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Ion Pairs

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Last updated date: 28th Feb 2024
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What are Ion Pairs?

In Physics and Chemistry, an ion pair is a duplex of charged particles (typically charged atoms or molecules), one positive and the other negative. For the physicist, an ion pair is the positively charged particle (positive ion) and the negatively charged particle (negative ion) produced simultaneously by adding enough energy to a neutral atom or molecule to cause it to dissociate into oppositely charged fragments.


Thus, an energetic electron passing near or through an oxygen molecule, $${{O}_{2}}$$, may force one of the molecule's electrons out. As a result, an ion pair is formed consisting of the positive oxygen ion, $${{O}^{2+}}$$, and the negative detached electron, $${{e}^{-}}$$.


Ion Pair Chromatography Principle

Ion chromatography principle is a technique for separating hydrophilic or charged analytes on columns using stationary phases that are reversed phase or “neutral”. It entails changing the polarity of the charged analytes via interaction with an ion-pairing reagent added to the mobile phase. These reagent molecules have charges that are diametrically opposed to those of the analyte ions with which they can form electrostatic bonds. The analyte-reagent ion pairs behave like neutral, hydrophobic moieties that can be separated on C18 or C8 columns. IPC is used to separate polar organic acids, bases, zwitterions, and inorganic ions.


Ion pairing reagents can also be referred to as ion pairing additives or hetaerons. These molecules resemble soap because they have a polar head group and hydrophobic hydrocarbon chains. As a result, when Göran Schill introduced this technique in 1973, it was dubbed "soap chromatography". It is also known as ion interaction chromatography because the reagent ion interacts with the stationary phase to control the retention of ions in the sample.


Ionic Compounds Definition

Ions of opposite charge are neatly packed together to form crystalline solids. Nonmetals and metals react to form an ionic compound. In other words, ionic compounds are those that are held together by ionic bonds. Elements can lose or gain electrons to achieve their closest noble gas configuration. The formation of ions for the completion of the octet (either by gaining or losing electrons), will give them stability.


Metals generally lose electrons to complete their octet in a reaction with nonmetals, while nonmetals gain electrons to complete their octet. Ionic compounds are formed when nonmetals and metals react.


Ionic Bond Formation

Ionic bonding can occur as a result of a redox reaction in which atoms of an element (usually metal) with a low ionisation energy give up some of their electrons in order to achieve a stable electron configuration. Cations are formed as a result. An atom of another element (usually a nonmetal) with a higher electron affinity accepts one or more electrons to achieve a stable electron configuration, and the atom becomes an anion after accepting electrons. For elements in the s-block and p-block, the stable electron configuration is typically one of the noble gases, with specific stable electron configurations for d-block and f-block elements.


The electrostatic attraction between anions and cations causes the formation of a solid with a crystallographic lattice in which the ions alternately stack. Since it is usually impossible to distinguish discrete molecular units in such a lattice, the compounds formed are not molecular in nature. The ions themselves, on the other hand, can be complex and form molecular ions such as the acetate anion or the ammonium cation.


Ionic Bond Examples

Below are some examples of Ionic bonds:

  • Lithium Chloride (LiCl)

  • Sodium Chloride (NaCl)

  • Potassium Chloride (KCl)

  • Cesium Chloride (CsCl)

  • Lithium Hydroxide (LiOH)

  • Silver Iodide (AgI)

  • Silver Hydroxide (AgOH)

  • Zinc Sulphide (ZnS)

Important Questions

1.What are the properties of ionic bonds?

Ans. The following properties are observed in ionic bonded molecules due to the presence of a strong force of attraction between cations and anions:

  • Ionic bond is the most powerful of all bonds.

  • Since ionic bonds have charge separation, they are the most reactive of all the bonds in the appropriate medium.

  • The boiling and melting points of ionic bonded molecules are extremely high.

  • Ionic bonded molecules in aqueous solutions or molten state are excellent conductors of electricity. This is because of the presence of ions, which act as charge carriers.

2. What are the solvents used in chromatography?

In most cases, liquid solvents are used in chromatography. Common liquid solvents used in fast protein liquid chromatography (FPLC), high-performance liquid chromatography (HPLC), and liquid chromatography-mass spectrometry include water, methanol, isopropanol, acetonitrile, and formic acid (LC-MS). These chromatography solvents extract, dissolve, and move samples without altering their chemical structure permanently, making them an essential component of standard separation techniques. Solvents are frequently used in conjunction with water or another solvent. However, some solvents are not miscible and must be used as a pure reagent. Only other polar solvents will dissolve water and other polar solvents.


Key Features

  • An ion pair is a duplex of charged particles (typically charged atoms or molecules), one positive and the other negative.

  • Ion chromatography (IPC) is a technique for separating hydrophilic or charged analytes on columns using reversed phase or "neutral" stationary phases that do not carry charges.

  • Ions of opposite charge are neatly packed together to form crystalline solids. Nonmetals and metals typically react to form ionic compounds. In other words, ionic compounds are those that are held together by ionic bonds.

Multiple Choice Questions

1.Which is true for ion pairs?

(a) Bigger organic anion and smaller cation

(b) Close association of cation and anion

(c) Non polar solvents

(d) All of the above

Answer: (b)

2.Which of the following types of chromatography involves the movement of the mobile phase through the stationary phase via gravity or capillary action?

(a) Column Chromatography

(b) High-Pressure Liquid Chromatography

(c) Gas Chromatography

(d) Paper Chromatography

Answer: (a)

Competitive Exams after 12th Science

FAQs on Ion Pairs

1.What is the structure of an ionic bond?

In the solid state, ionic compounds form lattice structure. The relative sizes and charges of the ions are the two most important factors in determining the lattice shape. Many alkali halides and binary oxides, such as magnesium oxide, adopt certain structures. For example, the structure of the salt (sodium chloride) is also adopted by many alkali halides and binary oxides. Pauling's rules provide guidelines for predicting and rationalising ionic crystal structures.

2.What is the application of chromatography?

Chromatography is most commonly used in bio analytical chemistry to separate, isolate, and purify proteins from complex sample matrices. Proteins, for example, coexist in cells with a wide range of other compounds, including lipids and nucleic acids. To be studied, these proteins must be isolated from the rest of the cell. It is possible that the proteins of interest will need to be separated from other proteins and purified further.


A variety of chromatographic techniques are used in protein purification and analysis. They are classified in accordance with the physical principle at work in the separation process. Some examples include reversed phase chromatography, ion exchange chromatography, affinity chromatography, and size exclusion chromatography.

3. What is the history of chromatography?

Mikhail Tsvet, an Italian-born scientist, invented chromatography in Russia in 1900. In the first decade of the twentieth century, he developed the technique and coined the term chromatography, primarily for the separation of plant pigments such as chlorophyll, carotenes, and xanthophylls. Because these components separate into different coloured bands (green, orange, and yellow, respectively), they directly inspired the technique's name. During the 1930s and 1940s, new types of chromatography were developed, making the technique useful for a wide range of separation processes.