Difference Between Alkalinity and pH

Alkalinity vs pH

pH is a most common term used in laboratories. It is associated with alkalinity measurement and acidity measurements.

Alkalinity

‘Alkalinity’ has alkali properties. Group 1 and group 2 elements, which are also known as alkali metals and alkaline earth metals, are considered to be alkaline when they dissolve in water. Sodium hydroxide, potassium hydroxide, magnesium hydroxide, and calcium carbonate are some of the examples. Arrhenius defines the bases as the substances that produce OH^–in solutions. The said molecules form OH^– when dissolved in water, therefore, act like bases. Alkalinity of a solution is measured by taking the sum of all the bases in that solution. Normally, when calculating the alkalinity, the sum of carbonate (CO₃^2-), bicarbonate (HCO₃–), and hydroxide alkalinity (OH^–) is taken. Alkaline solutions react easily with acids producing water and salt molecules. They show a pH value higher than 7 and turn red litmus to blue. There are other bases except alkaline bases like NH₃. They also have the same basic properties. Alkalinity is important in neutralizing acidity, removing fat and oils. Therefore, most detergents have an alkalinity.

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 represent acids. 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 is decreased. 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 aren’t 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 can be used to measure pH values directly. pH papers will give approximate pH values, whereas pH meters give more accurate values.

What is the difference between Alkalinity and pH?

• pH measures the total [H⁺] in a solution and it is a quantitative measurement of alkalinity. Alkalinity gives a qualitative indication of the degree of bases or basic salts present in a solution.

• When pH increases, alkalinity should not have to be necessarily increased, because alkalinity is different from the basicity.

• Alkalinity is the state of having a pH value higher than 7.

• pH also measures the acidity, not only alkalinity.