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Why is solubilty important?

We as pharmacists administer the drug in many forms. Capsules, tablets, etc.
This drug needs to be absorbed and thus in a soluble form

There are also problems when starting to formulate pharmaceutical solutions which understanding solubility can help to overcome


What do we need to consider when formulating a solution?

which substances are soluble in which solvent
if there are any incompatibilities


How can we predict which solvent is soluble with which type of drug?

We can look into the chemical structure of the drug


What is a solution?

a mixture of 2 or more components that form a homogenous molecular dispersion


what are the components in a solution refferred to as?

the solute and the solvent


What is the solute?

the dissolved agent and is dispersed as molecules or ions through the solvent.
Generally the less abundant part of the solution although this is not always the case


what is the solvent?

the component in which the solute is dissolved in and is generally the more abundant part of the solution although this is not always the case


in a solution of salt and water, what is the solute? what is the solvent?

salt = solute
water = solvent


What is dissolution?

the transfer of molecules or ions from a solid state into the solution
This is a rate, dynamic process.
It is an extensive material property, so is dependent on other physicochemical barriers


what is solubility?

the extent at which the amount of substance passes into the solution to establish equilbrium under a given set of environmental conditions

i.e. the maximum amount of solutes that will dissolve in a given quantity of solvent at a given temperature.

it is a static process


Why do solutions form?

molecules have a natural tendency to spread out
Consider the example of water and ethanol in a beaker
there will be a layer of water at the bottom, and a layer of ethanol at the top.
At the interface there will be a tendency for some water molecules to go into the ethanol phase and vice versa. When this happens, there is a breakage of water-water and ethanol-ethanol bonds and a formation of new bonds between ethanol and water


What allows the ethanol and water to mix easily?

the attractions between the ethanol and water molecules are of the same kind (hydrogen bonds)

as the bonds are similar, they have a tendency to form a homogenous solution


What is a saturated solution

one in which the maximum amount of solute is dissolved in the solvent.
The dissolved solute in the solution is in dynamic equilbirum with the undissolved state
the rate of dissolved solute = rate of precipitation


What is an unsaturated solution (sub saturated solution)

a solution which contains an amount of solute that is less than its solubility
All the solute is present in its dissolved state.

unsaturated = havent reached equilibrium


What is a supersaturated solution?

a solution which contains an amount of solute that is more than its solubility
this is a meta-stable state
If its disturbed in any way, the excess solute will precipitate out and eventually form a saturated solution.
The amount of solute is more than expected at room temperature


How is solubility defined according to the USP?

in terms of parts


What is classified as very soluble?

less than 1 part of solvent is required to dissolve 1 part of solute


What is freely soluble?

1-10 parts of solvent required for 1 part of solute


what is soluble?

10-30 parts of solvent required for 1 part of solute


what is sparingly soluble?

30-100 parts of solvent required for 1 part of solute


what is slightly soluble?

100-1000 parts of solvent required for 1 part of solute


what is very slightly soluble?

1000-10,000 parts of solvent required for 1 part of solute


what is practically insoluble?

more than 10,000 parts of solvent required for 1 part of solute


how is solubility epressed in general?

in terms of maximum mass or volume of solute that will dissolve a mass or volume of solvent


what is the unit of solubility?



what is the mass fraction?

calculated as mass of solute/total mass of solution


what is the weight percent?

mass fraction x 100


what can solubility also be expressed as?

percentage e.g. %w/v, %v/v and %w/w


What is molarity?

the number of moles of solute in 1L of solution
solutions of equal molarity contain the same number of solute molecules in a given volume of solution


What is the unit of molarity?



How is Molarity calculated?

weight of substances / molecular mass x L


What is molality?

the moles of solute / kg of solvent
water is easy as 1g = 1mL but othersolvents may be different so need to calculate weight using specific gravity


why is molality the more precise way of expressing solubility?

it is not altered by temperature


what is the mole fraction?

same as the mass fraction, but you are calculating the number of moles, not mass


when are ppn, ppb and ppt used?

for very dilute solutions for expressing solubility


what is ppm?

= mass of solute/mass of solution x 10^6


what is ppb?

= mass of solute/mass of solution x 10^9


what is ppt?

= mass of solute/mass of solution x 10^12


What does a solution form due to?

an interaction between the 2 components


what is a simple way to express this interaction phenomena that causes a solution to form?

like dissolves like


what happens if cohesive forces are greater than adhesive forces?

the solvent will be attracted to each other but the solute will be attracted out, so there is no chance of forming a solution


what happens if one of the cohesive forces are greater than the other? e.g. dissolving NaCl in benzene. the electrostatic forces of NaCl crystal cannot be broken by benzene

the bonds will not break between the solute or solvent molecules


what happens if the adhesive forces between the two components are greater than both cohesive forces of the two components?
e.g. NaCl in water

then the solute will disperse and form a solution


What are the 3 types of solvents?

polar solvents
non polar solvents
semi polar solvents


What are polar solvents?

dilute ionic solute and other polar substances
e.g. glycerol, methanol,
Other structural features which affect the solubility are:
-if the number of polar groups is more in the solute, it will dissolve in the polar solvent.
-chain length, shorter chains are more soluble
-functional groups, OH groups more soluble
-Branching of C chain. branching increases surface so increases solubility


What are non polar solvents?

they do not form H bonds
Are not able to induce ions due to low dielectric constants
can only dissolve non polar solutes


What are semi-polar solvents?

act as intermediate solvents to bring abut the miscibility of polar and non polar liquids e.g. acetone increasing the solubility of water