Isotonic solutions
Objective
At the end of this
lecture, student will be able to
• Explain osmosis phenomenon
• Predict the effect of paratonic solutions on blood cells
• Calculate quantities for preparation of isotonic solutions
by various methods
Introduction
• Osmosis –
movement of solvent molecules from Lower concentration to higher concentration
until equilibrium is established
• Pressure driving this movement – Osmotic pressure
• Osmotic pressure is driven by
– Number of particles of solute in solution
– Nonelectrolytes – Solute concentration
– Electrolytes – Solute conc and degree of dissociation
• Solutions with same conc of particles- solutions exerting
same osmotic pressure
• Isotonic or equal
tone – solution which is iso-osmotic with blood and tears
• Normal saline –
0.9% solution of NaCl Isotonic
• Hypotonic –
lower osmotic pressure than body fluids
• Hypertonic –
Higher osmotic pressure than body fluids
Effect of paratonic
(hypertonic/hypotonic) solutions on RBC
Freezing
point depression method
• The freezing point of body fluids is -0.52⁰C
• Dissolved substances in body fluids (plasma and tears)
depress the freezing point
• Any solution that has freezing point -52⁰C is isotonic
with body fluids
Percentage W/V of
adjusting substance needed = 0.52-a/b
a = freezing point of the unadjusted solution
b= freezing point of a 1%w/v solution of the adjusting
substance
Example:
What concentration of procaine hydrochloride will yield a
solution iso osmotic with blood plasma?
Freezing point of 1%w/v solution of procaine HCl is 0.122⁰C
%w/v of procaine HCl required = 0.52 – 0/0.122
= 4.26%w/v
Practice problems
• Find the concentration of sodium chloride required to
render a 1% solution of cocaine HCl iso osmotic with blood plasma. Freezing
point of 1%w/v solution of cocaine HCl & 1% NaCl is 0.09⁰C and -0.576⁰C
resp.
• Find the concentration of NaCl required to render a 1.5%
solution of procaine HCl iso osmotic with plasma. Freezing point of 1%w/v
solution of procaine HCl & 1% NaCl is -0.122⁰C and -0.576 ⁰C resp.
Molecular
concentration method
• At normal temperature and pressure, one gram molecule of a
non-ionizing solute in 22.4 litres has an osmotic pressure of one
atmosphere.
• A solution containing one gram molecule in one litre will
have an osmotic pressure of 22.4 atmospheres.
• Osmotic pressure of body fluids ≈ 6.7 atm
• Molarity of body fluids = 6.7/22.4 = 0.3M
I gm mol. wt in 22.4 liters
—- > 1 atm
1 gm mol. Wt in 1 litre —- > 22.4 atm
22.4 atm —- > 1 gm mol. Wt (1 M)
6.7 atm —-
> ?
i.e, 6.7 / 22.4 = 0.3 M
Therefore it can be said that, 0.3M solution of any
non-ionizing solution will be iso-osmotic with plasma and tears
W = 0.3 M / N
W = Concentration required in grams / litre
M = Mol. wt of the solute
N = No. of ions produced from each molecule of the solute
assuming that dissociation is complete
Example: Find the
concentration of NaCl required to produce a solution iso osmotic with blood
plasma. Mol. wt of NaCl is 58.5 and it dissociates into 2 ions.
0.3 X 58.5/2 = 8.8 g/Lt or 0.9% w/v
Practice problems
• Find the concentration of dextrose required to make a
0.12% solution of NaCl iso osmotic with blood plasma.
• Give the formula for 500 ml of an intravenous solution containing
2% of anhydrous dextrose and 0.5% KCl and made iso osmotic with blood plasma.
Mol. Wt of dextrose = 180;
Mol. Wt of KCl = 74.5
Mol. Wt of NaCl = 58.5
Sodium
chloride equivalent method
• Specified concentration of the medicament converted to concentration
of NaCl producing the same osmotic effect
Eg. The freezing point depression produced by 1% w/v
ascorbic acid solution is 0.105 ⁰C. Freezing point depression of 1%NaCl
solution = 0.576 ⁰C
NaCl equivalent of 1% ascorbic acid = 0.105/0.576 = 0.18
Percentage of NaCl for adjustment to isotonicity is given by
Practice problems
• Calculate the % of NaCl required to render a 0.5% solution
of KCl iso osmotic with with blood
plasma. NaCl equivalent of 1% KCl solution = 0.76
0.9 – (percentage strength of medicament X NaCl equivalent
of 1% medicament)
• Calculate the % of anhydrous dextrose required to render a
1% solution of ephedrine HCl iso osmotic with body fluids. NaCl equivalent of
1% ephedrine HCl solution = 0.3
Summary
• Osmosis –
movement of solvent molecules from Lower concentration to Higher concentration
– Untill equillibrium is established
• Pressure driving this movement – Osmotic pressure
• Isotonic – same
osmotic pressure than body fluids
• Hypertonic –
higher osmotic pressure than body fluids
• Hypotonic solutions
– lower osmotic presssure than body fluids
• Adjustment of
isotonicity by
Freezing point
depression method
Percentage W/V of adjusting substance needed = 0.52-a/b
Molecular
concentration method
W = 0.3 M / N
Sodium chloride
equivalent method
0.9 – (percentage strength of medicament X NaCl equivalent
of 1% medicament)