RP 9 - physical

what is RP 9

investigate how pH changes when a weak acid reacts with a strong base and when a strong acid reacts with weak base

pH probe calibration

1. rinse pH probe with distilled water and shake to remove XS water

2. place probe in standard pH 7.00 buffer solution and record reading

3. rinse with distilled water

4. repeat using standard pH 4.00 and 9.20 buffer solutions and record readings

5. plot graph of pH buffer solution (y) against recorded pH reading (x)

why do you need to calibrate the pH probe

over time (in storage) the pH meter readings become innaccurate

what does the bulb of probe need to be

fully immersed

what does the calibration curve allow

can be used to convert pH readings into more accurate pH values

measuring pH method

1. rinse and fill burette with acid and transfer some to a beaker

2. rinse and fill another burette with alkali

3. rinse pH probe with distilled water and clamp so bulb is fully immersed

4. use rod to stir and record pH reading

5. add 2cm3 portions, stir and record pH each time

6. when end point being reached add 0.20cm3 portions, stir and record pH each time

7. after continue adding 2cm3 portions until alkali in XS, stir and record pH each time

burette accuracy

ensure jet space filled without air bubbles

why does the portion value decrease

its near the endpoint where the pH changes rapidly - means you can accurately measure pH change

what is the change of most indicators

over a range of 2 pH units

definitions of = equivalence, half equivalence and end point

• equivalence = point when volume of one solution has reacted exactly with the volume of another (according to stoichiometric ratios)

• half equivalence = point where exactly half of a weak acid has been neutralised so that its conc is same as its conjugate base, [HA] = [A-]

• end point = point where the indicator changes colour

analysing data

use pH probe calibration graph to adjust pH readings obtained - correct values should be added into new table column

• plot graph corrected pH values (y) against vol alkali added (x)

• join points in most appropriate way which should be a curve

diagram

what does the periodic measurement of the pH of the reaction solution allow

the plotting of a graph of pH against volume of solution added - a pH titration curve