3. Self-ionization of water, ionic product of water (Kw). The pH scale, methods of measuring and calculating pH. Physiological pH and pH of some body fluids (saliva, blood etc.). Critical pH of saliva.
Topic 3 of 25 in the MU Plovdiv syllabus for Chemistry (Dental Medicine, Year 1). The free AceMed note covers it in 9 sections, about 782 words (6 min of reading), with 17 flashcards.
In this note
Autoproteolysis.
Ion product constant of water.
pH concept (рН).
Importance of pH homeostasis in body fluids.
Methods for pH measurement and biological significance.
Autoprotolysis, Ion Product of Water, and pH
Autoprotolysis (Self-Ionization)
pH Concept
Biological pH Homeostasis
Autoproteolysis.
Autoprotolysis is a type of chemical reaction occurring in amphiprotic solvents, where two molecules of the solvent interact and exchange a proton (H⁺). These solvents can act as both acids and bases, donating or accepting protons.
Reaction:
The general formula for an amphiprotic solvent is SH. The autoprotolysis reaction can be represented as:
The equilibrium constant (Kₛ) for autoprotolysis is defined as:
Kₛ = [SH₂⁺][S⁻] / [SH]²
However, due to the low concentration of ions compared to the neutral solvent molecules, the concentration of [SH] is often considered constant. This simplification leads to the ionic product of water (Kᵅw):
Kᵅw = [H⁺][OH⁻]
pKᵅw:
Due to the small values of Kᵅw (typically on the order of 10^-14 for water at 25°C), it is often expressed using the negative decimal logarithm:
pKᵅw = -log₁₀(Kᵅw)
Therefore, pKᵅw represents the tendency of an amphiprotic solvent to undergo autoprotolysis and is a measure of the solvent's self-ionization strength.
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