Ionization of Water and the pH Scale

Every aqueous solution contains hydronium (H₃O⁺) and hydroxide (OH^-) ions as a result of the water autoionization process,

          (1.1)

[H₂O] does not appear in the equilibrium constant expression since, in dilute solutions, H₂O is very close to the standard state condition of pure liquid. In pure water at 25°C, [H₃O⁺] = [OH^-] = Kw_1/2 = 1.00 × 10-7 M.

Because the water ionization is endothermic, Kw increases with increasing T. In 0.10 M HCl solution, HCl is completely ionized,

and [H₃O⁺] = 0.10 M, [OH-] = 1.0 × 10^-13 M. Similarly 0.10 M NaOH is completely ionized,

and [OH^-] = 0.10 M, [H₃O⁺] = 1.0 × 10^-13 M. Because of the enormous range in the concentrations of H₃O⁺ and OH- ions, we usually use a logarithmic scale to express these concentrations:

         (1. 2)

                (1. 3)

Since the H₃O⁺ and OH^- concentrations are related by Eq. (1.1),

                                 (1.4)

(In general, pX = -log₁₀[X].) Thus, for 0.10 M HCl, pH = 1.00, pOH = 13.00, and for 0.10 M NaOH, pH = 13.00, pOH = 1.00.

Water is not unique in undergoing auto-ionization. Several other solvents are capable of acting both as acids and bases, e.g., the following equilibrium occurs in liquid ammonia (bp -33°C):

مواضيع ذات صلة

نظام استخلاص الجزئيات البسيطة

اختيار الطور الجديد وعدد مرات الاستخلاص

ميكانيكية الاستخلاص

طبيعة الصنف المستخلص

الاستخلاص بالمذيب

امتصاص الشعاع الكهرومغناطيسي وطيف الامتصاص

الطرق الكيميائية الطيفية في التحليل.

طبيعة الكيمياء التحليلية

التحليل بالحقن الجرياني المتقطع Batch flow injection analysis

الحقن الجرياني متعدد التخفيف Multi-commutation in FIA

التحليل بالحقن المتعاقب Sequential Injection Analysis

التحليل بالجريان المتقطع Segment Flow Analysis