Precipitation qualitative analysis

Qualitative Analysis Selective Precipitation Qualitative Analysis... 

Section 16.2 ion product selective precipitation qualitative analysis... 

Separation of Ions Using Differences in Solubility Fractional Precipitation Qualitative Analysis of Metal Ions in Solution... 

Qualitative Analysis. Nitric acid may be detected by the classical brown-ring test, the copper-turnings test, the reduction of nitrate to ammonia by active metal or alloy, or the nitrogen precipitation test. Nitrous acid or nitrites interfere with most of these tests, but such interference may be eliminated by acidifying with sulfuric acid, adding ammonium sulfate crystals, and evaporating to alow volume. 

The history of the discovery of amino acids is closely related to advances ia analytical methods. Initially, quantitative and qualitative analysis depended exclusively upon crystallization from proteia hydrolysates. The quantitative precipitation of several basic amino acids including phosphotungstates, the separation of amino acid esters by vacuum distillation, and precipitation by sulfonic acid derivatives were developed successively duriag the last century. 

Equilibria such as these have applications in fields as diverse as geology, medicine, and agriculture. In chemistry you are most likely to meet up with precipitation equilibria in the laboratory when you carry out experiments in qualitative analysis. ... 

As shown in Section 2.15, in a solution of 0.25M hydrochloric acid saturated with hydrogen sulphide (this is the solution employed for the precipitation of the sulphides of the Group II metals in qualitative analysis),... 

Precipitation reactions have many applications. One is to make compounds. The strategy is to choose starting solutions that form a precipitate of the desired insoluble compound when they are mixed. Then we can separate the insoluble compound from the reaction mixture by filtration. Another application is in chemical analysis. In qualitative analysis—the determination of the substances present in a sample—the formation of a precipitate is used to confirm the identity of certain ions. In quantitative analysis, the aim is to determine the amount of each substance or element present. In particular, in gravimetric analysis, the amount of substance present is determined by measurements of mass. In this application, an insoluble compound is precipitated, the precipitate is filtered off and weighed, and from its mass the amount of a substance in one of the original solutions is calculated (Fig. 1.6). Gravimetric analysis can be used in environmental monitoring to find out how much of a heavy metal ion, such as lead or mercury, is in a sample of water. 

Section 11.14 describes the use of precipitates in qualitative analysis in more detail. 

When a precipitate has been formed during the qualitative analysis of the ions present in a solution, it may be necessary to dissolve the precipitate again to identify the cation or anion. One strategy is to remove one of the ions from the solubility equilibrium so that the precipitate will continue to dissolve in a fruitless chase for equilibrium. Suppose, for example, that a solid hydroxide such as iron(IIl) hydroxide is in equilibrium with its ions in solution ... 

Complex formation, selective precipitation, and control of the pH of a solution all play important roles in the qualitative analysis of the ions present in aqueous solutions. There are many different schemes of analysis, but they follow the same general principles. Let s think through a simple procedure for the identification of five cations by following the steps that might be used in the laboratory. We shall see how each step makes use of solubility equilibria. 

FIGURE 11.20 Part of a simple qualitative analysis scheme used to separate certain cations. In the first step, three cations are separated as insoluble chlorides. In the second step, cations that form highly insoluble sulfides are removed by precipitation at a low pH and, in the third step, the remaining cations are precipitated as the sulfides at a higher pH. 

Qualitative analysis involves the separation and identification of ions by selective precipitation, complex formation, and the control of pH. 

In 0-level qualitative analysis, unknown cations are identified by reacting them with aqueous sodium hydroxide and/or aqueous ammonia. For example, zinc salt solutions react with aqueous sodium hydroxide to form a white precipitate, zinc hydroxide, which in turn will react with excess aqueous sodium hydroxide to form a colourless solution of sodium zincate, a complex salt. However, 25% of the 915 students thought that when sodium hydroxide solution was added to a solution of a zinc salt, a displacement reaction resulted leading to the formation of a precipitate... 

The section on tests for eations is used to illustrate the QATP. Students need to have tacit knowledge of the phenomena involved in qualitative analysis, reagents and apparatus, and to eonstruet explanations of the phenomena at the sub-microscopic level and to write equations to deseribe them. To help students understand precipitate formation, they are instraeted to compare the behaviom of two solutions, sodium chloride and iron(lll) ehloride when aqueous sodium hydroxide is added to the solutions (Fig. 6.1). The students will observe that there is no visible reaction with the sodium chloride solution, but a brown precipitate will be formed in the... 

