Friday, April 17, 2020

Qualitative Analysis - Group 1 - Inorganic Chemistry


Qualitative Inorganic Analysis  

Objective To identify ions that are present in unknown solutions and solids using "wet chemical" separation methods.  These methods are based on the behavior of different ions when they react with certain reagents.  Reagents are substances chosen because of their chemical activity with the ions being analyzed.  Learning the chemistry that governs the identifications is an important part of this experiment.

Principles All measurements are all comparisons against references.  We normally talk about quantitative measurements of meters, kilograms, and second.  But qualitative measurements are no different.  In this experiment, you compare the chemical changes you observe in a known sample with observations on your unknown sample to determine the identity of the anions and cations in the unknown.  You’ll start with a sample known to contain all 9 cations and an unknown with 4-6 cations.

In the classical analytical scheme the chemical properties of the different ions, both positive ions (cations) and negative ions (anions), are used to separate a mixture of them into successively smaller groups of ions, until some characteristic reaction may be used to confirm the presence or absence of each specific ion.  In addition to analyzing the unknown for its component ions, the qualitative analysis scheme highlights some of the important chemical behavior of these metal salts in aqueous solution.  The concepts of chemical equilibrium are emphasized, as illustrated by precipitation reactions, acid-base reactions, complex-ion formation, and oxidation-reduction reactions.  Each experiment presents a puzzle that is solved "detective fashion" by assembling a collection of chemical clues into an airtight case for the correct identifications.  As a bonus, the clues often take the form of colorful solutions and precipitates.

The qualitative analytical scheme is divided into three parts: 

1. Separation and identification of cations.
2. Identification of anions.  
3. Identification of an unknown in which both a cation and anion are present.

Overview of cation separation process: 
1.  Separate cations that form insoluble chlorides (Ag+, Pb2+) 
2.  Separate cations that t have highly insoluble hydroxides that precipitate when the hydroxide ion          concentration is small, approximately 10-5 M.  (Fe3+, Cr3+, Al3+) 
3.  The remaining 4 cations (Ba2+, Mg2+, Cu2+, Ni2+) cations all precipitate at when the hydroxide        ion concentration increases to 0.01 M so we add SO42- to remove the Ba2+ as BaSO4.  Next add        ammonia to form complex ions (Cu(NH3)22+ and Ni(NH3)22+.  These complex ions are more          stable than are the hydroxides so we can add hydroxide ion to precipitate Mg2+ 
4.  We have a mixture of Cu2+, which we detect by adding iodide and Ni2+, which is detected by     
      adding a reagent called dimethylglyoxime.

 
In qualitative analysis, the ions in a mixture are separated by selective precipitation. Selective precipitation involves the addition of a carefully selected reagent to an aqueous mixture of ions, resulting in the precipitation of one or more of the ions, while leaving the rest in solution. Once each ion is isolated, its identity can be confirmed by using a chemical reaction specific to that ion.
Cations are typically divided into Groups, where each group shares a common reagent that can be used for selective precipitation. The classic qualitative analysis scheme used to separate various groups of cations is shown in the flow chart below.

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