Quantitative inorganic analysis

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.

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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 H_ZS 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

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.

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.

Date: 2015-12-11; view: 797