel electrode.
1- Glass electrode
•A glass electrode is a type of ion-selective electrode made of
glass membrane that is sensitive to a specific ion.
•Made of glass tube ended with small glass bubble. Inside of the electrode is usually
filled with buffered solution of chlorides in which silver wire covered with silver chloride is immersed.
•pH of internal solution it can be(different buffers).
•Active part of the electrode is the glass bubble.
•Bubble is made to be as thin as possible, while tube has strong and thick walls.
•Surface of the glass is
protonated by both internal and external solution till equilibrium is achieved (Both sides of the
glass are charged by the adsorbed protons).
•This charge is responsible for potential difference,
•The potential in turn is described by the Nernst equation.
• Is directly proportional to the pH difference between solutions on both sides of the glass.
•The electrode is represented as:
Ag|AgCl|Cl- and the electrode reaction is,
AgCl- --> Ag + Cl-
•Calomel electrodes
It consists of mercury at the bottom over which a paste of mercury-mercurous chloride is placed.
•A solution of potassium chloride is then placed over the paste.
•A platinum wire sealed in a glass tube helps in making the electrical contact.
•The electrode is connected with the
help of the side tube on the left through a salt bridge with the other electrode to make a complete cell
•The potential of the calomel electrode depends upon the concentration of the potassium chloride solution.
•If potassium chloride solution is
saturated, the electrode is known as saturated calomel electrode (SCE) and if the potassium chloride
solution is 1 N, the electrode is
known as normal calomel electrode (NCE).
•The electrode Hg2Cl2, "calomel, reaction of electrode acts as:
•1/2 Hg2Cl2 + e- <---> Hg + Cl-
Application
•The Calomel and glass electrode is used in pH measurement, work in the same way.
•In both electrodes, the activity of the metal ion is fixed by the solubility of the metal salt.
•The calomel electrode contains mercury, which poses much greater health hazards than the silver metal .
•The calomel electrode uses in biological systems.
