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Unformatted text preview: QUALITATIVE ANALYSIS QUALITATIVE Separation and Identification of Group I Cations OBJECTIVE To provide an overview of a general To scheme for separating and identifying cations cations To introduce the laboratory techniques used in qualitative analysis. used analysis Qualitative Analysis What’s in there – at an element or functional group level Quantitative Analysis Quantitative Analysis How much of particular element is in there Some modern methods Atomic absorption (AA) ICP NMR Chromatography (flame test) Atomic Absorption Cathode lamp Atoms in gas phase -Electrode in tube excite Absorption occurs when atom in cathode specific element to be tube and atomized atoms are the same. measured Measure absorption to quantify atoms in Produces emission (of sample light) of a frequency Typically 1s – 2p, 2s – 3p etc transitions characteristic to that element Orange color in some street lights due to Na emissions Copper in blue/green Inductively coupled plasma atomic emission spectroscopy (ICP-AES), Detect characteristic emissions in a plasma (when charged atom combines with electron) BACKGROUND INFORMATION BACKGROUND OF QUALTATIVE ANASIS OF CATIONS Classical Strategy: Split into groups through selective precipitation by the addition of various reagents called group reagents Whose anion forms a precipitate with only one or few of the cations in the mixture Then find individuals by doing confirmatory Tests In the classical qualitative scheme common In the classical qualitative scheme common cations are divided into 5 major groups based on their solubilities which makes the basics of the scheme Solubility: Homogeneous mixtures­solutions. Solutions may be gases, liquids, or solids. Each of the substances in a solution is called a component. Saturated Solution: One in which the solution is in equilibrium with an un­dissolved solute AgCl (s) ↔ Ag+ + Cl- Equilibrium Constant = Ksp = [Ag+] [Cl-] Equilibrium SOLUBILITY PRODUCT CONSTANT SOLUBILITY PRODUCT CONSTANT EXPRESSES THE DEGREE TO WHICH THE SOLID IS SOLUBLE IN SOLVENT Eg. KSp for AgCl is 1.6 x 10­10 mol/liter Therefore Solubility ( = X ) of AgCl is X = [Ag+] = [Cl­] X2 = 1.6 x 10­10 mol/liter or X = 1.265 x 10­5 mol/liter LET US CONSIDER REVERSE OF SOLUBILITY LET US CONSIDER REVERSE OF SOLUBILITY when solutions are mixed HOW TO PREDICT WHETHER A PRECIPITATE WILL BE FORMED WHEN TWO SOLUTIONS ARE MIXED WE FIND THE IONIC PRODUCT = Q [cation]xini [anion]yini = Q Sp IF Q IS GREATER THAN K , PRECIPITATION OCCURS 2+ in Pb How much 6 M HCl would you need to add in 2+ Reaction: PbCl2 (s) ↔ Pb+2 (aq) + 2 ClPb to precipitate 99.9 % of Pb ? to [Pb+2] [Cl-]2 = Ksp = 1.6 X 10-5 [Pb 1.6 Precipitation will begin when ionic product is greater than 1.6 x 10­5 To precipitate 99.9 percent of 0.10 M Pb+2 (0.0999 M) will leave 0.0001 M Pb+2 in solution +2 ini - ini2 Q = [Pb ] [Cl ] = = 1.6 X 10-5 1.6 [1 x 10­4] [Cl­]ini2 = 1.6 x 10­5 [Cl­]ini2 = 1.6 x 10­5 / 1 x 10­4 [Cl­]ini = 0.40 M Concentration of HCl should be O.4M +2 2 2 Hg Cl and AgCl are even less soluble Hg So they will precipitate 99.9 percent also at HCl = 0.4 M PbCl2 1.6 x 10­5 AgCl 1.6 x 10­10 Hg2Cl2 1.1 x 10­18 PbCrO4 2.0 x 10­16 Chemistry Behind the method Chemistry Behind the method Separation and identification of Pb 2+ Chemistry Behind the Method Chemistry Behind the Method Separation and Identification of Ag+ CHEMISTRY BEHIND THE CHEMISTRY BEHIND THE METHOD Separation and Identification of Hg2+2 Chemistry Behind the method Chemistry Behind the method Add 10 drops of 6M HCl to 20 drops of theunknown solution (a) Ag+ + Cl­ AgCl (b) Pb2+ + Cl­ PbCl2 (c) Hg22+ + 2Cl­ Hg2Cl2 N.b., [Hg­Hg]2+ 2+ N.b., [Hg­Hg] Why does this form? Hg 6s25d10; Hg+ 6s15d10 As you go down the periodic table for elements with complete d shells, the electrons in the s orbitals become more difficult to remove (the “inert pair effect”) Oxidizing power increases from Zn2+ to Cd2+ to Hg2+ 2 unpaired 6s1 electrons in 2 Hg+ atoms can combine to form a bond Confirmation of Ag+ Confirmation of Ag+ Add ~5 drops of 6 M NH3 (aq) to the white precipitate, after centrifuge. The solid should dissolve, but if any precipitate remains of grey color confirms Hg 2 2+ centrifuge and proceed using only the centrifugate. Add one drop of bromocresole green and 6 M HNO3 to the solution until the solution is acidified (yellow solution). A white precipitate (AgCl) confirms the presence of Ag+. The Cl­ needed for precipitation will be present from the prior dissolution of AgCl. Confirmation of Pb2+ Confirmation of . Lead chloride is almost three times more soluble in hot water than cold. We use this as a basis for separating it from silver and Mercurous chlorides. The presence of lead is then confirmed by precipitation of yellow lead chromate. Add ~40 drops of hot distilled water to the solid precipitate. Heat on a boiling water bath for 5 minutes. Centrifuge while hot, decant, and save both the solution and the solid. Wash 2 times with 10 drops of hot water add washings to the solution. Add 2­3 drops of 1 M K2CrO4 to the solution. A bright yellow precipitate confirms the presence of Pb2+. Techniques Techniques Use of Centrifuge Place test tube in centrifuge holder. Balance with another test tube filled to the same level in the opposite holder. Turn knob. Centrifugation takes a minute or more. Note that you must turn off the centrifuge with the switch and wait for it to stop spinning, to effectively separate the precipitate and solution. TECHNIQUES TECHNIQUES Heating in a water bath TECHNIQUES TECHNIQUES After centrifugation, the solution should be clear and the solid should be collected in the bottom of the test tube. Remove the solution by decanting, or pouring, it away from the solid. Tachniques Tachniques You can also remove the solution from a centrifuged precipitate by using a medicine dropper to carefully draw off the liquid. This is an effective alternative to decanting the solution NEXT WEEK NEXT WEEK Read SUSB­017 Complexometric Titration of Calcium in Antacids DO PRELAB ...
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