Complexation and Precipitation Titrimetry Flashcards
_____ uses complex-forming reagents for titrating cations.
Complex-Formation Titrations
____ are reagents that are organic compounds that have several electron-donor groups that form multiple covalent bonds with metal ions.
Complex-Forming Reagents
Most metal ions react with _____ to form coordination compounds or complexes.
electron-pair donors
The donor species or ligand must have ____ available for bond formation.
at least one pair of unshared electrons
The number of covalent bonds a cation tends to form with electron donors is its ____ typical values for coordination numbers are _____.
coordination number; two, four, and six
True or False
The species formed as a result of coordination can be electrically positive, neutral, or negative.
True
____ are titrimetric methods based on complex formation. They are based on a particular class of coordination compounds called chelates.
Complexometric Methods
___ are 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.
Chelates
Ligands can be:
Unidentate = 1 donor group
Bidentate = 2 donor groups
Tridentate = 3 donor groups
Tetradentate = 4 donor groups
Pentadentate = 5 donor groups
Hexadentate = 6 donor groups
….
Advantages of multidentate over unidentate ligands. (2)
- They generally react more completely with cations and provide sharper end points.
- They ordinarily react with metal ions in a single-step process where a unidentate ligand usually involves two or more intermediate species.
_____ is the most widely used complexometric titrant. It has six potential sites for bonding a metal ion; the four carboxyl groups and two amino groups. It has a hexadentate ligand.
Ethylenediaminetetraacetic Acid (EDTA)
Solutions of EDTA are particularly valuable as titrants because the EDTA combines with metal ions in a ____ ratio regardless of the charge on the cation.
1:1
Many cations form _____ when the pH is raised to the level required for their successful titration with EDTA.
hydrous oxide precipitates (hydroxides, oxides, or oxyhydroxides)
An _____ is needed to keep the cation in solution.
auxiliary complexing agent
____ 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
____ is typically a metal-ion indicator used in the titration of several common cations but behaves as an acid/base indicator. It forms red complexes with more than two dozen metal ions. But only a few are appropriate for endpoint detection.
Erichrome Black T (EBT)
_____ is the total calcium and magnesium ion concentration in a water sample and is expressed as the concentration of calcium carbonate.
Water Hardness
Historically, 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
Precipitation titrations are based on reactions that _____.
yield ionic compounds of limited solubility
Titrations with silver nitrate are sometimes called _____.
argentometric titrations
Most indicators for argentometric titrations respond to changes in the _____.
concentrations of silver ions
Endpoints can be observed by ____.
potentiometry/amperometry
In this method, silver ions are titrated with a standard solution of thiocyanate ions. 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+.
Volhard Method
In this 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
