Chem AS

• what is a titration?

  a method of chemical analysis where 2 solutions are reacted together. A known volume of the solution of unknown concentration is placed into a conical flask and the solution with a known concentration is added from a burette until the 'end point' of the reaction is reached.
• what is the end point for titrations?

  may be the point at which the supply of one of the reactants runs out or (in an acid-base titration) the point where the acid or base is neutralised.
• how is the point of neutralisation observed? (titrations)


  through the use of a chemical indicator.
• what is a chemical indicator (tittrations) ?

  one colour in the acid and a different colour in the base. The change of colour of the indicator is used to indicate the moment at which the solution has been neutralised and is no longer acidic or basic.
• what are the 3 common indicator solutions?

  methyl orange, phenolphthalein, bromothymol blue.
• what colour is methyl orange in acid? in bases? and what pH range does it cover?

  red in acid, yellow in bases. 4.8 to 6.0 pH.
• what colour is phenolphthalein in acid? in bases? and what pH range does it cover?

  colourless in acid, pink in bases. 8.2 to 10 pH.
• what colour is bromothymol blue in acid? in bases? and what pH range does it cover?

  yellow in acid, blue in bases. 3.2 to 4.2 pH.
• what is step 1 for an acid-base titration?

  pipette a known volume of the solution of unknown concentration into a clean and dry conical flask.
• what is step 2 for an acid-base titration?

  add a few drops of a suitable indicator solution to the contents of the conical flask.
• what is step 3 for an acid-base titration?

  rinse a clean and dry burette through with the solution of known concentration to remove any traces of water or other impurities which may affect the results.
• what is step 4 for an acid-base titration?

  fill the burette with the solution of known concentration so that the meniscus is as close to the zero mark as possible on the burette.
• what is step 5 for an acid-base titration?

  place the conical flask on top of a white tile directly below the burette. The white tile provides a background for the flask so that any colour change can be easily seen.
• what is step 6 for an acid-base titration?

  record the accurate initial volume from the burette
• what is step 7 for an acid-base titration?


  turn the tap on the burette and add the solution into the conical flask. Swirl the flask to ensure that the contents of the flask are mixed thoroughly.
• what is step 8 for an acid-base titration?

  continue to add the solution from the burette until the indicator changes colour. As soon as the colour change is observed close the tap on the burette so that no more solution is added. The colour change shows that the endpoint has been reached.
• what is step 9 for an acid-base titration?

  record the final volume from the burette and calculate the titre value as the difference between the finial and initial volumes. This is known as the rough titre.
• what is step 10 for an acid-base titration?

  pour away the contents of the conical flask and rinse the flask thoroughly using distilled water.
• what is step 11 for an acid-base titration?


  repeat steps 1 to 8 but you get close to the volume added to reach the endpoint in the previous titration you should slow the flow rate down to add the solution drop wise. This increases the accuracy of your results.
• what is step 12 for an acid-base titration?

  repeat the titration until you achieve at least 3 titre values which are within 0.1 cm3 of one another. The average titre can be calculated from these 3 values and used to calculate the concentration of the unknown solution
• how many decimal places should you quote your volume from the burette?

  2 decimal places.
• what is one way that the results from an acid-base titration can be used?

  to find out the concentration of an acidic or basic solution providing that the concentration of the other solution used is shown.
• what is the 2nd use of the titration technique? who is this important for?

  finding out the mass of an active ingredient present or the percentage purity of a tablet. This is important for scientists in the pharmaceutical industries.
• what would stop a scientist being able to do a standard titration?

  if the analysis involves a solid which is insoluble or only partially solid in water.
• what happens during a back titration?

  the substance is reacted with an excess volume of reactant. The amount of remaining unreacted excess reactant can be determined using titration against a second reaction.
• how many reactions are involved in back titrations?

  2 reactions.
• what happens in the 1st reaction in a back titration?

  the substance being analysed is reacted with an excess of a reactant
• what happens in the 2nd reaction in a back titration?


  the resulting solution is titrated against a 2nd reactant to determine the number of moles of the excess reactant which didn't react in the 1st reaction.
• when measuring mass, how is the precision dictated?

  by the number of decimal places the balance can read to.
• a measurement can be

  precise but not very accurate.
• how can we measure how much confidence we can have that our measurement is accurate?

  by calculating the percentage uncertainty associated with the measurement.
• what is the equation for percentage uncertainty?

  % uncertainty = equipment accepted error / quantity measured x 100
• what does the accepted error on any piece of equipment depend on?

  the precision with which it can make the measurement.
• the more precise the equipment can be

  the smaller the accepted error
• the higher the number of decimal places the balance reads to


  the smaller the error and the more precise the measurement will be and the smaller the percentage uncertainty.
• an error on a balance which reads to 1 decimal place is?


  ±0.1
• an error on a balance which reads to 2 decimal places?

  0.01
• an error on a Balance which reads to 3 decimal places?

  0.001
• an error on a balance which reads to 4 decimal places?

  0.0001
• how are titre values calculated?

  by subtracting the initial burette volume recorded from the final burette volume recorded.