Pharmaceutical Aids And Necessities - Acids And Bases

This Blog shows discussion of several topics important to the preparation,preservation and storage of pharmaceutical products.The major topics include ACIDS and BASES - frequently employed in the conversion of drugs to chemical forms convenient to their product formulation;BUFFERS - used to maintain the pH of various formulations within Prescribed Limits .

ACIDS AND BASES

Acids and Bases play a center role in chemistry because,with the exception of redox reaction,every chemical reaction can be classified as an ACID-BASE Reaction.Several different theories explained what composes an acid and base.All classical theories of acids and bases describe only properties of acids and bases but does not gives the idea about required structural properties of acids and base.Over the years,much more accurate definitions of acids and bases have been created.The concept of acid and base have undergone a considerable change.Some of these concept are described as follows:

1)EARLIER CONCEPT OF ACID AND BASE :

An Acid may be defined as any substance which as a sour taste and its aqueous solution turns Blue LITMUS Red.

A Base may be defined as any substance which as a bitter taste and its aqueous solution turns Red LITMUS Blue.

The Earlier Concept of Acid And Base could not explain the behaviour of all acids and bases.

2)ARRHENIUS CONCEPT :

The concept of acid and base was first of all introduced by Arrhenius. According to him, an acid may be defined as any hydrogen containing substance which gives H⁺ ions in aqueous solution and a base may be defined as a substance containing hydroxy groups or groups capable of providing hydroxide ions OH^- in aqueous solution. Thus, HCl is an acid while NaOH is a base.

(On adding H2O) - HCL ⇌  H⁺ + Cl^-

(On adding H2O) - NaOH ⇌ Na⁺ + OH^-

According to Arrhenius, the neutralisation process can be represented by a reaction involving the combination of H+ and OH- ions to form water.

 H⁺ + OH^- ⇌ H2O 

LIMITATIONS :- Although the ARREHENIUS based theory is most simple and convenient in explaining the reaction in aqueous solution,it has number of limitations.The limitation of ARRHENIUS THEORY are given below :

• The definition of acid and base are only in terms of aqueous solution and not in terms of substance.
• The theory is not able to explain the acidic and basic properties of substance in non-aqueous solution. e.g. NH₄NO₃  in liquid NH₃ acts as an acid though it does not give H⁺ions.
• The neutralization of acid and base in absence of solvent could not be explained.
• It cannot explain the acidic character of certain salts such as AlCl3 in aqueous solution.

3)BRONSTED-LOWRY CONCEPT : Bronsted in Copenhagen and Lowry in London independently in 1923 proposed a general definition of acids and bases.According to them,an acid may be defined as any Hydrogen containing material(a molecule or a cation or an anion) that is capable of donating a proton H⁺ to any other substance whereas a base may be defined as any substance that is capable of accepting a proton H⁺ from any other substance. In short an acid is a proton donor and a base is proton acceptor.

Some examples of ACIDS are as follows :

HCL ⇌  H⁺ + Cl^-

CH₃COOH ⇌ H⁺ + CH₃COO^-

H₃O⁺ ⇌ H⁺ + H2O 

Some examples of BASE are as follows :

 OH^-+ H⁺ ⇌ H2O 

H2O  + H⁺ ⇌ H₃O⁺ 

NH₃ + H⁺ ⇌ NH₄

Conjugate Acid-Base Pairs: 

Let us consider a reaction

HCL   +   H₂O   ⇌   H₃O⁺   +   Cl^-

ACID(1)         BASE(2)           ACID(2)          BASE(1)

In this reaction, HCl donates a proton to H₂0 and is therefore an acid. On the other hand, water accepts a proton from HCl and is therefore a base. In the reverse reaction which at equilibrium proceeds at the same rate as the forward reaction, the H₃0⁺ ion donates a proton to Cl^-ion and hence H₃0⁺ is an acid, while Cl^- ion which accepts a proton from H₃0⁺ is a base. The acid-base pairs,are called CONJUCATE ACID-BASE PAIRS.

Some examples of conjugate acid-base pairs are as follows: 

H₂SO₄ + H₂O ⇌ H₃O⁺ + HSO₄^-

CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO^-

LIMITATIONS : 

• The Bronsted-Lowry concept lays excessive emphasis on the proton-transfer. Although it is true that most common acids are protonic in nature, yet there are many which are not.
•  A large number of acid-base reactions are known in which no proton transfer can take place, e.g.,SO + SO ⇌ SO²⁺ + SO₃^2-

4)LEWIS CONCEPT(Electron-Donor-Acceptor System):

This theory explains the acid-base phenomena in terms of the acid and base along with the formation

of a coordinate bond. According to Lewis (1923), an acid may be defined as any species (molecule, radical or ion) that can accept an electron-pair to form a coordinate bond and a base may be defined as any species that can donate an electron pair to the formation of a coordinate bond. Thus, in Lewis System,an acid is an electron-pair acceptor and a base is an electron-pair donor.

Thus, according to Lewis theory, the process of neutralization is simply the formation of a coordinate bond between an acid and a base. Consider the reaction:

   ↓⟵↑

H⁺ + : NH₃ ⟶ H ⟵ NH₃

                                   _(Adduct)

In the above reaction,proton (H⁺) accepts a one electron pair from :NH₃ molecule and therefore is an acid,whereas :NH₃ moleclue which donates a electron is a base.The adduct is NH₄⁺ ion.

