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.