Unit 3 Pharmaceutical analysis

[Pages:13]Pharmaceutical Analysis

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Pharmaceutical Analysis

? Definition: Chemical Analysis is a method for determining the substance, its amount or impurity and plays a vital role in the pharmacy.

? Analytical chemistry ? Methods 1. Qualitative analysis 2. Quantitative analysis

? Qualitative or physical analysis Recognition of chemical species by means of: ? Colour, taste, Viscosity and solubility ? Reaction producing a colour ? Reaction producing a precipitate ? Reaction involving a change of a physical parameter.

? Quantitative chemical analysis Quantitative chemical analysis carried out by determining the volume of a solution of accurately known concentration which is required to react quantitatively with a measured volume of the substance to be determined.

? Classification

1. Neutralisation Reactions 2. Complex Formation Reactions 3. Redox Reactions 4. Precipitation Reactions

? Basics and definitions

A solution: ? Is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent.

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Concentrations: ? Is a measure how much of given substance there is mixed with another substance ? most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in a substance.

Standard solution: ? Is a chemical term which describes a solution of known concentration. ? The concentration of the solution is normally expressed in units of moles per Liter (mol/L, often abbreviated to M for morality). ? Standard solutions are normally used in titrations to determine the concentration of a substance in solution.

A molar solution: ? Is one that contains one mole of solute (molecular weight) per liter of solution. ? For example: ? A five molar solution of aqueous hydrochloric acid (written as "5M HCl (aq)") means there are 5 moles of HCl per liter of solution ? If one liter of a solution contain 98 g of H2SO4, this means that the concentration is 1M ? If one liter of a solution contain 98x2 (196) g of H2SO4, this means that the concentration is 2M

Normal solution: ? Is one that contains one equivalent weights of solute per liter of solution. ? For example: A five normal solution of aqueous hydrochloric acid (written as "5N HCl (aq)") means there are 5 equivalents of HCl per liter of solution

Normality = number of equivalent weights of solute = no. eq = no. meq

liter of solution

L

mL

? Calculation of equivalents: 1. For acids: ? One equivalent of an acid is that amount of an acid that will furnish one mole of hydrogen ions or that will react with one mole of hydroxide ions.

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? Example: the molecular weight of HCL is 36.5 g and contain one mole of hydrogen ions that will react with one mole of hydroxide ions: The equivalent weights of HCL is 36.5/1 = 36.5

? The molecular weight of H2SO4 is 98 and contain two mole of hydrogen ions that will react with one mole of hydroxide ions: The equivalent weights of H2SO4 is 98/2 = 49

The molecular weight To calculate the equivalent weights of acid =

Number of H ions

2. For bases: ? one equivalent of a base is that amount of a base that will furnish one mole of hydroxide ions or react with one mole of hydrogen ions.

The molecular weight To calculate the equivalent weights of base =

number of OH ions

? Example: NaOH + HCL

NaCL + H2O

The molecular weight of NaOH is 40 and contain one mole of hydroxide ions that will react

with one mole of hydrogen ions:

The equivalent weights of NaOH is 40/1 = 40

? Al(OH)3 + 3HCL

ALCL3 + H2O

The molecular weight of Al(OH)3 is 87 and contain three mole of hydroxide ions that will react

with three mole of hydrogen ions:

The equivalent weights of Al(OH)3 is 78/3 = 26

3. For Salts:

? one equivalent of a salt is that amount of a Salt that will result from replacement of one mole of

hydrogen ions of conjugated acid .

? NaOH + HCL

NaCL + H2O

The molecular weight of NaCL is 58.5 and result from replacement of one mole of hydrogen

ions of conjugated acid HCL.

The equivalent weights of NaCL is 85.5/1 = 85.5

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? H2CO3

HCO3- + H+

The molecular weight of NaHCO3 is 84 and result from replacement of one mole of hydrogen ions

of conjugated acid H2CO3

The equivalent weights of NaCL is 84/1 = 84

? Na2CO3: The molecular weight of Na2CO3 is 106 and result from replacement of two moles of hydrogen ions of conjugated acid H2CO3 The equivalent weights of NaCL is 106/2 = 53

1. Neutralisation Titrations (acid - base titration)

? Acid ? base titration terminology: Titration A process in which a solution of one reactant, the titrant, is carefully added to a solution of another reactant, and the volume of titrant required for complete reaction is measured.

