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An electrolyte is a substance containing free ions which behaves as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions, but molten electrolytes and solid electrolytes are also possible. They are sometimes referred to in abbreviated jargon as lytes.

An electrolytic process is the use of electrolysis industrially to refine metals or compounds at a high purity and low cost. Some examples are the Hall-Héroult process used for aluminium, or the production of hydrogen from water. Electrolysis is usually done in bulk using hundreds of sheets of metal connected to an electric power source. These sheets are then lowered into a solution such as copper, the ions from the copper solution get attracted to the positive sheet of metal and forms 99.999% pure copper.

Explanation

Electrolytes commonly exist as solutions such as acids, bases or salts. Furthermore, some gases may act as electrolytes under conditions of high temperature or low pressure.

Electrolytes are normally formed when a salt is placed into a solvent such as water and the individual atomic components are separated by the force applied upon the solute molecule, in a process called chemical dissociation in which the solution applies force to hold the ions apart. Salts are compounds that are linked by weak ionic bonds, and will separate into charged ions in the presence of a solution containing stronger covalent bonds.

An electrolyte 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 solution is strong; if most of the solute does not dissociate, the solution 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 (Ca++), magnesium (Mg++), chloride (Cl-), phosphate (PO4---), and bicarbonate (HCO3-). The electric charge symbols of plus (+) and minus (-) are used to indicate that the substance indicated is ionic in nature and has an imbalanced distribution of electrons. This is the result of chemical dissociation.

All 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, blood pH, and are critical for nerve and muscle function.

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 (Ca++), sodium (Na+), and potassium (K+). (See muscle contraction) Without sufficient levels of these key electrolytes, muscle weakness or severe muscle contractions may occur.

Electrolyte balance is maintained by oral, or in emergencies, intervenous (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 or urinalysis. 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, as 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.

Nutritional significance

In oral rehydration therapy, electrolyte drinks containing sodium and potassium salts are used to replenish the body's water and electrolyte levels after dehydration caused by exercise, diaphoresis, diarrhea, vomiting or starvation. Giving pure water to such a person is not the best way to restore fluid levels, because it dilutes the salts inside the body's cells and interferes with their chemical functions. This can lead to water intoxication.

Sports drinks such as Gatorade or Lucozade are electrolyte drinks with large amounts of added carbohydrates, such as glucose, to provide energy. The drinks commonly sold to the public are isotonic (with osmolality close to that of blood), with hypotonic (with a lower osmolality) and hypertonic (with a higher osmolality) varieties available to athletes, depending on their nutritional needs.[1] (http://www.disen.org/nutrition/pages-to-edit/fluids.htm)

Because sports drinks contain very high levels of sugar, they are not recommended for regular use by children. Rather, specially-formulated pediatric electrolyte solutions are recommended. Sports drinks are also not appropriate for replacing the fluid lost during diarrhea. The role of sports drinks are to inhibit electrolyte loss, but are insufficient to restore imbalance once it occurs. Medicinal rehydration sachets and drinks are available to replace the key electrolyte ions lost. Dentists recommend that regular consumers of sports drinks observe precautions against tooth decay.

Electrolyte and sports drinks can be home-made by using the correct proportions of sugar, salt and water. [2] (http://www.webmd.com/hw/health_guide_atoz/str2254.asp?navbar=hw86827)

Electrolytes in electrochemistry

When two 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 cathode, and another reaction occurs at the anode producing electrons to be taken up by the anode. 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 move to neutralize these charges so that the reactions can continue and the electrons can keep flowing.

For example, in a dilute solution of ordinary 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

2H₂O → O₂ + 4H⁺ + 4e⁻

and oxygen gas will be liberated. The positively charged sodium ions Na⁺ will move towards the cathode neutralizing the negative charge of OH⁻ there, and the negatively charged chlorine ions Cl⁻ will move towards the anode neutralizing the positive charge of H⁺ 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.

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 slowly use up the chemical energy stored in the electrolyte.
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 electric current through it.

Electrolysis

In chemistry and manufacturing, electrolysis is a method of separating bonded elements and compounds by passing an electric current through them.

Overview

An ionic compound is dissolved with an appropriate solvent, or otherwise melted by heat, so that its ions are available in the liquid. An electrical current is applied between a pair of inert electrodes immersed in the liquid. The negatively charged electrode is called the cathode, and the positively charged one the anode.

