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

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 ___________________________________________________________ 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) _____________________________________________________________________ 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. _____________________________________________________________________ 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. _____________________________________________________________________ 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? PLACE THIS ORDER OR A SIMILAR ORDER WITH US TODAY AND GET AN AMAZING DISCOUNT :)