Reduction-Oxidation Titrimetry

Question 1

____ is the vertical axis in oxidation/reduction titration curves and is generally an electrode potential instead of a logarithmic function.

Answer 1

Redox Titration Curves

Question 2

____ are readily obtained by taking advantage of the fact that the two reactant species have known conc. ratios at chemical equivalence.

Answer 2

Equivalence Point Potentials

Question 3

Derivation of Redox Titration Curves (4)

Answer 3

1. Initial Potential
2. Potential After Initial Addition
3. Equivalent-Point Potential
4. Potential After the Excess Addition

Question 4

_____ are substances that change color upon being oxidized or reduced.

Answer 4

General Redox Indicators

Question 5

The color change caused by redox indicators is independent of the chemical nature of the analyte and titrant and depends instead upon the _____ that occur as the titration progresses.

Answer 5

changes in the electrode potential of the system

Question 6

General Redox Indicators (3)

Answer 6

1. Iron(II) Complexes of Orthophenanthrolines
2. Diphenylamine and its Derivatives
3. Iodine Solutions

Question 7

_____, also known as phenanthroline, form stable complexes with iron(II) and certain other ions.

Answer 7

Iron(II) Complexes of Orthophenanthrolines

Question 8

____ are widely used in oxidation/reduction reactions involving iodine as an oxidant or iodide ion as the reductant.

Answer 8

Iodine Solutions

Question 9

___ is the best-known specific indicator which forms a dark blue complex with triiodide ion.

Answer 9

Starch Indicator

Question 10

Why is it necessary to filter an analyte solution?

Answer 10

To remove granular or powdered forms of metal.

Question 11

____ is a powerful oxidizing agent capable of converting manganese(II) quantitatively to permanganate ion.

Answer 11

Sodium Bismuthate (NaBiO3)

Question 12

____ is a powerful oxidizing agent that converts Cr(III) to dichromate, Ce(III) to Ce(IV), and Mn(II) to permanganate in acidic solutions.

Answer 12

Ammonium Peroxydisulfate

Question 13

____ is a convenient oxidizing agent either as a solid sodium salt or as a dilute solution of the acid.

Answer 13

Sodium Peroxide and Hydrogen Peroxide

Question 14

Standard solutions of most reducing agents tend to react with _____. For this reason, reductants are seldom used for the direct titrations of oxidizing analytes; indirect methods are used.

Answer 14

atmospheric oxygen

Question 15

The most common indirect methods are based upon: (2)

Answer 15

1. Iron(II) Solutions
2. Sodium Thiosulfate

Question 16

____ are readily prepared from iron(II) ammonium sulfate (Mohr’s salt) or form closely related iron(II) ethylenediamine sulfate (Oesper’s salt).

Answer 16

Iron(II) Solutions

Question 17

____ is a moderately strong reducing agent that has been widely used to determine oxidizing agents by an indirect procedure that involves iodine as an intermediate.

Answer 17

Sodium Thiosulfate

Question 18

In an iodine test, a negative test (starch is absent) should display a ____ color. While in a positive test, the color ____ is displayed.

Answer 18

brown/yellow; blue/purple

Question 19

Variables that influence the rate of thiosulfate reaction include: (5)

Answer 19

1. pH
2. presence of microorganisms
3. concentration of the solution
4. presence of copper (II)
5. exposure to sunlight

Question 20

_____ is an excellent primary standard for thiosulfate solutions.

Answer 20

Potassium Iodide

Question 21

The choice among the two (Potassium Permanganate and Cerium(IV) depends upon: (5)

Answer 21

1. the strength of the analyte as a reducing agent
2. the rate of reaction between oxidant and analyte
3. the stability of the standard oxidant solutions
4. the cost
5. the availability of an indicator

Question 22

The intense ____ color of potassium permanganate solution is sufficient to serve as an indicator for most titrations. The permanganate ions react slowly with the large conc. of manganese(II) ions present at the endpoint.

Answer 22

purple

Question 23

Solutions of Cerium(IV) are ____ but the color is not intense enough to act as an indicator in titrations. The most widely used indicator is the iron(II) complex of 1,10-phenanthroline or one of its derivatives.

Answer 23

yellow-orange

Question 24

Permanganate solutions when prepared properly, are stable because ____. It is catalyzed by light, heat, acids, bases, manganese(II), and manganese dioxide. Standardized permanganate solutions should be stored in the dark.

Answer 24

the decomposition reaction is slow

Question 25

____ is used to standardize permanganate and cerium(IV) solutions. In an acidic solution, the oxalate is converted to the undissociated acid.

Answer 25

Sodium Oxalate

Question 26

The disadvantage of potassium dichromate over cerium(IV) and permanganate ion is its _____ with certain reducing agents.

Answer 26

lower electrode potential and the slowness of its reaction

Question 27

Solutions of iodine are ____ that are used for the determination of strong reductants.

Answer 27

weak oxidizing agents

Question 28

The endpoint of a Karl Fischer reagent is 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; the change is from the ____ of the reaction products to the _____ of the excess reagent.

Answer 28

yellow; brown

Question 29

The Karl Fischer reagent is applied to the determination of ____ in numerous samples.

Answer 29

water