How many mL contain 1 mg of a solution made with 10 mg in 2 mL of bacteriostatic water?

Calculation of Volume for 1 mg from a 10 mg/2 mL Bacteriostatic Water Solution

To obtain 1 mg from a solution of 10 mg in 2 mL of bacteriostatic water (concentration 0.05 mg per unit), you would need 0.2 mL of the solution.

Understanding the Solution Concentration

• The given solution contains 10 mg of medication dissolved in 2 mL of bacteriostatic water 1
• This creates a concentration of 5 mg/mL (10 mg ÷ 2 mL = 5 mg/mL) 1
• Alternatively expressed as 0.05 mg per 0.01 mL (or per "unit" on an insulin syringe) 1

Calculation Method

• Using the formula C₁V₁ = C₂V₂ (where C = concentration and V = volume) 2:
  • C₁ = 5 mg/mL (original concentration)
  • C₂ = 1 mg (desired amount)
  • V₂ = ? (volume needed)
• Solving for V₂: V₂ = (C₂ × V₁) ÷ C₁ = (1 mg × 1) ÷ 5 mg/mL = 0.2 mL 3

Practical Application

• To measure 0.2 mL accurately:
  • Use a 1 mL syringe with 0.01 mL gradations for precise measurement 2
  • If using an insulin syringe marked in units, you would need 20 units (since each unit = 0.01 mL) 3
• Ensure proper sterile technique when withdrawing the solution to prevent contamination 2

Important Considerations

• Bacteriostatic water contains a preservative (typically 0.9% benzyl alcohol) that allows for multiple withdrawals while maintaining sterility 4, 5
• The solution should be stored according to manufacturer recommendations, typically refrigerated after reconstitution 6
• Check the solution visually before use to ensure there are no visible particles or discoloration 7