Our tests are focused on the qualitative analysis of inorganic ions. The aim of qualitative analysis is identification of the components present in the unknown compound. In contrast, quantitative analysis is interested in the exact amounts of known components. This means that quantitative analysis can be performed only after identification of the components via qualitative analysis. Of course, these branches of analysis are not always separated completely. For example, observations during qualitative analysis always contain some quantitative information. Different analytical reactions have different sensitivity to the target compound. Sensitivity is the smallest concentration required for identifying the target compound with a given reaction. Usually, it is characterized quantitatively by the concentration limit:
The concentration limit can be measured in ppm (pars pro million) too. 1 ppm means that mass of the target compound is one millionth of the mass of the sample. In the case of diluted solutions (ρ ≈ 1 g/ml) the value of the concentration limit in μg/ml and in ppm are practically the same. Dilution limit (106/concentration limit) is used too.
Notably, experiments can only determine whether the concentration of a compound exceeds the concentration limit or not. They cannot exclude the possibility that the compound is present below the concentration limit. Because sensitivity is influenced by various factors depending on the type of reaction, the analytical method had to be chosen according to the required concentration limit of detection.
From the viewpoint of analytical chemistry, only those reactions are useful where ions or ion groups which produce observable change with the reagent are known. If the reaction results in observable or unique change only with a certain ion, the reaction is specific. If the reaction produces observable change with a small well-defined group of ions, the reaction is selective.
From a quantitative view of point, selectivity and specificity are connected with the ratios of sensitivities. If the reagent reacts with a wide range of ions, it is called group reagent (and the reaction is a group reaction).
Selectivity of an analytical reaction can be improved without physically removing the interfering ions with separation methods. These techniques are called masking and they transform interfering ions to inert (non-interfering) ones by appropriate reactions (e.g.
precipitation or formation of stable complexes).
Instrumental methods of qualitative analysis are omitted from this curriculum, experiments will be performed with classic methods and simple equipment. As a result, the reactions used to identify components should produce changes which are easily observable with human senses like precipitation, dissolution of a precipitate, gas formation or change in the colour of the solution or precipitate. In the case of precipitation, important details are its colour, consistence, as well as the time and temperature required for precipitation. In the case of gas evolution, the intensity of the process and the smell of the formed gas can be important.
1.2. Role of dissolution
Although qualitative analytical tests can be performed with solids or aqueous solutions, most reactions are done in aqueous phase with ionic reagents. The role of the solvent is very important, since during dissolution the reagents and the unknown compound dissociate and become solvated. Thanks to its highly polar nature and hydrogen bonded structure, water readily dissolves ionic compounds and highly polar molecules.
Often, dissolution only involves secondary bonds (physical dissolution). When dissolution involves primary bonds (chemical dissolution), covalent bonds are rearranged during the process and water serves as both solvent and reactant (for example, aqua ion formation or dissolution of anhydrides, acids, or bases). As a consequence of the above facts, the studied ions are solvated in water and take part in chemical reactions independently. This enables considerable generalization and simplification: instead of examining the chemical properties of every possible cation – anion pair, it is enough to discover and systematize behaviour of individual cations and anions in qualitative analysis.
1.3. Stoichiometry and chemical equations
Thanks to the atomic structure of matter and the law of conservation of mass, the reactants react with each other during a chemical reaction in well-defined ratios. These ratios are called the stoichiometry of the reaction. Chemical reactions are described with chemical equations
where reactants are on the left side while products are on the right side. Reactants and products are represented with their chemical or structural formulas. The coefficients deduced from stoichiometric ratios are on the left side of the formulas.
Chemical equations should obey the law of conservation of mass and the law of conservation of charge. If the enthalpy of the reaction and the state of the compounds are also shown, the reaction equation also obeys the conservation of energy (in qualitative analysis this is not important). As a result of the above laws, the overall charge and the amounts of elements should be the same at the left and the right sides of the equation. This enables balancing chemical equations in a stoichiometrically correct way.
Some reactions used in qualitative analysis are the results of complex reaction systems. In these cases, determining the overall stoichiometry is not always possible. In the case of such non-stoichiometric reactions, the chemical equation only shows the reactants and the products (without coefficients).
Important observations are often noted in the chemical equation with symbols. Underlined products are precipitates and gas state is noted with an upward pointing arrow (↑). When analytical chemical equations are written, incorporating every component is usually not necessary. In most cases, the counterions are not participating in the reaction so showing relevant reactants and products is enough. For example, precipitation of silver chloride from silver nitrate solution with sodium chloride reagent can be written with all components or without the inert counterions:
2. Classification of reactions