Understanding Volume Percent Concentration (v/v %)

One of the simplest yet most confusing units of concentration is volume percent or volume-volume percent (v/v %). It is very easy to prepare a chemical solution using volume percent, but if you misunderstand the definition of this unit of concentration, you'll experience problems.

Volume percent is defined as:

v/v % = [(volume of solute)/(volume of solution)] x 100%

Note that volume percent is relative to volume of solution, not volume of solvent. This is the key! For example, wine is about 12% v/v ethanol. This means there are 12 ml ethanol for every 100 ml of wine. It is important to realize liqud and gas volumes are not necessarily additive. If you mix 12 ml of ethanol and 100 ml of wine, you will get less than 112 ml of solution.

As another example, 70% v/v rubbing alcohol may be prepared by taking 700 ml of isopropyl alcohol and adding sufficient water to obtain 1000 ml of solution (which will not be 300 ml). Volumetric flasks are great for this. Add your solute, then dilute to the mark with the solvent. If you need to know how many milliliters or grams of solvent you added you can keep track by dispensing the solvent with a biuret. Temperature comes into play when preparing a volume percent solution, so it's a good idea to either aim for a relatively constant lab temperature or note the temperature when then solution was prepared.

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