Osmosis and Iso-tonic Solution


Tonicity is the measure of effective osmotic pressure gradient.

In biology: how an extracellular solution can change the volume of a cell by affecting osmosis and it is directly correlates with the osmolarity of the solution.


If the solution of different concentration are separated by semi-permeable membrane, the solvent is move from the solution of lower solute concentration to the solution of higher solute concentration.

Semi-permeable membrane

The membrane with allow movement of solvent, but resist the passage of dissolved substances.

There are three types of osmosis solution;

1.      Isotonic solution

2.      Hypertonic solution

3.      Hypotonic solution

Isotonic solution

An isotonic solution has the same osmolarity or same solute concentration. The water flows in equal parts of each solution.

Example: if the red blood cell is in contact with a solution that has the same osmotic pressure as that of blood plasma, the cell wall will neither swell nor shrink. Therefore the solution is said to be “isotonic”.

Blood plasma contains 0.88% of inorganic salts, mainly sodium chloride (NaCl), which is the main reason to osmotic pressure.

A solution containing 0.9% of sodium chloride is practically isotonic with blood plasma and state that standard. Hence a solution containing more than 0.9% sodium chloride is called “Hypertonic”, and solution containing less than 0.9% than that is called “Hypotonic”.

Hypertonic solution

A solution containing higher concentration on the outside of the cell, than the inside of the cell called hypertonic solution. The water is liberated from the cell and the cell becomes shrink.

Hypotonic solution


The solute concentration of a solution is greater inside the cell, than that of the outside of the cell, so the solvent moves from outside of the cell to inside of the cell, this phenomenon is called hypotonic. Due to higher entry of water molecules the cell becomes turgid or burst.

General principle for adjustment to Iso-tonicity

1.      Solution for IV injection (Intravenous injection): Iso-tonicity must be required.

2.      Solution for SC injection (subcutaneous injection): this injections are injected into fatty tissue hence the Iso-tonicity is needed but, not essential.

3.      Solution for IM (intra muscular injection): In case aqueous solution should be slight hypertonic to promote rapid absorption.

4.      Solution for intra-cutaneous injection: The parenteral preparations which are meant for diagnosis purpose should be isotonic in order to avoid false reaction.

5.      Solution for intra-thecal injection: These must be isotonic because the volume of cerebro spinal fluid (C.S.F) is only 60 to 80ml. hence a small volume of a Paratonic (solution having not same osmotic pressure) solution will disturbed the osmotic pressure and may cause side effects.

6.      Solution used for nasal drops: Iso-tonicity is needed.

7.      Solution used as eye lotion and eye lotion: Eye lotion should be isotonic with lachrymal secretion, since a large volume is brought in contact with the eye. Eye drop may not be isotonic, because only a small volume is required which quickly get diluted by the lachrymal secretion.

Calculation for solution Iso-osmotic/isotonic with blood and tears

Some methods are used for the calculation:

1.      Method used on freezing point data.

2.      Method based on molecular concentration.

3.      Graphic method based on vapour pressure and freeze point depression.

4.      Method based on sodium chloride equivalent.

Freezing point depression or cryoscopic method

The temperature at which blood plasm and tears freeze is -0.520C. This means that the dissolved substances contained in them depress the freezing point 0.520C below that of pure water. So, any other solution which freezes at -0.520C will have the same osmotic pressure as blood plasma and tears.

In order to make this solution isotonic, an adjusting substance must be added to produce the freezing point to -0.520C. The general formula for calculation for solution to be made Iso-osmotic with blood serum is as follows:

%w/v of adjusting substance needed = 0.52 – a / b                

Where a = freezing point of the un-adjusted solution

b = freezing point of a 1% solution of the adjusting substance.

(Each solute have independent freezing point, if a solution contain more than one solute, then the value of “a” will be the sum of their depression)

Example: Find the concentration of sodium chloride required to make a 1.5% solution of cocaine hydrochloride Iso-osmotic with blood plasma,

(Given: the freezing point of 1% w/v solution of cocaine hydrochloride is -0.090C. The freezing point of 1% w/v solution of sodium chloride is -0.5760C)

By applying formula:

%w/v of sodium chloride required = 0.52 – (0.09 × 1.5) / 0.576 = 0.668% w/v                             

Method based on molecular concentration

Molecular concentration means the number of units that is molecules or ions or both present in a solution. A solution containing on gram molecule of a non-ionising solute in 22.4 liters at normal temperature and pressure (NTP) has an osmotic pressure of one atmosphere.

