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Analysis of Vinegar by Titration. Lab 10. Outline. Purpose Potentiometric Titrations Proticity Equivalence pK a Vinegar Titration Calculations Waste Following Lab Reminder pH Probe Calibration. Purpose.
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Outline • Purpose • Potentiometric Titrations • Proticity • Equivalence • pKa • Vinegar Titration • Calculations • Waste • Following Lab Reminder • pH Probe Calibration
Purpose Students will use a pH probe to perform potentiometric titrations to determine the mass percent of acetic acid in vinegar and the pKa of acetic acid.
Potentiometric Titrations Potentiometric titrations can be used to determine: • proticity (how many acidic hydrogens are donated to solution) • the amount or concentration of acid or base present (using MaVa= MbVb) • pKa (the “– log” of the acid dissociation constant)
Proticity Acetic acid is a monoprotic weak acid that reacts with NaOH or KOH in a 1:1 ratio and produces a single sigmoidal curve.
Equivalence The equivalence point volume is determined by plotting a derivative curve of the titration curve. The steepest point on the derivative curve corresponds to the equivalence point volume. (Find the exact value on your spreadsheet!)
Equivalence Point Volume • The concentration of acetic acid can be determined from: • the equivalence point volume of the base • the concentration of the base • the volume of the acid used in the titration • The equation to use is: • MaVa = MbVb
pKa The pKa of acetic acid is the pH at the half-equivalence point volume of the titration, because: For a weak acid: HA H+ + A- and Ka = At the half-equivalence point, half the acid has been converted to its salt, so: [HA] = [A-] Ka = [H+] pKa = pH
Vinegar Titration • Make the required dilution of vinegar. • Calibrate your pH probe. • Titrate the specified aliquots to obtain titration curves. • Determine the volume of base delivered at each time point, using the base delivery rate. • Graph the derivative of your titration curves following the instructions in the manual. • Make up a spreadsheet that will allow you to calculate the indicated values.
Spreadsheet pKa Half-equivalence Point Volume Highest derivative Equivalence Point Volume
Calculations • At the equivalence point: moles of base = moles of acid (because they react in a 1:1 ratio) • Moles of base = (Eq Pt Vol base, L) x ([base], M) • Mass of acetic acid = (molar mass of acetic acid, g/mol) x (moles of acid, moles) • Mass of vinegar = (density, g/mL) x (Vol vinegar, mL) • Mass % = (mass of acetic acid / mass of vinegar) x 100%
Safety Concerns • Reagents: • Acetic Acid (1 N) • Sodium Hydroxide (0.1 N) / Potassium Hydroxide (0.1 N) • Eye Contact: • Irritation, tearing, redness, pain, impaired vision, severe burns and irreversible eye injury. • Skin Contact: • Severe skin irritation, soreness, redness, destruction of skin (penetrating ulcers) . May cause sensitization and / or allergic reaction. • Inhalation: • May cause coughing, serious burns, pneumonitis, pulmonary edema, and coma. • Ingestion: • Toxic. Corrosive to mucous membranes. May cause perforation of the esophagus and stomach, abdominal pain, nausea, vomiting, diarrhea, general gastro-intestinal upset.
Waste • Only solutions with a pH between 6 and 8 can go down the drain. • All other solutions need to go in the acid/waste container in the fume hood.
Lab 11 Reminder • Lab 11 is next.
pH Probe Calibration • Calibrate your pH probe according to the instructions in your lab manual. • Be careful not to break the bulb. • Be careful not to contaminate the buffer solutions. • Ask your instructor if you need help.