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Analytical Chemistry > Complexation and Precipitation Reactions and Titrations > Flashcards

Complexation and Precipitation Reactions and Titrations Flashcards

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1
Q

What is a ligand?

A

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

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2
Q

The number of covalent bonds that a cation tends to form

A

coordination number

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3
Q

Define chelate.

A

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

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4
Q

Differentiate unidentate and bidentate

A

unidentate - ligand that has a single donor group

bidentate - ligand with two donor group

***tri-, tetra-, penta-

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5
Q

Explain the selectivity of ligand for one metal ion

A

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

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6
Q

Based on reactions that yield ionic compounds of limited solubility

A

Precipitation titrations

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7
Q

Precipitation titration is considered complete when
____

A

Ksp < 10^-8.

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8
Q

Factors decreasing solubility

A
  1. Common ion effect
  2. Organic solvent
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9
Q

Factors increasing solubility

A
  1. Complex formation
  2. Electrolyte effect
  3. Acid
  4. Temperature
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10
Q

most widely used and important precipitating reagent

A

AgNO3

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11
Q

Parameters for a feasible argentometric analysis (4)

A
  1. insoluble precipitate
  2. fast and rapid
  3. minimal co-precipitation
  4. visible equivalence point
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12
Q

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

A

pAg versus the volume of silver nitrated added

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13
Q

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

A

More concentrated solution has increased sharpness in the equivalence point

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14
Q

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

A

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

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15
Q

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

A

the one with lower Ksp

(I < Br < Cl < IO3 < BrO3)

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16
Q

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

A
  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
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17
Q

Requirements for Mohr Method (5)

A
  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
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18
Q

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

A

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

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19
Q

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

A

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

20
Q

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

A
  1. Indicator - ferric nitrate/ferric ammonium alum
  2. Red solution
  3. nitric acid solution
  4. standard potassium/ammonium thiocyanate
  5. Cl, Br, I (indirect)
21
Q

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

A

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

22
Q

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

A
  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
23
Q

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

A
  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
24
Q

How adsorption indicator works in Fajans method?

A

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

25
Q

Describe Liebig Method

A

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

26
Q

Stoichiometric ratio of CN- (NaCN) and AgNO3

A

2 mol NaCN = 1 mol AgNO3

27
Q

Describe EDTA and its binding sites

A

Ethylenediaminetetraacetic Acid

widely used complexometric titrant

has 6 binding sites, 4 -COOH, 2 amino groups

28
Q

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

A

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

29
Q

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

A

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

30
Q

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

A

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.

31
Q

Describe Eriochrome Black T and its color change

A

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

32
Q

: 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

A

Direct titration

33
Q

Describe back titration of EDTA

A
  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
34
Q

How did direct titration fail? (3)

A
  1. Analyte precipitates in the absence of EDTA
  2. Analyte reacts too slowly with EDTA
    c. Analyte blocks the indicator
35
Q

Describe displacement EDTA titration. When is it used?

A
  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.

36
Q

Describe indirect titration

A

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

37
Q

Describe auxiliary complexing agent

A

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

38
Q

Describe masking agent

A
  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
39
Q

releases metal ion from a masking agent.

A

demasking agent

40
Q

Describe blocking agent

A

Blocks the indicator (not analyte) from dissociating a metal

41
Q

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

A

water hardness
(capacity of water to produce soap)

42
Q

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

____ is used to mask the reduced ions

A

Ascorbic acid
hydroxylamine

cyanide

43
Q

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

A

pH 13 without ammonia

Mg(OH)2

44
Q

Differentiate temporary and permanent water hardness. How to determine?

A

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 hardnessPermanent 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
45
Q

Determined by titrating the water sample against the standard EDTA solution

A

total hardness of water

46
Q

How to classify water hardness (Sengupta, 2013)

A

Soft: < 60
Mod. Hard: 61 to 120
Hard: 121 to 180
Very hard: >180

values in mg/L CaCO3

Analytical Chemistry (12 decks)

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