pH Scale of Acid and Base
pH and pOH range differentiate between acidic, basic, and neutral solution in pure water which has very low dissociation constant. In chemistry, Sorensen define the term known as pH and pOH scale or chart for calculation of the concentration levels of H+ ion in acid solution and OH– ion value in the base solution.
The calculation of the concentrations of hydrogen ion and hydroxyl ion in pure water not much convenient and meaningful because of this low-value of dissociation constant. Hence the Sorenson definition of pH levels of solution
When a solution having H+ concentration range 10-1 M to 10-14, then the pH range of said solution 1 to 14. Hence acidity of the solution in terms of protonation is more convenient.
|[H] = 10-1 M||1|
|[H] = 10-14 M||14|
pOH Scale Value for Basic Solution
In chemistry, the concentration value of OH– ion define as the pOH scale of the basic solution as that of pH.
When the acidity of a solution goes down 100 fold, the pH goes up by two units.
- Therefore the solution of pH = 1, has H+ concentration 100 times greater than that of 3.
- Same way, when OH– ion concentration will go down by two units then pOH goes from 13 to 11.
pH Value Chart of Acid and Base
Now we can proceed to differentiate between neutral, acidic, and basic solutions on the basis of relative concentrations of H+ and OH– ions. When the concentration value based on the pH of the acid base solution, it constructed the pH and pOH chart.
pH Range of Neutral Solution
A neutral solution is one where the concentrations of H+ and OH– are equal.
∴ CH⁺ = COH⁻ = 10-7 M
or, -log (CH⁺) = -log (COH⁻) = -log (10-7) M
Agin from the definition
pH = -log (CH⁺) and pOH = -log (COH⁻)
∴ pH = pOH = 7
Therefore the pH and pOH range of neutral solution having the same value of 7.
pH Scale of Acidic and Basic Solutions
An acidic solution has the concentration of hydrogen ion greater then hydroxyl ion.
∴ CH⁺ > COH⁻
Thus, CH⁺ > 10-7 M and COH⁻ < 10-7 M
∴ pH < 7 and pOH > 7
Thus for an acidic solution pH range one to less than seven and pOH range greater than seven to fourteen.
In alkaline solution, the concentration of H+ less than OH–
CH⁺ < COH⁻
Thus COH⁻ > 10-7 M and CH⁺ < 10-7 M
or, pOH < 7 and pH > 7
Value of the Ionic Product of Water
Water molecule ionizing weakly to form H+ and OH– ions. Thus there will always be an equilibrium between hydrogen and hydroxyl ions in the water molecule.
H2O ⇄ H+ + OH-
At this dynamic equilibrium for dissociation of water will have its own equilibrium constant value.
or, k × CH2O = CH+ × COH–
where CH₂O = concentrations of water and CH⁺ and COH⁻ = concentration of hydrogen and hydroxyl ions.
But in any dilute aqueous solution, the concentration of water = 55.5 moles/liter, can be taken as a constant.
∴ k × CH₂O = Kw = CH⁺ × COH⁻
where Kw = ionic product of water.
Chemical reactions in which a specific heat absorbed by the system from the surroundings are known as endothermic reactions.
A specific heat absorbed from the surroundings for ionization of water. Thus it is an endothermic reaction.
H2O → H+ + OH– +13.7 kcal
Thus according to Le-Chatelier’s principle, increasing temperature will facilitate dissociation and giving higher values of Kw.
pH and pOH of Pure Water
Previously we see that the product of H+ and OH– has constant having the value 10-14.
∴ CH⁺ × COH⁻ = 10-14
or, – log(CH⁺ × COH⁻) = – log(10-14)
or, – log (CH⁺) – log(COH⁻) = 14
Again from the definition
– log CH⁺ = pH and -logCOH⁻ = pOH
∴ pH + pOH = 14
pH Levels of Pure Water
The concentration of H+ and OH– in pure water has been determined from the ionic product of water. Ionic product of pure water =14 and the concentration of H+ and OH– are the same.
pKw = pH + pOH =14
or, Kw = CH⁺ × COH⁻ =10-7 × 10-7
Therefore from the above relation shows the water pH levels and the concentration of hydrogen ion and hydroxyl ion in the solution are inversely proportional to each other.
To maintain constant Kw, if hydrogen ion concentration increases 100 fold than hydroxyl ion concentration decrease 100 fold.
For 0.01 m sulfuric acid
Sulfuric acid is a dibasic acid. Molarity of sulfuric acid = 2 × molarity of the sulfuric acid solution. Thus 0.1 m and 0.2 N sulfuric acids are the same.
∴ For 0.1 or 0.2 H2SO4 = – log (0.2)
For 0.02 M HCl solution
Hydrochloric acid is a strong electrolyte and completely dissociated in the solution.
∴ CH⁺ = CHCl = 0.002 = 2×10-3 M
For HCl 0.02 M HCl = – log CH⁺
= – log (2×10-3)
= (3 – log2) = 2.7
For 0.02 m acetic acid
Acetic acid is a weak acid and the concentration of hydrogen ion in 0.02 m acetic acid
∴ For 0.002 CH3COOH = – log (CH⁺)
= -log (2 × 10-4)
Calculation of pH and pOH scale
The mathematical calculation of pH scale and pOH scale provides a negative value when the concentration of H+ in acid-base exceeds 1 M.
Therefore in chemistry, the define pH and pOH scale value of such concentrated solutions are avoided. Because the solution not likely to be dissociated fully in pure water.