Each electrode attracts ions which are of the opposite charge. Therefore, positively charged ions (called cations) move towards the cathode, while negatively charged ions (termed anions) move toward the anode. The energy required to separate the ions, and cause them to gather at the respective electrodes, is provided by an electrical power supply. At the probes, electrons are absorbed or released by the ions, forming a collection of the desired element or compound.

The amount of electrical energy that must be added equals the change in Gibbs free energy of the reaction plus the losses in the system. The losses can (theoretically) be arbitrarily close to zero, so the maximum thermodynamic efficiency equals the enthalpy change divided by the free energy change of the reaction. In most cases the electric input is larger than the enthalpy change of the reaction, so some energy is released in the form of heat. In some cases, for instance in the electrolysis of steam into hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the surroundings, and the heating value of the produced hydrogen is higher than the electric input. In this case the efficiency can be said to be greater than 100%. (It is worth noting that the maximum theoretic efficiency of a fuel cell is the inverse of that of electrolysis. It is thus impossible to create a perpetual motion machine by combining the two processes. See water fuel cell for an example of such an attempt.)

In electrolysis, the anode is the positive electrode, meaning it has a deficit of electrons; species in contact with the anode can be stripped of electrons (i.e., they are oxidized). The cathode is the negative electrode, meaning it has a surplus of electrons. Species in contact with the cathode tend to gain electrons (i.e., they are reduced).

A higher current flow (amperage) through the cell means it will be passing more electrons through it at any given time. This means a faster rate of reduction at the cathode and a faster rate of oxidation at the anode. This corresponds to a greater number of moles of product. The amount of current that passes depends on the conductance of the electrodes and electrolyte, though it also depends on how much current the power source itself can generate. Current also makes a difference in that it can shift chemical equilibria by sheer mass action. The processes in an electrolytic cell with just two or three reactants can become very, very complex. Most of the time it's best to search the literature to see what current density works best for a desired process. For instance, metals plated at a certain current density might form a durable and shiny coating on the substrate, while some other current density might form an excessively grainy, dull coating.

A higher potential difference (voltage) applied to the cell means the cathode will have more energy to bring about reduction, and the anode will have more energy to bring about oxidation. Higher potential difference enables the electrolytic cell to oxidize and reduce energetically more "difficult" compounds. This can drastically change what products will form in a given experiment. On a practical level, both current and voltage determine what will form in a cell.

The following technologies are related to electrolysis:

Electrochemical cells, including the hydrogen fuel cell, use the reverse of this process.
Gel electrophoresis is an electrolysis where the solvent is a gel: it is used to separate substances, such as DNA strands, based on their electrical charge.

Electrolysis of water

Electrolysis of water is an electrolytic process which decomposes water into oxygen and hydrogen gas with the aid of an electric current, where a power source from a 6 volt battery is commonly used. The electrolysis cell consists of two electrodes (usually an inert metal such as platinum) submerged in an electrolyte and connected to opposite poles of a source of direct current.

One important use of electrolysis is to produce hydrogen. The reaction that occurs is

2H₂O_(aq) → 2H_2(g) + O_2(g)

This has been suggested as a way of shifting society toward using hydrogen as an energy carrier for powering electric motors and internal combustion engines. (See hydrogen economy.) Electrolysis of water can be achieved in a simple hands-on project, where electricity from a battery or low-voltage DC power supply (e.g. computer power supply 5 volt rail) is passed through a cup of water (in practice a saltwater solution or other electrolyte will need to be used otherwise no result will be observed). Using platinum electrodes, hydrogen gas will be seen to bubble up at the cathode, and oxygen will bubble at the anode. If, however, any other metal is utilised for the anode the oxygen will react with the anode instead of being released as a gas. For example using iron electrodes in a sodium chloride solution electrolyte, iron oxide will be produced at the anode, which will react to form iron hydroxide. When producing large quantites of hydrogen, this can significantly contaminate the electrolytic cell - which is why iron is not used for commercial electrolysis.

