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(2011-07-20)   A Brief History of Acidity
The slow maturation of a key chemical concept.

The term  acid  comes from the latin word  acetum  and the Greek word  oxein.  Both mean "sour",  which betrays the fact that, for many centuries, acidity was only measured using our sense of taste  (possibly dangerously so)  much like  pungency  today.

In 1777, Antoine Lavoisier (1743-1794) thought that the newly-discovered active component of air  (Sheele 1773, Priestley 1774)  was the long-sought-after  "universal acidifying principle".  Accordingly,  he coined the name  oxygen  for it  (i.e., "generator of acidity", using the aforementioned Greek etymology).  As it takes oxygen to turn wine into vinegar or to produce  organic acids  from other alcohols,  Lavoisier guessed that oxygen was essential to acidity.  This misguided theory survived long enough for the name  oxygen  to be universally accepted, in spite of its dubious etymology.

Jacob Berzelius (1779-1848)  upheld the same misconception in his theory of electrochemical dualism, as he argued that acids were oxides of non-metals and bases were oxides of metals...

In 1810 however,  Humphry Davy (1778-1829)  showed that muriatic acid doesn't contain any oxygen, by discovering the  elemental nature  of chlorine  (which had been thought to be an  "oxide of muriaticum" by Scheele, when he isolated it, in 1774).

Thereafter, the acids fitting the structure envisioned by Lavoisier were dubbed  oxacid  (French:  oxacide ) .  The modern term is oxoacid or oxy-acid.  The other acids were dubbed  hydracids.

In 1838, Justus von Liebig (1803-1873) defined acids as  compounds containing an hydrogen which can be replaced by a metal.

In 1884,  Svante Arrhenius (1859-1927)  suggested, in his doctoral dissertation, that some substances are ionized in solution.  The ideas of Arrhenius were ahead of his time  (he only earned a fourth-class degree with that thesis)  but he would eventually get a Nobel prize for the work  (1903).

In 1887, Arrhenius used the electrolytic theory of dissociation he had introduced in his thesis to formally define an acid as  a substance that dissociates in water into anions and hydrogen ions.  Conversely, he view a base as a substance dissociating into a cation and an hydroxide ion  (OH-).  Although this was a great breakthrough, this definition only applies to aqueous solutions.  It fails to recognize the acidity of  insoluble  substances.

In 1909,  using the definition of Arrhenius,  Søren Sørensen (1868-1939)  introduced the  pH scale  to quantify the acidity of aqueous solutions.

In 1923,  ... ...

Acid-Base History   |   A Brief History of Acidity  by  John W. Nicholson  (January 2004).
A Basic History of Acid  by  Mark S. Lesney  (American Chemical Society, 2003).
 
Wikipedia :   Acid-base chemistry   |   Acid-base reaction theories   |   Acid   |   Base   |   Hydronium Solvation   |   Acid-base titration

(2022-09-03)   Acid-base neutralization is  exothermic.  So is dilution.
Never  pour water into a concentrated acid!  (Water drops may boil.)

Two rhyming mnemonics for the safer way to dilute an acid:
 
French :   Acide dans l'eau,  bravo.
Eau dans l'acide,  suicide.
 
English :   Do like you oughta, add acid to water.

Enthalpy of neutralization   |   Enthalpy of neutralization

(2011-07-25)   Hydrochloric acid  =  muriatic acid :   HCl
A strong acid found in the stomach  (0.1 M).

Ferdinand Hoefer (1811-1878) has attributed the initial discovery of muriatic acid to the alchemist Mary the Jewess  (third century AD). 

Wikipedia :   Hydrochloric acid   |   Gastric acid

(2011-07-21)   Sulfuric acid  =  vitriolic acid :   H₂SO₄
The anhydrous oil is a solvent subject to  autoprotolysis.

Vitriol  was the name of the  salts  of  3  common metals  (M)  that dissolve in water as  metal aquo complexes  M(H₂O)₆⁺⁺  and  sulfate ions  SO₄^- -.

