practical 9

• pH at half neautralisation

  pKa = -log Ka
• describe a method used to calibrate a pH meter

  1. rinse pH probe with deionised water and remove excess water
  2. place in buffer solutions of different known pH values and determine the pH recorded by the probe
  3. produce a graph of known pH of buffer against measured pH and draw a line of best fit
• describe a method to investigate how pH changes as a weak acid reacts with a strong base

  1. add 20cm3 of ethanoic acid to a beaker using a burette
  2. add pH probe to the solution ensuring the bulb is fully immersed
  3. use a rod to stir and record initial pH
  4. add sodium hydroxide from a burette at 2cm3 intervals, stir and record pH between each addition
  5. add sodium hydroxide in smaller 0.2cm3 intervals around the end point and then 2cm3 until it is in excess
  6. adjust recorded pH values with the calibration curve and plot a graph of adjusted pH values against volume of NaOH added
• why must the pH probe be rinsed with deionised water?

  to remove any solution that may contaminate from previous recordings
• why do pH probes need to be calibrated?

  takes into account any error of the probe so that recorded values can be converted into actual values
• why must the Ph probe be clamped so that the bulb in fully immersed?

  ensures the whole surface area is in contact with the solution so that the pH recordings are accurate
• why must you use a rod to stir the mixture?

  so that the acid and base mix to ensure homogenous neutralisation
• why is the base added in smaller proportions around the endpoint?

  to ensure the neutralisation is not 'over-shot' and the pH at neutralisation is accurately determined