Calculations of Isotonicity

• To achieve isotonic conditions in a solution (matching the osmotic pressure of physiological fluids like blood or tears, ~290 mOsmol/kg), you need to calculate and adjust the solution's osmolarity using an isotonicity-adjusting substance.

Steps to Calculate Isotonicity

1) Determine the Osmolarity of the Test Solution

• Calculate the molar concentration of each solute and consider how many particles each solute forms when dissolved.

• Sum the osmolarities of all solutes to get the total osmolarity.

2) Identify Desired Osmolarity

• Reference the osmolarity of an isotonic solution (e.g., blood plasma or 0.9% w/v sodium chloride solution = 308 mOsmol/L).

3) Calculate the Osmolarity Difference

• Subtract the test solution's osmolarity from the desired osmolarity to find how much adjustment is needed.

4) Select an Isotonicity-Adjusting Substance

• Choose a compatible substance (e.g., sodium chloride, glucose, or mannitol) based on the intended application and solution components.

5) Calculate the Amount of Adjusting Substance Needed

• Based on the osmolarity difference, calculate the amount of the isotonicity-adjusting substance required.

Example Calculation

• Test Solution Osmolarity: 100 mOsmol/L

• Desired Osmolarity: 308 mOsmol/L

• Osmolarity Difference: 308 - 100 = 208 mOsmol/L

• Isotonicity-Adjusting Substance: Sodium chloride (NaCl)

Calculation for Sodium Chloride

• Molecular weight of NaCl: 58.44 g/mol

• NaCl contributes 1 osmole per mole. A 1 M NaCl solution has an osmolarity of 1000 mOsmol/L.

• To raise the osmolarity by 208 mOsmol/L:

NaCl 208/1000=0.208" mol of NaCl"

• Multiply by molecular weight:

0.208×58.44=12.15 g

• Therefore, 12.15 g of NaCl should be added to 1 L of solution to achieve isotonicity.