3rd Exam Redox Titration Flashcards
is based on an oxidation-reduction reaction between the analyte and titrant
Redox titration
agent and reaction
1. gains e-
2. loses e-
- Oxidizing Agent; Reduction
- Reducing Agent; Oxidation
Redox titration most commonly use a ______ or a ______ to determine the endpoint.
potentiometer or a redox indicato
The vertical axis in oxidation/reduction titration curves is generally an ________ instead of logarithmic functions
electrode potential
True or False
The conc. can be obtained from the stoichiometric reaction.
False
the conc. cannot be obtained from
the stoichiometric reaction. Fortunately, equivalence-point potentials are readily obtained by taking advantage of the fact that the two reactant species have known conc. ratios at chemical equivalence.
Derivation of Redox Titration Curves (4)
a. initial potential
b. potential after initial addition
c. equivalence-point potential
d. potential after the excess addition
Two Types of Indicator
General Redox Indicator
Specific Indicator
substances that change color upon being oxidized or reduced. The color change is independent of the chemical nature of the analyte and titrant and depend instead upon the changes in the electrode potential of the system that occur as the titration progresses.
General Redox Indicator
3 Examples of General Redox Indicator
a. Iron(II) Complexes of Orthophenanthrolines
b. Diphenylamine and its Derivatives
c. Iodine Solutions
known as phenanthroline that form stable complexes with iron(II) and certain other ions.
Iron(II) Complexes of Orthophenanthrolines
widely used in oxidation/reduction reactions involving iodine as an oxidant or iodide ion as the reductant.
Iodine Solutions
the best known specific indicator which forms a dark blue complex with triiodide ion.
Starch indicator
the analyte in an oxidation/reduction titration must be in a what oxidation state at the outset?
single oxidation state
however, steps that precede the titration frequently convert the analyte to a mixture of oxidation state
Is added on the solution to turn the analyte in single oxidation state?
Auxiliary Reducing and Oxidizing Agents
It is a number of metals are good auxiliary reducing agents and have been used for prediction of analytes?
What are the example of these metals?
Auxiliary Reducing Reagents
Zn, Al, Cd, Pb, Ni, Cu, and Ag (in the presence of chloride ion)
T or F
sticks or coils of the metals that are auxiliary reducing agent can be immersed directly in the analyte solution in order to covert the analyte into single oxidation state.
True
After removing the solid, it is necessary to filter the solution to remove granular or powdered forms of the metal.
why should we remove unreacted auxiliary reducing agent from the analyte?
any unreacted auxiliary reducing agent will react with the titrant, it is removed before we begin the titration by removing the coiled wire or by filtering
an alternative method for using an auxiliary reducing agent where the amount of the reductant is selected to ensure the analyte’s complete reduction
use of reductor or reduction column
Give the 2 examples of reductor as an alternative of filtration.
Walden Reductor: Ag(s) + Cl- -> AgCl(s) + e-
Jones Reductor: Zn(Hg) (s) -> Zn2+ + Hg + 2e-
Auxilliary Oxidizing Reagents (3)
- Sodium Bismuthate (NaBiO3)
- Ammonium Peroxydisulfate
- Sodium Peroxide and Hydrogen Peroxide
- powerful auxiliary oxidizing agent capable of converting manganese(II) quantitatively to permanganate ion.
- a sparingly soluble salt but its exact composition is uncertain.
Sodium Bismuthate (NaBiO3)
powerful auxiliary oxidizing agent that converts Cr(III) to dichromate, Ce(III) to Ce(IV), and Mn(II) to permanganate in acidic solutions
Ammonium Peroxydisulfate
a convenient auxiliary oxidizing agent either as a solid sodium salt or as a dilute solution of the acid.
Sodium Peroxide and Hydrogen Peroxide
➢Standard solutions of most reducing agents tend to react with atmospheric oxygen. For this reason, reductants are seldom used for the ________titrations of oxidizing analytes.
What methods are used to avoid reaction with atmospheric oxygen or air oxidation?
direct titrations;
indirect methods
(Standard Reductants)The two most common indirect methods are based upon:
a. Iron(II) Solutions
b. Sodium Thiosulfate
is readily prepared from iron(II) ammonium sulfate (_______) or form closely related iron(II) ethylenediamine sulfate (________)
Iron(II) Solutions
Mohr’s salt: iron(II) ammonium sulfate
Oesper’s salt: iron(II) ethylenediamine sulfate
air oxidation of iron(II) takes place rapidly in neutral solutions but is inhibited in the presence of ____with the most stable preparations being ____M in H2SO4
acid
0.5 M in H2SO4
moderately strong reducing agent that has been widely used to determine oxidizing agents by indirect procedure that involves iodine as an intermediate.
