Ex5: Back titrations - Antacid titration analysis

• when do you use back titrations

  active ingredients are not water soluble, method is diff for each antacid, samples have multiple active ingredients
• what is different about a back titration compared to a regular titration

  the analyte is not directly titrated but instead is reacted completely with an excess amount of a solution of a more suitable compound, whose concentration and volume is well known
• what is directly titrated

  excess amount of the first reactant
• Back titration


  dissolve the sample in excess acid, neutralize active ingredients and determine level of remaining acid
• titrant

  a reagent solution of precisely known concentration (NaOH)
• how do you know the amount of acid that reacted with carbonate

  comparing the amount of acid which was added to the carbonate to the amount of acid which is neutralized in the titration
• Steps of experiment:

  1. prepare both samples
  2. Add excess acid + heating
  3. heat to drive off CO2 and prevent a buffered system from forming
  4. titrate using NaOH, record end point
• what does it mean if more titrant is required

  tablet not fully reacted! indicates analyte has reacted w/ excess titrant but there is still some remaining unreacted titrant
• what does it mean if less titrant is required?

  products loss! indicates excess titrant has already reacted w/ analyte
• what do you do if... antacid has pink/non white colour?

  use appropriate indicator
• what do you do if... antacid tablet does not break down

  mechanically crush it
• what do you do if... tablet produces foaming layers

  add less acid than required
• calculate moles of acid neutralized

  n neut HCl = nHCl - nNaOH

  nHCL = cv

  nNaOH = cv
• calculate moles of acid neutralized/tablet/$/g


  n neut HCl x (# tablets / $cost)