Solubility of drugs

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  • Created by: cat
  • Created on: 11-03-14 13:42
Define solubility
The max amount of solute that dissolves completely in a given amount of solvent at a particular temperature
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Define dissolution
The process of dissolving a solute in a solvent to give a homogeneous solution.
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what does solvation lead to?
An increase in entropy. most endothermic solvation processes are spontaneous due to the rise in disorder
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Why is a drug'g reactivity with water important?
Determines drug solubility, affects the drug's stability. hydrolysis by water is a main metabolic route for the breakdown of Ester, amide and other group's that can be broken down.
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Two types of solvent important pharmaceutically?
Aqueous media and lipid based media.
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What does a drug require to transverse across a cell membrane?
The drug needs to be surrounded by hydrophobic alkyl chains.
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solubility of ions in water and lipids
ionic compounds are readily soluble in water due to ion-dipole interaction. ionic compounds are not soluble in lipids
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name three factors affecting solubility
1) molecular weight, 2) H-bonding, 3) hydrophobic or hydrophillic group
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how can you improve a drugs solubility
1) formation of salt 2) addition of water solubilising groups 3) use of special dosage forms
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why does formation of salt improve solubility?
As salts dissociate in water to form hydrated ions.
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Advantages to a salt form of the drug
advantages: oily and low melting point drugs can be given as a solid dosage, crystals have a higher melting point so smaller particles can be produced which aids dissolution.
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Disadvantage to salt form of the drug
Salt form of drug has a higher molecular weight so the proportion of active ingredient is reduced (due to reduced solubility) high doses are therefore required
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Factors to be considered when incorporating water soluble groups and consider why.
1) the type of group added 2) is the reaction reversible or irreversible? 3) the position of incorporation 4) the chemical route of incorporation
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Effect of pH on solubility?
biological fluids contain solutes that control the pH of the fluid. An acidic or basic environment will enhance or reduce the ionization of the drug, thus affecting solubility and absorption.
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what is the name of the equation that determines the degree of ionization?
Henderson-Hasselbalch Equation: pKa = pH + log [non-ionised form] / [ionised form]
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in which form are drugs more easily absorbed?
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state three special dosage forms
Micelles : deliver lipophillc drugs with poor water solubility. Mixed micelles: mixture of lipids used to optimize delivery. Lipososmes: liphophobic drugs via core or lippophillic drugs via bilayer
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Define partition coefficient.
the ratio of the concentration of unionised compound at equilibrium between organic and aqueous phases.
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Equation for the partition coefficient
log p = [unionised compound] octanol / [unionised compound] water
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what factors effect the partition coefficient?
pH, temperature and the nature of the solvent.
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what is Ksp?
It is the solubility product. salts with a Ksp values smaller than x10^-5 are generally insoluble in water.
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what is molar solubility?
molar concentration of a salt in its saturated solution. it can be used to determine Ksp assuming 100% dissociation in to its ions.
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Qsp is the ionic product. what is the difference between Ksp and Qsp?
Ksp applies only for saturated solutions, Qsp applies to all solutions of any concentration.
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what is the common ion effect?
any ionic salt will be less soluble in the presence of a common ion.
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Card 2


Define dissolution


The process of dissolving a solute in a solvent to give a homogeneous solution.

Card 3


what does solvation lead to?


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Card 4


Why is a drug'g reactivity with water important?


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Card 5


Two types of solvent important pharmaceutically?


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