Biochem Trans

WATER & pH

Raymond Oliver A. Cruz, MD, FPAFP

WATER IN THE BODY

▪ The total body water is roughly 50 – 60% of body weight in adults and 75% of body weight in children

▪ Obese people tend to have a lower percentage of body water than thin people

▪ Women tend to have lower percentage than men

▪ Older people tend to have lower percentage than younger people

▪ 60% - intracellular

▪ 40% - extracellular

Dehydration

– Occurs when salt and water intake is less than the combined rates of renal plus extra-renal (non-kidney-related) volume loss

THE “POWERS” OF WATER

▪ Can dissolve most organic and inorganic molecules because of its dipolar structure and ability to form hydrogen bonds

▪ Water is an excellent nucleophile, which enables it to participate in many chemical reactions

▪ It exhibits a slight but important tendency to dissociate, enabling it to act as either an acid or base and playing a role in maintaining pH

Water as a Tetrahedron

– With oxygen in the center, water assumes a tetrahedral shape

– 2 hydrogen atoms and the unshared electrons of the remaining spᶾ-hybridized orbital occupy the corners of the tetrahedron

– The 105° angle between the two hydrogen atoms differs slightly from the ideal tetrahedral angle, 109.5°

Water as a Dipole

– As a dipole, water’s electrical charge is distributed asymmetrically

– The strongly electronegative oxygen pulls electrons towards it

– Dielectric constant

- Also called permittivity

- A measure of the ability of a material to resist the formation of an electric field within it

- Water decreases the force of attraction between charged particles because of its high dielectric constant

- Enables water to dissolve salts

- Dielectric constants: Hexane – 1.9

Ethanol – 24.3

Water – 78.5

- Thus, a compound with a high dielectric constant can easily break ionic...