ChemLab - Chemistry 3/5 - Monoprotic and Polyprotic Acids

Acids, Bases, and Buffers 1: Monoprotic and Polyprotic Acids

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Techniques

Procedure

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Techniques
This week you will perform titrations using a pH meter. You will work with a partner and share data. The primary purpose of these titrations is to map out the titration curve accurately, not to determine the position of the endpoint of a reaction, as in previous titration experiments. Because of this, the titration is performed somewhat differently. The website ChemLab shows helpful photos of the techniques and procedures, on the pages for this week's lab. Here is a general procedure for a titration with a pH meter:

1. Prepare your notebook. You will need to plot pH vs. mL base added as you titrate. Make a full-page graph in your notebook that will cover the pH and mL ranges needed before you begin.

2. Condition and fill a 50 mL buret with standardized NaOH solution. Record its concentration in your notebook.

3. Titrate your solutions by adding larger aliquots of base (2 to 3 mL) during buffer regions, where the pH does not change rapidly and smaller amounts of base (0.1 to 0.5 mL) near the equivalence points. One partner should operate and read the buret and the other should read the pH meter and record the points on your plot. Be sure to switch jobs during the course of the lab, so each partner has the opportunity to do both things.

You will also learn to calibrate and operate a pH meter this week. Your TA will give you a demonstration and the calibration procedure is outlined in the Guide to Equipment and Instruments booklet at your lab station and in the introduction to this manual. The calibration procedure is illustrated in the Techniques section of the ChemLab website.

Procedure
Most of the experimental work for this laboratory will involve monitoring pH during titrations. You will work with another student as a team in collecting pH data as described in the Techniques section. Record primary data in one student's notebook and subsequently transfer them (with the requisite citation) to the other student's notebook.

Titration of Vinegar
Prepare a 50 mL buret with NaOH titrant and record the concentration of the solution. Accurately pipet 5 mL of vinegar into a beaker and dilute with water until the pH meter electrode is submerged and the stirrer will stir without splashing. Titrate the acid by adding base from the buret. Record the pH vs. mL titrant added at appropriate intervals. Readings should be taken more frequently near the equivalence point. Plot the pH vs. mL base added as you perform the titration.

Making a Buffer
We next focus on the buffer region of acetic acid. Accurately pipet 10 mL of vinegar into a clean beaker. Use the molarity of vinegar that you determined in the previous experiment to calculate the moles of acetic acid in this vinegar sample. You will add acetate ion to this solution, to make a buffer, by adding sodium acetate trihydrate, NaCH₃COO·3H₂O (MW = 136.080). Weigh a mass of sodium acetate trihydrate that contains the same number of moles of acetate ion as moles of acetic acid in your 10 mL of vinegar. Add the sodium acetate trihydrate to the vinegar and mix well. Transfer the acetate/acetic acid solution to a 100 mL graduated cylinder. Rinse the beaker with distilled water and add the rinsings to the graduated cylinder. Add distilled water to the graduated cylinder to dilute the solution to 100 mL. Return the solution to a clean beaker and mix well. Measure and record the pH of your buffer.

Titrate half of your buffer solution with standardized HCl and half with standardized NaOH. For the NaOH titration, titrate to the adjacent equivalence point in the basic direction. For the HCl titration, titrate to a limiting low pH.

Titration of Phosphoric Acid
Obtain 100 mL of 0.02 M phosphoric acid solution and a 100 mL graduated cylinder for use this week. Using the standardized NaOH solution, a 50 mL buret, and a magnetic stirrer, titrate a 100 mL aliquot of the phosphoric acid (carefully measured with the graduated cylinder) through the third equivalence region (to a high pH limit). Remember to use smaller intervals of base around the equivalence points and to calculate in advance where these points should occur. While you should calculate the position of the third equivalence point, you will not be able to detect an inflection point in pH which would signal it.