Difference Between pH and pKa

pH vs pKa
 

We normally identify an acid as a proton donor. Acids have a sour taste. Lime juice, vinegar are two acids we come across at our homes. They react with bases producing water, and react with metals to form H₂; thus, increase metal corrosion rate. The ability to donate a proton is characteristic of an acid, and pH, PKa values are calculated based on this characteristic.

pH

pH is a scale that can be used to measure the acidity or basicity in a solution. The scale has numbers from 1 to 14. pH 7 is considered as a neutral value. Pure water is said to be having pH 7. In the pH scale, from 1-6 acids are represented. Acids can be categorized into two, based on their ability to dissociate and produce protons. Strong acids like HCl, HNO₃ are completely ionized in a solution to give protons. Weak acids like CH₃COOH partially dissociate and give fewer amounts of protons. An acid with pH 1 is said to be very strong, and as the pH value increases, acidity decreases. Therefore, the pH values more than 7 indicate basicity. As the basicity increases, pH value will also increase and strong bases will have pH value 14.

pH scale is logarithmic. It can be written as below in relative to the H⁺ concentration in the solution.

pH= -log [H⁺]

In a basic solution, there is not any H⁺s. Therefore, in a situation like that, pOH can be determined from –log [OH^–] value.

Since, pH + pOH = 14; pH value of a basic solution can also be calculated. There are pH meters and pH papers in laboratories, which can be used to measure pH values directly. pH papers will give approximate pH values, whereas pH meters give more accurate values.

pKa

Acidity is the state of being an acid. This is related to the degree of being an acid. Acids can be categorized into two, based on their ability to dissociate and produce protons. Strong acids like HCl, HNO₃ are completely ionized in a solution, to give protons. Weak acids like CH₃COOH partially dissociate and give fewer amounts of protons. K_a is the acid dissociation constant. It gives an indication of the ability of a weak acid, to lose a proton . In an aqueous medium, a weak acid is in equilibrium with its conjugate base as shown in the example below.

CH₃COOH_(aq) + H₂O_(l)    CH₃COO^–_(aq) + H₃O⁺_(aq)

The equilibrium for the above can be written as,

 E = [CH3COO-] [H3O+] / [CH3COOH] [H2O]

This equation can be re-written as below changing the constant to acid dissociation constant.

 Ka = [CH3COO–] [H3O+] / [CH3COOH]

The reciprocal of the logarithm value of Ka is the pKa value. This is another way of expressing the acidity.

                                                           pKa = -log K_a

For a stronger acid, Ka value is larger, and pKa value is smaller. And for a weak acid, it is the opposite.

What is the difference between pH and pKa? • pH is the reciprocal of the logarithm of H+ concentration. pKa is the logarithm of Ka value. • pH gives an idea about the amount of H+ions present in the medium. pKa value gives an idea about to which side the equilibrium is favored (the degree of acid dissociation). • Both pH and pKa are related by Henderson-Hasselbalch equation: pH = pKa + log ([A–]/[HA])