Electrolyte

An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water. The dissolved electrolyte separates into cations and anions, which disperse uniformly through the solvent. Electrically, such a solution is neutral. If an electrical potential (voltage) is applied to such a solution, the cations of the solution would be drawn to the electrode that has an abundance of electrons, while the anions would be drawn to the electrode that has a deficit of electrons. The movement of anions and cations in opposite directions within the solution amounts to a current. 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 (e.g., DNA, polypeptides) and synthetic polymers (e.g., polystyrene sulfonate), termed polyelectrolytes, which contain charged functional groups. A substance that dissociates into ions in solution acquires the capacity to conduct electricity. Sodium, potassium, chloride, calcium, and phosphate are examples of electrolytes, informally known as lytes.

In medicine, electrolyte replacement is needed when a patient has prolonged vomiting or diarrhea, and as a response to strenuous athletic activity. Commercial electrolyte solutions are available, particularly for sick children (solutions such as Pedialyte) and athletes (sports drinks, such as Gatorade). Electrolyte monitoring is important in treatment of anorexia and bulimia.

Etymology

The word "electrolyte" derives from the Greek lytós, meaning "able to be untied or loosened".

History

Svante Arrhenius put forth, in his 1884 dissertation, his explanation of the fact that solid crystalline salts disassociate into paired charged particles when dissolved, for which he would win the 1903 Nobel Prize in Chemistry.[1][2][3][4]

Arrhenius' explanation was that in forming a solution, the salt dissociates into charged particles, to which Michael Faraday had given the name "ion" many years earlier. Faraday's belief had been that ions were produced in the process of electrolysis. Arrhenius proposed that, even in the absence of an electric current, solutions of salts contained ions. He thus proposed that chemical reactions in solution were reactions between ions.[2][3][4]

Formation

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 (sodium chloride), 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. For example, carbon dioxide gas dissolves in water to produce a solution that contains hydronium, carbonate, and hydrogen carbonate ions.

Molten salts can also be electrolytes, as for example, when sodium chloride is molten, the liquid conducts electricity. In particular, ionic liquids, which are molten salts with melting points below 100 °C, are a type of highly conductive non-aqueous electrolytes and thus have found more and more applications in fuel cells and batteries.[5]

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 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 (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, 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 (heavy sweating), diarrhea, vomiting, intoxication or starvation. Athletes exercising in extreme conditions (for three or more hours continuously, e.g. marathon or triathlon) that do not consume electrolytes risk dehydration (or hyponatremia).[6]

A home-made electrolyte drink can be made by using water, sugar and salt in precise proportions.[7] Pre-made preparations are also available commercially,[8] and also for veterinary use, like Electrovite.

Electrolytes are commonly found in fruit juices, coconut water, sports drinks, milk, nuts, 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 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.

Solid electrolytes

Solid electrolytes can be mostly divided into four groups:

See also

References

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  6. J,Estevez E,Baquero E,Mora-Rodriguez R (2008). "Anaerobic performance when rehydrating with water or commercially available sports drinks during prolonged exercise in the heat". Applied Physiology, Nutrition and Metabolism 33 (2): 290–298. doi:10.1139/H07-188. PMID 18347684.
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  8. "Oral Rehydration Salt Suppliers". Rehydrate.org. 2014-10-07. Retrieved 2014-12-04.
  9. 1 2 Kamil Perzyna, Regina Borkowska, Jaroslaw Syzdek, Aldona Zalewska, Wladyslaw Wieczorek (2011). "The effect of additive of Lewis acid type on lithium–gel electrolyte characteristics". Electrochimica Acta 57: 58–65. doi:10.1016/j.electacta.2011.06.014.
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  12. Syzdek Jaroslaw; et al. (2007). Journal of Power Sources 173: 712–720. doi:10.1016/j.jpowsour.2007.05.061. Missing or empty |title= (help)
  13. Syzdek Jaroslaw; et al. (2010). Electrochimica Acta 55: 1314–1322. doi:10.1016/j.electacta.2009.04.025. Missing or empty |title= (help)
  14. Syzdek Jaroslaw; et al. (2009). Journal of Power Sources 194: 66–72. doi:10.1016/j.jpowsour.2009.01.070. Missing or empty |title= (help)
  15. Jiangshui Luo, Olaf Conrad and Ivo F. J. Vankelecom (2013). "Imidazolium methanesulfonate as a high temperature proton conductor". Journal of Materials Chemistry A 1. doi:10.1039/C2TA00713D.

External links

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