Isotonic and buffer solutions

University of thi-qar
College of Pharmacy
Pharmaceutical calculation
Isotonic and Buffer Solutions
Lec (3)
Hussein kadhim alkufi
BSc pharmacy
MSc pharmaceutics

Isotonic and Buffer Solutions
It is generally accepted that for ophthalmic and
parenteral administration, isotonic solutions are
better tolerated by the patient than those at the
extremes of hypo- and hypertonicity. Most
ophthalmic preparations are formulated to be
isotonic. Injections that are not isotonic should be
administered slowly and in small quantities to
minimize tissue irritation, pain, and cell fluid
imbalance.

Osmosis
is a phenomenon of passing of a solvent
through a semipermeable membrane from a
dilute solution into a more concentrated one,
which makes the concentrations equalized.

Osmotic pressure
The pressure responsible for passing of a solvent
through a semipermeable membrane and is depend
on the number of particles of solute(s) in a solution;
the total number of particles of a solute in a solution
is the sum of the undissociated molecules and the
number of ions into which the molecules dissociated.

• isosmotic is a term use to describe two solutions
that have the same osmotic pressure.
• Isotonic solution: A solution having the same
osmotic pressure as a specific body fluid (meaning
of equal tone with that specific body fluid).
• Hypotonic Solution: is a solution of lower osmotic
pressure than that of a body fluid.
• Hypertonic solutions: the solutions having a higher
osmotic pressure than that of a body fluid.

• (b) Hemolysis is caused by water movement into a
cell in a hypotonic solution. Excess infusions of
hypotonic fluids can result in the osmotic hemolysis
of red blood cells and surpass the upper limits of
the body’s capacity to safely absorb excessive
fluids.
• (c)Shrinking is caused by water movement out of a
cell in a hypertonic solution. Large volumes of
hypertonic infusions containing dextrose, for
example, can result in hyperglycemia, osmotic
diuresis, and excessive loss of electrolytes.

Using of freezing points to determining
tonicity of solutions:
Like osmotic pressure, the other properties of
solutions (vapor pressure, boiling point, and freezing
point) depend on the number of particles in solution.
Therefore, these properties are interrelated and a
change in any one of them will result in a
corresponding change in the others. Theoretically, any
one of these properties may be used as a basis for
determining tonicity. Practically and most
conveniently, a comparison of freezing points is used
for this purpose.

For nonelectrolyte
1. The freezing point of both blood , serum and
lacrimal fluid is about -0.52˚C.
2. The scientist found When one gram molecular
weight of any nonelectrolyte, is dissolved in 1000 g
of water, the freezing point of the solution is about
(- 1.86 ˚C).for example boric acid, has a molecular
weight of 61.8; so one gram molecular weight is
61.8 g. thus (in theory),dissolved 61.8 g in 1000 g
of water should produce a freezing point of -
1.86˚C.
3. Therefore: by make comparison between
1(freezing point of body fluid) and 2 (freezing point
of boric acid, will find

According to theory when dissolved 17.3 g of
boric acid in 1000 g of water the result solution,
will has the same tonicity of body fluid (blood
serum and lacrimal fluid), this solution is
isotonic solution.

For electrolyte
Because the electrolyte particles dissociated into more
number of particles, the osmotic pressure depend on
degree of dissociation which represent by dissociated
factor (i) If the number of ions is known, we may use the
following values as dissociated factor, lacking better
information :
Nonelectrolytes and substances of slight dissociation :
1.0
Substances that dissociate into 2 ions: 1.8

To calculate the weight of electrolyte required to
prepare isotonic solution use following equation:
Example: how many gram of NaCl required to prepare
one liter of NaCl isotonic solution (m.w. 58.5 and
i=1.8)?

Example Calculations of the i Factor
Zinc sulfate is a 2-ion electrolyte, dissociating 40% in a
certain concentration.
Calculate its dissociation (i) factor.
On the basis of 40% dissociation, 100 particles of zinc
sulfate will yield:
40 zinc ions
40 sulfate ions
60 undissociated particles
=140 particles
Because 140 particles represent 1.4 times as many
particles as were present before dissociation, the
dissociation (i) factor is 1.4, answer.

Example Calculations of the Sodium
Chloride Equivalent
The sodium chloride equivalent of a substance may be
calculated as follows:
Papaverine hydrochloride (m.w. 376) is a 2-ion
electrolyte, dissociating 80% in a given concentration.
Calculate its sodium chloride equivalent.
Because papaverine hydrochloride is a 2-ion
electrolyte, dissociating 80%, its i factor is 1.8.

Calculate the sodium chloride equivalent for glycerin,
a nonelectrolyte with a molecular weight of 92.

Calculations of Tonic Agent Required
The procedure for the calculation of isotonic solutions
with sodium chloride equivalents may be outlined as
follows:
Step 1. Multiply the amount (in grams) of each substance
by its sodium chloride equivalent. The result is amount of
sodium chloride represented by the ingredients in the
prescription.
Step 2. Calculate the amount (in grams) of sodium
chloride, alone, that would be contained in an isotonic
solution of the volume specified in the prescription,
namely, the amount of sodium chloride in a 0.9% solution
of the specified volume. (Such a solution would contain
0.009 g/mL.)

