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 Potentiometric Titration of an Acid Mixture : Procedure

Operation of a pH meter.

Your TA will demonstrate the proper use of a pH meter and glass pH electrode. Important points to remember include the following:

  • For accurate pH measurements a pH meter must be calibrated using buffer solutions of known pH prior to measurement.

  • For a titration this is less important since it is the change in pH, not the actual pH value, that is used to detect the end point in the titration.

  • Always rinse the electrode with distilled water and wipe dry when transferring them from solution to solution.

  • When finished with the last titration, rinse the electrode and then immerse it in the buffer reservoir provided. NEVER ALLOW A GLASS ELECTRODE TO SIT IN THE ATMOSPHERE FOR ANY EXTENDED LENGTH OF TIME. A DRY GLASS ELECTRODE IS WORTHLESS.

Preparation of the 0.1 M NaOH Titrant.

  • Place about 900 mL of freshly distilled and deionized water in a clean polyethylene bottle.
  • Add sufficient NaOH to make an approximately 0.1 M solution. Keep tightly covered to minimize absorption of atmospheric CO2.

Titrations.

  • At the beginning of each lab period, BE SURE TO CALIBRATE THE pH METER WITH THE pH STANDARDS PROVIDED.
  • Alternate titrations of sulfamic acid and sample.
  • Sulfamic acid, H2NSO3H, is also known as amidosulfonic acid. It has a formula weight of 97.10 g.

Standardization of the 0.1 M NaOH Titrant.

  • Weigh to the nearest 0.1 mg about 2.5 g of sulfamic acid and then transfer it to a 100 mL volumetric flask.
  • Dissolve the acid in freshly distilled and deionized water, dilute to the mark, and mix thoroughly.
  • Pipet 10 mL portions into 250 mL beakers and add 40 to 50 mL of deionized water to each.
  • Insert the glass/reference electrode pair into the solution so that the tip is completely submerged.
  • Add a stirring bar, positioning it so that it does not strike the electrode.
  • Titrate with the 0.1 M NaOH solution, recording the initial buret reading, then adding titrant until the pH meter indicates a change of about 0.2 pH units.
  • Record the pH and buret readings, and then add titrant again until another change of 0.2 pH units is observed.
  • Continue in this manner until a pH of 3 is reached.
  • Between pH 3 and 8, record pH and buret readings every 0.1 mL of base added. Above pH 8, record pH and buret readings every 0.2 mL of base added. The titration may be terminated at pH 11.

For the second and subsequent titrations only the initial pH and data near the equivalence points (pH 3‑10) need be recorded, so these titrations can be performed rather quickly. MAKE SURE TO ALTERNATE SULFAMIC ACID AND SAMPLE TITRATIONS.

Titration of an Unknown Mixture of Acids.

  • Obtain a sample acid mixture from the TA in a 100 mL volumetric flask.
  • Dilute to volume with deionized water and mix thoroughly.
  • Pipet 10 mL portions of this solution into 250 mL beakers, add 40 to 50 mL of water to each, and titrate with 0.1 M NaOH.
  • Record the initial pH, then add titrant until the pH meter indicates a change of about 0.2 pH units.
  • Record the pH and buret readings.
  • Continue in this fashion until a pH of 3 is reached. Between pH 3 and 6 (region of the first end point), record the pH after the addition of 0.1 mL increments of base. Then, record the pH after addition of each 0.2 mL of base from pH 6 to 8, each 0.1 mL from pH 8 to 10.5 (region of the second end point), and finally each 0.2 mL until pH 11.
For the second and subsequent titrations only the initial pH and data near the equivalence points  need be recorded, so these titrations can be performed rather quickly. MAKE SURE TO ALTERNATE SULFAMIC ACID AND SAMPLE TITRATIONS.

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