COMPARISON OF METHODS FOR END-POINT DETECTION AVAILABLE FOR ACID-BASE TITRATIONS

Order Description

calculate ?pH/?V all tables given.

Note down the volume required to reach end point (by indicator, i.e. at what point does the colour change) and note down the volume required to reach end point (by pH meter, i.e. the point of inflection on your graphs)for all results.

In your conclusions comment on the ease of determining the end-point and discuss any differences between the values obtained.

Please draw all graphs on graph paper or use a computer program such as Excel or OfficeHD. Please add your graphs in the box below and the following page.

finally answer the questions:

Why does an acid-base titration curve (pH versus volume of titrant) have an abrupt change at the equivalence point for the strong acid – strong base titration system?

Why does a properly chosen indicator change colour near the equivalence point in a titration?

(calculations/graphs/answer the questions)

EXPERIMENT 5

COMPARISON OF METHODS FOR END-POINT DETECTION
AVAILABLE FOR ACID-BASE TITRATIONS

USafety information

The chemicals listed below will be used in this experiment. The likely hazards associated with each of the chemicals are noted and recommended procedures for handling are given. You must read this page and the experimental description carefully before starting the experiment and before coming into the laboratory. Note any potential hazards and adopt precautions as your safe lab practice. When you are satisfied that you understand any possible difficulties that might arise and the recommended procedures for dealing with them, sign the declaration and have it initialled by a demonstrator. This must be done prior commencing lab work. At the beginning of the lab session demonstrators will quiz you about the safety information and experimental procedure in order to identify your ability to work safely and efficiently. If you fail to prove the ability for safe and efficient work you will not be allowed to start lab practical. Please note that it is you own responsibility to complete the lab practical during time that is allocated to you. Be sure to request information or help if you are in doubt on any point.

Chemical    Hazard    Precautions
Sodium hydroxide    Corrosive, Toxic    Do not ingest, avoid skin/eye
contact, wear gloves
Aqueous ammonia    Corrosive            Do not ingest, avoid skin/eye
contact, wear gloves.
Dilute hydrochloric acid    Irritant    Do not ingest, avoid skin/eye
contact, wear gloves.
Ethanoic acid        Harmful, Corrosive    Do not ingest, avoid skin/eye
contact, wear gloves.

UNOTE:
•    Carefully observe the instructions regarding clean-up of the apparatus. Take great care when handling materials to avoid spillage, splashing etc. Do not ingest and do not breathe chemical vapours. In case of accident or if you feel unwell, report to a member of staff and seek medical advice immediately.

Declaration – I have read and understood the contents of the safety information sheet and the script for the experiment

Signed (student): ……………………………………………………..

Checked (demonstrator): …………………………………………..    Date: ………………………

EXPERIMENT 5

COMPARISON OF METHODS FOR END-POINT DETECTION
AVAILABLE FOR ACID-BASE TITRATIONS
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LEARNING AIMS

?    To compare indicators
?    To gain familiarity with the use of a pH meter
?    To develop the ability to detect the end-point of the reaction by using various approaches

LEARNING OUTCOMES

?    To critically discuss the use of pH meters in analysis
?    To manipulate pH equipment

DIRECTED READING

Pharmaceutical Analysis 2PndP Ed; Watson (2005) ISBN: 0443074453; Chapter 3, pp: 57-86
Quantitative Chemical Analysis 7PthP Ed; Harris (2007) ISBN: 9780716770411; Chapters (7-11), (Chapter 11 the most important; pp: 199-227)
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INTRODUCTION

A range of techniques is available to detect end-points in titrations. You have already used colour changes in the first experiment from this lab manual. In this experiment you will carry out strong acid – strong base, strong acid – weak base, weak acid – strong base and weak acid – weak base titrations and compare three different methods for detecting end-points.
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EXPERIMENTAL

1.    Calibrate the pH meter by following the standard operating procedure (SOP) that is in your kit for this experiment and using the standard buffer solutions provided.

2.    Pipette 25.00 cmP3 of the 0.01 mol dm-3 hydrochloric acid solution into a 100 cmP3P beaker. Place the pH electrode into the solution ensuring that the electrodes are covered and that they are not struck by the rotating stirrer magnet.  Record the colours observed at all times.