The osmotic pressure of blood plasma and lachrymal secretion is approximately 6.7 atmosphere.

Hence molarity of these fluid = 6.7 / 22.4 = 0.3 M (approx.)                           

Therefore, a 0.3 M solution of any non-ionising solution will be Iso-osmotic with blood plasma and tears. This may be used to calculate the concentrations of un-ionised medicaments needed to produce Iso-osmotic solutions by using the following formula.

W = 0.3 M

Where W = concentration required in g per liter

            M = molecular weight of the solute

If the solute ionises in solution, the following formula is used:

W = 0.3 M / N

Where N = Number of ions produce from each molecule of the solution assuming that dissociation is complete.

Example: find the proportion of dextrose needed to form a solution Iso-osmotic with blood plasma.

Calculation: Molecular weight of dextrose = 180

Dextrose is non-ionising substance

Hence the formula used is: W + 0.3 M

                                            W = 0.3 × 180

                                                 = 54.0 g/liter or W = 5.4 g/100ml

Those solutions which are not Iso-osmotic, can be made so by adding a suitable adjusting substance, such as, sodium chloride or dextrose in order to produce a total concentration of solutes equivalent to 0.3 M concentration of non-ionising solute. In such case the calculation must be:

1.      To find out the effective molar concentration (EMC)

2.      Subtract it from 0.3 to obtain the effective molar concentration of adjusting substance.

3.      The value is converted to g per liter

Example: find the proportion of sodium chloride required to make 1% solution of a cocaine hydrochloride Iso-osmotic with blood plasma.


Step – 1: Molecular weight of cocaine hydrochloride = 339.5

Cocaine hydrochloride is ionising substance and it gets dissociated into 2 ions.

            By applying the formula: W = 0.3 M / N            

            Let C is the effective molar concentration (EMC).

            Substituting C for 0.3 in the formula

                                                                        W = C × M / N

            Where W is the concentration required in g/liter = 10 g/liter

10 = C × 339.5 / 2

C = 20/339.5 = 0.059 g/liter.

Step -2: Effective molar concentration of adjusting substance i.e. sodium chloride = 0.3 – 0.059

                                                                                                                                    =0.241 g/liter

Step – 3: required concentration of sodium chloride is

W = 0.241 × M / N

Molecular weight of sodium chloride = 58.5

Sodium chloride is dissociated into 2 ions

W = 0.241 × 58.5 / 2 = 7.01 g/liter or 0.7g/100ml i.e. 0.7% w/v

Generally, an error occurs in both the methods of calculation, particularly, when ionising substances are used and also error increases with increase in concentration.

Graphical method used on vapour pressure and freezing-point depression

In this method various concentration of sodium chloride solution are prepared and their freezing point are determined. A graph is prepared from this data by taking percentage concentration against freezing points. Such graphs are prepared for each medicament. From these graphs the percentage of adjusting substances required to make any percentage of medicament isotonic with blood plasma or body fluids can be determined.

Method based on sodium chloride equivalents

Sodium chloride equivalent can be calculated from the following formula.

Sodium chloride equivalent of a medicament = Freezing point depression produced by a solution of a medicament / Freezing point depression produced by sodium chloride solution of the same strength.

            The percentage of sodium chloride required for adjustment to Iso-tonicity can be calculated using the following formula:

Percentage of sodium chloride for adjustment to iconicity = 0.9 – [percentage strength of medicament solution × sodium chloride equivalent of medicament]

Example: calculate the percentage of anhydrous dextrose required to make a 1% solution of ephedrine hydrochloride Iso-osmotic with body fluids.


            Sodium chloride equivalent of 1% ephedrine hydrochloride = 0.30

            Strength of ephedrine hydrochloride = 1% w/v

            Percentage of sodium chloride for adjustment to Iso-tonicity

                                                                        = 0.9 – (1 × 0.3)

                                                                        = 0.9 – 0.3

                                                                        = 0.6

When solution is made Iso-osmotic with body fluids by adding an adjusting substance other than sodium chloride, the above value is divided by the sodium chloride equivalent of the adjusting substance.


Equivalent percentage of anhydrous dextrose = 0.6/0.18 = 3.3

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