pH-HYDROGEN ION CONCENTRATION

 HYDROGEN ION CONCENTRATION (pH)

It is defined as the negative logarithm of hydrogen ion concentration to the base 10. The hydrogen ion concentration [H+ ] is expressed in moles/litre. 1

pH = –log [H+ ] = log 1/[H+ ]

Pure water or neutral aqueous solutions have [ ] = 1× mol/litre. Therefore their pH according to definition can be calculated to be equal to 7. pH can also be expressed as the index of the exponential term obtained by writing the molar conc. of as a power of 10, omitting the negative sign of the term. According to this, the pH scale ranges from 0 to 14. It was introduced by Sorensen. pH = 7 is neutral, while pH > 7 is alkaline and pH < 7 is acidic. A rise or fall in pH by 1 signifies a tenfold fall or rise in the conc. respectively. pOH is the negative logarithm of [OH- ] to the base 10. The ionization exponent of water (PKw) is the negative logarithm of Kw to the base 10 and equals the sum of pH and pOH values. PKw = pH + pOH. pKw of water at room temp. is 14

 

Significance of pK: pK of an acid group is the pH at which the protonated and unprotonated species are present at equal concentrations. Applications of Henderson-Hasselbalch Equation 1. Can be used to determine the pH of blood, if the concentration of salt, i.e. bicarbonate and acid (carbonic acid) is known. Example • Approx. the concentration of bicarbonate in normal health = 0.025 M • Approx. the concentration of carbonic acid = 0.00125 M. • pKa of carbonic acid = 6.1 Then applying HH equation

2. Determination of pH of an unknown buffer solution By using the HH equation, one may directly calculate the pH of a buffer solution if pKa of the buffer acid and the molar ratio of salt to acid in the solution is known. Examples (a) A solution containing 0.05 M Na-acetate and 0.1 N Acetic acid. The value of pKa of acetic acid is 4.73 at room temperature.

the pH of the solution

                                                                                               

(b) In the above example, if the solution contained 0.1 M Na-acetate and 0.1 N acetic acid. 0.1

pH = 4.73 + log  

    = 4.73 + log 1

         = 4.73 + 0 = 4.73

 

This shows that when the molar ratio of salt to acid in a buffer solution is unity, the pH of the solution becomes equal to the value of pKa for the buffer acid. Biological Importance of pH .pH plays an extremely important role in biological systems. It is already mentioned in the chapter on enzymes that enzymes have an optimum pH. Various body fluids and cell organelles, etc. maintain a specific pH to allow the activity of the enzymes located there. Gastric juice is highly acidic with pH = 1.2. The gastric juice contains pepsin whose optimum pH is around 2. The other functions of HCl are mentioned in gastric function tests. Osteoblasts possess a highly alkaline pH. The values of pH lie between 9 and 10. The alkaline phosphatase located there has the optimum pH in that range. The prostate has acidic pH (3-5) to allow acid phosphatase activity. the pH of arterial blood is 7.4 and is effectively regulated as described in Chapter on “Acid-base Balance and Imbalance”. • Tautomeric forms of purines and pyrimidines: Keto/ enol (lactam/lactim) forms of nucleic acid bases depend on pH. Specific tautomeric forms exist at pH 7.4. This helps in proper hydrogen bonding between the complementary base pairs. • Isoelectric pH and optimum pH of proteins: Amino acids and proteins exist as Zwitterions at isoelectric pH. The magnitude of the charge depends on the pH. By influencing the ionized states of proteins, pH considerably affects the formation and maintenance of ionic and hydrogen bonds. The solubility and biological activity of proteins depend on their 3-D structure. Hence protein has an optimum pH where it best can maintain its 3-dimensional conformation befitting its biological activity. • pH and Gibbs-Donnan effect: The physiological pH 7.4 renders body proteins to exist as anions. Since they are large molecules, they do not cross semipermeable biological membranes resulting in an unequal distribution of diffusible cations and anions on the two sides of membranes. The diffusible cations are more concentrated, on the side containing a higher concentration of the protein anions, while diffusible anions have a higher concentration on the opposite side of the membrane (Gibbs-Donnan membrane equilibrium). pH is responsible for this. Ionic states of nucleic acids, lipids, and mucopolysaccharides: It is observed that nucleic acids, MPS, phosphoglycerides, sphingolipids, etc. exist as ionized forms either as cations or anions depending on the pH. At pH 7, lysylphophatidylglycerols bear net positive charges, gangliosides carry a negative charge. By influencing the electrical charges of the polar head groups of membrane lipids, pH influences the membrane structure.  pH and Keq: pH influences the Keq product yield and spontaneity of metabolic oxidation-reduction and some nonenzymatic acid-base catalysis