RP1: Make up solution & Titrations

• How to set up a standard solution:

  1. Weigh the sample bottle containing solid on a balance
  2. Transfer to beaker and reweigh sample bottle
  3. Record difference in mass
  4. Add distilled water
  5. Stir with glass rod
  6. Until all solid dissolved
  7. Transfer to volumetric flask using funnel
  8. With washings
  9. Make up 250cm3
  10. Shake/invert/mix
• Mean titre = Sum of concordant titres / Number of concordant titres
• Remove air bubbles from the burette tap because air bubbles will increase burette reading
• Remove the funnel from the burette once starting the titration BECAUSE some solution may fall into the burette and affect the reading
• Swirl conical flask and rinse sides so that all reactants mix - adding water doesn't change the number of moles of reactant
• Percentage error = (error of equipment / amount measured) x 100
• To reduce percentage error you can only increase the measurements made from the burette by:

  • Increasing concentration of solution in conical flask
  • Decreasing concentration of the solution in the burette
• Conical flask used instead of beaker because it's easier to swirl the solution and nothing is lost
• Measuring cylinder is not used because it has a higher percentage error than a volumetric pipette
• Only use 3-5 drops of indicator SO that you can see the end-point (colour change)
• Do not rinse the burette with water then with reactant because the reactant will become diluted
• If too much indicator is added then this may react and affect the end-point reading
• Do not add reagent until the solution 'just' changes colour because the reagent may have not fully reacted (not swirled). Add reagent until a permanent colour change is seen
• Titration method:

  1. Fill a burette with the acid of known concentration
  2. Accurately measure 25cm3 of alkaline standard solution using a volumetric pipette and transfer to a conical flask
  3. Add a few drops of a suitable indicator to the alkali
  4. Slowly add the acid from the burette into the conical flask, swirling constantly
  5. Stop adding acid when indicator changes colour
  6. Repeat the process until you get two results which are concordant (calculate average titre from this)
• pH titrations using probe:

  1. Measure the pH of the acid solution and record
  2. Add 1cm3 of base solution
  3. Stir the mixture
  4. Measure the pH and record
  5. Repeat the process until the base is in excess
  6. Add the base in smaller increments near the end point
• Adding water to the conical flask does not affect the titre because the number of moles are not changed
• Do not wash the pipette with water because this may take up space and decrease the number of moles in the conical flask