Complexation and Precipitation Reactions and Titrations

Question 1

What is a ligand?

Answer 1

an ion or a molecule that forms a covalent bond with a cation or a neutral metal atom by donating a pair of electrons, which are then shared by the two

EX: water, ammonia, and halide ions

Question 2

The number of covalent bonds that a cation tends to form

Answer 2

coordination number

Question 3

Define chelate.

Answer 3

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

Question 4

Differentiate unidentate and bidentate

Answer 4

unidentate - ligand that has a single donor group

bidentate - ligand with two donor group

***tri-, tetra-, penta-

Question 5

Explain the selectivity of ligand for one metal ion

Answer 5

The higher the formation constant of the metal-ligand complex, the better the selectivity of the ligand for the metal relative to similar complexes formed with other metals

Question 6

Based on reactions that yield ionic compounds of limited solubility

Answer 6

Precipitation titrations

Question 7

Precipitation titration is considered complete when
____

Answer 7

Ksp < 10^-8.

Question 8

Factors decreasing solubility

Answer 8

1. Common ion effect
2. Organic solvent

Question 9

Factors increasing solubility

Answer 9

1. Complex formation
2. Electrolyte effect
3. Acid
4. Temperature

Question 10

most widely used and important precipitating reagent

Answer 10

AgNO3

Question 11

Parameters for a feasible argentometric analysis (4)

Answer 11

1. insoluble precipitate
2. fast and rapid
3. minimal co-precipitation
4. visible equivalence point

Question 12

The titration curve for Argentometric titration usually consists of a plot of _____

Answer 12

pAg versus the volume of silver nitrated added

Question 13

Explain the effect of concentration of AgNO3 (precipitating agent) on titration curve

Answer 13

More concentrated solution has increased sharpness in the equivalence point

Question 14

Explain the effect of reaction completeness (Ksp) on argentometric titration

Answer 14

The smaller Ksp (less soluble) give sharper breaks at the end point (more feasible titration)

Question 15

In argentometric titration with mixture of analyte, the first to be titrated will be ___

Answer 15

the one with lower Ksp

(I < Br < Cl < IO3 < BrO3)

Question 16

Describe Mohr Method
1. Indicator
2. Color change
3. Medium
4. Titrant
5. Used to determine

Answer 16

1. Indicator - chromate ion
2. yellow sol’n (CrO4 2-) to brick red precipitate (Ag2CrO4)
3. Neutral or faintly alkaline
4. AgNO3
5. Cl and Br

Question 17

Requirements for Mohr Method (5)

Answer 17

1. The reverse titration of silver ion with chloride ion using chromate as indicator is not feasible
2. Not successful for titration of iodide and thiocyanate
3. No cations that give insoluble chromate (eg. barium)
4. neutral or faintly alkaline medium only
5. right concentration of indicator

Question 18

What will happen to chromate in Mohr Method if done in acidic or alkaline medium?

Answer 18

Acid: decreased chromate concentration
Base: silver will precipitate as its oxide

Question 19

What will happen if not right amount of indicator was used in Mohr method?

Answer 19

Too little: higher Ag concentration at equivalence point before precipitation of AG2CrO4 occurs

Too much: color change is hard to notice due to intense yellow color

Question 20

Describe Volhard Method
1. Indicator
2. Color change
3. Medium
4. Titrant
5. Use to determine

Answer 20

1. Indicator - ferric nitrate/ferric ammonium alum
2. Red solution
3. nitric acid solution
4. standard potassium/ammonium thiocyanate
5. Cl, Br, I (indirect)

Question 21

In Volhard method, when the excess of silver has reacted, the thiocyanate may react with the silver chloride, since ___

Answer 21

silver thiocyanate is the less soluble until the ratio [Cl−] / [SCN−] in the solution is 170.

Question 22

How to prevent thiocyanate from reacting with silver chloride in Volhard method? (3)

Answer 22

1. filter the silver chloride before back titration
2. After the addition of the silver nitrate, the suspension is boiled for about 3 minutes, cooled and then titrated immediately
3. using nitrobenzene to coat silver chloride

Question 23

Describe Fajans Method
1. Indicator
2. Color change
3. Titrant
4. Use to determine

Answer 23

1. acid dyes
   fluorescein, dichlorofluorescein, eosin
2. Color change
   fluorescein/dichlorofluorescein = greenish yellow to pink
   eosin = yellowish red to reddish-violet
3. AgNO3
4. Cl (except eosin), Br, I, and SCN

Question 24

How adsorption indicator works in Fajans method?

Answer 24

Before equivalence point, the excess lattice ion is negative (Cl-) and hence the colloid is negatively charged.

