Encyclopedia of Chemistry

It constitutes the skeleton C 19 H 22 N 4 upon which various
B 12 vitamins, COFACTORs, and derivatives are based.

corrosion Oxidation of a metal in the presence of air
and moisture.

cosphere SeeCYBOTACTIC REGION.

cotransport A simultaneous transporting of two
solutes across a MEMBRANEby a transporter going one
way (symport) or in opposite directions (antiport).

coulomb Unit of electrical charge. A quantity of elec-
tricity when a current of one ampere flows for 1 sec-
ond; 1 coulomb is abbreviated as 1 C.

Coulomb’s law The force of attraction or repulsion
of two electric charges is proportional to the product of
the charges and inversely proportional to the square of
the distance between them. F= k(Q 4 Qb/ d^2 ), where F
represents the electrostatic force, krepresents a con-
stant of proportionality, Q 4 and Qbrepresent quanti-
ties of electrostatic charge, and d represents the
distance between the charges.

countercurrent exchange The effect caused when
two fluids move past each other in opposite directions

and facilitate the efficient exchange of heat, gas, or sub-

stance; for example, passage of heat from one blood

vessel to another; rete mirabile, the countercurrent

exchange structure of capillaries that allows gas uptake

in a fish swim bladder; the kidney nephron loop, a

tubular section of nephron between the proximal and

distal convoluted tubules where water is conserved and

urine concentrates by a countercurrent exchange sys-

tem; and the upper airway where, on expiration, heat

and moisture are retained and then given up to the rel-

atively cooler and drier inspired gases.

coupled system Two or more processes that affect

one another.

coupling constant(spin-spin coupling constant), J(SI

unit: Hz) A quantitative measure for nuclear spin-

spin, nuclear-electron (hyperfine coupling), and elec-

tron–electron (fine coupling in EPR) coupling in

magnetic resonance spectroscopy. The “indirect” or

scalar NMR coupling constants are in a first approxi-

mation independent of the external magnetic field and

are expressed in units of hertz (Hz).

See alsoELECTRONIC PARAMAGNETIC RESONANCE

SPECTROSCOPY; NUCLEAR MAGNETIC RESONANCE SPEC-

TROSCOPY.

covalent bond A region of relatively high electron

density between nuclei that arises at least partly from

sharing of electrons and gives rise to an attractive force

and characteristic internuclear distance.

See also AGOSTIC; COORDINATION; HYDROGEN

BOND; MULTICENTER BOND.

covalent compounds Compounds held together by

covalent bonds.

Cox-Yates equation A modification of the

BUNNETT-OLSEN EQUATIONof the form

lg ([SH+])/([S])–lg[H+] = m*X+ pKSH+

where Xis the activity function lg(γSγH+)/γSH+) for an

arbitrary reference base. The function Xis called the

62 corrosion