Complexation Flashcards
uses complex-forming reagents for titrating cations. These
reagents are organic compounds that contain several electron-
donor groups that form multiple covalent bonds with metal ions.
Complex-Formation Titrations
• Most metal ions react with electron-pair donors to form
coordination compounds or complexes.
• The donor species or ligand must have at least one pair or
unshared electrons available for bond formation.
• The number of covalent bonds that a cation tends to form with
electron donors is its coordination number typical values for
coordination number are two, four, and six.
• The species formed as a result of coordination can be
electrically positive, neutral, or negative.
Complex-Forming Reactions
= titrimetric methods based on complex formation; based upon a
particular class of coordination compounds called _____(1).
(1)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.
Complexometric methods
(1) chelates
Ligands ca be
Unidentate = 1 donor group
Bidentate = 2 donor groups
Tridentate, tetradentate, pentadentate, hexadentate
◦ they generally react more completely with cations and
provide sharper end points
◦ they ordinarily react with metal ions in a single step
process wherea unidentate ligands usually involves two
or more in termediate species.
Advantages of multidentate over unidentate ligands
Tertiary amines that contains carboxylic acid groups
form stable chelates with many metal ions.
Titrations with Aminocarboxylic Acids
Titrations with Aminocarboxylic Acids (3)
a. Ethylenediaminetetraacetic acid (EDTA)
b. Equilibrium Calculations Involving EDTA and EDTA
Titration Curves
c. Indicators for EDTA Titrations
= most widely used complexometric titrant
= has six potential sites for bonding a metal ion; the
four carboxyl groups and two amino groups.
= a hexadentate ligand.
a. Ethylenediaminetetraacetic acid (EDTA)
A titration curve for the reaction of a cation Mn+ with
EDTA consist of a plot of pM versus reagent volume
b. Equilibrium Calculations Involving EDTA and EDTA
Titration Curves
olutions of ___ are particularly valuable as titrants because
the ____ combines with metal ions in a ____ratio regardless
of the charge on the cation. For example, the silver and
aluminum complexes are formed by the reactions
EDTA,
1:1
most of these ____ are sufficiently stable for titrations
Chelates
is also used as a food preservative
EDTA
Many cations form hydrous oxide precipitates (hydroxides,
oxides, or oxyhydroxides) when the pH is raised to the level
required for their successful titration with EDTA.
an _________ is needed to keep the cation in
solution.
For example, zinc(II) is usually titrated in a medium that has
fairly high concentrations of ammonia and ammonium
chloride.
Auxiliary complexing Agents
= generally these are organic dyes that form
colored chelates with metal ions in a pM range
that is characteristic of a particular cation and
dye.
c. Indicators for EDTA Titrations
= typically a metal-ion indicator used in titration
of several common cations but behaves as an
acid /bae indicator. It forms red complexes with
more than two dozen metal ions. But only a few
are appropriate for end point detection.
Erichrome Black T
◦ ____(1) is the total calcium and magnesium ion concentration in a
water sample and is expressed as the concentration of calcium carbonate.
◦ Historically, water “____(2)” 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 “___(3)” in the sink or bathtub.
(1)Water hardness
(2) hardness
(3) scum
______are based on reactions
that yield ionic compounds of limited
solubility.
We limit our discussion here to the most
widely used and important precipitating
reagent, silver nitrate, which is used for the
determination of the halogens, the
halogenlike anions, mercaptans, fatty acids,
and several divalent inorganic anions.
Titrations with silver nitrate are sometimes
called _______
Precipitation titration
argentometric titrations.
Most indicators for _________ respond to changes in
the concentrations of silver ions. Because of this response, titration
curves for precipitation reactions usually consist of a plot of pAg
versus volume of the silver reagent (usually _____).
argentometric titrations
AgNO3
can be observed by potentiometry/amperometry
Endpoints
(1)______ - In this method, silver ions are titrated with a
standard solution of thiocyanate ion. Iron(III) serves as the
indicator. The solution turns red with the first slight excess of
thiocyanate ion due to the formation of Fe(SCN)2+.
Chemical indicators
(1) 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
Treatment of hydroxylamine (H2NOH) with an excess of Fe(III) results
in the formation of N2O and an equivalent amount of Fe(II):
2H2NOH + 4Fe3+➔ N2O(g) 1 + Fe2+ + 4H+ + H2O
Calculate the molar concentration of an H2NOH solution if the Fe(II)
produced by treatment of a 25.00-mL aliquot required 14.48 mL of
0.01528 M K2Cr2O7.
Ans. 0.0266 M
The Tl in a 9.57-g sample of rodenticide was oxidized to the trivalent state and
treated with an unmeasured excess of Mg/EDTA solution. The reaction is
Tl3+ + MgY2- ➔ TlY- + Mg2+
Titration of the liberated Mg21 required 12.77 mL of 0.03610 M EDTA. Calculate
the percent Tl2SO4 (504.8 g/mol) in the sample.
Ans. 1.216%