After the equivalence point, the excess lattice ion is positive (Ag+) and hence, the colloid is positively charged which makes the negatively charged indicator to bind and change color

Question 25

Describe Liebig Method

Answer 25

Turbidity method used for cyanides. Adding AgNO3 to solution with CN- ions will form white precipitate which can be redissolves when stirred to form soluble stable complex Further addition of AgNO3 will yield insoluble silver argento cyanide

Question 26

Stoichiometric ratio of CN- (NaCN) and AgNO3

Answer 26

2 mol NaCN = 1 mol AgNO3

Question 27

Describe EDTA and its binding sites

Answer 27

Ethylenediaminetetraacetic Acid widely used complexometric titrant has 6 binding sites, 4 -COOH, 2 amino groups

Question 28

The larger formation constant, the reaction of metal ion with EDTA is ____ and ___

Answer 28

more complete and larger change occurs in the equivalence-point region

Question 29

Discuss the influence of pH on titration of metal ion with EDTA

Answer 29

the end point becomes less sharp as the pH decreases because the complex-formation reaction is less complete under these circumstances However, cations with larger formation constants provide sharp end points even in acidic media

Question 30

Discuss the influence of ammonia concentration on the end point for the titration of 50.0 mL of 0.00500 M Zn2+

Answer 30

when the ammonia concentration is higher, the change in pZn near the equivalence point decreases. (do not affect pZn beyond equivalence point) the concentration should be kept minimum to prevent precipitation of analyte.

Question 31

Describe Eriochrome Black T and its color change

Answer 31

Eriochrome Black T is a typical metal-ion indicator. (also behave as acid/base indicator) H2In- -> HIn2- red to blue HIn2- -> In3- blue to orange

Question 32

: simplest and the most convenient method in which a standard solution of EDTA is slowly added to the metal ion solution till the endpoint is achieved

Answer 32

Direct titration

Question 33

Describe back titration of EDTA

Answer 33

1. An excess of a standard solution of EDTA is added to the metal solution being determined so as to complex all the metal ions present in the solution. 2. The excess of EDTA left after the complex formation with the metal is back titrated with a standard solution of a second metal ion

Question 34

How did direct titration fail? (3)

Answer 34

1. Analyte precipitates in the absence of EDTA 2. Analyte reacts too slowly with EDTA c. Analyte blocks the indicator

Question 35

Describe displacement EDTA titration. When is it used?

Answer 35

1. the metal to be analyzed is added to a metal-EDTA complex 2. The analyte ion displaces EDTA from the metal and the metal is subsequently titrated with standard EDTA used when the metal does not have a suitable indicator or possible side reaction during titration.

Question 36

Describe indirect titration

Answer 36

the anion is first precipitated with a metal cation and the precipitate is washed, dissolved and titrated with standard EDTA.

Question 37

Describe auxiliary complexing agent

Answer 37

a ligand, such as ammonia, tartrate, citrate, or triethanolamine, that binds the metal strongly enough to prevent its reaction with other reagent but weakly enough to give up the metal when EDTA is added

Question 38

Describe masking agent

Answer 38

1. reagent that protects some component of the analyte from reaction with the titrant. 2. prevents one species from interfering in the analysis of another

Question 39

releases metal ion from a masking agent.

Answer 39

demasking agent

Question 40

Describe blocking agent

Answer 40

Blocks the indicator (not analyte) from dissociating a metal

Question 41

measure of the total concentration of the calcium and magnesium ions expressed as calcium carbonate in water

Answer 41

water hardness (capacity of water to produce soap)

Question 42

____ or _____ are used to reduce the interferents (other ions than Ca and Mg) in determining water hardness ____ is used to mask the reduced ions

Answer 42

Ascorbic acid hydroxylamine cyanide

Question 43

In determining water hardness, Ca2+ can be determined separately if the titration is carried out at ____ without ____. At this pH, _____ precipitates.

Answer 43

pH 13 without ammonia Mg(OH)2

Question 44

Differentiate temporary and permanent water hardness. How to determine?

Answer 44

Temporary hardness is due to the presence of bicarbonate of calcium and magnesium; can be removed by boiling - determined by subtracting the permanent hardness from the total hardness ------- Permanent hardness is due to the presence of chlorides and sulfates of Ca and Mg; cannot be removed by boiling - determined by titrating boiled and filtered hardwater sample against EDTA solution

Question 45

Determined by titrating the water sample against the standard EDTA solution

Answer 45

total hardness of water

Question 46

How to classify water hardness (Sengupta, 2013)

Answer 46

Soft: < 60 Mod. Hard: 61 to 120 Hard: 121 to 180 Very hard: >180 values in mg/L CaCO3