Acid, Base, Buffer solutions & their theories

Acid, base,buffers solutions and their theories part2

Acid, Base, Buffers solutions & their theories

Arrhenius, Brönsted-Lowry, Lewis acid-base, HSAB & Usanovich concept

Acid, bases and buffer solutions play an important role in the pharmaceutical chemistry. These are required not only in the manufacturing and quality assurance of drugs, but are also required as pharmaceutical aids and necessities in the despensing pharmaceutical for their stability, compatibility and optimum distribution in various physiological systems.

Acid and Base

An acid is a molecule or ion capable of donating a proton (hydrogen ion H+) (a Brønsted–Lowry acid), or, alternatively, capable of forming a covalent bond with an electron pair (a Lewis acid) e.g. HCL, HNO3, H2SO4, CH3COOH.

A base is a chemical species that donates electrons, accepts protons, or releases hydroxide (OH-) ions in aqueous solution e.g. NaOH, KOH, NH4OH.

1) Arrhenius concept

Arrhenius theory was introduced in 1887 by the Swedish scientist Svante Arrhenius. It states that acids are substances that dissociate in water to yield electrically charged atoms or molecules, called ions, one of which is a hydrogen ion (H+), and that bases ionize in water to yield hydroxide ions (OH−). It is now known that the hydrogen ion cannot exist alone in water solution; rather, it exists in a combined state with a water molecule, as the hydronium ion (H3O+). In practice the hydronium ion is still customarily referred to as the hydrogen ion.

Examples:-

HCl  in Aq Solution               H+  +  Cl-  (An Acid)

H2O + H+                              H3O+

NaOH in Aq solution             Na+ + OH- (A Base)

HCl + NaOH                          NaCl + H2O (Neutralization)

Note:- If the degree of ionization is high than the compounds are strong acid or strong base, but if the ionization is lower than substance are referred as weak acid or weak base.

Limitations of Arrhenius concept.

1) The concept was not able to explain acidic and basic nature of compounds without aqueous medium and not applicable to non aqueous and gaseous reactions.

2) Another failure that it was not able to explain acidic nature of compounds like CO2 and SO2, similarly the basic nature of NH3,  Na2CO3  and many other compounds.

3) The theory does not consider the nature of solvent in the reaction e.g. HCl is a strong acid in H2O solvent but weak in benzene solvent.

4) Concept of Neutralization reaction was not explained in the reaction of HCl with NH3.

Bronstead & Lowry concept.

This  theory was introduced independently in 1923 by the Danish chemist Johannes Nicolaus Brønsted and the English chemist Thomas Martin Lowry, stating that any compound that can transfer a proton to any other compound is an acid, and the compound that accepts the proton is a base. A proton is a nuclear particle with a unit positive electrical charge; it is represented by the symbol H+ because it constitutes the nucleus of a hydrogen atom. It is also known as PROTON THEORY OF ACID & BASE.

When an acidic substance loses a proton, it forms a base, called the conjugate base of an acid, and when a basic substance gains a proton, it forms an acid called the conjugate acid of a base.

Examples:-

HCl      +      H2O               =                         H3O+           +            Cl—

Acid             Base                                 Conjugate Acid         Conjugate Base

HNO3 + C2H5OH             =                   C2H5OH2+       +             NO-3

Acid             Base                                 Conjugate Acid         Conjugate Base

Limitations of Bronstead Lowry concept.

1) This theory works for all protic solvents (acetic acid, water, liquid ammonia, etc.), not only for water as Arrhenius’ theory. But it does not explain the acid-base behavior in aprotic solvents such as benzene and dioxane.

2) Still, Bronsted-Lowry´s theory cannot explain the reactions between acidic oxides for instance like CO2, SO2, SO3 etc.  and also the basic oxides like CaO, BaO, MgO etc. which also take place even in the absence of the solvent, for instance show in this reaction:
CaO + SO3 --> CaSO4

3) Substances like BF3, AlCl3 and many more do not have hydrogen present and therefore they cannot donate a proton but are known to act as acid. 

Lewis Acid Base Concept

In 1923, the American chemist Gilbert Newton Lewis proposed generalized definitions for acids and bases that do not restrict them to compounds containing hydrogen but in terms of the structure of acids and bases.

Lewis defined an acid as any molecular or ionic species that contains an atom capable of sharing a pair of electrons furnished by a base. In the simplest terms, a Lewis acid is an electron acceptor or electrophilic species.

