Analysis— qualitative versus quantitative

Routine testing for quality control, particularly in the petrochemical and pharmaceutical industries, often employs the methods of classical analysis. The testing involves modern apparatus and laboratory facilities such as this enclosed cabinet, which can be used for hazardous materials.

Modern developments in electronics and materials science have led to a variety of advanced analytical instruments and procedures. However, many analyses are still carried out by what are termed “classical methods,” using chemical techniques and procedures that have proved reliable and reproducible over many years. Such methods are still used in small laboratories with limited equipment. Classical methods fall into two groups: qualitative analysis to find out what substances are present in a given compound, and quantitative analysis to determine how much of each substance is present in that compound.

Flame tests can reveal the presence of various metals. A sample (preferably a chlo ride) is burned in a Bunsen flame on a platinum wire. The color of the flame identifies the metal: Na = sodium; Li = lithium; Sr = strontium; Ca = calcium; K = potassium; Ba = barium; Cu = copper; and Pb = lead.

Qualitative inorganic analysis

Cations are atoms or groups of atoms with a positive charge, and anions are atoms or groups of atoms with a negative charge. Chemists identify them by the characteristic reactions that each type undergoes in solution. The first important step in the analysis is generally to dissolve any solid samples by treatment with water, acids, or bases. Individual ions in the solution are then identified by well-established chemical reactions.

Three types of reactions are used. The first is the formation of colored precipitates. (A precipitate is a substance that emerges from a solution as a solid.) The precipitate is formed by treating the ions in solution with chemical reagents under specified conditions of acidity or alkalinity. (Reagents are chemical substances that help identify other substances by the chemical reactions they cause.) The second identification process is the development of characteristic colors in solution, usually as a result of using selective and highly specialized reagents. The third is the evolution of easily identified gases from the solution following reaction with acids, alkalis, or selective reagents.

Tests of this nature, often referred to as “spot tests,” may be done on a small volume of the sample solution placed on a glazed white tile. The tile surface provides an ideal background for viewing any color changes occurring in the solutions. Identification of any individual ion, however, depends on obtaining positive reactions with a range of several reagents. While a single positive reaction can serve to indicate a possible identification, it is not in itself conclusive.

Qualitative organic analysis

The first steps in the identification of organic compounds are based on physical tests, including appearance, color, odor, and solubility. These are followed by tests to determine either melting point or boiling point. By comparing the results with published data, or known compounds, the analyst can rapidly narrow down the number of chemical possibilities.Further identification then follows a well-established procedure. The elements present in the compound are determined by decomposing it into inorganic substances to test whether the compound is acidic, alkaline, or neutral. Next, a series of tests are performed to determine the nature of the reactive groups in the compound. Many such tests produce colored precipitates or solutions when a positive result is obtained.

Qualitative inorganic analysis uses “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 (NHflOH). A further group is precipitated as hydroxides. Then H2S is again bubbled in, precipitating another group of sulfides (this time from alkaline, not acid, solution). Finally, ammonium carbonate l(NH4)2C03l 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

Quantitative inorganic analysis

To find out how much of a compound, element, or ion is present in a substance, two main classical procedures are available: gravimetry and titrimetry. Most gravimetric determinations involve converting the ion or element being studied into a pure, stable solid compound that can readily be isolated and weighed. Gravimetry thus involves three main steps: the weighed sample is dissolved in order to give a solution; the required element or ion is precipitated out in a new, pure chemical form using a very selective reagent; and the precipitate is filtered, dried, and weighed. By calculation, the percentage of the required ion or element present in the starting material can then be established.

Such determinations, which are usually carried out on less than 1 g of a substance, can reveal the components of a mixture with an accuracy of better than 1 per cent. In some gravimetric determinations, the precipitated solid is further reacted or strongly heated, providing a more stable chemical compound that is suitable for weighing.

In titrimetry, all determinations are carried out in solution. Carefully measured volumes of very pure chemical reagents are reacted with similar volumes made from the materials being studied. For instance, a carefully measured volume of 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. The volume of acid present in the fruit juice can then be calculated.

Volumetric analysis involves 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.