Lewis Acid: H⁺, NH₄⁺;Na⁺, K⁺ Cu²⁺, AI³⁺

Lewis Base : NH_3,H2O,OH^- ,Cl^-

Bronsted acid, i.e., the proton is also a Lewis acid but all Lewis acids are not Bronsted acids.Similarly in the case of Bronsted bases and Lewis bases.

LIMITATIONS :

• Since the strength of Lewis acids and bases is found to depend on the type reaction, it is not possible to arrange them in any order of their relative strength.
• As Lewis acid-base reactions involve electrons, they are expected to be very fast reaction. However, there are many Lewis acid-base reactions which are slow.

RELATIVE STRENGTH OF ACID AND BASES

Acids and bases can be identified as monoprotic or polyprotic depending upon the number of dissociable proton the acid contain or the number of protons with which the base can combine.Irrespective of whether the substance ids monoprotic or poyltic ,the strength of acid or base refers to the efficiency with which the substance demonstrates the properties of acid and base.For an acid it means the ease with which it accepts the proton;for base, it mean the ease with which it donates proton.These properties of a substance will be affected by the environment;it is easier for an acid to donate proton in a proton accepting medium.

With respect to class,there are two general class-STRONG and WEAK.Strong acids and bases are the ones which are almost completely dissociated in dilute aqueous media.Weak acids and bases are the ones which are partially dissociated in dilute aqueous media.The strength of acid and base are frequently determined by the dissociation constant K.If B is a strong base,it will have affinity for H⁺ And hence BH⁺ is a weak acid.Conversely,if B is a weak base,it will have low affinity for H⁺ and so BH⁺is a strong conjugate acid.

For this reason, the strength of base is frequently expressed in terms of acid dissociation constant of its conjugate acid  BH⁺,i.e

 K_a= [B][H⁺]/[BH⁺]

Therefore the strength of an acid is given by Kn and the strength of a base by Kb or Kn of its conjugate acid.

For weak acids and bases, there is a gradation of strength,i.e some are weaker than others as shown by Kn (or pK) or Kb (or pKb) values in table below.Biochemical interest in these substances lies in

their potential impact on biomolecules.

Many biochemicals possess functional groups that are weak acids or bases.One or more of these functional groups - carboxyl groups,amino groups,or the secondary phosphate dissociation of phosphate esters - are present in all proteins and nucleic acids, most coenzyme s, and most intermediary metabolites. The dissociation behaviour (protonic equilibria) of weakly acidic and weakly basic functional groups is therefore fundamental to understand the influence of intracellular pH on the structure and biochemical activity of these compounds. Their separation and identification in research and clinical laboratories are also facilitated by knowledge of the dissociation behaviour of their functional groups:

Substance	Formula	Ka	pKa	Kb	pKb	Acidity or  Basicity
Formic acid	HCOOH	1.77 x 10-4	3.75	-	-	↑
Acetic acid	CH3COOH	1.76 x 10-5	4.75	-	-	Increasing  acidity
Trimethylacetic acid	(CH3)3CCOOH	4 x 10-6	5.03	1.77 x 10-5	4.75	፧
Ammonia	NH3	5.62 x 10-10	9.25	8.32 x 10-5	4.19	Increasing  Basicity
Trimethylamine	(CH3)2NH	1.55 x 10-10	9.81	5.25x 10-5	3.28	↓

IMPORTANCE OF ACIDS AND BASES IN PHARMACY

Acids,Bases and thier actions play important/vital role in pharmacy.Some of their main application are as follows :

1. Acid-base neutralization reaction find use in preparative procedures for the preparation of suitable salts, and for conversion of suitable salt in to their suitable form,specific acid-base mixture are used for preparaing effervescent mixture.
2. Acids-bases are used in analytical procedures which are involving acid-base titrations.
3. Acids and bases find use as therapeutic agents in the control of and adjustment of the pH of the gastrointestinal tract, body fluids and urine.
4. The conjugate pairs of acids or bases are used as buffers, e.g., Buffer acid as a proton donor and buffer base as a proton acceptor.

For the safety of patients as well as pharmacists due percautions have to be undertaken during handling and storage of acids and bases. Storage problems are inadequate closures or exposure to heat and light.Inadequate closures give rise to vapourisation and loss of gaseous components and to exposure to moisture, oxygen and carbon dioxide. Oxygen may be able to oxidise acids or bases, e.g hydriodic acids.Carbon dioxide can react with bases such as sodium hydroxide, potassium hydroxide,sodalime and lime water. Some hygroscopic substances like sodium hydroxide are able to absorb moisture on exposure to air and become liquids showing deliquescent property.

Dehydrating agents like concentrated sulphuric acid and quick lime have to be protected from air by tight closures. Acids and bases in crystalline hydrate form such as citric acid, sodium carbonate which are having higher vapour pressures than the relative humidity of the atmosphere lose varying portions of _their water of crystallisation to the atmosphere exhibiting efflorescent property and has to be protected by tight closure. At higher temperatures such substances also lose water of crystallisation,so have to be protected from heat.

PHARMACIST THOUGHT

MEDICINE HEALS DOUBTS AS WELL AS DISEASES.

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