Titrations with Strong Acids and Bases

Neutralization Titrations

• Strong acids or strong bases are used as standard solutions in neutralization titrations because they react more completely with an analyte and produce a larger change in pH at the equivalence point.

Standard Acid Solutions

• Hydrochloric acid (HCl) is widely used as a standard solution for titrating bases.
• Dilute HCl solutions are stable indefinitely.
• Many chloride salts are soluble in aqueous solutions.
• Solutions of 0.1 M HCl can be boiled for up to an hour without acid loss as long as evaporated water is replaced.

Preparation of Standard Acid Solutions

• Steps to obtain a standard acid solution:
  • Prepare a solution of an approximate concentration by diluting the concentrated agent.
  • Standardize the diluted acid solution against a primary-standard base or occasionally use density measurements.
  • Weigh a quantity of the concentrated acid and dilute it to a known volume.

Preparation of Constant-Boiling HCl

• A stock solution with a known HCl concentration can be prepared by diluting a quantity of the concentrated reagent with an equal volume of water followed by distillation.
• The final quarter of the distillate (constant-boiling HCl) has a constant and known composition.
• The acid content of constant-boiling HCl depends only on atmospheric pressure.

Primary Standards for Acids

• Sodium carbonate is the most frequently used reagent for standardizing acids.
• Tris-(hydroxymethyl)aminomethane (TRIS) is also a common primary standard for acids.
• Sodium tetraborate decahydrate and mercury (II) oxide have also been recommended.

Preparation of Standard Solutions of Base

• Sodium hydroxide is the most common base for preparing standard solutions.
• Potassium hydroxide and barium hydroxide are also used.
• Absorption of carbon dioxide by a standardized solution of sodium or potassium hydroxide leads to a negative systematic error (the carbonate error) in analyses.

Removing Carbonate Ion from Standard Base Solutions

• Carbonate ion in standard base solutions decreases the sharpness of end points and is usually removed before standardization.
• Sparging is the process of removing a gas from a solution by bubbling an inert gas, such as air, through the solution.

Standardization of Bases

• Most primary standards used to standardize bases are weak organic acids that require the use of an indicator with a basic transition range.
• Potassium hydrogen phthalate is a nearly ideal standard.
• Other primary standards include benzoic acid and potassium hydrogen iodate.