RP9

• Obtaining a pH curve (eg ethanoic acid and sodium hydroxide)


  1. Rinse pH meter with deionised water and calibrate with solution of a known pH

  2. Plot calibration curve with recorded pH vs pH meter reading

  3. Titrate

  4. Use pipette to transfer 25cm3 of 0.1moldm-3 ethanoic acid to conical flask and fill burette with 0.1moldm-3 of NaOH

  5. Measure the pH of the acid

  6. Add alkaline in 1cm3 portions and stir

  7. Measure the pH after each addition

  8. Continue until alkaline is in excess adding in smaller increments closer to the end point

  9. Plot graph

  10. Results on y axis and vol of alkaline on x axis

  11. Smooth curve ignoring any anomalies
• Half neutralisation 


  pH = pKa
• Why is a burette more reliable than a pipette


  It can deliver variable volumes
• Why would it be difficult determining end points using an indicator

  Colour change is over a range of volumes of NaOH
• State why calibrating the pH meter improves accuracy

  Over time and storage, it does not give accurate readings
• Another way to improve accuracy of reading?

  water bath because pH changes with temp.
• When the strong alkali is added to a weak acid, there is little change in pH for some time. Why?

  Forms a buffer solution