Titrations

• The end point is when the indicator changes colour from one colour to another
• A burette is used as it allows accurate volumes of titrant to be measured out, which can then be recorded on the burette reading
• Titrations require specialised equipment to enable the reacting volumes to be measured as accurately as possible
• Conical flask has sloped slides to prevent any liquid escaping. It must be rinsed with deionised water and no other solution
• A Pipette accurately measures the volume of one of the solutions added to the conical flask.

  
  

  Most common sizes are 20cm3 and 25cm.

  
  

  These are so accurate that the temperature needs to be controlled for the most accurate work.
• A second solution is placed into a burette.

  Usually has a capacity of 50.0cm3 and is divided in to 0.10cm3 divisions with accurate markings.

  The volume should be recorded to 2 decimal places

  Supported by a retort stand and a burette clamp and we need to make sure it is not slanted.

  While titrating the tip should be just inside the neck of the conical flask.
• the equivalence point is when the concentration of the reactants and products are equal (moles acid = moles base)
• The equivalence point occurs when enough of the second solution has been added to completely react with the solution in the flask.
• The indicator changes colour when the reaction is complete which is the endpoint of the titration.

  Most titration reactions require the addition of an indicator.
• Phenolphthalein is an indicator. It is pink in alkali and colourless in acid.

  It is used in titrations between a strong acid and a strong base or a weak acid with a strong base.
• Methyl orange is an indicator. It is yellow in alkali and turns red in acid.

  It is used in a titration between a strong acid with a strong base or a weak base with a strong acid.
• Titration technique

  1. Rinse all glassware three times with distilled water before use to prevent contamination

  2. Rinse the pipette with the base (e.g. sodium hydroxide) to ensure the final contents match the concentration of the stock base

  3. Add 25.0cm3 to the pipette using a pipette filler

  4. Add the contents of the pipette to the conical flask

  5. Touch the tip of the pipette to the side of the conical flask to remove all drops

  6. Add three drops of indicator to observe a distinct color in the conical flask
• Steps to following titration problems:

  • write balanced equation
  • calculate the number of moles from known solution
  • use mole ratio to calculate number of moles of unknown solution
  • calculate concentration of the unknown using (c=n/v)
• 7. Rinse the burette with the acid (hydrochloric acid solution) to match the concentration of the stock hydrochloric acid solution

  8. Fill the burette with hydrochloric acid ensuring no air bubbles or space
• 9. Read the volume in the burette to the nearest 0.05cm3 at the bottom of the meniscus using a light background for accuracy

  10. Add hydrochloric acid slowly from the burette while swirling until the solution in the conical flask turns colorless

  11. Repeat the titration to obtain three accurate values for the volume of the acid, known as CONCORDANT VALUES, agreeing within 0.10cm3 of each other