Solutions Flashcards
What are some advantages of pharmaceutical liquids?
*easier administration
*increased rate of drug absorption
*high bioavailability if IV
*taste of medicines masked
What are some disadvantages of pharmaceutical liquids?
*difficult to formulate drugs that possess poor chemical stability
*difficult to formulate liquids for drugs with poor solubility
*require specialist formulation techniques
What factors affect drug solubility?
*crystal/liquid properties
*hydration
*molecular size
*properties of solvent
*pH of solvent
What are the effects of substituents on solubility?
*substituents can have effect on properties of the solid or liquid- molecular cohesion
*substituents have different interactions with water molecules
*substituents can be classified as hydrophobic or hydrophilic
*N, O containing molecules are hydrophilic
*position of substituents can effect stability- para has low solubility due to greater stability of crystalline structure
What does solvation mean?
Binding of solvent to solute
What is hydration?
Binding of water to solute
Are acids more soluble in acidic or basic solutions?
Basic
What is the equation for the dissociation of acid in water?
HA + H20 >< H3O+ +A-
What is the equation for dissociation constant in acids?
Ka= [H3O+][A-]/[HA]
What is the equation for solubility of a base in water?
RNH2 + H20 ><RNH3+ +OH-
What is the equation for solubility of a base?
Kb= [RNH3+][OH-]/[RNH2]
What is an amphoteric drug?
One that possesses both acidic and basic properties
What is the generalised structure of an amphoteric drug?
RXNH2COOH
What is the structure of RXNH2COOH at low pH?
RXNH3+COOH
What is the structure of RXNH2COOH at high pH?
RXNH2COOH-
What is the zwitterion of RXNH2COOH?
RNXH3+COO-
How do you calculate the % ionisation of a drug given pH and concentration?
- Convert pH to [H+
pH is -log[H+]
so do 1/10^pH
then x1000 to give in mmol - Use given concn or calculate [HA]
- Calculate %ionisation by doing [H+]/[HA] x 100
What is a co solvent?
Substances added to primary solvent (water) to increase the solubility of poorly water soluble drug.
Examples- ethanol, glycerol, propylene glycol
What is the solubility parameter?
Thermodynamic measure of cohesive forces in a substance
Maximum solubility= Solubility parameters of solvent and solute identical
What is the dielectric constant?
Maximum solubility is achieved in a solvent at defined dielectric constant.
Dielectric constant is Cx (material in condenser)/ Co (standard).
What is a surfactant and how does it improve solubility?
Surfactants reduce surface tension and increase spreading/wetting properties.
*examples= spans, tweens, phospholipids (in combination)
*used for vitamins and steroids
What is a hydrotrope?
Compound that solubilises hydrophobic compounds in aqueous solutions by means other than micellar solubilisation. Hydrotropy is the process of adding a secondary solute which increases aqueous solubility of the primary solute.
What are examples of hydrotropes?
Sodium benzoyl
Sodium salicylate
What are methods used to increase solubility of drugs?
*hydrotropy
*cyclodextrins
*surfactants
*co-solvents
What are added excipients in liquids?
*vehicle
*preservatives- parabens, benzoic/sorbic acid, benzyl alcohol
*sweetening agents- sucrose common but can cause cap locking.
, aspartame and saccharin.
*viscosity control-optimise pourability. Hydroxyethyl or …cellulose. Modify sugar conc
*flavours-menthol, chloroform
*colours
