Dictionary of Chemistry [6th Ed.]

Ürst sheet in the solution. In the

Helmholtz model the double layer is

regarded as consisting of two planes

of charge, with the inner plane of

ions from the solution being caused

by the charge on the electrode and

the outer plane being caused by op-

positely charged ions in the solution

responding to theÜrst layer of ions.

In the *Gouy–Chapman model (dif-

fuse double layer) thermal motion of

ions is taken into account. Neither

model is completely successful since

the Helmholtz model exaggerates the

rigidity of the structure of the

charges and the Gouy–Chapman

model underestimates the rigidity of

the structure. The Stern model im-

proves on both models by assuming

that the ions next to the electrode

have a rigid structure, while taking

the second layer to be as described

by the Gouy–Chapman model.

electric-arc furnace A furnace

used in melting metals to make al-

loys, especially in steel manufacture,

in which the heat source is an elec-

tric arc. In the direct-arc furnace,

such as the Héroult furnace, an arc is

formed between the metal and an

electrode. In the indirect-arc furnace,

such as the Stassano furnace, the arc

is formed between two electrodes

and the heat is radiated onto the

metal.

electrochemical cell See cell.

electrochemical equivalent

Symbol z. The mass of a given el-

ement liberated from a solution of its

ions in electrolysis by one coulomb

of charge. See faraday’s laws.

electrochemical series See elec-

tromotive series.

electrochemistry The study of

chemical properties and reactions in-

volving ions in solution, including

electrolysis and electric cells.

electrochromatography See elec-

trophoresis.

electrocyclic reaction A type of

cyclic rearrangement in which a

sigma bond is formed between two

terminal carbon atoms of a conju-

gated molecule, resulting in a de-

crease of one in the number of pi

bonds present.

electric-arc furnace 194

e

Electrocyclic reaction

electrode 1.A conductor that

emits or collects electrons in a cell,

thermionic valve, semiconductor de-

vice, etc. The anodeis the positive

electrode and the cathodeis the neg-

ative electrode. 2.See half cell.

electrodeposition The process of

depositing one metal on another by

electrolysis, as in *electroforming

and *electroplating.

electrode potential The potential

difference produced between the

electrode and the solution in a *half

cell. It is not possible to measure this

directly since any measurement in-

volves completing the circuit with

the electrolyte, thereby introducing

another half cell. Standard electrode

potentialsEŠare deÜned by measur-

ing the potential relative to a stan-

dard *hydrogen half cell using 1.0

molar solution at 25°C. The conven-

tion is to designate the cell so that

the oxidized form is writtenÜrst. For

example,

Pt(s)|H 2 (g)H+(aq)|Zn2+(aq)|Zn(s)

The e.m.f. of this cell is –0.76 volt (i.e.

the zinc electrode is negative). Thus

the standard electrode potential of

the Zn2+|Zn half cell is –0.76 V. Elec-

trode potentials are also called reduc-