The energy efficiency of water electrolysis varies widely. The efficiency is a measure of what fraction of electrical energy used is actually contained within the hydrogen. Some of the electrical energy is converted to heat, a useless by-product. Some reports quote efficiencies between 50–70%[3] (http://www.hyweb.de/Knowledge/w-i-energiew-eng3.html) This efficiency is based on the Lower Heating Value of Hydrogen. The Lower Heating Value of Hydrogen is thermal energy released when Hydrogen is combusted. This does not represent the total amount of energy within the Hydrogen, hence the efficiency is lower than a more strict definition. Other reports quote the theoretical maximum efficiency of electrolysis. The theoretical maximum efficiency is between 80–94%.[4] (http://bellona.org/filearchive/fil_Hydrogen_6-2002.pdf). The theoretical maximum considers the total amount of energy absorbed by both the hydrogen and oxygen. These values only refer to the efficiency of converting electrical energy into hydrogen's chemical energy. The energy lost in generating the electricity is not included. For instance, when considering a power plant that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency is more like 25–40%.[5] (http://www.uic.com.au/nip73.htm)

The electric current disassociates water molecule into hydroxide $(OH^{-})\,$ and hydrogen $H^{+}\,$ ions.

In the electrolytic cell, at the cathode, hydrogen ions accept electrons in a reduction reaction that forms hydrogen gas:

$\mbox{Cathode (reduction): }2H_{2}O(l) + 2e^{-} \rightarrow H_{2}(g) + 2OH^{-}(aq)\,$

At the anode, hydroxide ions undergo an oxidation reaction and give up electrons to the anode to complete the circuit and form water and oxygen gas:

$\mbox{Anode (oxidation): }2H_{2}O(l) \rightarrow O_{2}(g) + 4H^{+}(aq) + 4e^{-}\,$

hence decomposing water into Oxygen and Hydrogen;

$\mbox{Overall reaction: }2H_{2}O(l) \rightarrow 2H_{2}(g) + O_{2}(g)\,$

The volume of hydrogen gas produced is therefore twice the amount of oxygen gas. Assuming equal temperature and pressure for both gases, the hydrogen gas has twice the volume of the oxygen.

Spontaneity of the process

Decomposition of water into hydrogen and oxygen at standard temperature and pressure is not favorable in thermodynamical terms as half reactions standard potential are negative values,

$\mbox{Anode (oxidation): }2H_{2}O(l) \rightarrow O_{2}(g) + 4H^{+}(aq) + 4e^{-}\qquad E^{o}_{red}=-1.23 V\,$

$\mbox{Cathode (reduction): }2H_{2}O(g) + 2e^{-} \rightarrow H_{2}(g) + 2OH^{-}(aq)\qquad E^{o}_{red}=-0.83 V\,$

, on the other hand Gibbs free energy for the process at standard conditions is a higher positive value, about $474.4 kJ\,$ . Those considerations makes the process "impossible" to occur without adding electrolytes in the solution.

Electrolyte selection

As pure water conducts electricity very poorly, a water-soluble electrolyte must be added to the electrolysis cell to close the circuit. The electrolyte dissolves and disassociates into cations and anions (positive and negative ions) that carry the current. Electrolytes are normally acids, bases, or salts.

Care must be taken in choosing an electrolyte, since an anion from the electrolyte is in competition with the hydroxide ions to give up an electron. An electrolyte anion with less standard electrode potential than hydroxide will be oxidized instead of the hydroxide, and no oxygen gas will be produced. A cation with a greater standard electrode potential than a hydrogen ion will be reduced in its stead, and no hydrogen gas will be produced.

The following cations have lower electrode potential than H⁺ and are therefore suitable for use as electrolyte cations: Li⁺, Rb⁺, K⁺, Cs⁺, Ba²⁺, Sr²⁺, Ca²⁺, Na⁺, and Mg²⁺. Sodium and lithium are frequently used, as they form inexpensive, soluble salts.

If an acid is used as the electrolyte, the cation is H⁺, and there is no competitor for the H⁺ created by disassociating water.

The most commonly used anion is SO₄^2-, as it is very difficult to oxidize.

Standard potential for oxidation of this ion to the peroxydisulfate ion is −2.05 volts.

$\mbox{Anode (oxidation): } 2 SO^{2-}_{4} \rightarrow S_{2}O^{2-}_{8} + 2e^{-} \qquad E^{o}_{ox}=-2.05 V \,$

Strong acids such as Sulfuric acid (H₂SO₄) are frequently used as electrolytes.

