Isotonic Solutions Based on Freezing Point and Molecular weight

As the name implies, an isotonic solution cross a semipermeable membrane has the same osmotic pressure as two different solutions. Another solution with the same osmolarity (solute concentration) is also present.

• Isotonicity means that the molecular concentration of one solution is the same as the molecular concentration of another solution.
• Solutes on both sides of the membrane can remain steady at their concentrations without being affected by water being pushed across the membrane.
• For example, lactated ringers and normal saline 0.9% are examples of isotonic solutions.
• In hypertonic solutions, the solutes are present at a higher concentration than when they were in semipermeable solutions.

As a result of the movement of bodily fluid in the form of a biological membrane, an isotonic solution has the same internal pressure. It is normal for biological fluids, such as blood and lachrymal fluid, to have an osmotic pressure equivalent to the osmotic pressure associated with 0.9% w/v sodium chloride solution. Sodium chloride solution of 0.9% is therefore described as being isotonic with physiological fluids. Pharmaceutical solutions that will be applied to delicate body membranes should also be adjusted to an osmotic pressure consistent with that of body fluids, in addition to pH adjustments. As isotonic solutions come into contact with the tissue they do not cause swelling or contraction and do not produce discomfort when they are injected into the eye, nasal tract, or blood. As a familiar pharmaceutical example, isotonic sodium chloride is a preparation of this type.

The amounts of substances necessary for making isotonic solutions can be determined by using the following methods:

1. Based on molecular weight
2. Based on freezing point
3. Based on sodium chloride equivalent value
4. White-Vincent method

Based on molecular weight

The following molecular concentration is calculated based on 1% molecular concentration:

 
Assuming a substance consists of one gram of molecular weight, one gram will dissolve in 100 milliliters of water, leaving you with a solution whose molecular concentration is 1%. A gram of boric acid has a molecular weight of 62, so if 62 grams (30 mol) of boric acid are dissolved in 100 ml of water, the resultant solution has a molecular concentration of 1%. Aqueous solutions with a molecular concentration of 1% depress the freezing point of a non-ionizing substance to - 18.6 C °, while plasma freezes at - 0.52 °. This information means that it is easy to compute the molecular concentration of blood plasma; In solution, getting less than 1% molecular concentration results in a decrease in the boiling point of 18.6°C. It takes 18.6 % of the molecular concentration to lower the freezing point of a solution by 1 C°. Inability to freeze plasma at -0.52C° resulted in a 0.03% molecule-to-molecule increase in concentration. Plasma molecule concentration is therefore 0.03%. In the case of any solution with a molecular concentration of 0.03%, both solutions will have the same molecular concentration (for the same reason).

Based on freezing point

Lachrymal secretions and blood plasma, both of which are body fluids, have a freezing point of - 0.52C because of the solutes in them. In addition, NaCI solution with a 0.9% concentration has a freezing point of -0.52 C. Therefore, all liquids which are isotonic with these fluids are also liquids at - 0.52 C°. By knowing the freezing point of a 1% solution of the substance that needs to be adjusted and that of a 1% solution of the substance that will adjust it, the adjustment of the tonicity of solutions is streamlined. The general formula below can help you determine how much adjusting substance to add to the solution to make it isotonic with blood;

To adjust the substance required, subtract 0.52 from a/b

Where,
a = Temperature at which 1% of an unadjusted solution freeze.
b = at 1% of the adjusting substance's freezing point.

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