UNIT 11 Complexation and Precipitation Flashcards
What are complexation reactions used for
titrating cation
creating complexes that are colored or absorb
Sapringly soluble compounds that can be used in gravimetric analysis
Extracting ation from one solvent to another and dissolving insoluble precipitates
What do most metal ion react with?
electron pair donors or coordination compounds to from complexes
What is a Ligand?
Ion or molecule that forms a covalent bond with a bond with a cation or neutral metal atom by donateing a pair of electron which is shared
What is a coordination number?
Number of covalent bonds that a cation tends to form with electron donors
What do most metal ion exist as?
aquo-complexes
What are some typical values for coordination numbers?
2,4,6
What is the charge of a complex?
Positive, neutral or negative
What are titration based on complex formations called?
Complex-metric titrations
What are Chelates
Class is coordination compounds
–> metal ion coordinates with two or more donor groups of a ligand to form a 5/6 membered heterocyclic ring
What is a Unidentate?
“Single-toothed”
Ligand that has a single donor group
What is a Bidentate?
Ligand with two groups available for covalent bonding
What are Macrocycle?
Cyclic organic compounds which contain none or more atoms and contain at least three heteroatoms (usually O, N, S)
What are Cryptands?
Macrocyclic compunds that form 3D cavities that can accomodate appropriately sized ions
How do you show a complexation equilibria?
M= metal ion
L= ligand
ML= complex
M+LML
How does a complexation reaction occur?
Stepwise fashion
M+L ML2
What is the complexation of unidentate ligand?
Invariably add in series of steps ??
How do you satisfy the maximum coordination number ?
Only one or a few added ligands
How are equilibrium constants written as?
Formation constant
Symbol: Beta
Why are multidentate ligand prefereable to unidentate ligand for compleometeric titrations?
They form more stbale complexes than unidentate and they form a single complex with the cation which implifies their titration curves and makes end-point detectoin easier
When would a complexation reactions be treated as a dissociation reaction of a sparingly soluble salt?
If there is an insoluble species
What ligands can protonate?
Weak acid or conj base of weak acid
Ex. Oxalate Hox- and H2ox
Basic: oxalate dominante (stable)
Acid: oxalate protonates which causes dissociation of ferric complexes
What do complexometeric titration curves plot?
pM=-logM as a function of the volume of titrant added
What is the titrant and the analyte in a complexometric titration?
Ligand- titrant
Metal ion- analyte
Why are tetradentate or hexadentrate ligands more prefereable as titrants?
Because unidentates can lead to low complex stability and indistinct titration end points
Whereas, polydentates are better because their reaction with cation and more complex, giving a sharper end point and form 1:1 complexes
What are precipitation titration based on?
reactions that yeild ionic compounds of limited solubility
What is the most used precipitating agent and what is it used for?
Silver Nitrate
–> halogens, halogen like anions, mercaptans, FA, dialent inorganic anions
What are silver nitrtate titration called ?
argentometric titrations
What is the effect of concentration on titration curves?
Whent the analyte and the titrant are higher concentration, there is a steeper end poitn region
What Ksp will give a sharper end point?
Smaller Ksp
What endpoint are used in argentometric titrations?
Chemical, potentiometirc and amperometric
What endpoint do chemical indicators create?
Color change or occasionally the apperance of turbidity in the solution
What are the requirements of an indicator?
The color change should occur over a limited reange in the p function of the analyte titration curve
The color change should take place within the steep portion of the titration curve of the analyte
What is the Voldhard method?
Silver ion are titration with a standard solution of thiocyanate ion
Fe3+ serves as the indicator, The solution turns red with the first slight excess of thiocyanate ion due to the formation of FeSCN2+
What is the Mohr method?
Indicator: Sodium Chromate
For the argentometric titration of Cl-, Br- and CN-
Ag+ react with the chromate to form a brick red silver chromates (AgCrO4) precipitae when all the Cl ions have been used up
What is the Fajans Method?
Used adsorption indicator
Transfer of color from the solution to the precipitate at eq point
What is a adsorption indicator?
Organic compounds that absorbs onto the surface of the solid in a precipitation titration which resutls in a color change at eq point.
