Volume-Volume (v/v), Weight-Volume (w/v), and Weight-Weight (w/w) in Percent Solutions

While it may seem like a solution of 10% refers to only one concentration value, percentage could indicate different concentration values depending on the type: Volume-Volume (v/v), Weight-Volume (w/v), and Weight-Weight (w/w).

The most common variant is volume-volume (v/v). In v/v solutions, the noted product makes up a percentage of the final solution as a measure of liquid volume into liquid volume. For example, sodium hydroxide 10% v/v means that per 100ml of final solution, 10% of that volume (or 10ml) is made up of liquid sodium hydroxide concentrate. Volume-volume is used most often pertaining to dilutions from chemicals that are in liquid form, such as acid and base concentrates.

When indicating weight-volume (w/v), the percentage is based off of the weight of the dry form of chemical. In other words, instead of using the liquid form (usually in ml or L) as the base of the calculation, the weight of the powder, salt, or dry form of chemical is used in the calculation. Therefore, the noted product makes up a percentage of the final solution as a measure of concentrated weight into liquid volume. For example, a solution of sodium hydroxide 10% w/v contains 10g of sodium hydroxide per 100ml of solution. A liquid form of the chemical can still be used to create a w/v solution, but the weight-concentration of the starting solution (assay) needs to be taken into consideration as part of the final calculation. For the same 10% w/v sodium hydroxide solution, one would need 12.9ml of liquid concentrated sodium hydroxide. You can now see how this 10% w/v solution needing 12.9ml is different than the 10% v/v needing only 10ml.

Finally, weight-weight (w/w) is a measure of concentrated weight into the final total weight of solution. This notation is rarely used for chemicals that are found only in liquid form, so you will rarely see chemicals like hydrochloric acid, sulfuric acid, or ammonium hydroxide in w/w notation. For an example, 10% w/w sodium hydroxide is as simple as weighing 10g of dry sodium hydroxide and bringing the total weight up to 100g with your diluent. Using the liquid form as your starting material, however, tends to be more complicated, as the density/weight of chemical liquids vary and the density of the diluent and sodium hydroxide concentrate is required for the calculation. For this example, a 10% w/w solution of sodium hydroxide using the concentrated liquid form will need 14.2ml per 100g of solution.

Each of these different notations of percentage have their own uses in the manufacturing industry, so when you order a percent solution be sure you are ordering the correct type!