Inorganic Chemistry (18) Calculations and Method of Adjusting Isotonicity of Solution | Buffers

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Contents for 2nd Unit of Inorganic Chemistry -

(01) Acids, Bases and Buffers: Buffer equations and buffer capacity in general, buffers in pharmaceutical systems, preparation, stability, buffered isotonic solutions, measurements of tonicity, calculations and methods of adjusting is tonicity.

(02) Major extra and intracellular electrolytes: Functions of major physiological ions, Electrolytes used in the replacement therapy: Sodium chloride*, Potassium chloride, Calcium gluconate* and Oral Rehydration Salt (ORS), Physiological acid-base balance.

(03) Dental products: Dentifrices, the role of fluoride in the treatment of dentalcare, Desensitizing agents, Calcium carbonate, Sodium fluoride, and Zinc eugenol cement.

Methods of adjusting Tonicity and pH
Several methods are used to adjust the isotonicity of pharmaceutical solutions. Isotonicity can be calculated from the colligative properties of drug solutions. If solutions are injected or introduced in to eyes and nose, these are to be made isotonic in order to avoid haemolysis of RBC’s and to avoid pain and discomfort. This is possible for either manufactured or extemporaneous prepared solutions. By using the appropriate calculations based on colligative properties of solutions, it is easy to determine the amount of adjusting agents to be added. It helps to overcome the side effects caused from administering solutions which contain adjusting agents less or more
than isotonic solutions. The three frequently used methods to calculate isotonicity of the solutions are described below.

Class-1 Methods: Nacl or some other substances is added to the solution of the drug to lower the freezing point of the solution to -0.52ᵒC and thus make the solution isotonic.
Examples of this class-
1) Cryoscopic method
2) Sodium chloride equivalent method.

Class-2 Methods: Water is added to the drug in a sufficient amount to make it isotonic. Then the
preparation is brought to its final volume with an isotonic or buffered isotonic solution.
Examples of this class- White Vincent method

Class-3 Methods: Freezing point depression and L iso values for number of drugs are estimated theoretically from the molecular weight of the drug and can be used to calculate the amount of adjusting substance to be added in order to make the solution isotonic.

Cryoscopic method
In this method, the quantity of each substance required for an isotonic solution can be calculated from the freezing point depression values. A solution which is isotonic with blood has a of 0.52ᵒC. Therefore, the freezing point of drug solution must be adjusted tothis valve. In case of drug solutions, if it is not possible to adjust tonicity by altering the drug concentration, then an adjusting substance is added to achieve desired tonicity. The weight (in grams) of adjusting substance can be calculated in manner described below.
For example, the drug concentration in 100ml solution is a grams, then Sodium chloride equivalent method Tonicity equivalent or sodium chloride equivalent method is used to adjust the tonicity of pharmaceutical solutions.

Sodium chloride equivalent method
Sodium chloride equivalent (E) of a drug is the amount of sodium chloride that is equivalent to 1 gm of the drug. The percent of sodium chloride required for adjusting the isotonicity can be calculated using the following equation.
PSA = 0.9 – (PSM E of medicament)
Where,
PSM = Percent strength of medicament
PSA = Percent of sodium chloride for adjustment of isotonicity

The L ISO – Method
The E Nacl value of tonicity adjusting substances can also be calculated from the substances. The L iso values of the tonicity adjusting substances are given in table and are mentioned as constants in many references. In this method, the freezing point depression equation is used to calculate the amount of the isotonicity-adjusting substance that must be added to hypotonic solution of drug to bring to tonicity.

White-Vincent method: This method involves use of addition of water to the solution to make isotonic followed by final volume adjustment with addition of isotonic or isotonic buffered solution. White Vincent, from their study of need of Ph adjustment in addition to tonicity of ophthalmic solution, developed an equation.


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