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
are useful for the determination of cations that form stable EDTA complexes and for which a satisfactory indicator is not available
back titration
is also useful for analyzing samples that contain anions that could form precipitates with the analyte under the analytical conditions
back titration
an unmeasured excess of a solution containing the magnesium or zinc complex of EDTA is introduced into the analyte solution
displacement titrations
is the most widely used complexometric
titrant.
EDTA
is a typical metal-ion indicator that is used in the titration of several common cations. generally red in color of its metal complexes
Eriochrome Black T
forms red complexes with more than two dozen metal ions, but the formation constants of only a few are appropriate for end-point detection
Eriochrome Black T
contains calcium, magnesium, and heavy metal
ions that form precipitates with soap (but not detergents).
hard water
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
hard water
forms the least stable EDTA complex of
all of the common multivalent cations in typical water samples is not titrated until
enough reagent has been added to complex all of the other cations in the sample
magnesium
TRUE or FALSE
Water hardness is usually determined by an EDTA titration after the sample has
been buffered to pH 10
TRUE
TRUE or FALSE
The formation of soluble complexes can be used to control the concentration of free
metal ions in solution and thus control their reactivity
TRUE
One of the most common side reactions is that of a ligand that can protonate, that is, the ligand is a
weak acid or the conjugate base of a weak acid
a metal ion reacts with a suitable ligand to form a complex, and the equivalence point is determined by an indicator or an appropriate instrumental method. The formation of soluble inorganic complexes is not widely used for titrations, but the formation of precipitates, particularly with silver nitrate as the titrant, is the basis for many important determinations
complexometric titrations
Complexometric titration curves are usually a plot of
pM=-log[M] as a function of the volume of titrant added
