Why strength matters to a cell
A red blood cell is a tiny bag of fluid wrapped in a membrane that lets water pass freely. By osmosis, water moves across that membrane toward the side with more dissolved particles, trying to even out the concentration. The strength of that pull is the osmotic pressure, and it depends on the *number* of dissolved particles, not their identity — one of the colligative properties.
**Tonicity** describes how a solution behaves *next to a cell*. An isotonic solution has the same effective particle concentration as blood, so no net water moves and the cell is happy. A hypotonic (too-weak) solution lets water rush *into* the cell until it swells and bursts — for red cells this is haemolysis. A hypertonic (too-strong) solution pulls water *out*, leaving the cell shrunken and crinkled. Either extreme injected into a vein is painful and damaging, so a good parenteral aims for isotonicity.
The sodium chloride equivalent
Most drugs are not isotonic on their own, so we add a tonicity modifier — usually sodium chloride — to top the solution up to match. But how much? The elegant trick is the **sodium chloride equivalent (E value)**: the weight of sodium chloride that produces the *same osmotic effect* as 1 gram of the drug. If a drug has E = 0.20, then 1 g of it “acts like” 0.20 g of salt as far as the cell is concerned.
- Decide how much total salt would make the whole volume isotonic — for water it is 0.9 g of sodium chloride per 100 mL.
- Work out the “salt already contributed” by the drug: multiply the drug’s weight by its E value.
- Subtract the drug’s contribution from the total needed. The remainder is the sodium chloride you must actually add.
Make 30 mL of an isotonic injection of a drug, E = 0.20, drug dose = 0.3 g.
Step 1 Salt to make 30 mL isotonic by itself:
0.9 g / 100 mL -> 0.9 x (30/100) = 0.27 g NaCl
Step 2 Tonicity already supplied by the drug:
drug weight x E = 0.3 g x 0.20 = 0.06 g (NaCl-equivalent)
Step 3 NaCl to actually add:
0.27 g - 0.06 g = 0.21 g NaCl
Answer: dissolve 0.3 g drug + 0.21 g NaCl, make up to 30 mL.When you cannot reach isotonic
Sometimes the drug dose alone already overshoots — its own osmotic contribution exceeds isotonic, so there is nothing left to add and the solution is unavoidably hypertonic. That is acceptable for slow IV infusion into a large vein, where blood dilutes it quickly, but it should be flagged. Other times a drug is incompatible with chloride and a different tonicity modifier such as dextrose or mannitol is used; the same E-value bookkeeping applies, just with that agent’s own equivalent.