Chapter 15: Complexation and Precipitation Reactions and Titrations Flashcards
The number of covalent bonds that a cation tends to form with electron donors
is its
coordination number
Titrations based on complex formation, sometimes called
complexometric titrations
is 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
Another important type of complex is formed between metal ions and cyclic
organic compounds, known as macrocycles. These molecules contain nine or more
atoms in the cycle and include at least three heteroatoms, usually oxygen, nitrogen,
or sulfur.
macrocycles (e.g. crown ethers)
form three dimensional cavities that can just accommodate appropriately sized metal ions
macrocyclic compounds
are based on reactions that yield ionic compounds of limited solubility
precipitation titrations
Titrations with silver nitrate are sometimes called
argentometric titrations
end points used in argentometric titrations
chemical
potentiometric
amperometric
the potential difference between a silver electrode and a reference electrode is measured as a function of titrant volume
potentiometric
the current generated between a pair of silver electrodes is measured and plotted as a function
of titrant volume
amperometric
produce a color change or occasionally the appearance or disappearance of turbidity in the solution being titrated.
chemical indicators
this method, silver ions are titrated with a standard solution of thiocyanate ion:
volhard method
sodium chromate serves as the indicator for the argentometric titration of chloride, bromide, and cyanide ions. Silver ions react with chromate to form the brick-red silver chromate (Ag2CrO4) precipitate in the equivalence-point region.
Mohr method
uses an adsorption indicator, an organic compound that adsorbs onto or desorbs from the surface of the solid in a precipitation titration. Ideally, the adsorption or desorption occurs near the equivalence point and results not only in a
color change but also in the transfer of color from the solution to the solid or vice versa.
Fajans method
organic complexing agents extracts
8-hydroxyquinoline
diphenylthiocarbazone
acetylacetone
ammonium pyrrolidine dithiocarbamate
thenoyltrifluoroacetone
dibenzo-18-crown-6
Another important application of organic complexing agents is in forming stable
complexes that bind a metal and prevent it from interfering in a determination, which is also called as
masking agent
is the most widely used complexometric
titrant.
EDTA
ethylenediamminetetraacetic acid
When it is dissolved in water, EDTA behaves like an amino acid, such as
glycine
is the second most common
amino-polycarboxylic acid used
for titrations. It is a tetradentate
chelating agent and has the
structure
nitrilotriacetic acid
TRUE or FALSE
Solutions of EDTA are particularly valuable as titrants because the EDTA combines
with metal ions in a 1:1 ratio regardless of the charge on the cation.
TRUE
TRUE or FALSE
EDTA is a remarkable reagent not only because it forms chelates with nearly all cations but also because most of these chelates are sufficiently stable for titrations and themselves colorless
TRUE
TRUE or FALSE
The ability of EDTA to form complexes with metals is responsible for its widespread use as a preservative in foods and in biological samples
TRUE
TRUE or FALSE
EDTA is an excellent preservative for foods
and is a common ingredient of such commercial food products
as mayonnaise, salad dressings, and oils
TRUE
EDTA and other similar chelating agents are often called _______ because of their ability to remove or inactivate metal ions
sequestering agents