• White vitriol :
  Zinc sulfate  (ZnSO₄, 7 H₂O)  Goslarite mineral  (1847).
• Green vitriol,  kankatum or copperas  (used in iron-gall ink):
  Ferrous sulfate  (FeSO₄, 7 H₂O)  Melanterite mineral  (1850).
• Blue vitriol :
  Copper sulfate  (CuSO₄, 5 H₂O)  Chalcanthite mineral  (1853).

The corresponding acid  (H₂SO₄ )  was properly called  vitriolic acid  in aqueous solution, or  oil of vitriol  in its viscous concentrated form.  The latter was also improperly shortened to  vitriol.

The invention of vitriol is usually attributed to Geber (c. AD 721-815).  It may have been known to the Assyrians much earlier.

H₂SO₄  protonates  water molecules very easily.  In a dilute aqueous solution,  sulfuric acid  is essentially just a diprotic acid.  It's a strong acid with respect to its first ionization  (i.e., un-ionized sufuric acid is virtually nonexistent in water)  and a weak acid with respect to the second ionization:

H₂SO₄  +  H₂O   «   H₃O⁺  +  HSO₄^-         ( K = 2.4 10⁺⁶   at 25°C )
HSO₄^-  +  H₂O   «   H₃O⁺  +  SO₄^{- -}         ( K = 1.0 10^-2   at 25°C )

Without water, pure sulfuric acid is an oily substance which is a powerful solvent that undergoes an  autoprotolysis  similar to that of water:

2 H₂SO₄   «   H₃SO₄⁺  +  HSO₄^-         ( K = 2.7 10^-4   at 25°C )

However, the analogy stops here, since pure  H₂SO₄  actually contains many other chemical species in equilibrium, as listed below.  The first entry is the result of subtracting all the other concentrations from  10195.9 mmol/kg,  the reciprocal of the nominal molar mass of  H₂SO₄  (98.0785 g/mol).

H2SO4	10138.5 mmol/kg
HSO4-	15.0 mmol/kg		Bisulfate ion
H3SO4+	11.3 mmol/kg
H3O+	8.0 mmol/kg
HS2O7-	4.4 mmol/kg
H2S2O7	3.6 mmol/kg		Pyrosulfuric acid
H2O	0.1 mmol/kg
SO4- -	< 10-10 mmol/kg

The above concentrations at chemical equilibrium correspond to the following  dimensionless  equilibrium constants  (cf. mass-action law)  if we equate  chemical affinities  and  concentrations  (as an approximation).

A  +  B   «   C  +  D	[C] [D] / [A] [B]
H2SO4  +  H2SO4   «   H3SO4+  +  HSO4-	1.65 10-6
H2SO4  +  H2SO4   «   H3O+  +  HS2O7-	3.42 10-7
H2SO4  +  H2SO4   «   H2O  +  H2S2O7	3.50 10-9
H2SO4  +  H3SO4+   «   H3O+  +  H2S2O7	2.51 10-4
H2SO4  +  HSO4-   «   H2O  +  HS2O7-	2.89 10-6
H2SO4  +  H3O+   «   H2O  +  H3SO4+	1.39 10-5
H2SO4  +  H2O   «   H3O+  +  HSO4-	1.18 10-1
HSO4-  +  H2O   «   H3O+  +  SO4- -	4.9 10-10

The above equilibrium is properly described as  100% sulfuric acid.

H₂S₂O₇  can be viewed as sulfuric gas  (SO₃)  dissolved in  vitriol oil :

H₂SO₄  +  SO₃   «   H₂S₂O₇

If more trioxide is dissolved than called for by the stoichiometry of pure sulfuric acid, then we effectively have sulfuric acid rated at  more  than 100%, because adding water to it would yield 100% sulfuric acid!  Such stuff is called  oleum  or  fuming sulfuric acid  (the vapor is mostly sulfur trioxide).

For better stability in storage, commercial  concentrated sulfuric acid  is usually rated at 98%  (under 1 atm, there's an azeotrope at 98.3%, which boils at 338°C).

By definition,  20% oleum  is  20 kg  of  SO₃  (80.0632 g/mol)  dissolved in  80 kg  of  H₂SO₄  (98.0785 g/mol).