Seven chemical reactions were identified from the chemistry syllabus. These chemical reactions were selected because they were frequently encountered during the 2-year chemistiy course and based on their importance in understanding concepts associated with three topics, namely, acids, bases and salts, metal reactivity series and inorganic chemistry qualitative analysis. The seven types of chemical reactions were combustion of reactive metals in air, chemical reactions between dilute acids and reactive metals, neutralisation reactions between strong acids and strong alkalis, neutralisation reactions between dilute acids and metal oxides, chemical reactions between dilute acids and metal carbonates, ionic precipitation reactions and metal ion displacement reactions. Although two of the chemical reactions involved oxidation and reduction, it was decided not to include the concept of redox in this study as students had only recently been introduced to ion-electron... 

Howard [772] has been amongst the first to show the usefulness of conventional SEC for polymer/additive systems. Coupek el al. [773] have also reported results with this technique in an early stage their work was limited to synthetic mixtures of additives. The use of open-column SEC in the analysis of plastics additives has been reported [774], Qualitative analysis of additives has been performed by stopped-flow SEC with IR detection [775]. Polypropylene oligomers were isolated from a PP/(Irganox 1010, Irgafos 168, DBS) matrix by dissolution (toluene)/precipitation (methanol) and Soxhlet... 

Coordinate bonds between metals and ligands result in the formation of complexes under many different types of conditions. In some cases, complexes form in the gas phase, and the number of known solid complexes is enormous. However, it is in solutions that many of the effects of complex formation are so important. For example, in qualitative analysis, AgCl precipitates when a solution of HC1 is added to one containing Ag+. When aqueous ammonia is added, the precipitate dissolves as a result of the formation of a complex,... 

These methods may prove useful in the qualitative analysis of organic compounds, once the selectivities of the precipitants are understood. The metallic oxides suffer from the disadvantage of producing a precipitate which is difficult to filter, while calcite and zirconium phosphates produce relatively well-mannered precipitates. Even when the efficiencies of collection of various model compounds in seawater is known, the immense variety of organic compounds in seawater will keep this technique largely qualitative. 

As well as separating similar ions out of solution, chemists can also use their understanding of solubility and precipitation reactions to identify unknown ions in solution. Qualitative analysis is the hranch of analytical chemistry that involves identifying elements, compounds, and ions in samples of unknown or uncertain composition. The other hranch of analytical chemistry is quantitative analysis. In quantitative analysis, analytical chemists determine how much of a compound, element, or ion is in a sample. 

The scheme that is shown in Figure 9.9 is very simple. More complex qualitative analyses involve many more steps of isolation and identification, including some steps that are not precipitation reactions. For example, some ions, such as sodium, Na", and potassium, K, cannot he precipitated out of an aqueous solution, because the ionic compounds that contain them are always soluble. Instead, chemists identify these ions using a flame test. In the following ThoughtLab, you will simulate a qualitative analysis that includes a flame test. 

A student was performing an experiment in qualitative analysis. Upon addition of HCl to her unknown, she noticed that a white precipitate formed. However, upon heating the solution, the white precipitate dissolved. The precipitate most likely contained... 

Hydrogen sulfide causes the precipitation of sulfides from many heavy-metal salts. The classical qualitative analysis scheme depends on precipitation of the sulfides of Hg, Ph, Bi, Cu, Cd, As, Sh, and Sn under acid conditions and the sulfides of Co, Ni, Mn, Zn, and Fe under ammoniacal conditions. 

The solution and precipitate were separated by filtration. Qualitative analysis of the precipitate showed that it contained cerium(III) and sulfate ion. [No precipitate appeared when m-bisoxalatodiaquochromate(111) ion and cerium(IV) were allowed to react at this acidity under conditions where the sulfate and/or cerium concentrations were somewhat lower.] The filtrate, including washings, was diluted to a measured volume, and two measured aliquots of this solution were withdrawn for study. The first aliquot was passed onto a cation exchange column in the hydrogen form. Elution with 1M sulfuric acid left a small dark violet band of hexaaquochromium(III) ion at the top of the column, while the rest of the chromium passed through the column. [In separate experiments it was confirmed that the Cr(OH2)fe+3 ion is retained as a dark violet band at the top of the resin when the resin is eluted with 1M sulfuric acid. This is in agreement with the observations of King and Dismukes (23).] Analysis of the effluent solution showed that 4 to 5% of the chromium had been retained by the column. 

To prepare compounds, you must use reactions that (by the principles just outlined) are predicted to "go." Not all reactions that take place, however, are suitable for synthesis, and attention must be paid to purity of product, cost of reactants, and ease of preparation. We comment on these factors in the answers to the A problems, and in the following discussion of the preparation of a compound X. If your laboratory work includes qualitative analysis, you might well consider each evolved gas and each precipitate to be a chemical preparation, even though the prime purpose of the project before you is separation or identification. 

Qualitative analysis is a procedure for identifying the ions present in an unknown solution. The ions are identified by specific chemical tests, but because one ion can interfere with the test for another, the ions must first be separated. In the traditional scheme of analysis for metal cations, some 20 cations are separated initially into five groups by selective precipitation (Figure 16.17). 

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