Titrations

1
Titrations

• Main Idea Titrations are an application of
  acid-base neutralization reactions that require
  the use of an indicator.

2
Stoichiometry

• The stoichiometry of an acid-base neutralization
  reaction is the same as that of any other
  reaction that occurs in solution (they are double
  displacement reactions, after all).
• For example, in the reaction of sodium hydroxide
  and hydrogen chloride, 1 mol of NaOH neutralizes
  1 mol of HCl
• NaOH (aq) HCl (aq) ? NaCl (aq) H2O (l)
• Stoichiometry provides the basis for a procedure
  called titration, which is used to determine the
  concentrations of acidic and basic solutions.

3
Titration

• Titration is a method for determining the
  concentration of a solution by reacting a known
  volume of that solution with a solution of known
  concentration.
• If you wish to find the concentration of an acid
  solution, you would titrate the acid solution
  with a solution of a base of known concentration.
• You could also titrate a base of unknown
  concentration with an acid of known concentration.

4
In the titration of an acid by a base, the pH
meter measures the pH of the acid solution in the
beaker as a solution of a base with a known
concentration is added from the buret.
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5
How is an acid-base titration performed?

• The figure on the previous slide illustrates one
  type of setup for the titration procedure
  outlined on the next slide.
• In the procedure pictured on Slide 4, a pH meter
  is used to monitor the change in pH as the
  titration progresses.

6
Titration Procedure

1. A measured volume of an acidic or basic solution
   of unknown concentration is placed in a beaker.
   The electrodes of a pH meter are immersed in this
   solution, and the initial pH of the solution is
   read and recorded.
2. A buret is filled with the titrating solution of
   known concentration. This is called the standard
   solution, or titrant.
3. Measured volumes of the standard solution are
   added slowly and mixed into the solution in the
   beaker. The pH is read and recorded after each
   addition. This process continues until the
   reaction reaches the equivalence point, which is
   the point at which moles of H ion from the acid
   equal moles of OH- ion from the base.

7
In the titration of a strong acid by a strong
base, a steep rise in the pH of the acid solution
indicates that all of the H ions from the acid
have been neutralized by the OH- ions of the
base. The point at which the curve flexes is the
equivalence point of the titration. Bromthymol
blue is an indicator that changes color at
thisequivalence point. Notice that
phenolphthalein andmethyl red dont match the
exact equivalence point, but the slope is so
steep that it doesntmatter.
8
Strong-Strong Titration

• The previous slide shows how the pH of the
  solution changes during the titration of 50.0 mL
  of 0.100 M HCl, a strong acid with 0.100 M NaOH,
  a strong base.
• The inital pH of the 0.100 M HCl is 1.00.
• As NaOH is added, the acid is neutralized and the
  solutions pH increases gradually.
• When nearly all of the H ions from the acid have
  been used up, the pH increases dramatically with
  the addition of an exceedingly small volume of
  NaOH.
• This abrupt change in pH occurs at the
  equivalence point of the titration.
• Beyond the equivalence point, the addition of
  more NaOH again results in the gradual increase
  in pH.

9
The equivalence point here is not at a pH of 7.
Phenolphthalein is an indicator that changes
color at this equivalence point. Notice that the
starting pH is different and the region of change
is smaller.
10
Strong-Weak Titrations

• You might think that all titrations must have an
  equivalence point at pH 7 because that is the
  point at which concentrations of hydrogen ions
  and hydroxide ions are equal and the solution is
  neutral.
• This is not the case some titrations have
  equivalence points at pH values less than 7, and
  some have equivalence points at pH values greater
  than 7.
• These differences occur because of reactions
  between the newly formed salts and water salt
  hydrolysis.
• Some salts are basic (weak acid, strong base) and
  some salts are acidic (strong acid, weak base).
• The previous slide shows that the equivalence
  point for the titration of HPr (a weak acid) with
  NaOH (a strong base) lies at pH 8.80.

