AcidBase Titration

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Acid-Base Titration

• Math-Science Partnership Workshop
• Maryville College-Pellissippi State
• July 07-11

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Definition of Titration

• Titration is a common analytical procedure where
  a standard solution is added to an analyte
  solution until some chemical change occurs at the
  point where the volume of the standard solution
  is equivalent to the amount of the analyte. The
  equivalence point is determined by precipitation,
  color change in an indicator, or electrometric
  methods. This procedure is also a form of
  volumetric analysis.
• The volume of the titrant is determined using a
  burette or by using a calibrated drop counter.

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Acids and Bases

• Acids and bases can be classified as strong when
  they completely dissociate in water. When HCl or
  NaOH is added to water, the acid/base dissciates
  into its component hydrated ions
• HCl H2O ?? H30 Cl-
• NaOH H2O ?? Na(aq) OH-(aq)

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Weak Acids and Bases

• Other acids are weak acids and do not completely
  dissociate in water. They remain almost
  completely undissociated when added to water.
  The mathematical acid dissociation constant can
  show the extent of dissociation.
• CH3COOH H2O ?? CH3COO- H3O
• Ka CH3COO- H3O / CH3COOH

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Equivalence Point

• When a strong base is added to a strong acid,
  there is a point at which the number of moles of
  acid and base are equal, the equivalence point.
  If you know the volume and concentration of the
  standard base solution added and the volume of
  the strong acid you started with, the
  concentration of the strong acid can be
  determined.
• The equivalence point (EP) for such a titration
  is usually determined by using an indicator which
  changes color near the EP.

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Titration Strong Acid with Strong Base
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Titration Weak Acid with Strong Base
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First Derivative
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Second Derivative
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Vernier Equipment

• The titration of a weak or strong acid or base
  can be accomplished using a Vernier drop counter
  and pH probe.
• The end point (equivalence point) can be
  ascertained using the light probe and an
  appropriate LED to detect the color change
  endpoint using an indicator.
• A video camera can be used to see the color
  change if a constant flow of titrant is delivered
  as the camera records the color of the stirred
  analyte solution

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Drop Counter Setup
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Drop Counter Data
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Other Possible Probes

• Titration on strong acid with strong base (HCl
  with NaOH) can be followed with the temperature
  probe to avoid damaging the pH probe with the
  strong acid conditions in the beginning. The
  heat generated in the neutralization reaction can
  be followed and the equivalence point determined
  from a temperature versus tritrant added plot.

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Other Titrations

• The conductivity probe can be used for a
  conductometric titration. If the reaction
  between the analyte and titrant results in a
  change in conductivity. The curve will be
  similar to the pH curves the equivalence point
  is normally where the conductivity drops to a
  minimum as some insoluble precipitate forms in
  the reaction.

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Physiological Buffer Systems
Phosphate is one of the most common physiological
buffers in living systems. Phosphoric acid is a
tri-protic acid and as such has three Pkas. The
pKa closest to the region of physiological range
is the second, 7.2. That means that the buffer
is made from two forms of phosphate, the diprotic
acid and the monoprotic base.
H2PO4-1 ? H HPO4 -2
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What is a Buffer??

• The basic chemical definition of a buffer is a
  solution of salts of weak acids that resists
  changes in pH with added acid or base.
• A buffer solution is prepared from a weak acid
  and its conjugate base acetic acid and sodium
  acetate for example
• The pH of the buffer can be predicted
  mathematically from the pKa of the acid and the
  ratio of the conjugate base and free acid
  concentrations.

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Common Buffers

• Blood pH Buffers
• Carbonic acid / Bicarbonate
• Phosphate buffers
• Protein buffers (amino acids)
• Many different buffers are used in commercial
  products to maintain pH and therefore preserve
  the quality of the product over a longer period
  of time.

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Possible Class Activities

• Commercial products could be titrated with acid
  and base and the pH change recorded. The plot of
  pH versus added base or acid would allow the
  students to see the pH change being small in the
  buffering region of the solution.
• Various antacids could be reacted with added acid
  and the pH change recorded to see which one is
  more efficient