METHODS OF ADJUSTING TONICITY:
The methods for adjusting tonicity subdivided into two classes: Class I methods, which employ sodium chloride or some other substance to the drug solution to lower the freezing point of the solution to – 0.52° and thus make it isotonic with body fluids. Under this method is included the cryoscopic method and the sodium chloride equivalent method. Class II methods use water that is added to the drug in a sufficient amount to form an isotonic solution. The preparation is then made up to its final volume with an isotonic or a buffered isotonic dilution solution. They include the White–Vincent method and the Sprowls method.
Class I Methods:
Isotonic solutions may be made in terms of data relating to colligative properties of solutions. Colligative properties include osmotic pressure, elevation in boiling point, depression in freezing point, and lowering of vapour pressure. Depression in freezing point is a colligative property which is practical and most convenient for adjusting tonicity. The freezing point of human blood and lacrimal fluids is – 0.52°C. This temperature corresponds to freezing point of 0.90% (w/v) sodium chloride solution. This is considered to be isotonic to blood and lacrimal fluids. The freezing point depression of 1% (w/v) sodium chloride (∆Tf1%) is 0.58°C. In this method, an amount of tonicity adjuster (e.g. sodium chloride) is added to drug solution such that the final freezing point lowering is that of blood or serum (0.52°C).
Sodium chloride equivalent method-
This method is based on calculating the E-value, i.e. the sodium chloride equivalent or tonicity equivalent of a drug. It is the amount of sodium chloride that has the same osmotic effect (i.e. is equivalent to) as 1 g of the drug.
Derivation of E-value: Since freezing point depression is a colligative property, it depends on the number of particles, dissociation and association of particles.
Therefore, the equation ∆Tf = KfC
can be replaced with ∆Tf = LisoC
where, ∆T is the depression in freezing point, Kf is the freezing point depression constant, c is the concentration. Liso is a factor that is equal to iKf, where i is the vant Hoff factor.
Liso = ∆Tf / C
Class II Methods:
The Class II methods of computing tonicity involve the addition of water to the drugs to prepare an isotonic solution, followed by the addition of an isotonic or isotonic-buffered diluting vehicle to make up the solution up to the final volume. White and Vincent developed a simplified method for performing such calculations. The equation is derived as shown below:
To prepare 30 mL of a 1% (w/v) solution of procaine hydrochloride isotonic with body fluid (=0.3 g), weight of the drug w is multiplied by the sodium chloride equivalent E.
This is the quantity of sodium chloride osmotically equivalent to 0.3 g of procaine hydrochloride
= weight of drug (g) × E of drug (1)
= 0.3 × 0.21 = 0.063 g
It is known that 0.9 g of sodium chloride when dissolved in sufficient water sufficient to make a final volume of 100 mL yields an isotonic solution. The volume V of isotonic solution that can be prepared from 0.063 g of sodium chloride (equivalent to 0.3 g of procaine hydrochloride) is obtained by solving the following proportion:
0.9g / 100ml = 0.063g / V
V = 0.063 X 100 / 0.9 = 7.0ml (2)
Accordingly, Eq. (2) can be written as
V = w × E × 111.1 (3)
where V is the volume of isotonic solution (in mL) that may be prepared by mixing the drug with water, w the weight of the drug (in grams) and E the sodium chloride equivalent of the drug. The constant, 111.1, represents the volume of isotonic solution in millilitres obtained by dissolving 1 g of sodium chloride in water.
The isotonic and isotonic-buffered diluting solutions all have isotonicity values of 0.9% NaCl.
A further simplification of the method of White and Vincent was prepared by Sprowls. He recognized that the Eq. (3) given by White and Vincent could be utilized to make a table of values of V when the weight of the drug w was arbitrarily fixed. Sprowls chose 0.3 g as the weight of drug, the quantity for one fluid ounce of a 1% solution. The volume V of isotonic solution that can be prepared by mixing 0.3 g of a drug with sufficient water may be computed for drugs commonly formulate as ophthalmic and parenteral solutions.
The quantity of isotonic solution is finally brought to the specific volume with the desired isotonic or isotonic-buffered diluting solutions.
Multiple choice questions (MCQs)
1.Which can act as buffer?
b)CH3 COOH+H2 CO3
c)40ml of 0.1M NaCN+20ml of 0.1MHCN
d) NaCl + NaOH
2.Which of the following mixture in aqueous solution of equimolar concentration acts as a buffer solution?
b)H2 SO4 +KOH
d) CH3 COOH + NaOH(excess)
3.For an acid buffer solution the pH is 3. The pH can be increased by ____________
a)Increasing the concentration of salt
b)Increasing the concentration of acid
c)Decreasing the concentration of salt
d)Independent of concentration of acid & salt
4.The buffer capacity is equal to __________
d) ± 2pKa
5.Buffer capacity is maximum when __________
a) One mole of NH4Cl is added to two moles of NH4OH
b) One mole of NH4Cl is added to one moles of NH4OH
c) One mole of NH4Cl is added to one mole of NaOH
d) One mole of NaCl is added to one mole of NaOH
6.In term pH, H indicates
7.Maximum buffer capacity equals to
8.In which method tonicity is calculated by adding water to the drugs to make an isotonic solution
a)Sodium chloride equivalent method
c)White Vincent method
9.The tonicity of solution can be determined by
d)Both a and b
10.Cryoscopic method for adjusting tonicity and pH comes under
a)Class 1 method
b)Class 2 method
c)Class 3 method
d)Class 4 method
11.The solution having an osmotic pressure greater than that of 0.9% w/v sodium chloride is called
c)Iso osmotic solution
12.Which of the following methods are used to measure pH value?
d)all of the above
13.The number of osmoles of solute in a liter of solution is called as
14.The colligative properties of a solution are related to
a)pH of the solution
c)total number of solute particles in the solution
d)total number of ions in the solution
15.The pH of the pharmaceutical buffer system can be calculated by
a)pH partition theory
b)Michaelis Menten equation
c)Henderson Hasselbalch equation
- c)40ml of 0.1M NaCN+20ml of 0.1MHCN
- c)NH4 OH(excess)+HCl
- a)Increasing the concentration of salt
- b) One mole of NH4Cl is added to one moles of NH4OH
- a) hydrogen
- a) 0.576C
- c) white vincent method
- d) both a and b
- a) class 1 method
- a) hypertonic solution
- d) all of the above
- a) osmolarity
- c) total number of solute particles in the solution
- c) Henderson Hasselbalch equation
- GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 154-159.
- Martins Physical Pharmacy, 6th edition 2011, page no. 324-331.