Home

• mixtures and solutions
• water & glassware
• terms & units
• formulas
• solution examples
• making dilutions
• working with stock solutions

Water and Glassware for Solution Making

Water is by far the most commonly used solvent in biology because it is the major component of all living organisms. Most known biochemical reactions take place in an aqueous environment and water is frequently a reactant in or is a product of biochemical reactions. Biologically important macromolecules, organelles, cells, and organs are all designed to function in an aqueous environment.

Water quality is highly variable, and for any task an appropriate grade of water must be chosen. For example, tap water is fine for washing dishes. It is not recommended for making solutions because the quality of such water is unknown. Tap water typically contains sediments (suspended particles), metal and other ions, deliberately added chemicals such as chlorine or fluoride, and/or traces of organic solvents. Although tap water is generally safe for drinking and other personal uses, materials in tap water can be toxic to some cells or may interfere with assays or biochemical reactions. It is recommended that glassware that has been washed and rinsed in tap water be thoroughly rinsed with a higher quality water.

Distilled water, obtained from the condensation of steam, is of better quality because distillation eliminates all of the sediment and most of the inorganic solutes. Organic contaminants and some of the inorganic contaminants remain.

Deionized water is produced by running tap water through a resin cartridge or series of them. A home deionizing system might simply replace divalent cations with sodium ions, producing what is commonly known as “soft” water. Laboratory deionized water is usually treated so as to remove both cations and anions, which are exchanged for hydrogen and hydroxyl ions respectively. Deionized water is often of better quality than distilled water although on the downside, the resins used in the cartridges may release organic contaminants into the water.

The highest grade of water is called 18 megohm water. Eighteen megohms is 18 million ohms, which are units representing resistance to the flow of electricity. Eighteen meghoms is more than a million times the electrical resistance of a typical household electric circuit. Very pure water does not conduct electricity well compared with contaminated water because it contains no inorganic ions with which to carry electric current. Eighteen megohm water is usually produced in multiple steps, including reverse osmosis and the passage of product through ion exchange resins, activated carbon beds and filters.

Pure water is somewhat acidic, with pH close to 5. It is also what we call an aggressive reagent, meaning that it will leech ions from plastic or glass containers. It does so because of the polar nature of water molecules. Ions dissolve most readily in 18 megohm water because the system (water plus dissolved ions) is more stable than when pure water is separated from soluble materials. Because very pure water accumulates contaminants during storage, it should be freshly prepared. The use of plastic tubing, funnels, and especially metal containers, should be avoided.

Beakers are suitable for mixing solutions because they have large open tops for pouring in solvent or large amounts of dry chemicals. Flasks are a bit easier to handle and solution is less likely to splash out of a flask. The narrow opening discourages evaporative loss and contamination from the outside. A powder funnel can be used to add dry chemicals to a flask and a glass funnel can be used to add liquid. For measurement of liquid volumes from 10 milliters on up, graduated cylinders are usually the practical choice. Cylinders are accurate to perhaps 1% of total volume, which is more than sufficient for most solutions. We seldom have a need for volumetric flasks in biology, since we don’t need such a high level of precision.

It is good practice to choose graduated cylinders and containers that are as close as possible to the intended volume of the contents. For example, it is not very accurate to use a 2 liter cylinder to measure out 100 ml of water. The same principle holds for weighing materials. It does not make sense to weigh out one hundredth of a gram of substance in a container that weighs 100 grams.

A magnetic stirring rod is useful when it takes some time for a solute to go into solution, although it is possible to introduce additional contamination into the solution. Use heat only if a formula calls for it.