HYPOTONIC SOLUTION: COMPONENTS, PREPARATION, EXAMPLES - CHEMISTRY - 2025

A hypotonic solution is one that has a lower solute concentration than a solution separated or isolated by a semi-permeable barrier. This barrier allows the solvent to pass through it, water in the case of biological systems, but not all solute particles.

The body fluids of intracellular and extracellular vertebrates have an osmolarity of about 300 mOsm / L. While a hypotonic liquid is considered to have an osmolarity less than 280 mOsm / L. Therefore, a solution of this osmolarity is hypotonic in relation to the cellular environment.

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

An example of a hypotonic solution is that of 0.45% sodium chloride. But how does the cell or a compartment behave in the face of this type of solution? The image above answers this question.

The concentration of solute particles (yellow dots) is higher inside the cell than outside. As there is less solute around the cell, there are more free water molecules, which is why it is represented with a more intense blue color compared to the interior of the cell.

Water flows from the outside to the inside through osmosis to level the concentrations. As a result, the cell expands or swells by absorbing water that passes through its cell membrane.

Components of hypotonic solutions

Hypotonic solutions consist of a solvent that, unless otherwise indicated, consists of water, and solutes dissolved therein such as salts, sugars, etc., in pure or mixed form. But this solution will not have any tonicity if there is no semi-permeable barrier involved, which is the cell membrane.

There must be few dissolved salts so that their concentration is small, while the "concentration" of the water is high. As there is more free water outside the cell, that is, it is not solving or hydrating solute particles, the greater its pressure on the cell membrane and the more it will tend to cross it to dilute the intracellular fluid.

Preparation of a hypotonic solution

For the preparation of these solutions, the same protocol is followed as that followed for other solutions. Make the appropriate calculations of the mass of the solutes. These are then weighed, dissolved in water and taken to a volumetric flask to the corresponding volume.

The hypotonic solution has a low osmolarity, generally less than 280 mOsm / L. So when preparing a hypotonic solution we must calculate its osmolarity in such a way that its value is less than 280 mOsm / L. Osmolarity can be calculated with the following equation:

Osmolarity = m v g

Where m is the molarity of the solute, and v is the number of particles into which a compound dissociates in solution. Non-electrolytic substances do not dissociate, so the value of v equals 1. This is the case for glucose and other sugars.

While g is the osmotic coefficient. This is a correction factor for the interaction of electrically charged particles (ions) in solution. For dilute solutions and non-dissociable substances, for example and again glucose, a value of g is taken equal to 1. It is then said that the molarity is identical to its osmolarity.

Example 1

The 0.5% NaCl solution is brought to gram per liter:

NaCl in g / l = (0.5 g ÷ 100 mL) 1,000 mL

= 5 g / L

And we proceed to calculate its molarity and then determine its osmolarity:

Molarity = mass (g / L) ÷ molecular weight (g / mol)

= 5 g / L ÷ 58.5 g / mol

= 0.085 mol / L

NaCl dissociates into two particles: Na ⁺ (cation) and Cl ^- (anion). Therefore, the value of v = 2. Also, since it is a dilute solution of 0.5% NaCl, it can be assumed that the value of g (osmotic coefficient) is 1. We then have:

Osmolarity (NaCl) = molarity · v · g

= 0.085 M · 2 · 1

= 0.170 Osm / L or 170 mOsm / L

This is a hypotonic solution, since its osmolarity is much lower than the reference osmolarity for body fluids, which is the plasma osmolarity whose value is around 300 mOsm / L.

Example 2

We calculate the molarity having the concentrations of the respective solutes at 0.55 g / L and 40 g / L:

Molarity (CaCl ₂) = 0.55 g / L ÷ 111 g / mol

= 4.95 10 ^-3 M

= 4.95 mM

Molarity (C ₆ H ₁₂ O ₆) = 40 g / L ÷ 180 g / mol

= 0.222 M

= 222 mM

And in the same way we calculate the osmolarities, knowing that CaCl ₂ dissociates into three ions, two Cl ^- and one Ca ²⁺, and assuming that they are very dilute solutions, so the value of v is 1. We have then:

Osmolarity (CaCl ₂) = 4.95 mM 3 1

= 14.85 mOsm / L

Osmolarity of (C ₆ H ₁₂ O ₆) = 222 mM · 1 · 1

= 222 mOsm / L

Finally, the total osmolarity of the solution becomes the sum of the individual osmolarities; that is, of those of NaCl and glucose. This is therefore:

Total osmolarity of the solution = CaCl ₂ osmolarity + C ₆ H ₁₂ O ₆ osmolarity

= 222 mOsm / L + 14.85 mOsm / L

= 236.85 mOsm / L

The solution of the mixture of calcium chloride and glucose is hypotonic, since its osmolarity (236.85 mOsm / L) is much lower than the osmolarity of plasma (300 mOsm / L), which is taken as a reference.

Examples of hypotonic solutions

Sodium chloride solution

The 0.45% sodium chloride (NaCl) solution is administered intravenously to patients with diabetic ketosis who develop dehydration in the interstitial and intracellular compartments. Water flows from the plasma into these compartments.

Lactate Ringer's Solution

Lactate Ringer's Solution # 19 is another example of a hypotonic solution. Its composition is 0.6 g of sodium chloride, 0.03 g of potassium chloride, 0.02 g of calcium chloride, 0.31 g of sodium lactate, and 100 mL of distilled water. It is a solution used for the rehydration of patients and is slightly hypotonic (274 mosm / L).

References

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2. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
3. Wikipedia. (2020). Tonicity. Recovered from: en.wikipedia.org
4. Union Media LLC. (2020). Isotonic, Hypotonic, and Hypertonic Solutions. Recovered from: uniontestprep.com
5. Lodish H, Berk A, Zipursky SL, et al. (2000). Section 15.8 Osmosis, Water Channels, and the Regulation of Cell Volume. NCBI Bookshelf. Recovered from: ncbi.nlm.nih.gov
6. John Brennan. (March 13, 2018). How to Calculate Isotonicity. Recovered from: sciencing.com