pK of an Indicator

Year One Semester Two

INTRODUCTION:

The aim of the experiment is to determine the pK values of an acid/base indicator using colorimetric and pH measurements

Part D of the experiment requires pH and absorbance measurements to be taken of an initially basic form of the indicator as aliquots of acid are added (in order to achieve the aim)

The addition of the HCl will cause the solution to turn green and more NaOH will need to be added to turn the solution blue again

When more NaOH is added this is considered to be a separate experimental run

USEFUL DATA POINTS – Part D of the experiment

• There must be at least eight useful data points from all the runs by the end of the experiment

• A data point is considered useful if it follows ALL the selection rules

• When applying the selection rules, the data should be compared only to the other data points from that particular run and need not obey the selection rules between the runs

• Each run must be completed, i.e. HCl should be added until the solution turns green, before any data points are considered useful from that run

SELECTION RULES:• The data must be from solutions that are still in the blue phase

• The pH readings must increase from the first valid data point up the table to the top of the experimental run.

• All the data points within a run must be within one pH unit of each other

• Data with absorbance greater than 1.5 must be discarded

• The absorbance readings must increase from the first valid data point up the table to the top of the experimental run

• If any data points do not obey ALL of these rules they must be discarded

Apply the selection rules by starting with the last reading before the solution turned Green (called the ‘first valid reading’) and work upwards in the table:

EXAMPLE OF RUN ONE:

The run was complete after four aliquot additions of HCl as the solution turned green. Then the selection rules were applied:

Total amount of 0.5M HCl added/μL

Colour of

solution

pH Absorbance Total Volume of

solution /

mL

Valid

0 Blue 8.34 0.76 50.000 No

10 Blue 8.26 0.76 50.010 Yes

20 Blue 8.16 0.74 50.020 Yes

30 Green -- -- -- No

Therefore two data points are valid from this run

Note: the data for these examples were collected from a different indicator to the one you will be using!

Last reading before solution turned green, so always valid. Start from this point = First valid reading

This reading is valid as both the absorbance and pH increase compared to the first valid reading

This reading is invalid as the absorbance reading did not increase from the last valid reading

All of the readings are within one pH unit of the first valid reading

EXAMPLE OF RUN TWO:

The run was complete after four aliquot additions of HCl as the solution turned green. Then the selection rules were applied:

Total amount of 0.5M HCl added/μL

Colour of

solution

pH Absorbance Total Volume of

solution /

mL

Valid

0 Blue 8.34 0.76 50.000 No

10 Blue 8.26 0.76 50.010 Yes

20 Blue 8.16 0.74 50.020 Yes

30 Blue/ Green

-- -- -- No

Therefore two data points are valid from this run

Note: the data for these examples were collected from a different indicator to the one you will be using!

Last reading before solution turned green, so always valid. Start from this point = First valid reading

This reading is valid as both the absorbance and pH increase compared to the first valid reading

This reading is invalid as the absorbance reading did not increase from the last valid reading

All of the readings are within one pH unit of the first valid readingThe solution was no

longer blue so the reading is invalid

EXAMPLE OF RUN THREE:

The run was complete after five aliquot additions of HCl as the solution turned green. Then the selection rules were applied:

Total amount of 0.5M HCl added/μL

Colour of

solution

pH Absorbance Total Volume of

solution /

mL

Valid

0 Blue 8.34 1.56 50.000 No

10 Blue 8.26 1.32 50.010 No

20 Blue 8.16 1.34 50.020 Yes

30 Blue 8.05 1.27 5.030 Yes

40 Green -- -- -- No

Therefore two data points are valid from this run

Note: the data for these examples were collected from a different indicator to the one you will be using!

Last reading before solution turned green, so always valid. Start from this point = First valid reading

This reading is invalid as the pH decreases compared to the previous valid reading

This reading is invalid as the Absorbance reading is greater than 1.5

All of the readings are within one pH unit of the first valid reading

EXAMPLE OF RUN FOUR:

The run was complete after four aliquot additions of HCl as the solution turned green. Then the selection rules were applied:

Total amount of 0.5M HCl added/μL

Colour of

solution

pH Absorbance Total Volume of

solution /

mL

Valid

0 Blue 9.14 0.76 50.000 No

10 Blue 8.66 0.76 50.010 Yes

20 Blue 8.16 0.74 50.020 Yes

30 Green -- -- -- No

Therefore two data points are valid from this run

Note: the data for these examples were collected from a different indicator to the one you will be using!

Last reading before solution turned green, so always valid. Start from this point = First valid reading

This reading is valid as both the Absorbance and pH increase compared to the first valid reading

This reading is invalid as the pH reading is more than one unit larger than the first valid reading

Invalid because the solution is no longer blue

NEXT:

• Continue to carry out experimental runs until you have eight useful data points

• Collate all the useful data points and treat as one data set for the analysis section of the main report