Quantitative Analysis

Classical methods, employed since the beginning of modern chemistry in the nineteenth century, use balances and calibrated glass containers to directly measure the amounts of chemicals combined with an unknown substance. A classical gravimetric analysis utilizes an appropriate chemical reagent to combine with the analyte in a sample solution to form an insoluble substance, a precipitate. The precipitate is filtered, washed, dried, and weighed. From the weight of the precipitate and sample and from the known chemical composition of the precipitate, the analyst calculates the percent of analyte in the sample. A classical titrimetric, or volumetric, analysis uses titration, a procedure in which a solution of exactly known concentration reacts with the analyte in a sample solution. A chemical solution of known concentration, the titrant, is placed in a buret, a long calibrated tube with a valve at one end capable of dispensing variable known volumes of liquid. An indicator solution, a colored dye, is added to the unknown sample. Titrant is then delivered slowly from the buret. The indicator dye is chosen so that a color change occurs when exactly the proper amount of titrant to combine with the unknown has been added. This amount is called the equivalent point volume. From the strength of the titrant solution, the equivalent point volume, and the volume of unknown sample in the titration flask, the amount or percent of an analyte can be calculated.