- Formula
- Definition of variables in the osmolarity formula
- How to calculate it?
- Differences between osmolarity and osmolality
- References
The osmolarity is the parameter that measures the concentration which is of a chemical compound in one liter of solution, provided that this contributes to the colligative property known as osmotic pressure of said solution.
In this sense, the osmotic pressure of a solution refers to the amount of pressure that is needed to slow down the osmosis process, which is defined as the selective passage of solvent particles through a semi-permeable or porous membrane from a solution. from a lower concentration to a more concentrated one.
Likewise, the unit used to express the amount of solute particles is osmol (whose symbol is Osm), which is not part of the International System of Units (SI) that is used in most of the world. So the concentration of the solute in the solution is defined in units of Osmoles per liter (Osm / l).
Formula
As mentioned previously, osmolarity (also known as osmotic concentration) is expressed in units defined as Osm / L. This is due to its relationship with the determination of osmotic pressure and the measurement of solvent diffusion by osmosis.
In practice, the osmotic concentration can be determined as a physical quantity with the use of an osmometer.
The osmometer is an instrument used to measure the osmotic pressure of a solution, as well as the determination of other colligative properties (such as vapor pressure, an increase in the boiling point, or a decrease in the freezing point) to obtain the value of the osmolarity of the solution.
In this way, to calculate this measurement parameter, the following formula is used, which takes into account all the factors that can affect this property.
Osmolarity = Σφ i n i C i
In this equation, osmolarity is established as the sum resulting from multiplying all the values obtained from three different parameters, which will be defined below.
Definition of variables in the osmolarity formula
In the first place, there is the osmotic coefficient, represented by the Greek letter φ (phi), which explains how far the solution is away from the ideal behavior or, in other words, the degree of non-ideality that the solute manifests in the solution.
In the simplest way, φ refers to the degree of dissociation of the solute, which can have a value between zero and one, where the maximum value that is the unit represents a dissociation of 100%; that is, absolute.
In some cases -such as that of sucrose- this value exceeds unity; While in other cases, such as salts, the influence of electrostatic interactions or forces cause an osmotic coefficient with a value lower than unity, even though absolute dissociation occurs.
On the other hand, the value of n indicates the number of particles in which a molecule can dissociate. In the case of ionic species, the example is sodium chloride (NaCl), whose value of n is equal to two; whereas in the non-ionized glucose molecule the value of n is equal to one.
Finally, the value of c represents the concentration of the solute, expressed in molar units; and the subscript i refers to the identity of a specific solute, but which must be the same at the time of multiplying the three factors mentioned above and thus obtaining the osmolarity.
How to calculate it?
In the case of the ionic compound KBr (known as potassium bromide), if you have a solution with a concentration equal to 1 mol / l of KBr in water, it is inferred that it has an osmolarity equal to 2 osmol / l.
This is due to its character as a strong electrolyte, which favors its complete dissociation in water and allows the release of two independent ions (K + and Br -) that have a certain electrical charge, so that each mole of KBr is equivalent to two osmoles. in solution.
Similarly, for a solution with a concentration equal to 1 mol / l of BaCl 2 (known as barium chloride) in water, there is an osmolarity equal to 3 osmol / l.
This is because three independent ions are released: one Ba 2+ ion and two Cl - ions. So each mole of BaCl 2 is equivalent to three osmoles in solution.
On the other hand, nonionic species do not undergo such dissociation and produce a single osmol for each mole of solute. In the case of a glucose solution with a concentration equal to 1 mol / l, this is equivalent to 1 osmol / l of the solution.
Differences between osmolarity and osmolality
An osmol is defined as the number of particles that are dissolved in a volume equal to 22.4 l of solvent, subjected to a temperature of 0 ° C and causing the generation of an osmotic pressure equal to 1 atm. It should be noted that these particles are considered osmotically active.
In this sense, the properties known as osmolarity and osmolality refer to the same measurement: the concentration of solute in a solution or, in other words, the content of total particles of solute in solution.
The fundamental difference that is established between osmolarity and osmolality is in the units in which each is represented:
Osmolality is expressed in terms of amount of substance per volume of solution (i.e. osmol / L), while osmolality is expressed in amount of substance per mass of solvent (i.e. osmol / kg of solution).
In practice, both parameters are used indifferently, even manifesting themselves in different units, due to the fact that there is a negligible difference between the total magnitudes of the different measurements.
References
- Wikipedia. (sf). Osmotic concentration. Recovered from es.wikipedia.org
- Chang, R. (2007). Chemistry, Ninth edition. Mexico: McGraw-Hill.
- Evans, DH (2008). Osmotic and Ionic Regulation: Cells and Animals. Obtained from books.google.co.ve
- Potts, WT, and Parry, W. (2016). Osmotic and Ionic Regulation in Animals. Recovered from books.google.co.ve
- Armitage, K. (2012). Investigations in general biology. Obtained from books.google.co.ve