The key difference between tonicity and osmolarity is that the tonicity measures only the concentration of non-penetrating solutes through a semipermeable membrane while the osmolarity measures the total concentration of penetrating and non-penetrating solutes.
Osmolarity is the measure of osmotic pressure of a solution. In simpler terms, it is roughly the measure of the amount of solute in the solution. On the contrary, tonicity refers to the relative concentration of solute particles inside a cell with respect to concentration outside the cell. Thus, both tonicity and osmolarity seem to be similar concepts. However, there are differences that we will be discussing in this article.
What is Tonicity?
Tonicity is a measure of the osmotic pressure gradient by means of water potential of two solutions separated by a semipermeable membrane. It means; the term tonicity describes the relative concentration of solutes I solution which determines the direction and extent of diffusion. This measurement is important in determining the response of cells that are immersed in an external solution.
Unlike the osmotic pressure, the tonicity is only influenced by solutes that cannot pass through the membrane. The solutes that can freely go through the membrane has no influence on the tonicity. It is because, the concentration of these solutes will always remain the same in both sides of the membrane. Usually, we express the tonicity with respect to another solution. Accordingly, there are three types of solutions based on the tonicity; hypertonic solutions, hypotonic solutions, and isotonic solutions. The hypertonic solutions have a high solute concentration than another solution while the hypotonic solution has a lower solute concentration. A solution becomes isotonic if the effective osmole concentration of that solution is the same as that of another solution.
What is Osmolarity?
Osmolarity or osmotic concentration is a measure of solute concentration given by the unit osmoles of solutes per litre of solution. We can denote the unit as Osm/L. Likewise, we can use this value to measure the osmotic pressure of a solution. Thus, the tonicity of the solution as well. The equation that we can use to measure this parameter is as follows:
Osmolarity = ∑ψiniCi
Here, ψ is the osmotic coefficient, n is the number of particles into which a molecule dissociates, and C is the molar concentration of the solute. Likewise, there are three types of solutions according to the osmolarity; isosmotic, hyperosmotic and hypoosmotic.
What is the Difference Between Tonicity and Osmolarity?
The terms tonicity and osmolarity are related but distinct concepts. The reason why they are related to each other is that both these terms compare the solute concentrations of two solutions separated from a semipermeable membrane. These terms differ from each other according to the type of solute they take into account when measuring. Therefore, the key difference between tonicity and osmolarity is that tonicity measures only the concentration of non-penetrating solutes through a semipermeable membrane whereas osmolarity measures the total concentration of penetrating and non-penetrating solutes.
The below infographic gives further facts on the difference between tonicity and osmolarity.
Summary – Tonicity vs Osmolarity
The terms osmolarity and tonicity are inter-related as both these terms compare the solute concentrations in a solution. But, at the same time, the terms are distinct chemical concepts according to the types of solutes that they take into account in their measurements. Hence, the key difference between tonicity and osmolarity is that the tonicity measures only the concentration of non-penetrating solutes through a semipermeable membrane whereas the osmolarity measures the total concentration of penetrating and non-penetrating solutes.
1.”Osmotic pressure on blood cells diagram”By LadyofHats – Own work (Public Domain) via Commons Wikimedia