Examples

Electrolysis of an aqueous solution of table salt (NaCl, or sodium chloride) produces aqueous sodium hydroxide and chlorine, although usually only in minute amounts. NaCl(aq) can be reliably electrolysed to produce hydrogen. In order to produce chlorine commercially, molten sodium chloride is electrolysed to produce sodium metal and chlorine gas. These will react violently, so a mercury cell is used to ensure they do not come into contact with each other.

Techniques

A reaction is simple to replicate. Two leads running from the terminals of a battery into a cup of water and electrolyte are sufficient to produce a visible stream of oxygen or hydrogen bubbles at either electrode. The presence of hydroxide (OH^-) ions can be detected with a pH indicator such as phenolphthalein or Bromothymol blue.

Hofmann voltameter

The Hofmann voltameter is often used as a small-scale electrolytic cell. It consists of three joined upright cylinders. The inner cylinder is open at the top to allow the addition of water and the electrolyte. A platinum electrode is placed at the bottom of each of the two side cylinders, connected to the positive and negative terminals of a source of electricity. When current is run through the hofmann voltameter, gaseous oxygen forms at the anode and gaseous hydrogen at the cathode. Each gas displaces water and collects at the top of the two outer tubes, where it can be drawn off with a stopcock.

Industrial electrolysis

Many industrial electrolysis cells are very similar to Hofmann voltameters, with complex platinum plates or honeycombs as electrodes. Hydrogen gas is usually created, collected, and burned on the premises, as its energy density per volume is too low to make transporting or storing it economically feasible. Oxygen gas is treated as a byproduct.

High-temperature electrolysis

High-temperature electrolysis (also HTE or steam electrolysis) is a method currently being investigated for the production of hydrogen from water with oxygen as a by-product. High-temperature electrolysis (also HTE or steam electrolysis) is a method currently being investigated for water electrolysis with a heat engine. High temperature electrolysis is more efficient than traditional room-temperature electrolysis because some of the energy is supplied as heat, which is cheaper than electricity, and because the electrolysis reaction is more efficient at higher temperatures.

Efficiency

High temperature electrolysis is more efficient than traditional room-temperature electrolysis because some of the energy is supplied as heat, which is cheaper than electricity, and because the electrolysis reaction is more efficient at higher temperatures. In fact, at 2500°C, electrical input is unnecessary because water breaks down to hydrogen and oxygen through thermolysis. Such temperatures are impractical; proposed HTE systems operate at 100 to 850°C.

The efficiency improvement of high-temperature electrolysis is best appreciated by assuming the electricity used comes from a heat engine, and then considering the amount of heat energy necessary to produce one kg hydrogen (145 megajoules), both in the HTE process itself and also in producing the electricity used. At 100°C, 350 megajoules of thermal energy are required (41% efficient). At 850°C, 225 megajoules are required (64% efficient).

Economic potential

HTE does not provide a means to bypass the inherent inefficiency of a heat engine, by producing hydrogen which is then converted back to electricity in a fuel cell. (Any such efficiency improvement would allow the theoretical construction of a perpetual motion machine, which is impossible.) Thus any economic advantage to be gained from using HTE must come from supplying chemical processes which use hydrogen as a feedstock and not as a power source (such as the petrochemical or fertilizer industries), or motive processes for which hydrogen is a better energy carrier than electricity (rockets are an example, cars are not yet an example).

High-temperature electrolysis cannot compete with the chemical conversion of hydrocarbon or coal energy into hydrogen, as none of those conversions are limited by heat engine efficiency. Thus the possible supplies of cheap high-temperature heat for HTE are all nonchemical, including nuclear reactors, concentrating solar thermal collectors, and geothermal sources.

Electrolysis and thermodynamics

During electrolysis, the amount of electrical energy that must be added equals the change in Gibbs free energy of the reaction plus the losses in the system. The losses can (theoretically) be arbitrarily close to zero, so the maximum thermodynamic efficiency of any electrochemical process equals 100%. In practice, the efficiency is given by electrical work achieved divided by the Gibbs free energy change of the reaction.