Sodium Thiosulfate
When using sodium thiosulfate as standard reductant, the end point is the disappearance of the _____?
iodine color
True or False
Sodium thiosulfate solutions is stable since thiosulfate tend to decompose to give sulfur and hydrogen sulfite ion: S2O32- + H+ HSO3- + S(s).
True
Most often, iodine titrations are performed with a suspension of _____ as an indicator.
starch
Iodine test is an example of iodine titrations that uses starch as indicator. Give the result of negative test and positive test.
Negative test: brown / yellow
Positive test: blue / purple
Variables that influence the rate of thiosulfate reaction include (5)
- pH
- presence of microorganisms
- concentration of the solution
- presence of copper (II)
- exposure to sunlight
In standardization of thiosulfate solutions,
______ is an excellent primary standard for thiosulfate solutions.
Potassium iodate
COMMON OXIDANTS USED AS STANDARD SOLUTIONS (5)
KMnO4 -> Mn2+
KBrO3 -> Br-
Ce4+ -> Ce3+
K2Cr2O7 -> Cr3+
I2 -> I-
The strong Oxidants (2)
Potassium Permanganate, KMnO4
Cerium(IV), Ce4+
The choice among the 2 strong oxidants (KMnO4, Ce4+) upon: (5)
- the strength of the analyte as a reducing agent,
- the rate of reaction between oxidant and analye,
- the stability of the standard oxidant solutions,
- the cost
- the availability of indicator
The ___________ of potassium permanganate solution is sufficient to serve as an indicator for most titrations.
intense purple color
Solutions of Cerium(IV) are _________(color) but the color is not intense enough to act as an indicator in titrations. The most widely used indicator is the __________ or one of its derivatives.
yellow-orange,
iron(II) complex of 1,10- phenanthroline
Permanganate solutions when prepared properly, are stable because decomposition reaction is slow. It is catalyzed by: (6)
light, heat, acids, bases, manganese(II) and manganese dioxide.
True or False
Standardized permanganate solutions should be stored in the sunny area.
False
Standardized permanganate solutions should be stored in the dark.
STANDARD OXIDANTS (3)
- Permanganate and Cerium(IV)
- Potassium Dichromate
- Iodine
Primary Standards for Ce4+ (4)
- Cerium(IV) ammonium nitrate
- Cerium(IV) ammonium sulfate
- Cerium(IV) hydroxide
- Cerium(IV) hydrogen sulfate
Primary Standards for KMnO4 and Ce4+
used to standardize permanganate and
cerium(IV) solutions. In acidic solution, it is converted to the undissociated acid.
Sodium Oxalate (Na2C2O4)
what are the oxidation products that are formed with permanganate and cerium(IV) ?
CO2(g) and H2O
When Potassium Dichromate is used as an oxidant, dichromate ion is reduced to __________, carried out in solutions that are 1 M in ____ or ____.
green chromium(III) ion
1 M HCl or H2SO4
the disadvantage of potassium dichromate over cerium(IV) and permanganate ion is: (2)
- its lower electrode potential and
- the slowness of its reaction with certain reducing agents
standardization of potassium dichromate solutions
reagent of potassium dichromate is pure
Potassium dichromate has an _____ color and turns into ______chromium(III) ion when reduced.
orange -> green (when reduced)
has been widely used for indirect determination of nitrate, chlorate, permanganate, and dichromate ions as well as organic peroxides and several oxidizing agents.
reaction of dichromate
In dichromate solutions, the color change observed at the end point in a direct titration is from _____ of chromium(III) to _______.
green to violet
True or False
The principal use of dichromate is for the gravimetric titration of iron(II) in the presence of moderate conc. of H2O.
False
Dichromate is for the volumetric titration of iron(II) in the presence of moderate conc. of HCl.
are weak oxidizing agents that are used for the determination of strong reductants.
iodine solutions
iodine solutions although it has __________ is that it imparts a degree of _____ that makes possible the determination of strong reducing agents in the presence of weak ones.
low potential ;
selectivity
an important advantage of iodine is the _______________ for the titrations.
availability of sensitive or reversible indicator for the titrations
True or False
iodine solutions lack stability and must be restandardized regularly
True.
