Alligation alternate is a method used in pharmacy to determine the ratio in which two solutions of different strengths must be combined to produce a desired intermediate concentration.
How It Works
For two solutions (one stronger, one weaker) and a desired target:
- Parts of stronger = Desired − Weaker
- Parts of weaker = Stronger − Desired
Example
Mix 70% alcohol with 10% alcohol to make 25% alcohol:
- Parts of 70% = 25 − 10 = 15
- Parts of 10% = 70 − 25 = 45
- Ratio = 15 : 45 = 1 : 3
For 500 mL final volume: 125 mL of 70% + 375 mL of 10%.
Common Uses
- IV fluid concentration mixing
- Compounding ointments and creams
- Dilution of stock solutions
- Pediatric dose preparation
- Total parenteral nutrition (TPN) preparation