HYPERTONIC SOLUTION: COMPONENTS, PREPARATION, EXAMPLES - CHEMISTRY - 2024

A hypertonic solution is one that when put in contact with another solution, separated by a membrane permeable to water but impervious to solutes, a net flow of water occurs towards it, until an equal osmolarity (concentration) is reached in the two compartments.

A very representative example is when red blood cells are placed in a solution that is considered hypertonic. The osmolarity of erythrocytes, like that of all extra and intracellular body fluids, is approximately 300 mOsm / L.

Interaction of a cell with a hypertonic solution. Source: Gabriel Bolívar.

Therefore, the osmolarity of the hypertonic solution must be greater than 300 mOsm / L. Under this circumstance, a flow of water occurs from inside the erythrocytes into the surrounding solution. The same behavior can be seen in any type of cell and is generally represented in the image above.

Outside the cell there is a greater amount of dissolved solute (yellow circles), so the molecules are busy hydrating them; that is, there are fewer "free" water molecules. The cell yields water to its surroundings, reducing its volume and wrinkling like a raisin. Hence, the water within the cell is more "concentrated" than in the extracellular medium.

Components of hypertonic solutions

A hypertonic solution is composed of a solvent, usually water, and solutes that can be pure salts or sugars, or a mixture of them. The usual way of expressing the concentration of a solution, as a function of the number of particles and not so much of their individual concentrations, is through osmolarity.

Also, there must be a compartment that is separated by a semipermeable barrier, which in the case of cells is a lipid double layer membrane. Water molecules, as well as other neutral molecules, manage to sneak through the cell membrane, but the same does not happen with ions.

The aqueous medium that surrounds the cell must be more concentrated in solute, and consequently more “diluted” in water. This is so because the water molecules are surrounding the solute particles, with few that diffuse freely in the middle.

This variation of free water inside and outside the cell causes a gradient by which osmosis is generated, that is, the variation in concentrations due to displacement of the solvent through a barrier, without the solute diffusing.

Preparation

A hypertonic solution is prepared the same as all solutions: the components of the solution are weighed and brought to a certain volume by dissolving them in water. But to know if the solution is hypertonic in relation to the cells, its osmolarity must first be calculated and see if it is greater than 300 mOsm / L:

Osmolarity = m v g

Where m is the molarity of the solute, v the number of particles into which a compound dissociates, and g the osmotic coefficient. The latter is a factor that corrects the interaction of electrically charged particles (ions) and its value is 1 for dilute solutions and for substances that do not dissociate; like glucose.

The total osmolarity of a solution is calculated by adding the osmolarity provided by each of the compounds present in the solution.

- Example

Determine the osmolarity of a solution containing 5% glucose (MW = 180 g / mol) and 0.9% sodium chloride (MW = 58.5 g / mol) and conclude whether the solution is hypertonic or not.

First step

First you have to calculate the molarity of the glucose. The concentration of glucose is 5 g / 100 mL, and it is expressed in units of g / L:

(5 g ÷ 100 mL) 1,000 mL

Glucose concentration = 50 g / L

Glucose molarity (moles / L) = (50 g / L) ÷ (180 g / mol)

= 0.277 moles / L

Osmolarity provided by glucose = molarity · number of particle in which it dissociates · osmotic coefficient (g).

In this case, the value of the osmotic coefficient is equal to 1 and can be discontinued. Glucose only has covalent bonds in its structure that do not dissociate in aqueous solution, and therefore v is equal to 1. Thus, the osmolarity of glucose is equal to its molarity.

Osmolarity provided by glucose = 0.277 Osm / L

= 277 mOsm / L

Second step

We calculate the molarity and osmolarity of the second solute, which is NaCl. We also express its concentration in g / L:

Expressed in g / L = (0.9 g ÷ 100 mL) 1,000 mL

= 9 g NaCl / L

Molarity (moles / L) = (9 g / L) ÷ (58.5 g / mol)

= 0.153 mol / L

And we calculate its osmolarity:

Osmolarity = molarity 2 1

Sodium chloride dissociates into two particles: a Na ⁺ and a Cl ^-. It is for this reason that v has a value of 2.

Osmolarity = 0.153 mol / L · 2 · 1

Osmolarity = 0.306 Osm / L

= 306 mOsm / L

Third step

Finally we calculate the osmolarity of the solution and decide whether or not it is hypertonic. For this, we must add the osmolarity provided by glucose and the osmolarity provided by NaCl:

Total osmolarity of the solution = 0.277 osm / L + 0.306 osm / L

Osmolarity of the solution = 0.583 Osm / L or 583 mOsm / L

The osmolarity of the cells and the fluids that bathe them: plasma and interstitial fluid, is around 300 mOsm / L. Therefore, it can be considered that the glucose and sodium chloride solution, with an osmolarity of 583 mOsm / L, is a hypertonic solution in relation to the cellular environment.

Examples of hypertonic solutions

10% dextrose No. 2 (hypertonic glucose solution)

This hypertonic solution consists of 10 g of dextrose and distilled water in sufficient quantity for 100 mL. Its osmolarity is 504 mOsm / L.

This solution is used to treat a decrease in liver glycogen, a drop in plasma glucose concentration, and other metabolic disorders.

0.45% dextrose

This solution is composed of 5 g of dextrose, 0.45 g of NaCl, and sufficient distilled water for a volume of 100 mL. Its osmolarity is 406 mOsm / L

It is used in the decrease of hepatic glycogen and in the deficiency of sodium chloride.

10% mannitol

This solution consists of 10 g of mannitol and distilled water in sufficient quantity for 100 mL. Its osmolarity is 549 mOsm / L.

It is used to increase the renal excretion of water (osmotic diuretic) and to treat kidney failure.

References

1. De Lehr Spilva, A. and Muktans, Y. (1999). Guide to Pharmaceutical Specialties in Venezuela. XXXVª Edition. Global Editions.
2. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
3. Helmenstine, Anne Marie, Ph.D. (February 11, 2020). What Is a Hypertonic Solution? Recovered from: thoughtco.com
4. Wikipedia. (2020). Tonicity. Recovered from: en.wikipedia.org
5. Kevin Beck. (September 21, 2018). What is Hypertonic Solution. Recovered from: sciencing.com