Iodine solutions are not very soluble in water; it is ordinarily dissolved in moderately concentrated solutions of ____
KI
the volatility of iodine solute makes the iodine solutions _______
lack stability
iodine slowly attacks most _______ materials; cork and rubber stoppers are never used to close containers of reagent
organic
air oxidation of iodide ions can cause changes in the molarity of an iodine solution; causes molarity of iodine solution to _________. Air oxidation is promoted by ____, ______, and _________.
increase;
acids, heat and light
Iodine Solutions can be standardized against ______________ or ______________, both which are available commercially.
anhydrous sodium thiosulfate or
barium thiosulfate monohydrate
SOME SPECIALIZED OXIDANTS
- Potassium bromate
- Periodic acid
- Karl Fischer reagent
used for the determination of organic compounds that contain oleffinic and certain type of aromatic functional groups
Potassium bromate
reacts selectively with organic compounds having hydroxyl, carbonyl, or amine groups on adjacent carbon atoms.
Periodic acid
widely employed for the determination of water in a variety of organic or inorganic samples.
Karl Fischer reagent
- available for commercial uses and can be used to prepare standard solutions that are stable indefinitely.
- widely used as a stable source of bromine
Potassium Bromate
How does the Potassium Bromate as a Source of Bromine work and oxidant for titration?
- standard bromine solutions lack stability
BrO3- + 5Br + 6H+ <—> 3Br2 + 3 H2O - an excess potassium iodide is introduced to determine the excess bromine, the reaction occurring is 2I- + Br2 <—> 2Br- + I2
- the liberated iodine is then titrated with standard sodium thiosulfate.
- bromine is incorporated into an organic molecule either by substitution or by addition
is available for the preparation of standard solutions of periodic acid;
it is a crystalline, readily soluble, hygroscopic solid.
paraperiodic acid
for preparation of periodic acid solutions (3)
paraperiodic acid
sodium metaperiodate
sodium paraperiodate
sodium paraperiodate, (Na5IO6) is not sufficiently soluble but converted to ________ by recrystallization from concentrated nitric acid
soluble metaperiodate
T or F
periodate solutions vary in stability, depending on their mode of preparation
True
Periodate Solutions can be standardized by ___________ of the reagent with standard ________ or _______ to ensure they remain slightly alkaline.
buffering aliquots
borax or sodium hydrogen carbonate;
in standardization of periodate solutions, an excess _______ is introduced in the formation of a mol of iodine for each mole of periodate
iodide (I-)
- it is widely used because it reacts remarkably selectively with organic compounds containing certain combinations of functional groups
.
Periodic Acid
periodate oxidations are usually carried out in aqueous solutions, although __________ may be added to enhance the solubility of the sample
solvents such as methanol, ethanol or dioxane
is relatively specific for water
Karl Fischer Reagent for Water Determination
Karl Fischer Reaction composed: (5)
iodine, sulfur dioxide, pyridine and methanol
Karl Fischer stoichiometry
1 mol of iodine = 1 mol of sulfur dioxide = 3 mol of pyridine = 1 mol of water
preparation of Karl Fischer reagent
- prepare the reagent a day or two before
- its strength must be established at least daily
- keep atmospheric pressure away
- all glassware must be carefully dried
- Karl Fischer reagent decomposes on standing that’s why it is common practice to prepare the reagent a day or two before it is to be used.
- its strength must be established at least daily against a standard solution of water in methanol.
- keep atmospheric pressure from containing the Karl Fischer reagent and the sample.
- all glassware must be carefully dried before use and the standard solution must be stored out of contact with air to minimize the contact between the atmosphere and the solution during titration.
KARL FISCHER REAGENT End-Point Detection
signaled by the appearance of the first excess of pyridine/iodine complex when all water has been consumed.
- the color of the reagent is intense enough for the visual end-point: yellow -> brown
- applied to the determination of water in numerous samples.
KARL FISCHER REAGENT
- usually feasible, if the sample can be completely dissolved in methanol
- This method has been applied to the determination of water in many organic acids, alcohols, esters, ethers, anhydrides and halides. The hydrated salt of most organic acids as well as the hydrates of a number of inorganic salts that are soluble in methanol can be also determined by this method.
direct and rapid titration of KARL FISCHER REAGENT
several variations of the basic technique using Karl Fischef reagents are applied depending upon (3)
- the solubility of the material
- the state in which the water is retained
- the physical state of the sample.
thiosulfate solution stoichiometry
1 mol IO3- = 3 mol I2 = 6 mol S2O32-
