Inorganic Qualitative Analysis1

Inorganic Qualitative Analysis1 - Inorganic Qualitative...

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Inorganic Qualitative Analysis 1,2,3 Authors: B. K. Kramer* and J. M. McCormick Last Update: January 8, 2007 Introduction Qualitative analysis is the identification a sample's component(s). Unlike a quantitative analysis, we are not concerned with the amount of a substance present in a sample but only with its identity. In this exercise we will focus on identifying the cations and anions that make up ionic compounds, both solid and in solution. Ideally there would be chemical tests that could be used to identify individual ions without interference by any other ions. Unfortunately, there are often complications. For example, the formation of a yellow precipitate upon addition of aqueous S 2- confirms the presence of Cd 2+ in a solution. The color of this compound, however, will be hidden if any Pb 2+ or Cu 2+ are present in solution since they will form a black precipitate with added S 2- . In order to test for cadmium, then, any interfering ions must first be removed. This will be the case for most ions in a mixture: before their identities can be confirmed, they must be isolated from the remaining solution. The separation of ions in solution can be accomplished by the addition of a precipitating agent that will selectively react with an ion in the solution and not with others that may be present. The solid that is produced can then be removed from the liquid by centrifugation and decanting. Because many ions may behave similarly, separation of individual ions from a complex mixture is not usually possible. Instead, a group of ions with similar reactivity may be separated by precipitation from a larger mixture. After they are isolated in a solid, they must be further separated and reacted to confirm each one’s identity. There are several types of reactions that can be used to confirm the presence of ions in solution. The most common are precipitation and complexation . In a precipitation reaction, an ion in solution reacts with an added reagent to form a solid. Whether a solid will form from a given reaction can be predicted by the solubility product constant (K sp ) of the solid under the given conditions. Solubility product constants are the equilibrium constants for the dissolution of an "insoluble" ionic solid in water. A low K sp implies that the compound does not dissolve to an appreciable degree in water. If the two ions are mixed in solution, a precipitate will tend to form. If steps have been taken to remove ions that form competing precipitates, the presence of a properly colored solid can be used to confirm the presence of a given ion. If several different precipitates remain, the conditions of the solution can be manipulated to selectively redissolve one or more of the solids. When the equilibria involved are well understood, selective precipitation can be a powerful tool in the identification of unknown ions. Complexation can also be used to determine the presence of an ion in solution. In a
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Inorganic Qualitative Analysis1 - Inorganic Qualitative...

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