Electrolyte
From Wikipedia, the free encyclopedia
This article is about the ionic solution. For the R.E.M. song, see
Electrolite.
In
chemistry, an
electrolyte is any substance containing free
ions that make the substance
electrically conductive. The most typical electrolyte is an
ionic solution, but molten electrolytes and
solid electrolytes are also possible.
Commonly, electrolytes are solutions of acids, bases or salts. Furthermore, some gases may act as electrolytes under conditions of high temperature or low pressure. Electrolyte solutions can also result from the dissolution of some biological (e.g., DNA, polypeptides) and synthetic polymers (e.g., polystyrene sulfonate), 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 (a solid) 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, e.g., carbon dioxide gas dissolves in water to produce a solution which 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 (Na+),
potassium (K+),
calcium (Ca2+),
magnesium (Mg2+),
chloride (Cl−),
hydrogen phosphate (HPO42−), and
hydrogen carbonate (HCO3−). The electric charge symbols of plus (+) and minus (−) indicate that the substance in question is ionic in nature and has an imbalanced distribution of electrons, the result of
chemical dissociation.
All known higher lifeforms require a subtle and complex electrolyte balance between the
intracellular and
extracellular milieu. 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 (Ca2+), sodium (Na+), and potassium (K+). 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 (IV) intake of electrolyte-containing substances, and is regulated by
hormones, generally 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 (for three or more hours continuously e.g.
marathon or
triathlon) who do not consume electrolytes risk dehydration (or
hyponatremia).
[1]
A simple electrolyte drink can be home-made by using the correct proportions of water, sugar, salt, salt substitute for potassium, and baking soda.
[2]
Electrolytes are commonly found in
fruit juices,
coconut water,
sports drinks, milk, and many fruits and vegetables (whole or in juice form) (e.g.
potatoes,
avocados).
Electrochemistry
Main article:
electrolysis
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 (
sodium chloride, NaCl) 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 towards the cathode neutralizing the negative charge of OH− there, and the negatively charged hydroxide ions OH− will react towards 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.
Also: 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 metals 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.
Dry electrolyte
Dry electrolytes are essentially gels in a flexible lattice framework.
[3]
OK, that's clear as mud.
Thanks Andy, I'm all sorted now.
