Qualitative cation analysis

Group, functional See Functional group Groups I, II, III, IV Cation groups in qualitative analysis, 443t... 

Qualitative analysis The determination of the nature of the species present in a sample most often applied to cations, 364-365... 

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. 

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... 

Experiment 14 Separation and Qualitative Analysis of Cations and Anions... 

Chatteijee R, Dey AK (1967) Schematic qualitative analysis of cations by the ring oven technique. Microchem J 12 151-156. 

Ghose AK, Dey AK (1970) A scheme of qualitative analysis for twenty common cations. Analyst 95 698-701. 

In a laboratory environment, the method of qualitative analysis of cations is often required. The objective of qualitative analysis is to separate and identify the cations present in an unkonwn solution. Cations are first separated into five main groups depending on their solubilities ... 

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. 

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). 

FIGURE 16.17 Flowchart for separation of metal cations in qualitative analysis. 

When solutions of soluble ionic compounds are mixed, an insoluble compound will precipitate if the ion product (IP) for the insoluble compound exceeds its fCsp. The IP is defined in the same way as /equilibrium concentrations. Certain metal cations can be separated by selective precipitation of metal sulfides. Selective precipitation is important in qualitative analysis, a procedure for identifying the ions present in an unknown solution. 

The process is followed by the appearance of a bright blue, violet or green color of the reaction mixture and can be applied for qualitative analysis of pyrazolines. At the present time, it is proven that this dimerization passes via the preliminary formation of cation-radical particles 106, which are responsible for the deep color of pyrazolines under acidic conditions in the presence of oxidizing agents. 

Qualitative chemistry is an area of chemistry concerned with identifying substances. In Activity 9.1 you will perform a qualitative analysis to detect the presence of certain ions that, in turn, may reveal an art forgery. The ions could come from paints that were not available at the time of the artwork. In this qualitative analysis, metal ions (cations) and nonmetal ions (anions) are reacted with solvents and with each other. Then the cations and anions present are identified by the products produced. In addition, flame tests and pH determinations are used to identify ions. Qualitative analysis is an engaging opportunity for you to develop experience with chemical change and review solubility principles. Nowadays, however, most of the time a chemist analyzes a substance to detect ion content using quantitative analytical computerized instruments. 

Separation and qualitative analysis of cations and anions test tubes, beaker, evaporating dish, funnel, watch glass, mortar and pestle, centrifuge, Pt or Ni test wire... 

Chemists use a range of techniques for qualitative analysis. For example, the colour of an aqueous solution can help to identify one of the ions that it contains. Examine Table 9.3. However, the intensity of ion colour varies with its concentration in the solution. Also keep in mind that many ions are colourless in aqueous solution. For example, the cations of elements from Groups 1 (IA) and 2 (IIA), as well as aluminum, zinc, and most anions, are colourless. So there are limits to the inferences you can make if you rely on solution colour alone. 

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