3rd Exam Complexation and Precipitation Titrimetry Flashcards
uses complex-forming reagents for titrating cations.
Complex-Formation Titrations
are organic compounds that contain several electron-donor groups that form multiple covalent bonds with metal ions.
complex-forming reagents
Most metal ions react with electron-pair donors to form _________ or _________.
coordination compounds or complexes
The donor species or ligand must have __________________ available for bond formation.
at least one pair or unshared electrons
The number of covalent bonds that a cation tends to form with electron donors
coordination number
typical values for coordination number are
two, four, and six.
True or False
The species formed as a result of coordination can be electrically positive.
False
The species formed as a result of coordination can be electrically positive, neutral, or negative.
True or False
Almost all metal ions are found as complex ions or coordination complexes
True
_______ are titrimetric methods based on complex formation which is based upon a particular class of coordination compounds called _____.
Complexometric methods;
chelates
produced when a metal ion coordinates with two or more donor groups of a single ligand to form a five- or six- membered heterocyclic ring.
Chelate
Ligands can be: (6)
◦ Unidentate = 1 donor group
◦ Bidentate = 2 donor groups
◦ Tridentate, tetradentate, pentadentate, hexadentate
Advantages of multidentate over unidentate ligands: (2)
◦ react more completely with cations and provide sharper end points
◦ react with metal ions in a single step process where as unidentate ligands usually involves two or more intermediate species.
Tertiary amines that contains carboxylic acid groups which form stable chelates with many metal ions.
Aminocarboxylic Acids
= most widely used complexometric titrant
= has six potential sites for bonding a metal ion
= a hexadentate ligand.
Ethylenediaminetetraacetic acid (EDTA)
A titration curve for the reaction of a cation Mn+ with EDTA consist of a plot of _____ versus __________
pM versus reagent volume
coordination compounds part (3)
- metal ion
- ligand
- counter ion (for charge balance)
6 potential sites of EDTA
four carboxyl groups and two amino groups
Cu-EDTA complex charge
2-
the EDTa combines sith metal ions in a ______ ratio regardless of the charge of cation
1:1 ratio
is also used as food preservative
EDTA
many cations form _________ when the pH is raised to the level required for their successful titration with EDTA
hydrous oxide precipitate
hydrous oxide precipitate (3)
hydroxides, oxides, or oxyhydroxides
is needed to keep thr cation in solution
auxiliary complexing agent
an example of auxiliary agent when titrating zinc (II) with EDTA
zinc(Ii) is usually titrated in a medium that has fairly high concentrations of ammonia and ammonium chloride
are organic dyes that form colored chelates with metal ions in a pM range that is characteristic of a particular cation and dye.
Indicators for EDTA Titrations
example of Indicator for EDTA
Erichrome Black T
typically a metal-ion indicator used in titration of several common cations but behaves as an acid /base indicator.
Erichrome Black T
Erichrome Black T forms _______ complexes with kore than two dozen metal ions. But only a few are appropriate for end point detection.
red complexes
is the total calcium and magnesium ion concentration in a water sample and is expressed as the concentration of calcium carbonate.
Water hardness
was defined in terms of the capacity of cations in the water to replace the sodium or potassium ions in soaps and form sparingly soluble products that cause “scum” in the sink or bathtub
water “hardness”
are based on reactions that yield ionic compounds of limited solubility.
precipitation titrations
- most widely use and important precipitating reagent
- is used for the detrmination of halogens, the halogenlike anions, mercaptans, fatty acids, and several divalent inorganic anions.
silver nitrate
titrations with silver nitrate are sometimes
argentometric titrations
Most indicators for argentometric titrations respond to changes in the ______________
concentrations of silver ions
titration curves for precipitation reactions usually consist of a plot of _________ versus ___________
plot of pAg versus volume of the silver reagent (usually AgNO3).
Endpoints pf srgentometric titrations can be observed by ?
potentiometry/amperometry
Chemical indicators (2)
Volhard Method
Mohr Method
In this method, silver ions are titrated with a standard solution of thiocyanate ion.
The solution turns _______ with the first slight excess of thiocyanate ion due to the formation of Fe(SCN)2+.
Volhard Method; Red
Volhard Method indicator
Iron (III)
sodium chromate serves as the indicator for the argentometric titration of chloride, bromide, and cyanide ions.
Silver ions react with chromate to form the ____________ precipitate in the equivalence-point region.
Mohr method; brick-red silver chromate (Ag2CrO4)
