Solutions consist of a liquid which has a solid dissolved or dispersed throughout it. The liquid medium is known as the solvent and the solid is known as the solute. The concentration of a solution is expressed in terms of how many milligrams of solids are contained per liter of liquid (mg/L). A standard solution is a solution for which the concentration is known. Standards are often made up so that 1 mL = 1 mg/L. Lab analysts can easily dilute standard solutions to a lower strength, such as when preparing a calibration curve.
There are two main types of standards: primary standards and secondary standards. A primary standard is a standard that is prepared by dissolving a weighed amount of a substance of a known composition in a measured final volume. A secondary standard is a standard for which the concentration is derived by comparison, such as by titration. For the highest level of accuracy, the solids used to make up a primary standard should be weighed using an analytical balance and the liquid volumes should be measured using Class “A” volumetric glassware.
Sometimes it is convenient to purchase and use solutions that have a specific concentration, which is based on the molecular weigh of the solute. These are known as Molar Solutions (M). A molar solution consists of one gram (molecular weight) of a compound made up to one liter with distilled water. For example, the molecular weight of CaSO4 is 136. If you create a solution that contains 136 grams of CaSO4 made up to 1 liter, you will have a 1M CaSO4 solution.
Another solution that is sometimes convenient is the Normal Solution (N). A normal solution contains one gram equivalent weight of reagent per liter. An equivalent weight of a substance is defined as that weight which releases or accepts 1 mole of electrons. It takes equivalent volumes of acidic 1N solutions to neutralize equivalent volumes of caustic 1N solutions. (Caution, never directly mix strong acids and bases). Most laboratories maintain 0.05N, 0.1N and 1.0N working solutions of the most commonly used acids and bases.
The solutions used in the lab on an everyday basis are known as Stock Solutions. Stock solutions are made stronger than those used in various lab tests because stronger solutions are generally more stable than weak solutions. Stock solutions are easily diluted to give a desired working concentration. Stock solutions are considered standards when the concentration is very accurately known.