Wikipedia :   Sulfuric acid   |   Strong acids   |   Diprotic acids   |   Mineral acids

(2015-09-16)   Nitric acid  =  aqua fortis :   HNO₃
A strong acid and a powerful oxidizing agent.

Commercial  concentrated nitric acid  is  68%  HNO₃  which actually forms an  azeotrope with water, boiling at  120.5 °C  under  1 atm.

One of the earliest extant recipes for the preparation of  nitric acid  is the distillation of a mixture of  niter  (saltpeter;  KNO₃ )  green vitriol  ( FeSO₄ )  and alum.  This appears in  De Inventione Veritatis  (part of the  Pseudo-Geber corpus,  attributed to the Franciscan alchemist Paul of Tarento, who was active in the second half of the thirteenth century).  Related early recipes  (heating niter and clay together)  are described by Albertus (1205-1280)  and  Ramon Llull (c.1232-1316).

Wikipedia :   Nitric acid   |   Oxidizing acid

(2015-09-21)   Perchloric acid :   HClO₄
Stronger than nitric or sulfuric acid...

Commercially available at a 72.5% concentrantion which forms an azeotrope with water at  1 atm,  boiling at 203°C  (indefinitely stable but  hygroscopic).

Perchloric acid was first obtained in 1816,  by the Austrian Count Friedrich von Stadion, who observed the formation of potassium perchlorate by the action of sulfuric acid on potassium chlorate.  An aqueous solution of perchloric acid results from the distillation of that mixture.

Wikipedia :   Perchloric acid   |   Oxidizing acid

(2011-07-21)   Carboxylic Acids
Organic acids which owe their acidity to the  -COOH  group.

Simplest examples are  formic acid  HCOOH  and  acetic acid  CH₃COOH.

Carboxylic Anhydrides :

By dehydration, a carboxylic acid  RCOOH  gives a symmetrical anhydride  (RCO)₂O.   Acetic anhydride  (CH₃CO)₂O  is the simplest stable example at room temperature,  since  formic anhydride  decomposes at  24°C.

The simplest example of an  asymmetrical  carboxylic anhydride is  acetic-formic anhydride.

Wikipedia :   Carboxylic acids   |   Organic acid anhydride

(2015-08-31)   Picric Acid  =  Trinitrophenol  (TNP) :   (NO₂)₃CH₂OH
The most acidic phenol  (pKa = 0.38)  is an explosive soluble in water.

Wikipedia :   Picric acid

(2011-07-21)   Tosylic acid and other sulfonic acids
Sulfonic acids owe their acidity to the  -SO₂(OH)  group.

Para-toluene-sulfonic acid  or  PTSA  (CAS 104-15-4)  is also called  tosylic acid.  It is a non-oxydizing strong acid  (pKa = -2.8  in water)  which is well-suited to the preparation of standard titration solutions because it's a soluble  solid  which can be conveniently weighed  (172.202 g/mol).

The  negative  pKa  indicates a high  dissociation constant  in water:

[ H₃O⁺ ]  [ CH₃C₆H₄SO₃^- ]  /  [ CH₃C₆H₄SO₂(OH) ]   =   Ka   =   630

Many other sulfonic acids are even much stronger than that:

Halosulfuric Acids  =  Halosulfonic Acids :

Being combinations of an halogen atom with a sulfonic group,  these are best called  halosulfonic acids.  The alternate  -sulfuric  suffix merely evokes the structural similarity with the molecule of  sulfuric acid.

Fluorosulfuric acid  FSO₂(OH)  is the strongest sulfonic superacid  and one of the  strongest acids  commercially available.

Chlorosulfuric acid  ClSO₂(OH)  reacts violently with water to yield  hydrochloric acid  and  sulfuric acid:

ClSO₃H  +  H₂O   ®   HCl  +  H₂SO₄

Bromosulfonic acid  is a solid which decomposes at  8°C :

2 BrSO₃H   ®   Br₂  +  H₂SO₄  +  SO₂

Iodosulfonic acid  has apparently never been synthesized.

Triflic Acid   (TFMS, TFSA, HOTf or TfOH) :

Triflic acid  (trifluoromethanesulfonic acid)  CF₃SO₂(OH)   is a  superacid  first synthesized in 1954 by  Robert N. Haszeldine (b. 1925)  and  J.M. Kidd.