Record all of your readings and observations on the pages after these instructions.

3.    Add two drops of phenolphthalein indicator.

4.    Run in known volumes of 0.01 mol dm-3 sodium hydroxide from a burette and record the pH values given by the meters. Add about 5 cm3 aliquots initially, but when the pH starts to rise sharply, or the indicator starts to change colour, add 1 cm3 or 0.1 cm3 (or smaller) volumes when close to the end-point (which will be near 25 cm3). When the rapid pH rise slows, resume 1 cm3 additions until the pH curve levels out.

5.    Plot a graph of pH against volume of alkali added and also ?pH/?V against volume and determine the end-point from the graph.  Compare this to the end-point that you have determined by using the indicator.

6.    Repeat steps 2 to 6 replacing the hydrochloric acid, sodium hydroxide and indicator as shown in the table below:

Acid    Base    Indicator
hydrochloric acid    ammonia    methyl orange
ethanoic acid    sodium hydroxide    phenolphthalein
ethanoic acid    ammonia    methylene blue

If there is time, repeat the four titrations using 0.0001 mol dm-3 solutions prepared by accurately diluting 1 cmP3 aliquots of the original solutions to 100 cm3 Pin volumetric flasks.
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EVALUATION OF RESULTS

We suggest that you record your results for each of the titrations in the form:
Acid:    0.01 mol dm-3 hydrochloric acid    Alkali:    0.01 mol dm-3 ammonia

Vol. of base added  (cmP3P)    pH    ?pH/?V    colour
0.00    1.97        Light pink
5.00    2.02        Pink
10.00    2.23        Pink
15.00    2.44        Pink
20.00    2.53        Pink
25.00    2.80        Pink
30.00    3.52        Orange
31.00    4.07        Orange
32.00    5.57        Orange
33.00    6.25        Yellow
34.00    6.83        Yellow
35.00    7.50        Yellow
36.00    7.92        Yellow
37.00    8.12        Yellow
38.00    8.28        Yellow
39.00    8.40        Yellow
40.00    8.49        Yellow

Note down the volume required to reach end point (by indicator, i.e. at what point does the colour change) and note down the volume required to reach end point (by pH meter, i.e. the point of inflection on your graphs).

We suggest that you record your results for each of the titrations in the form:
Acid:    0.01 mol dm-3 ethanoic acid    Alkali:    0.01 mol dm-3 sodium hydroxide

Vol. of base added  (cmP3P)    pH    ?pH/?V    colour
0.00    1.96        Colourless
5.00    2.08        Colourless
10.00    2.22        Colourless
15.00    2.43        Colourless
20.00    2.74        Colourless
25.00    4.61        Pink
26.00    5.97        Pink
27.00    6.77        Pink
28.00    9.10        Pink
29.00    9.82        Pink
30.00    10.17        Pink
38.00    8.28        Pink
39.00    8.40        Pink
40.00    8.49        Pink

Note down the volume required to reach end point (by indicator, i.e. at what point does the colour change) and note down the volume required to reach end point (by pH meter, i.e. the point of inflection on your graphs).

We suggest that you record your results for each of the titrations in the form:
Acid:    0.01 mol dm-3 ethanoic acid    Alkali:    0.01 mol dm-3 ammonia

Vol. of base added  (cmP3P)    pH    ?pH/?V    colour
0.00    3.29        Light pink
5.00    3.96
10.00    4.36
15.00    4.66
16.00    4.72
17.00    4.78
18.00    4.84
19.00    4.90
20.00    4.97
21.00    5.04
22.00    5.11
23.00    5.19
24.00    5.30
25.00    5.38

Note down the volume required to reach end point (by indicator, i.e. at what point does the colour change) and note down the volume required to reach end point (by pH meter, i.e. the point of inflection on your graphs).

In your conclusions comment on the ease of determining the end-point and discuss any differences between the values obtained.

Please draw all graphs on graph paper or use a computer program such as Excel or OfficeHD. Please add your graphs in the box below and the following page.

_____________________________________________________________________
QUESTIONS

1.    Why does an acid-base titration curve (pH versus volume of titrant) have an abrupt change at the equivalence point for the strong acid – strong base titration system?

2.    Why does a properly chosen indicator change colour near the equivalence point in a titration?

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