Isotonic solution definition effect and calculation

Isotonic solutions, Definition and Effect of paratonic solution, Hypertonic solution, and hypotonic solution Calculation to prepare an isotonic solution using freezing point methods, and molecular weight methods.

PHARMACEUTICS

Alok Bains

9/14/20235 min read

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Isotonic solutions, Calculation based on freezing point and molecular weight

Isotonic Solutions: A solution with osmotic pressure equal to the osmotic pressure of blood plasma or body fluid is called isotonic solution. Two solutions with equal osmotic pressure are called iso-osmotic solutions. But, if the osmotic pressure of a solution is compared with the osmotic pressure of body fluid and is equal then it is called an isotonic solution.

A solution with osmotic pressure not equal to the osmotic pressure of body fluid is called a paratonic solution. A solution with osmotic pressure more than the osmotic pressure of body fluid is called a Hypertonic solution. A solution with osmotic pressure less than the osmotic pressure of body fluid is called a Hypotonic solution.

Effects of Paratonic Solutions: Injectables, ophthalmic preparations, etc. that come in direct contact with the blood plasma or body fluids should be the isotonic solution. Paratonic solutions cause discomfort or irritation upon administration parenteral route or ophthalmic route.

Effect of Hypertonic Solutions: Hypertonic solutions have a higher solute concentration than in body fluids. Direct contact of hypertonic solution with body fluids affects the overall physiology of the human body. The following are the Primary effects.

Osmosis: Hypertonic solution inside body fluids develops a higher concentration of solutes around cells present in the body fluids such as blood cells. This develops osmotic pressure. Water inside cells comes out to maintain equilibrium under osmotic pressure. This causes cell dehydration.
Cellular dehydration: Injection of hypertonic solution develops cellular dehydration. This decreases or blocks the normal physiological functions of the cells.
Increased blood volume: Injection of hypertonic solutions draws fluids from the cells. This increases the blood volume. However, this effect is useful to treat hypovolemia (Low blood volume) or shock.
Electrolyte imbalance: Injection of the hypertonic solution increases the electrolyte concentration in body fluid. This leads to electrolyte imbalance.
Increased urine output: The body tries to excrete excess electrolytes through urine. This leads to an increase in urine output. This also causes loss of water from the body. This further develops dehydration.

Effect of hypotonic solutions: Hypotonic solutions have a lower concentration of solutes than in body fluids. Direct contact of the hypotonic solution with the body fluids affects the normal physiology cells inside body fluids. Direct contact of hypotonic solution with body fluids is more dangerous than hypertonic solutions. The following are the Primary effects.

Osmosis: Hypotonic solutions inside body fluid develop low concentrations of solutes around cells. Water will move from the body fluid into cells to equalize solute concentration under osmotic pressure. This results in swelling in cells.
Cell swelling: Excessive swelling in cells is dangerous. Cells become non-functional.
Cell burst: Excess swelling of cells may lead to a cell burst. Burst of blood cells is called hemolysis. This results in anemia and kidney damage.
Electrolyte balance disruption: Hypotonic solutions dilute the body fluid. This leads to electrolyte imbalance in the body fluid. This disrupts the vital functions of the body such as nerve function, muscle function, heart function, etc.
Cerebral edema: Hypotonic solutions may lead to swelling in brain tissues. It is mainly due to the influx of water into brain cells. It is a life-threatening situation.

The following pharmaceutical preparations should be isotonic solutions.

Nasal drops, the paratonic solution cause irritation in the nasal cavity,
Ophthalmic preparations such as eye lotion. Paratonic solution develops irritation in the eyes that may damage eye cells. Ophthalmic preparations should be isotonic with the Lachrymal secretion (tear) in the eyes.
Parenteral preparations especially large volume parenteral (Large volume injection).
- i. Intravenous injection should be an isotonic solution,
- ii. Subcutaneous injection should be isotonic. But slight variation in isotonicity is allowed because very small amounts of medicines are injected through subcutaneous injection,
- iii. Intra-muscular injection should be slightly hypertonic. Hypertonic injection promotes the absorption of drugs from the muscles.
- iv. Intrathecal injection should be isotonic.

Calculation for preparation of isotonic solution:

The following two methods are used to prepare isotonic solution

i. Calculation based on the freezing point of the solution
ii. Calculation based on the molecular weight of medicine.

Calculation based on freezing point of the solution: Dissolution of solute affects colligative properties of the solution. Depression of freezing point and osmotic pressure are colligative properties of solutions. Both are interlinked. Thus freezing point of the solution is used for calculation to prepare isotonic solution. Solutions with the same freezing point will have the same osmotic pressure. Lachrumal secretion has solutes. The freezing point of lachrymal secretion is -0.52 degrees C. Any solution with a freezing point of -0.52 degrees C will be isotonic with the lachrymal secretion. Human blood plasma has solutes. The freezing point of human blood plasma is -0.52 degrees C. Any solution with a freezing point of -0.52 will be isotonic with the blood.

The hypotonic solution will have a higher freezing point than blood plasma and tears. The hypertonic solution will have a lower freezing point than blood plasma and tears. The Addition of solute into water or hypertonic solution to adjust the freezing point of -0.52 degrees C makes the solution isotonic or iso-osmotic with blood plasma and tears.

The following formula is used to adjust the freezing point of the solution is -0.52 degrees C or to adjust the isotonicity of the solutions.

W = (0.52-a)/b

· W = Weight of solute in gm to be added in 100 mL solution (% w/v of the adjusting substance).
· a = Freezing point of solution to make isotonic
· b = Freezing point of a 1% w/v solution of adjusting solute.

Q. Calculate the amount of sodium chloride to make an isotonic solution in distilled water. OR Calculate the amount of sodium chloride to make a normal saline solution. The freezing point of 1% sodium chloride solution is -0.576 degrees C.