% volume per volume (v/v), % weight per volume (w/v) and % weight per weight (w/w)
These variations on percentage concentration are used in chemistry and biology when making up solutions and have the following meanings.
volume per volume
|used where both chemicals are liquids (e.g. if I dilute 50 mL of acetic acid by adding it to 50 mL of water there is now 50 mL of acetic acid in a total volume of 100 mL, hence the acetic acid concentration is now 50% v/v)
weight per volume
|used where a solid chemical is dissolved in liquid (e.g. if I dissolve 10 g of table salt, sodium chloride, to make up a total volume of 100 mL of solution then I have made a 10% w/v solution of sodium chloride)
weight per weight
|used where the weight of each chemical is used and not the volume (e.g. If I dissolve 10 g of fat in 90 g ethanol so the total mass of the whole solution is 100 g, then I have made a 10% w/w solution of fat)
An example of a different way of expressing percent - making dilutions of ethanol in water.
Ethanol diluted in water is used as a disinfectant in microbiology laboratories because diluted ethanol is more reactive than 100% ethanol (up to a point)!
But the percent is often expressed in two ways:
If I add 800 mL of ethanol to 200 mL of water the resulting solution is 80% v/v. (800 mL in a total volume of 1000 mL)
But if I take into account that the mass of ethanol is less than the mass of water then for the same volumes we get:
700 g of ethanol diluted to make up a total volume of 1000 mL. Hence the percent now becomes 70% w/v.
So the concentration 80% v/v equals 70% w/v.
Both 70% and 80% ethanol are used in laboratories but often the v/v or w/v is left off - watch out for this. It can confuse new people to the laboratory!
Another trap for inexperienced technicians is that when you mix things together sometimes the volume changes in unpredictable ways. That is why we say we dissolved 'to make up a total volume of 100 mL'. You need to make up the solution to a slightly smaller volume first and then when dissolved/mixed completely adjust to exactly 100 mL by adding more solvent.