An electrolyte is a substance that ionizes when dissolved in suitable ionizing solvents
such as water.
This includes most soluble salts, acids, and bases.
Some gases, such as hydrogen chloride, under conditions of high temperature or low pressure
can also function as electrolytes.
Electrolyte solutions can also result from the dissolution of some biological and synthetic
polymers, termed polyelectrolytes, which contain charged functional groups.
Electrolyte solutions are normally formed when a salt is placed into a solvent such
as water and the individual components dissociate due to the thermodynamic interactions between
solvent and solute molecules, in a process called solvation.
For example, when table salt, NaCl, is placed in water, the salt dissolves into its component
ions, according to the dissociation reaction NaCl(s) → Na+(aq) + Cl−(aq)
It is also possible for substances to react with water, producing ions.
For example, carbon dioxide gas dissolves in water to produce a solution that contains
hydronium, carbonate, and hydrogen carbonate ions.
Note that molten salts can be electrolytes, as well.
For instance, when sodium chloride is molten, the liquid conducts electricity.
An electrolyte in a solution may be described as concentrated if it has a high concentration
of ions, or dilute if it has a low concentration.
If a high proportion of the solute dissociates to form free ions, the electrolyte is strong;
if most of the solute does not dissociate, the electrolyte is weak.
The properties of electrolytes may be exploited using electrolysis to extract constituent
elements and compounds contained within the solution.
Physiological importance In physiology, the primary ions of electrolytes
are sodium, potassium, calcium, magnesium, chloride, hydrogen phosphate, and hydrogen
carbonate.
The electric charge symbols of plus and minus indicate that the substance is ionic in nature
and has an imbalanced distribution of electrons, the result of chemical dissociation.
Sodium is the main electrolyte found in extracellular fluid and is involved in fluid balance and
blood pressure control.
All known higher lifeforms require a subtle and complex electrolyte balance between the
intracellular and extracellular environment.
In particular, the maintenance of precise osmotic gradients of electrolytes is important.
Such gradients affect and regulate the hydration of the body as well as blood pH, and are critical
for nerve and muscle function.
Various mechanisms exist in living species that keep the concentrations of different
electrolytes under tight control.
Both muscle tissue and neurons are considered electric tissues of the body.
Muscles and neurons are activated by electrolyte activity between the extracellular fluid or
interstitial fluid, and intracellular fluid.
Electrolytes may enter or leave the cell membrane through specialized protein structures embedded
in the plasma membrane called ion channels.
For example, muscle contraction is dependent upon the presence of calcium, sodium, and
potassium.
Without sufficient levels of these key electrolytes, muscle weakness or severe muscle contractions
may occur.
Electrolyte balance is maintained by oral, or in emergencies, intravenous intake of electrolyte-containing
substances, and is regulated by hormones, in general with the kidneys flushing out excess
levels.
In humans, electrolyte homeostasis is regulated by hormones such as antidiuretic hormone,
aldosterone and parathyroid hormone.
Serious electrolyte disturbances, such as dehydration and overhydration, may lead to
cardiac and neurological complications and, unless they are rapidly resolved, will result
in a medical emergency.
Measurement Measurement of electrolytes is a commonly
performed diagnostic procedure, performed via blood testing with ion-selective electrodes
or urinalysis by medical technologists.
The interpretation of these values is somewhat meaningless without analysis of the clinical
history and is often impossible without parallel measurement of renal function.
Electrolytes measured most often are sodium and potassium.
Chloride levels are rarely measured except for arterial blood gas interpretation, since
they are inherently linked to sodium levels.
One important test conducted on urine is the specific gravity test to determine the occurrence
of electrolyte imbalance.
Rehydration In oral rehydration therapy, electrolyte drinks
containing sodium and potassium salts replenish the body's water and electrolyte levels after
dehydration caused by exercise, excessive alcohol consumption, diaphoresis, diarrhea,
vomiting, intoxication or starvation.
