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Analytical Chemistry > Titrimetry > Flashcards

Titrimetry Flashcards

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1
Q

What is a standard solution?

A

Also known as standard titrant, a reagent with known concentration used in volumetric titration.

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2
Q

Explain how titration is performed

A

slowly adding a standard solution from a buret or other liquid-dispensing device to a solution of the analyte until the reaction between the two is judged complete. the difference between final and initial reading will be calculated

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3
Q

What is back-titration and when it is required?

A

a process in which the excess of a standard solution used to consume an analyte is determined by titration with a second standard solution

often required when the rate of reaction between the analyte and reagent is slow or when the standard solution lacks stability.

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4
Q

Define equivalence point and end point.

A
  1. equivalence point
    - the point in a titration when the amount of added
    standard reagent (titrant) is equivalent to the amount of analyte
  2. end point
    - point in titration when a physical change occurs that is associated with the condition of chemical equivalence.
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5
Q

What is a titration error?

A

The difference between the actual and final volume/mass necessary to reach equivalence point

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6
Q

Requirements for primary standards (6)

A
  1. High purity
  2. Atmospheric stability
  3. Absence of hydrate water
  4. Modest cost
  5. Reasonable solubility in titration medium
  6. Reasonably large molar mass
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7
Q

Why absence of hydrate water and large molar mass are required in a primary standard?

A

Absence of hydrate water = the composition of solid does not change with variation in humidity

Large molar mass = relative error associated with weighing the standard is minimized

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8
Q

A compound whose purity has been determined by standardization

A

secondary standard

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9
Q

The ideal standard solution for a titrimetric method will ___ (4)

A
  1. sufficiently stable
    - determine the concentration only once
  2. react rapidly with analyte
    - time required between additions of reagent is minimized
  3. react more or less completely with analyte
    - satisfactory end points are realized
  4. undergo selective reaction with analyte that can be described by a balanced equation
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10
Q

Methods of determining the concentration of standard solution. (2)

Explain

A
  1. Direct method
    - primary standard is weighed and diluted to a known volume
  2. Standardization
    - titrating the titrant against a known mass of (1) a primary or (2) secondary standard or (3) an exactly known volume of another standard solution.
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11
Q

Advantages of Gravimetric Titrations (4)

A
  1. No/less calibration/cleaning of glassware
  2. Unnecessary temperature corrections
  3. Greater precision and accuracy in mass measurement
  4. Easily automated
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12
Q

Define titration curve

A

plots of a concentration-related variable versus titrant volume

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13
Q

Two types of titration curve

A
  1. sigmoidal curve
  2. linear segment curve
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14
Q

Why strong acids/bases are used as standard solutions in acid-base titration?

A

these substances react more completely with an analyte than weak acids and bases and hence, produces sharper end points

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15
Q

Why HNO3 is seldom used as standard solution in acid-base titration despite being strong acid?

A

Has oxidizing properties that can produce undesirable side reactions

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16
Q

The pH transition range of most acid type indicators is roughly _____

A

indicator pH range = pKa ± 1

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17
Q

How to minimize titration error when the pH when the indicator changes color and the pH at the equivalence point is different?

A

Choose different indicator or perform blank correction

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18
Q

Variables that influence the behavior of indicators (3)

A
  1. Temperature
  2. ionic strength of medium
  3. presence of organic solvents and colloidal particles
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19
Q

Phenolphthalein = ____ to ____
Methyl Orange = _____ to ____

A

Phenolphthalein = acid to base (8.2-10)
Methyl Orange = base to acid (3.0-4.4)

20
Q

Differentiate acidimetry and alkalimetry

A

Acidimetry – an acid titrant is used to titrate a base solution
Alkalimetry – an alkaline titrant is used to titrate an acid solution

21
Q

Explain the effect of concentration in strong acid-strong base titration.

A

Change in pH is larger with higher concentration of strong base/acid as titrant (0.1 M NaOH than 0.001 M NaOH)

22
Q

At the half-titration point in a weak-acid titration, pH = ___

At the half-titration point in a weak-base titration, pOH = ___

A

pH = pKa
pOH = pKb

23
Q

In weak acid titration, the initial pH values are ____ and the equivalence-point pH is ____ for the more dilute solution

A

higher
lower

24
Q

In weak acid titration, the pH change in the equivalence-point region becomes ___ as the acid becomes weaker. Why?

A

smaller — as the reaction between the acid and the base become less complete

25
Q

When you titrate a weak base, use an indicator with a _____ transition range.

When titrating a weak acid, use an
indicator with a _____ transition range.

