/ \ . . .

Water ha s othe r uniqu e propertie s a s wel l . I t is a permanent dipole , a n d i t

has a rathe r high dielectri c constant . Thes e propertie s o f wate r an d othe r sub -

stances ar e ampl y describe d i n genera l chemistr y textbooks . Th e formatio n o f

hydrogen bonds , th e dipola r natur e o f water , an d it s othe r propertie s allo w

water t o ac t a s a solven t fo r a numbe r o f ioni c an d nonioni c substances . Suc h

water-soluble substance s ar e terme d hydrophilic. Thos e substance s tha t d o no t

interact wit h wate r (e.g. , th e hydrocarbons ) ar e terme d hydrophobic. A m o n g

substances tha t are hydrophil ic because o f hydrogen bon d formatio n ar e amines ,

ketones, aldehydes , mercaptans , an d alcohols . Ioni c compounds , bot h organi c

and inorganic , are generally wate r soluble becaus e o f th e dipola r nature of water :

positive ion s (e.g. , N a + ) interac t wi t h th e negativel y charge d portion s o f th e

water dipol e (th e O atom) , wherea s negativel y charge d ion s (e.g. , C l ~ ) interact

wi th th e positivel y charge d H atom s o f th e wate r dipole :

A H \ / H

\ Q / H O H

8 + 8 "

Body wate r itsel f i s poorl y ionized , bu t i t contain s numerou s solute s tha t

maintain th e organis m i n a stabl e environment , homeostasis. Wate r is dissociate d

as follows : 2 H 2 0 ^ H 3 0 + + O H " . H3 0

+ i s th e hydronium ion. However , fo r th e

sake o f simplicity , i t i s customar y i n th e biomedica l science s t o refe r t o H 3 0 + a s

simply H + o r a proton, s o tha t th e dissociatio n o f wate r ca n also b e expresse d a s

H 2 0 ^ ± H + 4 - O H " . Writing a dissociatio n constan t fo r thi s a s K d = [ H

+ ] [ O H " ] /

[ H 2 0 ] , w e obtai n [H 20]Kd = [ H + ] [ O H " ] = K w. Becaus e K d an d [ H 2 0 ] ar e constant s

( [ H 2 0 ] = 55. 5 M ) , K w i s als o a constant an d i s equa l t o 10" 1 4 a t 25°C , s o tha t

[ H + ] [ O H ~ ] = 10" 1 4 (3.3 )

If log s ar e taken on bot h side s o f Equatio n (3.3), w e obtai n — 14 = - l o g [ H + ]

+ lo g [OH"] ; changin g signs , w e obtai n 1 4 = lo g [ H + ] — log [ O H " ] . Th e ter m

- l o g [ H + ] is the p H , whereas - l o g [OH" ] i s p O H . T h u s , p H + p O H = 14 . W h e n

[ H + ] = [OH" ] = 10" 7, th e p H and p OH are both 7 and the aqueou s solutio n i s sai d

to b e a t neutrality . I f pH < p O H , we hav e a n acidi c solution, an d w h e n p O H <

p H , w e hav e a n alkaline or basic solution. Th e pH of blood plasm a is close t o 7.4 ,

whereas tha t o f othe r extra - and intracellula r fluids ma y deviat e quit e drasticall y

from neutrality : the p H of gastri c juice is between 1 and 2 , tha t o f pancreati c juice

is between 7. 5 an d 8, an d that of urine , depending o n diet , ma y var y between 4. 5

and 7.5 .