pH Meter: A pH meter consists of an electrode. pH reading displayed. pH meter more accurate
than indicator paper, as pH meter does to 2 dp compared to whole number. If data logger
attached to pH meter, can plot pH titration curve automatically.
Experiment Monitoring pH: Do a titration and monitor pH after each small addition of base from
burette added to acid.
pH Titration Curve: y axis is pH and x axis is volume of base.
Inverse- acid
from burette
added to the
base.
Equivalence Point: The equivalence point of titration is volume of one solution that reacts
exactly with volume of the other solution- matches stoichiometry of reaction. Vertical section =
titre cm3
, use this in calculations. Equivalence point = centre of vertical section.
Acid- Base Indicator: The acid- base indicator has a weak acid HA form and conjugate base Aform, which both have distinctly different colours. E.g. for indicator methyl orange, HA red and
A-yellow. Phenolphthalein- goes colourless (HA) to pink (A-).
End Point: The end point of titration, indicator contains equal concentrations of HA and A- at
end point so colour will be in between colours e.g. orange.
Equilibrium for an Acid- Base Indicator: Indicator is a weak acid. Similar to equilibrium of buffer
solution. All acid- base indicator work in same way as below.
Adding basic solution containing OH-:
– OH- react with H+ in indicator to form H2O
– Weak acid HA dissociates, shifting equilibrium position to the right, towards A- and H+.
– Colour changes to orange at end point. Then changes to yellow as equilibrium position
shifted to right.
Adding acid solution to basic solution:
– H+ react with A- conjugate base.
– Shifts equilibrium position to the left, towards HA acid.
– Colour changes to orange at end point. Then changes to red as equilibrium position shifted
to left.
pH Ranges: Different indicators change colour over different pH ranges. Also most indicators
change colour over range of two pH units- so watch out for sensitivity of end point.
At the End Point:
pH of end point same as pKa of HA.
Choosing Indicator: In titration, must use indicator that has colour change/ end point that
coincides with vertical section of pH titration curve. Sometimes end point may give slightly
different volume to actual equivalence point.
