Experiment 4 Qualitative Analysis of Cations Chemical Solutions Incorporated CSI has earned a contract from the City of Augusta which is trying to identify the source of metal contamination detected in the Kennebec River Excessively high levels of the heavy metal cations Ag Cu 2 Fe 3 Cr 3 Zn 2 and Ba 2 have been detected in the region of the Augusta State House which has concerned legislators thinking about the possible impact of the contamination on tourism in the area especially the Great Whatever Race The businesses that are likely suspects for contributing one or more heavy metal cations to the river water Figure 1 are Smiley s Photo Studio 1 Callie s Computer Chips 2 Central Maine Medical Testing 3 Peaceful Valley Funeral Home 4 Axel s Auto Body Repair 5 Evergreen Paper Mill 6 Monet s Art Restoration 7 Stay Dry Cleaners 8 and Call of the Wild Taxidermy 9 CSI s research team has compiled a useful summary below on inorganic qualitative analysis which can be used to detect ions present in a water sample We will use these methods to separate and identify the presence or absence of the relevant cations in river water samples collected near each of the suspect facilities that have been provided by the City of Augusta Additionally the City would like an easy to follow effective qualitative analysis scheme that they can use to monitor the river water for these cations from now on Therefore every CSI investigator will develop a qualitative analysis scheme to separate each cation and confirm its identity This scheme will be used to test water samples taken from two of the nine possible pollution sites For the separation and detection of the cations in question you will use reactions that involve different tendencies of these ions to precipitate form complex ions or display amphoteric behavior To familiarize yourself with these different chemical properties you will first do standard tests to observe the characteristic behavior of each cation before testing the river samples Computer Chips 2 Photo Studio 1 Medical Testing 3 Funeral Home 4 Kennebec River Auto Body 5 Paper Mill 6 Art Restoration 7 Dry Cleaner 8 Taxidermy 9 Figure 1 Map of the contaminated area of the Kennebec River with possible rogue polluters Qualitative Analysis Summary I Sparingly Soluble Salts Some cations form relatively insoluble salts that precipitate out of aqueous solution For example chloride salts are generally soluble with the notable exceptions of the salts containing Ag Pb 2 and Hg2 2 Similarly most sulfate salts are soluble except BaSO4 PbSO4 Hg2SO4 and CaSO4 The formation of a precipitate upon introduction of chloride or sulfate ions into solution would therefore be a positive test for Ag or Ba 2 respectively Most hydroxide salts are insoluble For example the solubility of Fe 3 in basic solution is governed by the reaction shown in eq 1 from which the corresponding equilibrium constant expression can be derived eq 2 The equilibrium constant for the reaction Ksp is 6 3x10 38 which says that Fe OH 3 is sparingly soluble under Experiment 4 Spring 2012 page 2 of 11 most conditions Equation 2 can be solved for the solubility of Fe 3 in solution eq 3 Equation 3 says that the solubility of Fe 3 rapidly decreases as the solution becomes more basic OH increases Indeed for hydroxide concentrations greater than 10 11 M pH 3 Fe 3 is relatively insoluble Therefore in a qualitative analysis scheme or qualitative analysis scheme Fe 3 may be removed from solution by making the solution basic thereby precipitating Fe OH 3 This precipitate can be removed from solution by centrifugation Fe OH 3 s Fe 3 aq 3 OH aq Ksp Fe 3 OH 3 solubility Fe 3 1 2 Ksp OH 3 3 However other cations for which you are testing also form insoluble hydroxides Therefore the addition of base to a mixture of cations may precipitate other metal hydroxides in addition to Fe OH 3 For example if Zn 2 is also present the analogous reaction and relationships hold eqs 4 6 Zn OH 2 s Zn 2 aq 2 OH aq Ksp Zn 2 OH 2 4 5 x 10 17 solubility Zn 2 Ksp OH 2 4 5 6 At hydroxide concentrations greater than 10 6 M pH 8 both Fe 3 and Zn 2 are insoluble and would coprecipitate as their hydroxides In theory one might be able to adjust the pH to the range where Fe 3 will precipitate but Zn 2 will not but in practice this approach works poorly Alternative approaches to separating Fe 3 and Zn 2 are either to use the amphoteric nature of Zn OH 2 or the ability of Zn 2 to form complex ions as described below II Amphoteric Hydroxides Some metal hydroxide precipitates dissolve in excess hydroxide solutions because of the formation of soluble hydroxide complex ions as seen for Zn2 in eq 7 Such hydroxides are called amphoteric hydroxides because they will react with and dissolve in both acid and base The equilibrium constant for this reaction is very product favored K 2 x 1020 Therefore Zn OH 2 solids will form when small amounts of a strong base are added to a Zn 2 containing solution but the continued addition of strong base will dissolve that precipitate as Zn OH 4 2 is formed This amphoteric behavior is useful in qualitative analysis Thus if excess base is added to a mixture of Fe 3 and Zn 2 ions Fe OH 3 will precipitate but the soluble Zn OH 4 2 will remain in solution to separate Fe 3 and Zn 2 Note that Cr 3 forms a similar soluble species Cr OH 4 in the presence of excess base Zn OH 2 s 2 OH aq Zn OH 4 2 aq 7 Experiment 4 Spring 2012 page 3 of 11 III Complex Ion Formation The formation of other complexes ions can also be useful For example Cu 2 and ammonia react to form the complex ion Cu NH3 4 2 eq 8 with a large equilibrium constant K 4 8 x 1012 showing that the formation of Cu NH3 4 2 is very favorable The utility of ammonia in qualitative analysis schemes can be shown by the behavior of a mixture of Fe 3 and Cu 2 ions If concentrated ammonia a basic solution because ammonia is a weak base eq 9 is added to a solution containing Fe 3 and Cu 2 ions Fe OH 3 will precipitate in the presence of base eq 1 and the soluble complex ion Cu NH3 4 2 will remain in solution separating the Fe 3 and Cu 2 ions Cu 2 aq 4 NH3 aq NH3 aq H2O Cu NH3 4 2 aq NH4 aq OH aq 8 9 IV Oxidation Reduction Reactions Oxidation reactions can also play a role in qualitative analysis All of the ions for which you are testing are in their maximum aqueous oxidation state except Cr 3 Therefore we can use a basic solution containing hydrogen peroxide to generate a good oxidizing agent OOH eq 10 which can