pH Scale Definition in Science
pH scale define and measure the acidity and basicity of neutral, acidic, and basic pure water solutions based on the relative concentration of hydrogen and hydroxyl ion mainly in chemistry, biology, and soil science. The term pH and pOH are widely used and important for chemical, biological, and soil health measurement by translating the values of the concentration of hydrogen or hydroxyl ion or using pH paper and meter. Sorensen first invented or used the term known as pH scale or chart for measuring the concentration of hydrogen and hydroxyl ion in the pure water solution of acid and alkaline.
Sorensen defines pH or in full form potential of hydrogen is the negative of the logarithm of the activity or concentration of hydrogen ion. Therefore, pH = – log aH+ or, aH+ = 10-pH. When the solution is very dilute, aH+ = CH+, therefore, pH = – log CH+. The Sorensen definition largely uses in chemistry, biology, and agronomy for the simple mathematical calculation of pH and pOH scale value but technically we use the instrument like pH paper and meter for range level determination.
pOH Scale Definition in Science
In learning chemistry, the concentration value of hydroxyl ion represents the term known as the pOH scale in place of pH. Therefore, pOH = – log (COH–). If a solution of pH levels = 1 and the acidity of the said solution goes down 100 fold, the pH scale range goes up by two units. In the same way, when hydroxyl ion will go down by two units means pOH values go from 13 to 11.
pH Scale Chart in Acid Chemistry
Now we can proceed to differentiate between neutral, acidic, and basic pure water solutions based on relative concentrations of H+ and OH– ions or pH scale values. The acidity level meaning the pH and pOH scale chart value of the neutral, acidic, or alkaline pure water solution in chemistry.
pH Scale for Neutral, Acidic, and Alkaline Solution
A neutral solution or pure water solution meaning the solution where the concentrations of hydrogen and hydroxyl ions are equal or pH values of acids and bases are equal. Therefore, CH+ = COH– = 10-7 M. From the definition we can write simply, pH = pOH = 7. Therefore, this scale range or level explained that the neutral water solution having the same value of pH and pOH which stand for 7.
An acidic solution has the hydrogen ion greater than hydroxyl ion or CH+ > COH–. Therefore from the definition, pH < 7 and pOH > 7. From this equation, an acidic pure water solution pH scale range stands from one to less than seven, and the pOH range stands from greater than seven to fourteen. But in alkaline or basic solution, the concentration of hydrogen less than hydroxyl ion or CH+ < COH–. Therefore, the measures pOH < 7 and pH > 7.
Value of Ionic Product of Pure Water
Water molecule ionizing weakly to form conjugate acid hydrogen and base hydroxyl ion in solution. Thus there will always be an equilibrium between hydrogen and hydroxyl ions in the water molecule. Therefore, H2O ⇄ H+ + OH–. At this equilibrium for dissociation of water will have its equilibrium constant value, k × CH2O = CH+ × COH–, where CH2O, CH⁺, and COH⁻ = concentrations of water, hydrogen, and hydroxyl ions. But in any dilute aqueous solution, the concentration of water = 55.5 moles/liter, can be taken as a constant. Therefore, 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 or energy (13.7 kcal) is absorbed from the surroundings for the ionization of water. Therefore, according to Le-Chatelier’s principle, increasing temperature will facilitate dissociation and giving higher values of Kw. The product of hydrogen and hydroxyl ion has constant having the value 10-14. Therefore, from the Sorensen definition in chemistry, pH + pOH = 14
pH of Pure Water Solution in Science
The concentration of hydrogen and hydroxyl ion or pH scale in pure water has been defined by the ionic product of water. The ionic product of pure water =14 and the concentration of hydrogen and hydroxyl are the same. Hence, Kw = CH+ × COH– =10-7 × 10-7 = 10-14. Therefore, the above facts describe the pure water solution pH scale level. This equation shows that the concentration of hydrogen ion and hydroxyl ion are inversely proportional to each other. To maintain constant Kw, if hydrogen ion concentration increases 100 fold then hydroxyl ion decrease 100 fold.
pH Scale Value Measurement
For 0.01 m sulfuric acid: Sulfuric acid causes acid rain, 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. Therefore, the pH scale value stand for 0.1 M or 0.2 N sulfuric acids in chemistry = – log (0.2) = 0.699.
For 0.02 M hydrochloric acid: Hydrochloric acid is a strong electrolyte and completely dissociated in the solution. Therefore the measured pH value of 0.02 M hydrochloric acid (HCl) solution = – log (2×10-3) = (3 – log2) = 2.7.
For 0.02 m acetic acid: Acetic acid is a weak organic acid and the concentration of hydrogen ion in 0.02 m acetic acid. Therefore the measured pH value define for 0.002 Acetic acid (CH3COOH) = – log (CH+) = -log (2 × 10-4) = 3.7.
Define pH 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 definition or application of pH meter provides the negative scale value when the concentration of hydrogen ion in some content exceeds 1 gm equivalent. Therefore, measure pH and pOH scales value for such polar acid, base, or salt solution avoided because these solutions are not likely to dissociate fully. The absolute concentration of such a strong acid solution is measure in terms of relative molarity rather than the potential of hydrogen or pH scale value.