Inorganic chemical analysis

Vogel A1 (1989) Quantitative Inorganic Chemical Analysis. 5th edn. Longman, Harlow, Essex, UK p 424... 

Numerous approaches, both physical and chemical, are recommended in the literature for the preservation of water samples for inorganic chemicals analysis. In particular, the significance of freezing, pasteurization, as well as the use of chemical preservatives such as acids, sodium hydroxide, chloroform, formaldehyde, or mercuric chloride as alternative techniques has been emphasized.810 Table 2.1 describes some recommended stabilization methods. 

Chemical analysis is used to identify and determine the amount of chemical substances (Chapter 1) present in a sample. It is often used for forensic, medical and environmental purposes, and for quahty control in the food and chemical industries. Inorganic chemical analysis in secondary school involves the identification of cations such as zinc and lead(II), anions such as sulfate(VI) and iodide, and gases such as carbon dioxide and ammonia. It provides a context for the learning of the topics Acids, bases and s ts (covered in depth in Chapter 6) and Redox (Chapter 7), as the reactions in these two topics are relevant to the identification of cations, anions and gases. Chemical analysis also enables students to develop their process skills, such as manipulation, observation and the making of inferences. 

Research has shown that secondary students find inorganic chemical analysis difficult. This is mainly because they do not rmderstand the purpose of the procedures and lack mastery of the skills required. Many students do not know how to carry out the procedures in the correct manner, what to observe and what valid... 

Chapter 7) before starting on inorganic chemical analysis, you may want to skip this section and go directly to sections 9.2 Cations, 9.3 Gases and 9.4 Anions. AU the relevant properties of substances and reactions that they have already learnt should be highlighted to students so that appropriate links can be made to what they do in inorganic chemical analysis. 

Students will need to be familiar with the main reactions involved in inorganic chemical analysis. These are ionic precipitation, acid-base, thermal decomposition and redox reactions. It is hoped that the formation and colour of products in the activities below will capture students attention and interest and motivate them to learn the reactions involved. 

In basic inorganic chemical analysis, the cations that students may be required to identify are aluminium, ammonium, calcium, copper(II), iron(II), iron(III), lead(II) and zinc. The reagents used are usually sodium hydroxide and ammonia solutions. Whether a precipitate (an insoluble hydroxide) is formed, the colour of the precipitate and further reaction, if any, of the precipitate with excess sodium hydroxide and/or ammonia solution, aU give clues to the identity of the cations present. The liberation of ammonia when sodium hydroxide solution is added to an unknown compound and gendy heated indicates the presence of the ammonium ion. Flame tests are also used to identify certain cations, for example, sodium, potassium and copper(II), as they give distinctive coloured flames when heated in a non-luminous flame. 

The anions that are involved in secondary inorganic chemical analysis are, in general, carbonate, chloride, iodide, nitrate(V), sulfate(IV) and sulfate(VI). Students may be familiar with the carbonate ion from lower secondary science the reactions of the remaining anions have been introduced in the tests for gases or ionic precipitation (iodide). Silver nitrate(V) solution, barium chloride/nitrate(V) solution, lead(II) nitrate(V) solution and dilute acids are generally used to test for anions. 

You may want students to draw similar flowcharts for the use of silver nitrate(V) and lead(ll) nitrate(V) solutions followed by the addition of acid. You may also want students to draw flowcharts for the addition of acid followed by silver, barium and lead(ll) solutions and compare them with the flowcharts in which the acid was added after the silver, barium and lead(ll) solutions. This is to help them to understand the impact of the procedures on the results obtained so that they may better understand what they do in the inorganic chemical analysis practical work research has shown that students often do not understand the purpose of the procedures. If students have difficulty, you may want them to carry out the relevant experiments. 

A study of students perceptions of learning inorganic chemical analysis can be found in ... 

Division I.l Inorganic Chemical Analysis Reference Materials,... 

Sommer, L., Ackermann, G. and Burns, D.T., Present and Future Status of Organic Analytical Reagents - Part II Inorganic Chemical Analysis Classical Methods, Molecular Spectroscopy (Absorption and Emission) and Solvent Extraction, Pure Appl Chem., 1990, 62, 2323. 

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