In most cases, such as room temperature water electrolysis, the electric input is larger than the enthalpy change of the reaction, so some energy is released as waste heat. In some other cases however, for instance in the electrolysis of steam into hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the surroundings, and the heating value of the produced hydrogen is higher than the electric input. In this case the efficiency relative to electric energy input can be said to be greater than 100%. It is worth noting that the maximum theoretic efficiency of a fuel cell is the inverse of that of electrolysis. It is thus impossible to create a perpetual motion machine by combining the two processes.

General energy efficiency

The energy efficiency of water electrolysis varies widely. Some report 50–70%[6] (http://www.hyweb.de/Knowledge/w-i-energiew-eng3.html), while others report 80–94%.[7] (http://www.bellona.no/en/energy/hydrogen/report_6-2002/22871.html) These values refer only to the efficiency of converting electrical energy into hydrogen's chemical energy. The energy lost in generating the electricity is not included. For instance, when considering a power plant that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency is more like 25–40%.[8] (http://www.uic.com.au/nip73.htm)

Laws of electrolysis

In 1832, Michael Faraday reported that the quantity of elements separated by passing an electrical current through a molten or dissolved salt was proportional to the quantity of electric charge passed through the circuit. This became the basis of the first law of electrolysis. Faraday also discovered that the mass of the resulting separated elements was directly proportional to the atomic masses of the elements when an appropriate integral divisor was applied. This provided strong evidence that discrete particles of electricity existed as parts of the atoms of elements.

Experimenters

Scientific pioneers of electrolysis included:

More recently, electrolysis of heavy water was performed by Fleischmann and Pons in their famous experiment, resulting in anomalous heat generation and the controversial claim of cold fusion.

Applications

About four percent of hydrogen gas produced worldwide is created by electrolysis, and normally used onsite. Hydrogen is used for the creation of ammonia for fertilizer via the Haber process, and converting heavy petroleum sources to lighter fractions via hydrocracking. There is some speculation about future development of hydrogen as an energy carrier.

The market for hydrogen production

Given a cheap, high-temperature heat source, other hydrogen production methods are possible. In particular, see the thermochemical sulfur-iodine cycle. Thermochemical production might reach higher efficiencies than HTE because no heat engine is required. However, large-scale thermochemical production will require significant advances in materials that can withstand high-temperature, high-pressure, highly-corrosive environments.

The market for hydrogen is large (50 million metric tons/year in 2004, worth about $135 billion/year) and growing at about 10% per year (see hydrogen economy). The two major consumers are currently oil refineries and fertilizer plants (each consume about half of all production). Should hydrogen-powered cars become widespread, their consumption would greatly increase the demand for hydrogen.

Industrial uses

Manufacture of aluminium, lithium, sodium, potassium, aspirin.
Manufacture of hydrogen for hydrogen cars and fuel cells.
High-temperature electrolysis is also being used for this.
Coulometric techniques can be used to determine the amount of matter transformed during electrolysis by measuring the amount of electricity required to perform the electrolysis.
Manufacture of chlorine and sodium hydroxide.
Manufacture of sodium and potassium chlorate.
Manufacture of perfluorinated organic compounds like trifluoroacetic acid.

Military uses

As well as producing hydrogen, electrolysis also produces oxygen. Nuclear submarines are able to generate breathable oxygen from the water around them, so can remain underwater for as long as their fuel lasts. Space stations can also use electrolysis to produce amounts of extra oxygen from waste water or surplus water produced from the Space Shuttle fuel cells. Both these applications depend on having an abundant electrical supply, from either the reactor or solar panels.

Related concepts

External articles and references

Fluids and Hydration in Sport (http://www.disen.org/nutrition/pages-to-edit/fluids.htm) - includes a discussion of the role of hypotonic, isotonic and hypertonic drinks.
Wikipedia contributors (http://en.wikipedia.org/wiki/Special:Recentchanges), Wikipedia: The Free Encyclopedia. Wikimedia Foundation. <http://en.wikipedia.org>.
Electrolysis of Water (http://www.pc.chemie.uni-siegen.de/pci/versuche/english/v21-2.html), Experiments on Electrochemistry
Electrolysis of Water (http://chemmovies.unl.edu/Chemistry/DoChem/DoChem044.html), Do Chem 044
U.S. DOE factsheet for high-temperature electrolysis (http://www.ne.doe.gov/hydrogen/HTE.pdf)

See also

Directory:Electrolysis

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