Factors Affecting pH Solution – Formula – Principle

Acid, bases. Both groups have their own characteristics. As we have known, acid solution is typical of acidic solutions, example of acidic solutions are such as lemons and limes containing citric acid. While typical base solutions have bitter and slick solution, sodium hydroxide in detergent for example. However, if only determined by these characteristics, it is certainly ineffective, right? There are many harmful strong acid and strong bases solutions if we accidentally swallow them up. Thus, how do we address a solution as acid or base? Also read here: Different Types of pH Meters

Yes, the simplest and safest way to determine amount of acid or base of a solution is by considering pH of the solution. pH is essentially derived from the French words ‘pouvoir hydrogène‘, which is translated in English as ‘hydrogen power’. In general, the pH can be defined as numerical scale used to determine acidity and basicity of a solution. This pH concept was first introduced by Danish chemists, Søren Peder Lauritz Sørensen in 1909.

The pH determination can be performed using natural indicators, artificial indicators, such as red and blue litmus papers, or pH indicator paper. The pH of a solution ranges from 0-14, in which the smaller the pH (= < 7), the more acidic it becomes. The larger the pH (= > 7), the more base it is. While a solution with pH of 7 is called as neutral. In addition, calculating pH of the solution can also be done by computing with the formula.

Well, after recognizing pH, we may wonder. If so, what exactly affects magnitude of the pH? What factors affecting pH solution?

Let’s briefly pay attention to the formula used to calculate the pH below!

• Acid solution

pH = -log [H⁺]

• Strong acids

[H⁺] = M x acid valence

• Weak acid

[H⁺] =

[H⁺] = M x α

Note:

[H⁺] = H⁺ ion concentration

K_a = acidic acid constant

M = acid concentration

α = degree of ionization

• Base solution

pOH = -log [OH^–]

pH = 14 – pOH

• Strong base

[OH^–] = M x base valence

• Weak base

[OH^–] =

[OH^–] = M x α

Note:

[OH^–] = OH^– ion concentration

K_b = base ion constants

M = base concentration

α = degree of ionization

Also read here: Electrolyte and Non Electrolyte Solutions

Concentrations of H⁺ and OH^– ions

From a series of formulas above, we can observe that pH of a solution is influenced by its concentration of H⁺ and OH^– ions. Concentration of H⁺ ions explains the number of moles of H⁺ ions in every 1 liter of water. Whereas the OH^– ion concentration explains the number of moles of OH^– ions in every 1 liter of water. Which means the greater concentration of H⁺ in a solution, the less acidity the solution is.  Also read here: Uses of Acetic Acid in Our Daily Life

For example, a solution with H⁺ ion concentration of 10^-3 would have pH of 3. However, if concentration of H⁺ ions is smaller such as 10^-6, pH will become 6. The solution with pH 3 is more acidic than the one with pH of 6.

1. If [H⁺] = 10^-3

pH = -log [10^-3]

pH = 3

2. If [H⁺] = 10^-6

pH = -log [10^-6]

pH = 6

Also read here: Differences between Acid, Base and Salt

From these calculations, it is evident that pH of a solution can be affected by concentration of H⁺ ion and OH^– ion in solution.

• Formula of concentration or polarity of the solution:

M = n / V

Note:

M = molarity (M)

n = mol of solute (mole)

v = volume of solution (Liter)

Since concentration of H⁺ and OH^– ions in solution is influenced by one of the solution molarity, thus its concentration is affected by the mole of solute and volume of solution.

If we want to increase or strengthen the acidic properties of a solution, we can do so by increasing concentration of H⁺ ions and adding moles of solute. The greater the value of acid or base solution, the greater concentration of H⁺ ions. The greater concentration of H⁺ ions, the smaller pH of the solution, the more acidic solution it becomes.

Conversely, if we want to weaken the acidity of a solution, we can reduce concentration of H⁺ ions by minimizing solubility of the solution. The smaller concentration of H⁺ ions, the greater is pH of the solution, which means the acidity of solution is weakened.

This principle does not only applies to chemistry laboratory, but also in a daily life. For example, for those who are pet lovers that tend the fish in ponds and aquariums. Addition of strong HNO₃ acid solution is usually conducted if it aims to lower the water pH. 

Degree of ionization

If pH of a solution is influenced by concentration of H⁺ or OH^– ions, thus why can an acid or base solution have the same pH for the same concentration? This may occur due to concentration of acid or base molar being ionized in water solvent. The amount of ionizable substances in aqueous solvents is the degree of ionization (α)

α = moles amount of ionized substance/initial mole quantity

The degree of ionization in addition to be used to differentiate electrolyte and non-electrolyte solutions is also used to describe the strength of acids and bases in units of 0-1 or percent. If the value of α = 1, it is classified as strong acid or strong base. The value of α = 0 means that moles of the substance are not ionized by any means. While if the value of α is between 0-1, it is classified as weak acid or weak base.

Based on this principle, we can conclude that if the pH of strong acid is compared to weak acid of the same concentration, strong acidic pH indicates a smaller number (stronger acidity). So it is the same case with base solution. If we have a solution of NaOH (strong base) with concentration of 0.1 and NH₃ solution of the same concentration, the pH of NaOH solution will be greater, which means stronger basicity.

Ionization Constant

Furthermore, the degree of ionization also affects Ka and Kb of a solution. As we know by calculation formula of the solution pH, Ka and Kb also influence [H⁺] and [OH^–]. If the formula is redefined, the degree of ionization of the solution can be found by rooting Ka/Kb divided by Ma/Mb. The greater is degree of ionization, the greater value of ionization constant. The larger is ionization constant, the greater value of [H⁺] or [OH^–]. The greater is value of [H⁺], the greater pH of a solution, the weaker is its acidity, and vice versa. Also read here: Acid Solution Examples

Thus, that’s all the complete theory about factors affecting pH solution. Let’s study chemistry well!