Buffered isotonic solutions Flashcards
Define buffers
Acid (or base) and its salt.
Used to minimise the change of pH upon the addition of a small amount of acid or base to the solution
Define osmosis
the spontaneous diffusion of water from the low solute concentration reservoir into a more concentrated solution at constant temperature and pressure
Leads to a decrease in solute concentration
Define colligative properites
properties of solutions that depend upon the number of solvent molecules in a solution and NOT on the type of chemical species present. The number ratio can be related to the various units for concentration of solution
Explain the role of buffers, osmosis and colligative properties in pharmaceutical formulations
Increase in drugs solubility will increase its stability over time and decrease any physiological difference with the site of delivery which may result in undesired secondary effects
What is the buffer range
The range of pH values for which the efficiency of a buffer is the highest
0.1
Write the Hendersson-Hasselbach equations
pH=pKa+log(salt/acid)
pH=pKa+log(base/salt)
Explain the role of buffers in oral solutions
usually controlled by the salt form or by- -Hydrochloric acid -tartaric acid benzoic acid citric acid
Explain the role of buffers in injectable solutions
Drug solubilised at desired concentration by pH adjustment (provided pKa is far enough from the formulation pH)
What is the acceptable pH range for IV and IM injection solutions?
2-12
What is the acceptable pH range for subcutaneous injection solutions?
2.7-9 because dilution rate is reduced and to reduce irritation at the injection site
How is the pH of injectable solutions controlled?
either
- a salt form of the drug
- strong acids/bases such as HCl and NaOH
- or a buffer
Name 8 buffers used for injectable solutions
Glycine Citrate Acetate Histidine Phosphate Borate TRIS Carbonate
Why does a buffer maintain the solubility of the therapeutic agent in a drug solution?
Because solubility is pH dependent
Chemical stability is dependant on what 2 factors?
pH and hence buffer dependent
The pH of a buffer extends +/- 1 pH unit around the..
pKa
What is an isotonic solution?
Solution with the same osmolarity either part of a membrane
How are solutions made isotonic?
Via addition of an adjusting substance e.g. NaCl
Define osmotic pressure
Pressure applied to oppose swelling of a membrane caused by water movement
osmotic pressure is a colligative property
What does isotonic correct depend upon? (2 factors)
Type of membrane
Type of drug
An osmotically active particle may be…
an ion or a molecule. Basically a particle dissolved regardless of its charge, weight or size
When does osmolarity=molarity?
for uncharged particles
For NaCl 1M, what is the osmolarity and why?
2 osmolar as two ions present
Osmolarity is a property of…
independent of…
a solution
type of membrane
Human plasma osmolarity =
320mOsmol
Define tonicity
measure of osmotic pressure exerted on a membrane
thus specific to the type of membrane
Two solutions on either side of a membrane with the same tonicity are…
isotonic
As it relates to the osmotic pressure… what kind of molecules are the only ones considered in a calculation for tonicity
non-permeant molecules
3 Methods to alter tonicity?
freezing point depression
sodium chloride equivalent method (E value)
white-vincent method/USP method
What is freezing point depression?
The difference between the freezing point (fp) of water and the fp of the solution
What is the freezing point of blood?
0.52 degrees celcius
If you adjust the tonicity of a solution for the body to that of blood you get
an isotonic solution
What adjusting substance can be added when adjusting the tonicity of a drug solution?
NaCl
Equation for freezing point (fp) depression of an isotonic solution
fp(drug) + fp(adjusting substance) = fp(blood)
if equation is correct then solution is isotonic
