Titrimetry Flashcards
What is a standard solution?
Also known as standard titrant, a reagent with known concentration used in volumetric titration.
Explain how titration is performed
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
What is back-titration and when it is required?
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.
Define equivalence point and end point.
- equivalence point
- the point in a titration when the amount of added
standard reagent (titrant) is equivalent to the amount of analyte - end point
- point in titration when a physical change occurs that is associated with the condition of chemical equivalence.
What is a titration error?
The difference between the actual and final volume/mass necessary to reach equivalence point
Requirements for primary standards (6)
- High purity
- Atmospheric stability
- Absence of hydrate water
- Modest cost
- Reasonable solubility in titration medium
- Reasonably large molar mass
Why absence of hydrate water and large molar mass are required in a primary standard?
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
A compound whose purity has been determined by standardization
secondary standard
The ideal standard solution for a titrimetric method will ___ (4)
- sufficiently stable
- determine the concentration only once - react rapidly with analyte
- time required between additions of reagent is minimized - react more or less completely with analyte
- satisfactory end points are realized - undergo selective reaction with analyte that can be described by a balanced equation
Methods of determining the concentration of standard solution. (2)
Explain
- Direct method
- primary standard is weighed and diluted to a known volume - 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.
Advantages of Gravimetric Titrations (4)
- No/less calibration/cleaning of glassware
- Unnecessary temperature corrections
- Greater precision and accuracy in mass measurement
- Easily automated
Define titration curve
plots of a concentration-related variable versus titrant volume
Two types of titration curve
- sigmoidal curve
- linear segment curve
Why strong acids/bases are used as standard solutions in acid-base titration?
these substances react more completely with an analyte than weak acids and bases and hence, produces sharper end points
Why HNO3 is seldom used as standard solution in acid-base titration despite being strong acid?
Has oxidizing properties that can produce undesirable side reactions
The pH transition range of most acid type indicators is roughly _____
indicator pH range = pKa ± 1
How to minimize titration error when the pH when the indicator changes color and the pH at the equivalence point is different?
Choose different indicator or perform blank correction
Variables that influence the behavior of indicators (3)
- Temperature
- ionic strength of medium
- presence of organic solvents and colloidal particles
Phenolphthalein = ____ to ____
Methyl Orange = _____ to ____
Phenolphthalein = acid to base (8.2-10)
Methyl Orange = base to acid (3.0-4.4)
Differentiate acidimetry and alkalimetry
Acidimetry – an acid titrant is used to titrate a base solution
Alkalimetry – an alkaline titrant is used to titrate an acid solution
Explain the effect of concentration in strong acid-strong base titration.
Change in pH is larger with higher concentration of strong base/acid as titrant (0.1 M NaOH than 0.001 M NaOH)
At the half-titration point in a weak-acid titration, pH = ___
At the half-titration point in a weak-base titration, pOH = ___
pH = pKa
pOH = pKb
In weak acid titration, the initial pH values are ____ and the equivalence-point pH is ____ for the more dilute solution
higher
lower
In weak acid titration, the pH change in the equivalence-point region becomes ___ as the acid becomes weaker. Why?
smaller — as the reaction between the acid and the base become less complete
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.
WB = mostly acidic
WA = mostly basic
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)
- concentrations of the two are of the same order of magnitude (x 10^n)
- dissociation constant for the weak acid or base is between 10^-4 and 10^-8
When weak acid has a relatively large dissociation constant, only ____ can be determined accurately
the total number of millimoles of weak and strong acid
When the weak acid has a very small dissociation constant, only the ___ content can be determined.
strong acid
For weak acids with of intermediate strength (Ka somewhat less than 10^-4
but greater than 10^-8), ______
There are usually two useful end points
Generally, Ka1 ____ Ka2
Why? (2)
Ka1 > Ka2
- 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)
- a proton can be removed from more locations than in the second and third steps
How large should be the ratio of Ka1/Ka2 to use calculated more than one endpoint?
greater than 10^3
Kb1 = Kw/?
Kb2 = Kw/?
Kb1 = Kw/Ka2
Kb2 = Kw/Ka1
A high mass per proton (molar mass; g/mol) consumed is desirable in a primary standard because ____
larger mass of reagent must be used, thus decreasing the relative weighing error.
Common primary standards
- sodium carbonate (Na2CO3)
- TRIS/THAM
- sodium tetraborate decahydrate (Borax; Na2B4O7 * 10H2O)
Absorption of carbon dioxide by a standardized solution of sodium or potassium hydroxide leads to _____.
How to prevent this? Why?
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+)
Describe carbonate error. How to minimize?
Error occurs when OH- absorbed atmospheric CO2 and base-range indicator is used.
Use same indicator for both standardization and analyis
Effect of carbonate ion in standard base solution.
Decreases the sharpness of end points. Must be removed before standardization.
How to remove carbon dioxide in water in preparing carbonate-free NaOH solution?
- boiling then cooling to room temperature. Hot alkali water absorbs CO2 rapidly
- sparging - bubbling inert gas through a solution to remove gas
Why the base strength of NaOH decreases when stored in glass bottles?
Reaction of base with glass to form sodium silicates.
Common primary standards for standardizing bases.
- Potassium hydrogen phthalate (KHP; KHC8H4O4)
- Benzoic acid (C6H5COOH)
- Potassium hydrogen iodate (KH(IO3)2)
Describe Kjeldahl method
- 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.
Main problem of Kjeldahl method. Solution.
Decomposition of sample is time-consuming.
Catalysts such as potassium sulfate and solution of hydrogen peroxide.
Describe Winkler method.
- Used for the analysis of carbonate/hydroxide mixtures
- Both components are titrated with standard acid solution with acid-range indicator
- unmeasured excess of neutral barium chloride is then added to a second aliquot of the sample solution to precipitate the carbonate ion
- Hydroxide ion is titrated to a phenolphthalein end point.
What happens when NaOH and NaHCO3 are mixed? How much titrant is used? (3)
- equimolar NaOH + NaHCO3 = Na2CO3
V phth = 1/2 V bcg - NaOH is exhausted = Na2CO3 + NaHCO3
V phth < 1/2 V bcg - 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