Athletes exercising in extreme conditions that do not consume electrolytes risk dehydration.
A home-made electrolyte drink can be made by using water, sugar and salt in precise
proportions.
Electrolytes are commonly found in fruit juices, coconut water, sports drinks, milk, nuts,
and many fruits and vegetables.
Electrochemistry
When electrodes are placed in an electrolyte and a voltage is applied, the electrolyte
will conduct electricity.
Lone electrons normally cannot pass through the electrolyte; instead, a chemical reaction
occurs at the cathode, consuming electrons from the anode.
Another reaction occurs at the anode, producing electrons that are eventually transferred
to the cathode.
As a result, a negative charge cloud develops in the electrolyte around the cathode, and
a positive charge develops around the anode.
The ions in the electrolyte neutralize these charges, enabling the electrons to keep flowing
and the reactions to continue.
For example, in a solution of ordinary table salt in water, the cathode reaction will be
2H2O + 2e− → 2OH− + H2 and hydrogen gas will bubble up; the anode
reaction is 2NaCl → 2 Na+ + Cl2 + 2e−
and chlorine gas will be liberated.
The positively charged sodium ions Na+ will react toward the cathode, neutralizing the
negative charge of OH− there, and the negatively charged hydroxide ions OH− will react toward
the anode, neutralizing the positive charge of Na+ there.
Without the ions from the electrolyte, the charges around the electrode would slow down
continued electron flow; diffusion of H+ and OH− through water to the other electrode
takes longer than movement of the much more prevalent salt ions.
Electrolytes dissociate in water because water molecules are dipoles and the dipoles orient
in an energetically favorable manner to solvate the ions.
In other systems, the electrode reactions can involve the metals of the electrodes as
well as the ions of the electrolyte.
Electrolytic conductors are used in electronic devices where the chemical reaction at a metal/electrolyte
interface yields useful effects.
In batteries, two materials with different electron affinities are used as electrodes;
electrons flow from one electrode to the other outside of the battery, while inside the battery
the circuit is closed by the electrolyte's ions.
Here, the electrode reactions convert chemical energy to electrical energy.
In some fuel cells, a solid electrolyte or proton conductor connects the plates electrically
while keeping the hydrogen and oxygen fuel gases separated.
In electroplating tanks, the electrolyte simultaneously deposits metal onto the object to be plated,
and electrically connects that object in the circuit.
In operation-hours gauges, two thin columns of mercury are separated by a small electrolyte-filled
gap, and, as charge is passed through the device, the metal dissolves on one side and
plates out on the other, causing the visible gap to slowly move along.
In electrolytic capacitors the chemical effect is used to produce an extremely thin 'dielectric'
or insulating coating, while the electrolyte layer behaves as one capacitor plate.
In some hygrometers the humidity of air is sensed by measuring the conductivity of a
nearly dry electrolyte.
Hot, softened glass is an electrolytic conductor, and some glass manufacturers keep the glass
molten by passing a large current through it.
Solid electrolytes Solid electrolytes can be mostly divided into
three groups: Gel electrolytes - they closely resemble liquid
electrolytes.
In essence, they are liquids in a flexible lattice framework.
Various additives are often applied to increase the conductivity of such systems.
Dry polymer electrolytes - they differ from liquid and gel electrolytes in the sense that
salt is dissolved directly into the solid medium.
Usually it is a relatively high dielectric constant polymer and a salt with low lattice
energy.
In order to increase the mechanical strength and conductivity of such electrolytes, very
often composites are used, and inert ceramic phase is introduced.
There are two major classes of such electrolytes: polymer-in-ceramic, and ceramic-in-polymer.
Solid ceramic electrolytes - ions migrate through the ceramic phase by means of vacancies
or interstitials within the lattice.
There are also glassy-ceramic electrolytes.
See also Strong electrolyte
ITIES References
External links electrolyte mixtures