pH Scale of Acid and Base Solutions

pH scale chart for acid, base, and neutral solutions defines the concentration of hydrogen ion or H+ ion pure water or aqueous medium. For the measurement of pH scale value, the concentration of hydrogen ion in acid solution is high, and in alkaline solutions low. Therefore, Sorensen defines pH is the negative of the logarithm of the activity or concentration of H+ ion. Similarly, pOH is the negative logarithm of the concentration of hydroxyl ion in study chemistry.

Therefore, pH = – log aH+ or, aH+ = 10-pH. When the solution is very dilute, aH+ = CH+, therefore, pH = – log CH+. Now for a solution pH = 3, CH+ = 10-3 and for a solution pH scale value = 8, CH+ = 10-8.

pH scale chart for acid base and neutral solution in pure water

Ion Product of Water Solution

In the pure state, the water dissociates to very small extent and behaves as a weak electrolyte. The equilibrium constant for the dissociation of the water molecule, H2O → H+ + OH given by, k = (aH+ × aOH)/ CH2O.  But in the pure state or dilute solution, the activity of water taken as unity. Therefore, aH+ × aOH = kw. The constant kw is called ionic activity product of water. But if we replace the activity with concentration, kw = CH+ × COH. In pure water or dilute solution activity coefficient is almost unity. Therefore kw is termed as the ionic product of pure water solution uses for determination of pH scale in learning chemistry.

pH of Acid Base and Neutral Solution

We can differentiate acid, base properties, and neutral solution on the basis of the relative concentration of hydrogen or hydroxyl ion or pH scale value.

  • A neutral solution having the concentration of hydrogen and hydroxyl ion equal. Therefore, [H+] = [OH] = 10-7; or, pH = pOH = 7.
  • An acid solution having the concentration of hydrogen ion exceeds that of hydroxyl ion or [H+] > [OH].  Therefore, the pH scale value of acid solution less than seven.
  • A base solution having the concentration of hydroxyl ion exceeds that of hydrogen ion or [H+] < [OH].  Therefore, the pH scale value of base solution greater than seven.