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Strong Acid


An acid that is completely dissociated in an aqueous solution and loses a proton, which is captured by water to produce hydronium ion [1].

                             HA (aq) → H+ + A

HA= protonated acid

H+ = free acidic proton

A- = conjugate base

                          HA (aq) + H2O → H3O+(aq) + A(aq)

Strong acid produces weak conjugate bases. For example, when sulfuric acid dissociate which is strong acid it produces HSO4, a conjugate base and gains a negative charge.

                           H2SO4 → H+ + HSO4

Arrhenius Concept of Acids

According to Arrhenius concept (1787):

Acid is a substance which dissociates in aqueous solution to give hydrogen ions. For example, substances such as HCl, HNO3, CH3COOH, HCN, etc. are acids because they ionize in aqueous solution to provide H+ ions.

                              HCl (aq) ⇋ H+ (aq) + Cl(aq)

Bronsted- Lowry Concept

In 1923 the Danish chemist Bronsted and the English chemist Lowry independently present their theories of acids and base on the base of proton transfer. According to this concept:

            An acid is a substance (molecule or ion) that can denote a proton (H+)to another substance.

A base is a substance that can accept a proton (H+)from another substance.

For example, HCl acts as an acid while Nh3 act as a base.

                                 HCl (aq) +   NH3 (aq)    ⇋   NH4+    +   Cl (aq)

Similarly when HCl dissolves in water, HCl act as an acid and H2O as a base.

                             HCl (aq) +   H2O    ⇋            H3O+ (aq) + Cl

Strong acid have small pka (logarithmic constant) and large ka (acid dissociation constant). An acid which have less than -2 pka value they are called as strong acid. The pka value of acid cannot be measured experimentally. The pka value of strong acids as follow:

  • Hydroiodic acid (HI): pka = -9.3
  • Hydrobromic acid (HBr): pka = -8.7
  • Perchloroic acid (HCIO4) : pka ≈ -8
  • Hydrochloric acid (HCl) : pka 1≈ -3 (first dissociation only)
  • Ρ- Toluenesulfonic acid : pka =-2.8
  • Nitric acid (HNO3) : pka ≈ -1.4
  • Chloric acid (HCIO3) : pka ≈ 1.0

Factors of Acid strength determination

There are some factors on which strong acid dissociate in water so well and weak acid do not completely dissociate [2]. These factors are as follow:

Atomic radius

Acidity of an acid increases with the increase in atomic radius. Like: HI is strong acid than HCl.


Acidity of an acid is increases from left to right in the periodic table.

Electrical charge

The atom which have positive charge has higher acidity as compare to the atom which carry negative charge.


An anions which have resonance structure are more acidic in nature than anion which don’t have.


Strong acid follows the following hybridization sequence sp ˃ sp2 ˃ sp3. If arrangement of electron is different it will affect the acidity.


Mostly strong acid dissociate in water but acetic acid ionize completely in liquid ammonia as compare to water, it is weak acid in water.

Hydroiodic Acid

It is a highly acidic aqueous solution of hydrogen and iodide. It is strong acid that ionize completely in an aqueous solution. Hydroiodic acid quickly reacts with oxygen in the air and produce water and iodide. It is also carried out addition reactions with unsaturated hydrocarbons i.e alkene like halogens. Hydroiodic acid is major component of cativa process which is used as co-catalyst for the production of acetic acid through carbonylation of methanol [3]. It is also used as a reducing agent in the production of methamphetamine from pseudoephedrine.

Hydrobromic Acid

It is formed by dissolving hydrogen bromide in water. Due to constant boiling in aqueous solution at 124.3o C it contain 47.6% HBr by mass. It is also known as mineral acid. It is used for the production of inorganic bromides like bromides of calcium, zinc and sodium. It also carried out catalyzation of alkylation and used in extraction of ores. It is main reagent for preparation of organobromine compounds. It is prepared by reacting bromine with either sulfur or phosphorus and water [4]. It can be prepared by diluting sulfuric acid and potassium bromide.

                           H2SO4 +KBr → KHSO4 + HBr

Perchloric Acid

It is also mineral acid. It is colorless and stronger than sulfuric acid and nitric acid. When hot it is a strong powerful oxidizer. But in aqueous solutions it shows only strong acid features and no oxidizing properties. It is useful in preparation of perchlorate salts like ammonium perchlorate. It is dangerously corrosive in nature and form explosive mixtures. It is prepared by two methods one is traditional method in which aqueous solubility of sodium perchlorate is treated with hydrochloric acid which gives perchloric acid and sodium chloride. Other method is anodic oxidation of aqueous chlorine at platinum electrode. It is used in rocket fuel. It is also useful in etching of chrome.

Hydrochloric Acid

It is a colorless inorganic compound. It has distinctive pungent smell. It is main component of gastric acid which is produced in digestive system of animal species including human being. It is used in leather processing. Hydrochloric acid was discovered by Jabir ibn Hayyan in 800 AD. It is prepared by dissolving hydrogen chloride in water. It is used in industrial processes like refining metal. It is also used in purification of common salt. In concentrated form it forms acidic mist which has corrosive effect on human tissue, respiratory organs, eyes, skin and intestines.

ρ- Toluenesulfonic acid

It is white solid soluble in water, alcohol and polar organic solvent. TsOH is one million times stronger than benzoic acid. It is prepared on industrial scale through sulfonation of toluene. It is used in esterification of carboxylic acid. Also used in Trans- esterification of an ester. Tosylates is used for protection for alcohols. Reduction of toyslate ester produce hydrocarbons [5].

Nitric Acid

It is also known as aqua fortis (strong water). Nitric acid is highly corrosive mineral acid. In pure form it is colorless but acquire a yellow cast in older sample due to decomposition into oxides of nitrogen and water. Commercially its concentration in water is 68%. When its concentration is 68% in a solution it is called as fuming nitric acid It is primary reagent for nitration, some nitro compounds are stable enough so can be used in munitions and demolition. While some are sensitive shock and thermally explosives, other are more stable to be used as pigments, inks and dyes. Nitric acid react with metals like magnesium, manganese and zinc liberate H2. Some metals gold and platinum do not react with nitric acid. Nitric acid is prepared by reacting nitrogen dioxide with water. Its main use is production of fertilizers. Nitric acid give ammonium nitrate through neutralization with ammonia. Also used in production of explosives, nylon precursors and organic compounds. It is used as oxidizer in liquid fueled rockets. Nitric acid used in food finishing. Its strong corrosive nature and oxidizing power it produce acid hydrolysis with protein and fats which decompose living tissue like skin and flesh.

Chloric Acid

Chloric acid is strong unstable oxidizing acid in nature. It is oxoacids of chlorine. Chloric acid is colorless and its density is similar with water but it has pungent smell. It is strong acid, react with base and form chlorate salts. Chloric acid is prepared by reacting barium chloride with sulfuric acid. Reducing agents like sodium or potassium metabisulfite is used to neutralize chloric acid. Chloric acid is used to perchloric acid. It is also used for fireworks i.e. strontium chlorate for red, calcium chlorate for orange [6].

pH Scale

Concentration of hydrogen ion [H+] in pure water is the basis for the pH scale. Water is a weak electrolyte because it ionizes very slightly into ions in a process called auto-ionization or self-ionization.

                       H2O      ⇋    H+    +     OH_

          The equilibrium expression of this reaction may be written as

                              KC =    [H+] [OH]





3. Jones, J. H. (2000). “The CativaTM Process for the Manufacture of Acetic Acid” (PDF). Platinum Metals Rev. 44 (3): 94–105.




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