In order to find out how much of a compound, element, or ion is present in a substance, two main classical procedures are available. These are referred to as gravimetry and titrimetry.
Most gravimetric determinations are concerned with converting the ion or element being studied into a pure, stable compound that can be readily weighed. Gravimetry thus involves three main steps. In the first step, the weighed sample is dissolved in order to give a solution. In the second step, the required element or ion is precipitated out in a new, pure chemical form using a very selective reagent. In the third stage, the precipitate is filtered, dried, and weighed. By calculation, the percentage of the required ion or element present in the starting material can be established.
Such determinations are usually carried out on as little as one-thirtieth of an ounce (less than one gram) of a substance. This can give an accuracy of better than one per cent. In some gravimetric determinations, the precipitated solid is further reacted or strongly heated. This provides a stable chemical compound suitable for weighing.
In titrimetry, all determinations are carried out in solution. The technique is based on reacting carefully measured volumes of very pure chemical reagents with similar volumes made from the materials being studied. For instance, a carefully measured volume of a fruit juice can be reacted with a steadily increasing volume of an appropriate base. This is done until all the acid in the juice has reacted. It is then possible to calculate how much acid was present in the sample of fruit juice.
Qualitative inorganic analysisuses "wet chemistry" on a large or small scale. It depends on precipitating groups of metals out of a mixture as insoluble compounds. The diagram outlines the principle of the method for a mixture of 22 common metals. These metals would normally be in the form of their salts or other simple compounds. A sample is dissolved or suspended in water, and dilute hydrochloric acid (HCI) added. Any silver, mercury, or lead present is precipitated as the insoluble chloride. Hydrogen sulfide (H2S) gas is bubbled through the remaining solution to precipitate the next group of metals as their sulfides. The solution is made alkaline with ammonium hydroxide (NH4OH). A further group is precipitated as hydroxides. Then HZS is again bubbled in, precipitating another group of sulfides (this time from alkaline, not acid, solution). Finally, ammonium carbonate [(NH„)2C03] is added to precipitate barium, calcium, and strontium as their carbonates. The only metals of the original mixture left in solution at this stage are potassium, magnesium, and sodium. The individual metals in each group are identified by specific tests. Some salts, such as phosphates, interfere with the method. They have to first be removed if they are present.
Potassium, magnesium and sodium remain
Solution for analysis (unknown concentration)
Volumetric analysisinvolves a standard solution of known concentration. It is prepared by accurate weighing and dissolution in a volumetric flask (A). This standard solution is placed in a burette (B). A sample of the unknown concentration is carefully measured using a pipette (C). It is placed in a flask below the burette. The standard solution is slowly reacted with the unknown solution. The end point, when the reaction between the two solutions is complete, is denoted by an indicator.
Among the most useful scientific instruments for chemical analysis are those involving the radiation and absorption of energy from different regions of the electromagnetic spectrum. This spectrum (pattern) is the entire range of the different types of electromagnetic waves. Electromagnetism is different types of electric and magnetic energy that travels in waves. Wavelength is the distance from the crest of one wave to the crest of the next. These lengths vary from about 10 trillionths of a meter for the shortest cosmic rays to 10,000 meters for the longest radio waves. Different types of electromagnetic radiation (from short to long wavelengths) include cosmic rays, gamma rays, X rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves.
By using different wavelengths of energy, it
Modern methodsof spectroscopy employ the whole range of the electromagnetic spectrum.This includes all the spectra from short-wavelength gamma rays and X rays to the long-wavelength radio waves used in nuclear magnetic resonance techniques.