11
Acid-Base Indicators

• Chemists often use a chemical dye rather than a
  pH meter to detect the equivalence point of an
  acid-base titration.
• Chemical dyes whose colors are affected by acidic
  and basic solutions are called acid-base
  indicators.
• Many natural substances act as indicators.
• If you use lemon juice in your tea, you might
  have noticed that the brown color of tea gets
  lighter when lemon juice is added.
• Tea contains compounds called polyphenols that
  have slightly ionizable hydrogen atoms and
  therefore are weak acids.
• Adding acid in the form of lemon juice to a cup
  of tea lessens the degree of ionization, and the
  color of the un-ionized polyphenols becomes more
  apparent.
• Chemists have several choices in selecting
  indicators.
• Bromthymol blue is a good choice for the
  titration of a strong acid with a strong base,
  and phenolphthalein changes color at the
  equivalence point of a titration of a weak acid
  with a strong base.

12
Indicators and Titration End Point

• Many indicators used for titrations are weak
  acids.
• Each has its own particular pH or pH ranges over
  which it changes color.
• The point at which the indicator used in a
  titration changes color is called the end point
  of the titration.
• It is important to choose an indicator for a
  titration that will change color at the
  equivalence point of the titration.
• Remember that the role of the indicator is to
  indicate to you, by means of a color change, that
  just enough of the titrating solution has been
  added to neutralize the unknown solution.
• Equivalence point ? End point!
• BUT for strong-strong titrations, the pH change
  is so steep and so large, that the are
  approximately equal.

13
Titration with an Indicator
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Whats the Point of a Titration Again?

• To find the unknown concentration of an acid or a
  base.
• So you perform the actual titration noting the
  volume you started with and how much volume of
  the titrant you added and then...
• Math! (Oh no! Not math! Anything but math!)

15
Titration Calculations An Example

• The balanced equation of a titration reaction is
  the key calculating the unknown molarity. For
  example, sulfuric acid is titrated with sodium
  hydroxide according to this equation
• H2SO4 (aq) 2 NaOH (aq) ? Na2SO4 (aq) 2 H2O
  (l)
• Calculate the moles of NaOH in the standard from
  the titration data molarity of the base (MB) and
  the volume of the base (VB). In other words, MB
  VB (mol/L)(L) mol NaOH in standard
• From the equation, you know that the mole ratio
  of NaOH to H2SO4 is 21. Two moles of NaOH are
  required to neutralize 1 mol of H2SO4. mol H2SO4
  titrated mol NaOH in standard x (1 mol H2SO4 /
  2 mol NaOH)
• MA represents the molarity of the acid and VA
  represents the volume of the acid in liters. MA
  mol H2SO4 titrated/VA

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In Short Form...

• MAVA MBVB (mol acid/mol base)
• This is the mole
  ratio
• Does this make sense? Lets find out using the
  definition of molarity (mol/L) and dimensional
  analysis...
• MAVA MBVB (mol acid/mol base)
• (mol acid/L acid)(L acid) (mol base/L base)(L
  base) (mol acid/mol base)
• mol acid mol base (mol acid/mol base)
• mol acid mol acid

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HOMEWORK QUESTIONS

1. What is the purpose of a titration? How is it
   performed (in general)?
2. What is the difference between the equivalence
   point and the end point of a titration?
3. Describe the differences in the titration curve
   of a strong-strong titration vs. a strong-weak
   titration.
4. When is the equivalence point of a titration not
   at pH 7? Why does this occur?

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MORE HOMEWORK

• 5) What is the molarity of a nitric acid solution
  if 43.33 mL of 0.1000 M KOH solution is needed to
  neutralize 20.00 mL of the acid solution?
• 6) What is the concentration of a household
  ammonia cleaning solution if 49.90 mL of 0.5900 M
  HCl is required to neutralize 25.00 mL of the
  solution?
• 7) How many milliliters of 0.500 M NaOH would
  neutralize 25.00 mL of 0.100 M H2SO4?