Equivalence point The point at which stoichiometrically equivalent amounts of an acid and base have reacted.

Indicator A compound that exhibits its different colors in solutions of different acidities. Used to determine the point at which an acid-base reaction is complete. (phenolphthalein)

End point The point at which an indicator changes color and a titration is stopped. The end point should coincide with the equivalence point

? Acid ? base titration

? The neutralisation reactions between acids and bases used in chemical analysis.

? These reactions involve the combination of hydrogen and hydroxide ions to form water.

? Ex:

NaOH + HCL

NaCL + H2O

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To perform neutralisation titration is necessary to have a solution of base for example at known concentration a glass vessel, a burette (calibrated tube able to measure volume, equipped with a tap) and an indicator (chemical species able to change colour when change the composition of the solution.

Change of pH

? The standard solution for the titration must be strong base or acid (for complete dissociation and

reaction)

? Weak acids and bases dissociate and react partially so can not be used by titration as standard

reagent.

? Ex:

NH4OH + HCL

NH4CL + H2O

? NH4OH dissociates partially in a solution into (NH4OH

NH4+ + H2O). Thus by

titration ammonia ions will be not consumed by reaction with HCL and still free.

? Color change of the indicator will appear only if the whole chloride ions are consumed.

? Requirements of standard reagents: 1. Not volatile and stable 2. Complete dissociation and ionization

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3. Have no oxidation properties (to prevent oxidation of indicator) 4. Are not insoluble salts because this make detection of end point difficult.

? Types of acid- base titration:

1. Non-Aqueous

? Take place in a solvent other than water (Glycerin, Polyethelene glycol, alcohol)

? Used for weak substances which dissociate partially in water making difficulties in

determining change of pH

? Called non ? aqueous titration

? Ex: CH3COOH

CH3COO- + H+

2. Aqueous ? Take place in water ? Used for substances which dissociate completely and rapidly in water ? Determination of change of pH is easy ? Called aqueous titration

? Requirements of indicators: 1. The change of color is at equivalent point is clear 2. Not volatile 3. Chemically inert 4. Do not form insoluble complexes

? pH Indicators 1. are organic dyes with either acid or basic character 2. are organic dyes that change their colour on acceptance or release of protons 3. are used for rapid determination of an endpoint in an acid-base titration 4. are used for the quantitative determination of a given acid or base content of a solution 5. does not change the colour immediately but continuously within a given pH range

? Application of acid base titrations in pharmacy : ? Salicylic acid ? Benzoic acid

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? General law used in calculations of neutralization titrations:

No. of equivalents in a solution = Volume of the solution x Normality No. of eq. = V x N

? Example: NaOH Solution, Volume = 200 ml, concentration = 0.1 N Calculate the weight of NaOH in the solution?

No. of eq. = V x N

No. of eq. = 200/ 1000 x 0.1 = 0.02 Equivalents

1 Equivalent

40 g

0.02 Equivalents

Xg

Xg = (40 x 0.02)/ 1 = 0.8 g

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2. Precipitation Titrations

? Titrations between analytes and reagents resulting in the formation of a precipitate.

? The most useful of these precipitating reagents is silver nitrate AgNO3.

? Titrimetric methods based upon the use of silver nitrate are sometimes called Argentometric

titrations.

? Used for the determination of many anions including: 1. Halides: Br-, CL-, I2. Divalent anions S2-

3. Mercaptans CH3SH

4. Certain fatty acids

? Ex: HCL + AgNO3

AgCL + HNO3

? The precipitate will begin to form as the reaction take place and the end point will be determined

at the end of reaction (after completion of precipitate)

? Endpoint determination is by coloured indicators (usually back titrations) or turbidity methods.

? Examples of solutions used in precipitation titrations

1. Silver nitrate AgNO3: For titration of Br-, CL-, I- and CN-

HCL + AgNO3

AgCL + HNO3

2. Potassium thiocyanate KSCN:

For titration of Ag+ as AgSCN and Hg2+ as Hg(SCN)2

KSCN + Ag+

AgSCN

KSCN + Hg2+

Hg(SCN)2

3. Potassium cyanate KCN: For titration of Copper Cu and Nickel Ni

4. Hg(NO3)2 Mercury(II) nitrate: For titration of Copper Cl- , Br- , I-, SCN-

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