A

WB = mostly acidic

WA = mostly basic

26
Q

Each of the components in a mixture containing a strong acid and a weak acid (or a strong base and a weak base) can be determined provided that ____ (2)

A
  1. concentrations of the two are of the same order of magnitude (x 10^n)
  2. dissociation constant for the weak acid or base is between 10^-4 and 10^-8
27
Q

When weak acid has a relatively large dissociation constant, only ____ can be determined accurately

A

the total number of millimoles of weak and strong acid

28
Q

When the weak acid has a very small dissociation constant, only the ___ content can be determined.

A

strong acid

29
Q

For weak acids with of intermediate strength (Ka somewhat less than 10^-4
but greater than 10^-8), ______

A

There are usually two useful end points

30
Q

Generally, Ka1 ____ Ka2
Why? (2)

A

Ka1 > Ka2

  1. first dissociation involves separating a single positively charged hydronium ion from a singly charged anion. In the second step, the hydronium ion must be separated from a doubly charged anion (more energy)
  2. a proton can be removed from more locations than in the second and third steps
31
Q

How large should be the ratio of Ka1/Ka2 to use calculated more than one endpoint?

A

greater than 10^3

32
Q

Kb1 = Kw/?
Kb2 = Kw/?

A

Kb1 = Kw/Ka2
Kb2 = Kw/Ka1

33
Q

A high mass per proton (molar mass; g/mol) consumed is desirable in a primary standard because ____

A

larger mass of reagent must be used, thus decreasing the relative weighing error.

34
Q

Common primary standards

A
  1. sodium carbonate (Na2CO3)
  2. TRIS/THAM
  3. sodium tetraborate decahydrate (Borax; Na2B4O7 * 10H2O)
35
Q

Absorption of carbon dioxide by a standardized solution of sodium or potassium hydroxide leads to _____.
How to prevent this? Why?

A

negative systematic error.

no systematic error when an indicator with an acidic range is used.

same amount of OH- and H+ ions are consumed during the formation of carbonate ion (CO2 + 2OH-) and carbonic acid (CO3 2- + 2H3O+)

36
Q

Describe carbonate error. How to minimize?

A

Error occurs when OH- absorbed atmospheric CO2 and base-range indicator is used.

Use same indicator for both standardization and analyis

37
Q

Effect of carbonate ion in standard base solution.

A

Decreases the sharpness of end points. Must be removed before standardization.

38
Q

How to remove carbon dioxide in water in preparing carbonate-free NaOH solution?

A
  1. boiling then cooling to room temperature. Hot alkali water absorbs CO2 rapidly
  2. sparging - bubbling inert gas through a solution to remove gas
39
Q

Why the base strength of NaOH decreases when stored in glass bottles?

A

Reaction of base with glass to form sodium silicates.

40
Q

Common primary standards for standardizing bases.

A
  1. Potassium hydrogen phthalate (KHP; KHC8H4O4)
  2. Benzoic acid (C6H5COOH)
  3. Potassium hydrogen iodate (KH(IO3)2)
41
Q

Describe Kjeldahl method

A
  1. Used for the determination of organic nitrogen based on acid/base titration and a standard process of determining protein content

Process:
1. Nitrogen (amides and amines) to ammonium ion
- sample is decomposed in hot, concentrated H2SO4
2. Ammonium ion to ammonia
- resulting solution is cooled, diluted, and made basic
3. Titration
- ammonia is distilled from the basic solution, collected iin acidic solution, and determined by titration.

42
Q

Main problem of Kjeldahl method. Solution.

A

Decomposition of sample is time-consuming.

Catalysts such as potassium sulfate and solution of hydrogen peroxide.

43
Q

Describe Winkler method.

A
  1. Used for the analysis of carbonate/hydroxide mixtures
  2. Both components are titrated with standard acid solution with acid-range indicator
  3. unmeasured excess of neutral barium chloride is then added to a second aliquot of the sample solution to precipitate the carbonate ion
  4. Hydroxide ion is titrated to a phenolphthalein end point.
44
Q

What happens when NaOH and NaHCO3 are mixed? How much titrant is used? (3)

A
  1. equimolar NaOH + NaHCO3 = Na2CO3
    V phth = 1/2 V bcg
  2. NaOH is exhausted = Na2CO3 + NaHCO3
    V phth < 1/2 V bcg
  3. NaHCO3 is exhausted = Na2CO3 + NaOH
    V phth > 1/2 V bcg

NaOH only
V phth = V bcg

NaHCO3 only
V phth = 0
V bcg > 0

45
Q
A

Analytical Chemistry (12 decks)

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