CHE134 Summer Lecture 7

CHE134 Summer Lecture 7 - HO O OH SA A ASPIRIN PURITY BY T...

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ASPIRIN PURITY BY TITRATION pH O HO OH ASA SA NaOH C OH O O C O CH 3
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Finishing Last Exercise Your synthesized aspirin should be completely dry by now. You can weigh it ( on the top-loading balance ) and calculate your percent yield. If your melting point last week was less than 120 oC, you should REPEAT the melting point determination
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Purpose: To study pH variation during an acid/base titration and use this to determine the purity of your synthesized aspirin Concepts: pH Nernst Equation Calibration and use of the pH meter Molar Mass Effective Molar Mass Titration Stoichiometry Equivalence Point Titration of a mixture Techniques: Weighing by Difference Using a pH Meter Graphing Apparatus: pH Meter Buret
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pH and the pH METER pH Definition pH = -log [ H+ ] pH METER : Measures voltage difference between two electrodes each in contact with the same aqueous solution Electrodes are selected so that relative voltage depends only on HYDROGEN ION concentration, [ H+ ] ( and temperature ) Other electrodes can measure concentration of other ions. { Have read SUPL-006 which discusses types, use and care of pH electrodes }
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NERNST EQUATION describes dependence of electrode voltages on concentration and temperature For normal pH electrodes, 2.303 R T E = E0 - ----------- log [ H+ ] NA e where: E = VOLTAGE DIFFERENCE between reference and indicator electrodes E0 = CONSTANT depending only on TYPES of ELECTRODES T is Absolute Temperature R is gas constant NA is Avogadro’s number e is absolute value of electron charge 2.303 R T = E0 + ------------ pH NA e
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At 298 K, 2.303 R T / NA e = 0.0592 volts Obj108 0 0.2 0.4 0.6 0.8 2 4 6 8 10 12 E0 + E0 + E0 + E0 + 298 K 273 K At constant T , voltage varies linearly with pH. Voltage, \ (volts) pH E0 + E = E0 + B x pH At 273 K, = 0.0542 volts
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CALIBRATION ( STANDARDIZATION ) Like all measuring devices, must calibrate pH METER before using it. Immerse pH electrode into one or more (buffer) solutions of accurately known pH. Set Calibration Control on meter so reading agrees with pH of standard buffer pH 7.00 Buffer How? Remember to rinse electrode after standardization
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Obj1 0 How does pH vary with added NaOH for a weak acid? How sharp is the pH rise at the equivalence (end) point?
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Obj1 1 Full Range Note Scale End Point
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Obj1 2 ~6 mL Range Note Scale Change Note Range
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Obj1 3 ~1 mL Range Note Scale Change Note Range 1 drop
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Obj1 4 ~0.5 mL Range Note Scale Change Note Range 1 drop
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SAMPLE TABLE FOR pH TITRATION Buret Rdg Cum Vol pH Incr Buret Rdg Cum Vol pH Incr 4.35 0.00 2.94 27.10 22.75 6.35 6.46 2.11 3.15 2 mL 27.20 22.85 7.00 8.72 4.37 3.31
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This note was uploaded on 01/09/2011 for the course CHE 134 taught by Professor Staff during the Summer '08 term at SUNY Stony Brook.

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CHE134 Summer Lecture 7 - HO O OH SA A ASPIRIN PURITY BY T...

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