Lewis base was to be considered according to Lewis as an electron pair donor and Lewis acid as an electron pair acceptor or nucleophilic species.

an acid must have a vacant orbital into which an electron pair donated by a base can be accommodated. A base, then, is a substance which has at least one unshared (lone) pair of electrons.

According to the lewis, the neutralization reaction is simply the formation of a coordinate chemical bond between an acid and a base.

Below are some examples:-

Advantages of Lewis Acid Base Concept

1. Lewis acid base theory able to explain the acidic nature of compound's like CO2, BF3, AlCl3 etc. In the similar manner basic nature of CaO, BaO, MgO etc.
2. It shows the acid and base behavior independent of Solvent behavior.

Limitations of Lewis Acid Base Concept

Though Lewis concept of acids and bases is more general than Arrhenius as well as Bronsted-Lowry concepts, yet it has several drawbacks which are discussed below:

1. Lewis concept gives the general idea and includes all the coordination reactions and coordination compounds. This may not be true always.

2. Lewis concept doesn’t explain the behavior of well known protonic acids like HCl, H2SO4, etc. which do not form coordination bonds with bases. Therefore, according to Lewis, these are not regarded as acids.

3. Relative strengths of acids and bases are not explained by Lewis.

4. Many Lewis acids do not possess catalytic property.

5. Lewis Concept does not fit in the acid-base reaction concept.

Hard & Soft Acid-Base concept

This concept was developed by R.G. Pearson and is often called as Pearson’s principle of hard and soft acid and bases. It categories the acid and base according to the properties of charge , size (eg ionic radius ), polarizability, etc.

As a general rule, more stable adducts are formed (Ionic bond) between hard acid and hard base  or (Covalent bond) between soft acid and soft base. 'Hard' applies to species which are small, have high charge states (the charge criterion applies mainly to acids, to a lesser extent to bases), and are weakly polarizable. 'Soft' applies to species which are big, have low charge states and are strongly polarizable.

Please note some Lewis acid and base cannot rigidly fit in the criteria for hard & soft acids or base & therefore they are placed in borderline area of acid and base. 

Uzanovich concept.

This concept tries to include all concepts of acid and base. Mikhail Usanovich developed a general theory that does not restrict acidity to hydrogen-containing compounds, and his approach, published in 1938, was even more general than the Lewis theory. An acid is a chemical entity which is capable of combining with  negative species, anions or electrons or donates positive ones, cations, and a base is a chemical entity capable of giving up anion or electrons or combining with cations. This definition could even be applied to the concept of redox reactions (oxidation-reduction) as a special case of acid-base reactions.

Some examples of Usanovich acid-base reactions include:

Na2O (base) + SO3 (acid) → 2Na+ + SO42- (species exchanged: anion O2- )

3(NH4)2S (base) + Sb2S5 (acid) → 6NH4+ + 2SbS43- (species exchanged: anion S2- )

2Na (base) + Cl2 (acid) → 2Na+ + 2Cl- (species exchanged: electron)

A comparison of the above definitions of Acids and Bases shows that the Usanovich concept encompasses all of the others but some feel that because of this it is too general to be useful.

Buffer solution & its types.

A buffer is a solution whose pH does not alter to a great extend by the addition of the small quantities of an acid or base.

General characteristics of a buffer solution.

1)It has a definite pH.

2)If a solution left in environment for long time its pH does not change.

3)Its pH does not change on dilution

4)Solution's pH may slightly change if acid or base is mixed with it.

Types of buffer solution.

1)Acidic buffer solution:- Acidic buffers are solutions that have a pH below 7 and contain a weak acid and one of its salts. For example, a mixture of acetic acid and sodium acetate acts as a buffer solution with a pH of about 4.75.

2)Basic buffer solution:- Basic buffer solution have a pH above 7 and contain a weak base and one of its salts. For example, a mixture of ammonium chloride and ammonium hydroxide acts as a buffer solution with a pH of about 9.25.

Buffer Capacity:-

               Buffer capacity quantifies the ability of a solution to resist changes in pH by either absorbing or desorbing H+ and OH- ions. When an acid or base is added to a buffer system, the effect on pH change can be large or small, depending on both the initial pH and the capacity of the buffer to resist change in pH.  Buffer capacity (β) is defined as the moles of an acid or base necessary to change the pH of a solution by 1, divided by the pH change and the volume of buffer in liters; it is a unitless number.  A buffer resists changes in pH due to the addition of an acid or base though consumption of the buffer.  As long as the buffer has not been completely reacted, the pH will not change drastically.  The pH change will increase (or decrease) more drastically as the buffer is depleted: it becomes less resistant to change.

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