**Buffered isotonic solutions**

Tonicity is a measure of effective osmolarity or osmolality in biological cell. The solutions having same osmotic pressure is known as isotonic solutions. Pharmaceutical solutions pass through delicate membranes of body; hence their osmotic pressure is to be adjusted with body fluids. Isotonic solutions cause no swelling or contraction of tissues when administered in the eye, nasal tract, blood or any other body tissues. Osmotic pressure of 0.9% NaCl and 5% dextrose solution is isotonic with blood and tears.

Buffered isotonic solutions are solutions that have been adjusted to the same osmotic pressure as body fluids and contain buffering agents to maintain stable pH. Depending on osmotic pressure of solutions, buffered isotonic solutions can be divided in three types.

**Types of buffered isotonic solutions**

**Isotonic:** When the osmotic pressure of the solution is same with the body fluids, it is known as isotonic solutions. A small quantity of blood is mixed with solution containing 0.9% solution of sodium chloride, has same osmotic pressure. Blood cells retain their normal shape and size in isotonic solution. Some examples of the standard isotonic solutions are,

- 0.9% w/v normal saline solution (sodium chloride)
- 5.0% w/v solution of dextrose
- 2.0% w/v solution of boric acid

**Hypertonic: **When the osmotic pressure of the solution is more than the body fluids, it is known as hypertonic solutions. A small quantity of blood is mixed with 2% solution of sodium chloride, the water from cells passes through semi permeable membrane to dilute the surrounding hypertonic solution. This outward passage of water causes cell shrinkage and cells become wrinkled or crenated. Some examples of the hypertonic solutions are,

- 2.0% w/v normal saline solution (sodium chloride)
- 10.0% w/v solution of dextrose
- 4.0% w/v solution of boric acid

**Hypotonic solutions: **When the osmotic pressure of the solution is less than the body fluids, it is known as hypotonic solutions. A small quantity of blood is mixed with 0.2% solution of sodium chloride, water cross the semi permeable membrane and enters into the cell, causing cell swelling. When RBCs are swelled, they finally burst and release haemoglobin, this phenomenon is known as haemolysis. Some examples of the hypotonic solutions are,

- 0.2% w/v normal saline solution (sodium chloride)
- 3.0% w/v solution of dextrose
- 1.0% w/v solution of boric acid

**Isosmotic solutions**

Isosmotic solutions have same osmotic pressure as that of cell contents, but the solute is permeable through cell membrane which alters the tonicity of the cell. Example of isosmotic solution is 1.8% solution of urea. Iso-osmocity does not necessarily mean isotonicity. For example, 1.8% solution of urea have same osmotic pressure as that of 0.9% solution of sodium chloride, but 1.8% solution of urea produces haemolysis due to permeability of solute.

**Applications of buffered isotonic solutions**

**Injection vehicles:** Buffered isotonic solutions and Ringer’s solutions are widely used as vehicles for injection of drugs, vaccines and other purposes. The isotonicity of the solution helps to prevent damage to tissues by maintaining pH of the solution.

**Cell culture media: **Buffered isotonic solutions are regularly used as culture media to provide the necessary nutrients and ions for cell growth and maintenance. For example, phosphate buffer solution.

**Protein and nucleic acid assays: **Buffer systems maintain the pH which is essential for the accuracy and reproducibility of the assays. Buffered isotonic solutions are used as reagents for protein and nucleic acid assays such as western blotting, ELISA and PCR.

**Medical procedures: **These solutions are used for cleansing purpose during medical procedures such as eye rinsing and dental procedures.

**Pharmaceutical formulations: **These solutions are commonly used in nasal sprays, topical solutions and ophthalmic preparations. In ophthalmic preparations these solutions help to treat various eye conditions such as dry eye and glaucoma. The isotonicity of solution helps to prevent irritation and damage to eye.

**Measurement of tonicity**

Isotonicity value is the concentration of an aqueous Sodium chloride solution having same colligative properties with body fluids. Many chemical compounds used in pharmaceutical formulations contribute to the tonicity of the solution. Hence it is important to measure and adjust the isotonicity. There are two methods to measure tonicity of solution; hemolytic method and cryoscopic method.

