Difference Between pH and Acidity

pH vs Acidity
 

Acidity and pH are two closely related terms in chemistry. pH is a most common term used in laboratories. It is associated with acidity measurement and basicity measurements.

Acidity

Acids are defined in several ways by various scientists. Arrhenius defines an acid as a substance that donates H3O⁺ ions in the solution. Bronsted- Lowry defines a base as a substance that can accept a proton. Lewis acid definition is far common than the above two. According to it, any electron pair donator is a base. According to the Arrhenius or Bronsted-Lowry definition, a compound should have a hydrogen and the ability to donate it as a proton to be an acid. But according to Lewis, there can be molecules, which don’t possess hydrogen, but can act as an acid. For example, BCl₃ is a Lewis acid, because it can accept an electron pair. An alcohol can be a Bronsted-Lowry acid, because it can donate a proton; however, according to Lewis, it will be a base. Regardless of the above definitions, 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 with metals to form H₂; thus, increase metal corrosion rate. Acidity is the state of being an acid. This is related to the degree of being an acid (strong or weak acid).

pH

pH is a scale which 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 are partially dissociated 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. So 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, from -log [OH^–] value pOH can be determined.

Since, pH + pOH = 14

Therefore, pH value of a basic solution can also be calculated. There are pH meters and pH papers in laboratories, which are used to measure pH values directly. pH papers will give approximate pH values, whereas pH meters give more accurate values than pH papers.