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Unformatted text preview: QUALITATIVE ANALYSIS
Separation and Identification of Group I Cations OBJECTIVE
To provide an overview of a general
scheme for separating and identifying
To introduce the laboratory techniques
used in qualitative analysis.
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.
Measure absorption to quantify atoms in
Produces emission (of
light) of a frequency
Typically 1s – 2p, 2s – 3p etc transitions
characteristic to that
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
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
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
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
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
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
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