**Hemolytic method:** In this method the various solutions of drugs is observed on the appearance of RBC suspended in solution. The suspended RBC in various solutions is observed for swelling, bursting, shrinking and wrinkling of the blood cells. In presence of hypotonic solutions, oxyhaemoglobin is released in proportion to the number of cells haemolysed. In presence hypertonic solutions the RBCs shrink and become wrinkled or crenated. In case of isotonic solutions, the RBCs do not change their morphology.

**Cryoscopic method (colligative property): **Isotonicity values can be measured from the colligative properties of the solution. There are four colligative properties, boiling point elevation, freezing point depression (ΔT_{f}), relative lowering of vapour pressure ad osmotic pressure. In the measurement of tonicity, we use freezing point depression colligative property. When some substances (like sodium chloride) are added to water, the freezing point of water (0^{0} C) decreases. Freezing point depression value for blood and tears is -0.52^{0 }C. Hence the value of 0.9% w/v NaCl solution should be -0.52^{0 }C. this solution shows same osmotic pressure equal to the blood.

**Methods of adjusting the tonicity**

Pharmaceutical formulations contain several drugs and excipients. In order to make their solution isotonic, sodium chloride, dextrose or boric acid used. There are two methods to adjust the tonicity of solutions.

**Class I method**

**Cryoscopic method**

In this method isotonic solution is prepared by using the freezing point depression method. The freezing point depression value (ΔT_{f}) for isotonic solution is adjusted to -0.52^{0}C, which is ΔT_{f }value of blood. Using this value isotonicity of the solution is achieved. If it is not possible to adjust the tonicity by changing drug concentration, then adjusting substance is added to achieve tonicity. The quantity of adjusting substance can be calculated as follows,

Total drug solution ΔT_{f }= ΔT_{f }of drug + ΔT_{f }of adjusting substance

ΔT_{f} for adjusting solution = w X a

Where w = weight of adjusting substance in grams per 100 ml

a = ΔT_{f} of the adjusting substance (sodium chloride 0.58)

for an isotonic solution weight of the adjusting substance (w) is calculated as

-0.52^{0}C = d+wa (where d = x X ΔT_{f }of 1% drug solution)

w = 0.52- d/a

w = 0.52- d/0.58

This equation is valid if 1% of drug solution is used. For any other given percentage strength of medicament (PSM), the modified equation is

w = [0.52- (PSM X d)]/0.58

if there are two or more substances are present, sum of their freezing point depression is considered.

**Sodium chloride equivalent method**

Sodium chloride equivalent (E) of drug means amount of sodium chloride equivalent to 1 gram of drug having same osmotic pressure as that of the drug.

E = (17 X L_{iso}) / M

Where, L_{iso} = freezing point depression of drug solution for showing isotonicity

M = molecular mass

After calculating E value, the percentage of sodium chloride require for adjusting tonicity can be calculated by following equation (valid for 100 ml),

PSA = 0.9 – (PSM E of drug)

Where, PSM = percentage strength of medicament

PSA = percentage of sodium chloride for adjustment of isotonicity

**Class II method**

In this type of methods water is added as solvent to make the preparation isotonic. Then this preparation is diluted with buffered isotonic solution to make final volume. There are two types of this method, White-Vincent method and Sprowls method.

**White-Vincent method**

White and Vincent developed a mathematical equation for calculating isotonicity. The equation is derived as,

To prepare 30 ml of 1% (w/v) solution of procaine hydrochloride isotonic with body fluid (0.3 gm),

V = weight of drug X E of drug

V = 0.3 X 0.21 =0.063 gm

The volume (V) of isotonic solution that can be prepared from 0.063 gm of NaCl is obtained by,

0.9 / 100 ml = 0.063 / V

V = 0.063 X 100 / 0.9 = 7 ml

**Sprowls method**

It is the simplification of the White-Vincent method. In this method table of values of V and weight (W) of the drug is prepared. The volume of isotonic solution, that can be prepared by mixing 0.3 gm of drug with sufficient water can be calculated. The quantity of isotonic solution is diluted to final volume desired isotonic solution.

**Conclusion**

The balance between isotonicity and buffer capacity is essential for ensuring the safety and efficacy of drugs. There are various methods to adjust isotonicity like, cryoscopic method, sodium chloride equivalent method, White-Vincent method and Sprowls method. These methods are used to adjust the isotonicity which ensures the physiological compatibility in pharmaceutical formulations.

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