pH, pK, ACID, BASE, BUFFER, WEAK BONDS
Water molecules have a have a slightly tendency to undergo reversible ionization to yield a hydrogen ion (a proton) and a hydroxide ion, giving the equilibrium. Although we commonly show the dissociation product of water is hydrogen ion, free proton do not exist in solution; hydrogen ion formed in water are immediately hydrated to form hydronium ion (H30+).
EXPRESSION OF WATER IONIZATION
The degree of ionization of water at equilibrium is small at 25◦C. The equilibrium constant for reversible ionisation of water is
Thus the product of [H+] [OH–]in aqueous solution at 25◦C is always equal to 1×10-14m2. When there are exactly equal concentration of H+ and OH– as in pure water, the solution the solution is said to neutral pH.
WHAT IS pH?
It is a convenient means of designating the concentration of H+ (and thus of OH–) in any aqueous solution in the range between 1.0m H + and 1.0m OH –. The term pH defined by the expression
The symbol p denotes “negative logarithm of. “ For a precisely neutral solution at 25◦C in which concentration of hydrogen ion is 1.0×10-7 m, the pH can be calculated as follow
HOW TO MEASURE pH OF THE SOLUTION?
The pH of an aqueous solution can be approximately measured with various indicator dyes, including litmus, phenolphthalein, and phenol red. These dyes undergo colour changes as a proton dissociates from the dye molecule. Accurate determination of pH in chemical or clinical laboratory is made with glass electrode that is selectively sensitive to hydrogen ion but insensitive to sodium and potassium ion, and other cations. In a pH meter, the signal from the glass electrode placed in the test solution is amplified and compared with signal generated by a solution of accurately known pH.
SIGNIFICANCE OF pH
Measurement of pH is one of the important and frequently used procedures in biochemistry. The pH affects the structure and activity of biological macromolecules; for example, catalytic activity of enzyme is strongly depends on pH. Measurement of the pH of blood and urine are commonly used in medical diagnosis. The pH of blood plasma of people with severe, uncontrolled diabetes, for example, is often below the normal value this condition is called acidosis. In certain other diseases the pH of the blood is higher than normal, a condition known as alkalosis. Extreme acidosis or alkalosis can be life threatening.
pK OF ACIDS
Equilibrium constant for ionization reaction are usually called ionization constant or acid dissociation constant, often designated by Ka which is given in the following equation
THUS, pKa ARE VALUE WHICH ARE ANALOGUE TO pH AND DEFINED BY THE EQUATION
- The stronger the tendency to dissociate a proton the stronger is the acid and the lower its pKa.
- Hence pKa of any weak acid is higher and can be determined easily.
pK OF BASES
Base dissociation constant can be written as
- Larger the value of Kb indicates weak base.
- whereas, stronger bases are indicated by lower value.
- Both pK of acid and pK of base are related to simple equation.
- pKa+pKb =14.
ACIDS AND BASES
Substances that turn blue litmus paper red are Said to be acid whereas substance which have ability to red litmus paper blue are base. Further different definitions of acid and base are given below.
THEORIES OF ACID AND BASES
Arrhenius theory of acid and base
- Any substances which have ability to produce H+ ion in solution can be defined as acid.
- Any substances which have ability to produce OH– ion in a solution is said to be base.
- Neutralisation can occur when hydrogen ion and hydroxide ion react to produce water.
Brownsted Lowry acid base concept
- It is also known as proton theory of acid and base.
- An acid is substance that loses a proton.
- Whereas base is a substance that accepts the proton.
Here Bronsted acid can be considered as HCL which donating an proton, And brownsted base is NH+4 ammonia which accepting a proton.
lewis acid and base
According to lewis;
Any substance that has tendency to accept pair of electron to form a covalent bond can be termed as acid.
Any substances which has tendency to donate a pair of electron to form a covalent bond is said to be lewis base.
Here BF3 is lewis acid and F– is lewis base.