Step 3. Subtract the amount of sodium chloride
represented by the ingredients in the prescription
(Step 1) from the amount of sodium chloride, alone,
that would be represented in the specific volume of
an isotonic solution (Step 2). The answer represents
the amount (in grams) of sodium chloride to be
added to make the solution isotonic.
Step 4. If an agent other than sodium chloride, such
as boric acid, dextrose, or potassium nitrate, is to be
used to make a solution isotonic, divide the amount
of sodium chloride (Step 3) by the sodium chloride
equivalent of the other substance.

How many grams of sodium chloride should be used in compounding the following
prescription?
Rx
Pilocarpine Nitrate 0.3 g
Sodium Chloride q.s.
Purified Water ad 30 mL
Make isoton. sol.
Sig. For the eye.

How many grams of boric acid should be used in compounding the following prescription?
Rx
Phenacaine Hydrochloride 1%
Chlorobutanol 1⁄2%
Boric Acid q.s.
Purified Water ad 60
Make isoton. sol.
Sig. One drop in each eye.
The prescription calls for 0.6 g of phenacaine hydrochloride and 0.3 g of chlorobutanol.

Using an Isotonic Sodium Chloride Solution
to Prepare Other Isotonic Solutions
A 0.9% w/v sodium chloride solution may be used to compound
isotonic solutions of other drug substances as follows:
Step 1. Calculate the quantity of the drug substance needed to
fill the prescription or medication order.
Step 2. Use the following equation to calculate the volume of
water needed to render a solution of the drug substance
isotonic:
Step 3. Add 0.9% w/v sodium chloride solution to complete the
required volume of the prescription or medication order.

Ex : Using this method, determine the volume of
purified water and 0.9% w/v sodium chloride solution
needed to prepare 20 mL of a 1% w/v solution of
hydromorphone hydrochloride (E = 0.22)

Use of Freezing Point Data in Isotonicity
Calculations
Freezing point data (ΔTf) can be used in isotonicity
calculations when the agent has a tonicic effect and
does not penetrate the biologic membranes in
question (e.g., red blood cells). As stated previously,
the freezing point of both blood and lacrimal fluid is -
0.52°C. Thus, a pharmaceutical solution that has a
freezing point of -0.52°C is considered isotonic.
Freezing point depression data may be used in
isotonicity calculations

Buffers and Buffer Solutions
Buffer solution: is a system, usually an aqueous
solution, that possesses the property of resisting
changes in pH with the addition of small
amounts of a strong acid or base.
Their ability to resist changes in pH is referred to
as buffer action; their efficiency is measured by
the function known as buffer capacity.
Buffers are used to establish and maintain an ion
activity within rather narrow limits.

In pharmacy, the most common buffer systems are
used in:
1- the preparation of such dosage forms as injections
and ophthalmic solutions, which are placed directly
into pH-sensitive body fluids.
2- the manufacture of formulations in which the pH
must be maintained at a relatively constant level to
ensure maximum product stability.
3- pharmaceutical tests and assays requiring
adjustment or maintenance of a specific pH for
analytic purposes.

• A buffer solution is usually composed of a weak
acid and a salt of the acid, such as acetic acid and
sodium acetate, or a weak base and a salt of the
base, such as ammonium hydroxide and
ammonium chloride.
• Typical buffer systems that may be used in
pharmaceutical formulations include the following
pairs: "acetic acid and sodium acetate"," boric acid
and sodium borate", and "disodium phosphate and
sodium acid phosphate".

Derivation of The Buffer equation from dissociation
constant, or Ka value equation.
Ka equation of a weak acid:
Kb for weak base

pKa Value of a Weak Acid with Known
Dissociation Constant

pH Value of a Salt/Acid Buffer System
Calculating the pH value:
What is the pH of a buffer solution prepared with 0.05 M sodium
borate and 0.005 M boric acid? The pKa value of boric acid is 9.24 at
25°C.
Note that the ratio of the components of the buffer solution is given in
molar concentrations. Using the buffer equation for weak acids:

pH Value of a Base/Salt Buffer System

Molar Ratio of Salt/Acid for a Buffer System
of Desired pH

Quantity of Components in a Buffer
Solution to Yield a Specific Volume
Calculating the amounts of the components of a
buffer solution required to prepare a desired volume,
given the molar ratio of the components and the
total buffer concentration:

The efficiency of buffer solutions—that is, their specific
ability to resist changes in pH—is measured in terms of
buffer capacity; the smaller the pH change with the
addition of a given amount of acid or base, the greater
the buffer capacity of the system.
Among other factors, the buffer capacity of a system
depends on:
1. the relative concentration of the buffer components
2. the ratio of the components. For example, a 0.5 M-
acetate buffer at a pH of 4.76 would have a higher
buffer capacity than a 0.05 M-buffer.

If a strong base such as sodium hydroxide is added to
a buffer system consisting of equimolar
concentrations of sodium acetate and acetic acid, the
base is neutralized by the acetic acid forming more
sodium acetate, and the resulting increase in pH is
slight.
Actually, the addition of the base increases the
concentration of sodium acetate and decreases by an
equal amount the concentration of acetic acid. In a
similar manner, the addition of a strong acid to a
buffer system consisting of a weak base and its salt
would produce only a small decrease in pH.

Change in pH with Addition of an Acid or
Base

Isotonic and buffer solutions

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