Calculations of Isotonicity

• For calculations of Isotonicity 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 for Calculations of 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

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:
• $NaCl 208/1000=0.208molof NaCl$
• 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.

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