Liquids can be a pain. Once mixed, you can't remove an over-added ingredient. This is a common frustration when trying to precisely pH a liquid, or when trying to get an exact mixture right. Liquids, due to this characteristic of component inseparability, can also be difficult to assess when trying to determine concentrations.
If, for example, one is mixing a salty liquid and isn't sure if water evaporated or if the concentration changed, it's impossible to assess visually. There are a few options available, like mass spectroscopy or gas chromatography, both of which require very expensive pieces of equipment. There's another way, though.
This isn't a good method if you want to know the precise molecular composition of a solution, but if you simply want to determine whether or not a concentration is "about right" you can measure the density of a given volume of the liquid by weighing it. It's important to keep in mind that this method isn't going to tell you what components are in a liquid, but it will tell you the final density of that liquid. By calculating the expected density of the solution, you'll be able to tell if it's approximately correct in density.
Example problem:
For example, in electron microscopy, many people work with a 4% solution of osmium tetroxide. Osmium tetroxide is, however, highly volatile and evaporates more readily than water. It also takes more than a day to dissolve, so most people need to keep a stock solution on hand, but it also needs to be kept air-tight. Because osmium is so reactive, especially with plastics, many people opt to store osmium tetroxide solutions in glass containers, but glass containers don't always seal tightly. In essence, there are a lot of things that can go wrong with a stored stock solution of 4% osmium tetroxide.
Solution:
Using an analytical balance, we can estimate the concentration of osmium tetroxide, though, because there are characteristics, we can look up and measure. We know that one cubic centimeter has the same volume as one milliliter. By looking up the density of solid osmium tetroxide, we know that it weighs 4.91 grams per cubic centimeter. We also know that one milliliter of water weighs one gram (by definition). That means a 4% solution of osmium tetroxide should weigh 1.164 grams. By using a pipetman, a laboratory worker can measure the density of a solution and estimate the concentration of known ingredients.
Analytical balances are more useful in this case than laboratory scales because measurements need to be precise. Repeated measurements increase confidence, too, so it's a good idea to measure the volume and density three or four times for reliability. Keep in mind that it's also important to tare the machine for each measurement and to measure 1 milliliter of water as a control in case the pipetman isn't calibrated properly. For more tips on how to use analytical balances or advice on selecting the perfect instrument for your needs, feel free to contact us.