Isotonic solutions - Pharmaceutics - I B. Pharma 1st Semester

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)

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