pH Scale for Acids and Bases
pH scale and pOH scales differentiate between neutral, acidic, or basic in pure or distilled water solution on the basis of the relative concentration range of ionic hydrogen and hydroxyl ion in acid and alkaline chemistry. Sorensen define these term known as pH and pOH scale or chart for measuring the concentration of hydrogen and hydroxyl ion in the pure water solution of acid and alkaline. Therefore, the acidity and basicity level chart of acid and base measure by pH scale value in pure or distilled water solution. The calculation of the concentrations of hydrogen ion and hydroxyl ion in pure water not much convenient and meaningful because this shows the low-value of the dissociation constant.
Hence Sorenson measure the neutral, acidic, or base properties of solution in pure or distilled water in terms of pH level chart value of acid and alkaline. Therefore, pH = – log (CH+). When a solution having hydrogen ion 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 hydrogen ion is more convenient.
pOH Scale Value for Alkaline Solution
In learning chemistry, the concentration value of hydroxyl ion define the pOH scale of the basic or alkaline solution as that of the pH of the acid solution. Therefore, pOH = – log (COH–). When the acidity of a solution goes down 100 fold, the pH goes up by two units.
- Therefore, the solution of pH = 1, has hydrogen ion concentration 100 times greater than that of 3.
- Same way, when hydroxyl ion concentration will go down by two units then pOH goes from 13 to 11.
pH Scale Chart of Acid and Alkaline Solution
Now we can proceed to differentiate between neutral, acidic, and basic solutions on the basis of relative concentrations of hydrogen and hydroxyl ions in water. When the concentration level based on the pH value of the neutral, acidic, and alkaline pure water solution, it defines the pH and pOH scales chart.
pH Level of Neutral Solution
A neutral solution is one where the concentrations of hydrogen and hydroxyl ions 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 water solution having the same value of 7.
pH Level of Acidic and Alkaline Solution
An acidic solution has the concentration of hydrogen ion greater then hydroxyl ion.
CH+ > COH–
or, CH+ > 10-7 M and COH– < 10-7 M
∴ pH < 7 and pOH > 7
Therefore, for an acidic pure water solution pH range one to less than seven and pOH range greater than seven to fourteen. In alkaline or basic solution, the concentration of hydrogen less than hydroxyl ion.
CH+ < COH–
or, COH– > 10-7 M and CH+ < 10-7 M
∴ pOH < 7 and pH > 7
Value of the Ionic Product of Pure Water
H2O ⇄ H+ + OH–
At this equilibrium for dissociation of water will have its own equilibrium constant value.
k = (CH+ × COH–)/CH2O
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 absorbed a specific heat by the system from the surroundings are known as endothermic reactions in thermodynamics. A specific heat absorbed from the surroundings for ionization of water. Thus it is an endothermic reaction.
H2O → H+ + OH– +13.7 kcal
Therefore, according to Le-Chatelier’s principle, increasing temperature will facilitate dissociation and giving higher values of Kw.
Measuring pH and pOH Scales of Pure Water
The product of hydrogen and hydroxyl ion 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
Acidity Scale of Pure Water
The concentration of hydrogen and hydroxyl ion or acidity scale in pure water has been determined by the ionic product of water. Ionic product of pure water =14 and the concentration of hydrogen and hydroxyl are the same.
pKw = pH + pOH =14
or, Kw = CH+ × COH– =10-7 × 10-7 = 10-14
Therefore, the above relation measure the pure water pH level and the concentration of hydrogen ion and hydroxyl ion in the neutral solution which is 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 acidity scale 0.1 m and 0.2 N sulfuric acids are the same. Thus, pH of 0.1 or 0.2 sulfuric acids = – log (0.2) = 0.699.
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 Solution
Acetic acid is a weak organic acid and the concentration of hydrogen ion in 0.02 m acetic acid
∴ For 0.002 CH3COOH = – log (CH+)
= -log (2 × 10-4) = 3.7
Measuring pH and pOH Scale and Acidity
Nitric acid, sulfuric acid, hydrochloric acid in pure solution are completely dissociated and regarded as an almost equally strong acid. But the mathematical measuring of pH scales provides the negative value when the concentration of hydrogen ion exceeds 1 M. Therefore, measure pH and pOH scale value for such polar solution avoided because these solutions are not likely to dissociate fully. The concentration of such strongly acid solution is expressed best in terms of molarity rather than pH.