Wikipedia :   Sulfonic acids   |   P-Toluenesulfonic acid

(2015-08-31)   Sulfamic Acid   H₃NSO₃
A moderately strong acid used in acidimetry  (non-hygroscopic crystals).

This is a moderately strong acid:   pKa = 1.05

In its solid form,  sulfamic acid  can be conserved indefinitely at room temperature.  Because the crystals are not hygroscopic, they can be weighed accurately to prepare standard solutions used in acidimetry.  For best accuracy, such solutions must be used promptly, as sulfamic acid degrades  (slowly)  when dissolved in water.

The  standard procedure  is to calibrate a loosely prepared solution of a strong base  (e.g.  NaOH)  by titration of a solution containing a precisely-measured mass of sulfamic acid crystals.  (The volume of water used to dissolve the sulfamic acid is largely irrelevant but the crystals should be as pure as possible.)  The basic solution so calibrated cam then be used for the  titration  of other acidic solutions.

Sulfamic acid is considered a safe acidic cleaning agent for household use, mostly because  (unlike stronger acids)  it will not produce dangerous chlorine gas when accidentaly mixed with hypochlorite  products.

Wikipedia :   Sulfamic acid

(2011-07-20)   Brønsted-Lowry Theory  (1923)
A Brønsted-Lowry acid is a  proton donor.

Wikipedia :   Brønsted-Lowry acid-base theory   |   Protonation   |   Deprotonation   |   Hydronium

(2011-07-20)   Lewis Acids and Bases  (1923)
An acid is an electron-pair acceptor.  A base is an electron-pair donor.

Arguing that only hydrogen compounds can be acids would be
like arguing that only oxygen compounds can be oxidizing agents.
Gilbert N. Lewis  (1940, paraphrased)

According to G.N. Lewis,  an acid  (A)  and a base  (B:)  form an  adduct  AB,  where the bond between  A  and  B  is a  dative covalent bond  (both electrons come from the base).

Lewis was nominated 35 times for the Nobel Prize in Chemistry.

Wikipedia :   Lewis acids and bases   |   Octet rule   |   Gilbert N. Lewis (1875-1946)

(2011-07-20)   Hammett Acidity Function
Going beyond the pH scale of dilute aqueous solutions.

Wikipedia :   Acidity functions   |   Hammett acidity function   |   Louis Plack Hammett (1894-1987)

(2011-07-20)   Superacids
Acids that are stronger than  pure sulfuric acid.

Wikipedia :   Superacids   |   Carborane
Superacid Chemistry  by George Andrew Olah (2009)

(2011-07-20)   Superbases
Great affinities for protons.

In aqueous solution, the hydroxide ion  OH^-  has the highest basicity but there are compound that have greater proton affinity in dry environments.

Wikipedia :   Superbases   |   Hydroxide ion   |   Lithium diisopropylamide (LDA)

(2011-07-20)   What are the strongest acids known?
Perchloric acid  (HClO₄)  fluorosulfuric acid  (HSO₃F)  and beyond.

The strongest known superacid is  fluoroantimonic acid   H₂F(SbF₆).

Some of the Strongest Known Acids   (H₀ = Hammett function)

Date	Formula	Name	H0
1990	H2F(SbF6)	Fluoroantimonic acid	-31.3
1974	FSO3H + SbF5	Magic acid	-19.2
1969	H(CHB11Cl11)	Carborane superacid	-18.0
1944	FSO3H	Fluorosulfuric acid	-15.1
	HF	Hydrofluoric acid	-15
	H2SO4 + 3 SO3		-14.96
1954	CF3SO3H	Triflic acid	-14.9
	H2SO4 + SO3	Pyrosulfuric acid	-14.44
	H2SO4 + HB(HSO4)4		-13.6
1927	HClO4	Perchloric acid	-13
	H2SO4	Sulfuric acid	-12

Wikipedia :   Fluorosulfuric acid   |   Triflic acid   |   Leveling effect
 
Strongest acid known? (Science Forums)
 
The strongest acid in the world:  Fluoroantimonic acid (26:35)  by  ChemicalForce  (2020-12-17).