STRONG ACID AND BASE
- Hydrochloric, sulphuric, and nitric acids commonly called strong acids, as they are completely ionized in dilute aqueous solution.
- The strong base NaOH and KOH are also completely ionized hence known as strong base.
WEAK ACID AND BASE
Weak acid and bases are not completely ionized when dissolve in water. Weak acids play important role in metabolism and its regulation.
CONJUGATED ACID AND BASE
Conjugated acid and base pair consist of proton donor and proton acceptor in a same chemical reaction. It is related to brownsted Lowry acid and base pair.
Initially CH3COOH is behaving as acid as it is donating a proton but the product formed which is CH3COO– will act as conjugated base as it will need a proton for its reversible reaction.
Similarly H2O is acting as weak base as it will accept the proton but H3o+ in its reversible reaction will act proton donor and will become conjugated acid.
What are buffers?
Almost every biological process is pH dependent a small change in pH produces a large change in a rate of process. Buffers are aqueous system that tends to resist change in pH when small amount of acid or base are added. A buffer system consist of weak acid and its conjugated base as an example, a mixture of equal concentration of acetic acid and acetate ion, found at midpoint of titration curve, is buffer system. Buffer can be acidic or alkaline. System which is responsible for maintenance of acidic environment is said to be acid buffer system. System that maintains basic environment of solution is said to be alkaline buffers.
How buffer system works?
Whenever H+ or OH– is added to a buffer, the result is a small change in the ratio of the relative concentration of the weak acid and its anion and thus a small change in pH. The decrease in concentration of one component of the system is balanced exactly by an increase in the other. The sum of the buffer components does not change, only their ratio changes.
Each conjugate acid-base pair has a characteristic pH zone in which it is an effective buffer. The H2PO4–/ HPO2-4 pair has a pKa of 6.86 and thus can serve as an effective buffer system between approximately pH 5.9 and pH7.9 the NH4+/NH3 pair with a pKa of 9.25 can act as a buffer between approximately pH 8.3 and pH 10.3
Significance of buffer
There are certain area which can resist large pH changes in our body such as dissolved protein, organic substances and inorganic salts present in the cytoplasmic fluid .On the other hand blood plasma can only work in between 7.2 to 7.3 pH. For maintaining its pH bicarbonate buffer system is present in our body. Buffer is vital for maintaining constant changes produces due to metabolism.
Weak interaction can also be known as non covalent interaction. This includes following bonds
- Hydrogen bonds
- Hydrophobic interaction
- Ionic bonds
- Van der waal forces
This bonds are term as weak bonds as it needs less energy to break their interaction which is about 1-5 kcal/mol, Which is very less than covalent interaction.
When hydrogen atom [H] is covalently bond to very electronegative atom[X] weak electrostatic force of attraction is generated between hydrogen atom and electronegative atom of another molecule [Y].
Where, X-H represent covalent polarised bond.
H…Y represents hydrogen bond.
The bond is generated as hydrogen atom achieves partially positive charge when bond to very electronegative atom.
In this interaction non polar molecules are cluster together in such a way that their contact to water can be minimized. Example arrangement of phospholipid in plasma membrane.
Ionic bonds are formed due to electrostatic attraction between positively charge and negatively charge ion.
Hydration of ion takes place in aqueous solutions because polar water molecules can attract charge ions when ions becomes hydrated ion can easily dissolve in the water hence attraction between the ions decreases.
VAN DER WAAL FORCES
It is inter molecular weak force. It can be define as short range electrostatic attractive force which can be arise due to interaction of permanent or transient electric charge separation i.e., dipole movement.
Water is a polar solvent which has capacity to dissolve many solutes in it. Depending upon nature of solute tremendous amount of bond can be formed or broken which leads to different chemical reaction including ionization. All above mention properties help water and other molecules to perform various functions in our body.
- Lehninger principles of biochemistry seventh edition By David L. Nelson and Michael M. Cox
- Life sciences fundamental and practices sixth edition, pathfinder publication By Pranav Kumar and Usha Mina