| Country : | United States America |
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Assignment Task:
Goals
To calibrate a pH probe. To become familiar with acid-base titration curves. To determine the concentration of an unknown acid.
Equipment and Materials 0.10 M ammonium hydroxide solution, 0.10 M sodium hydroxide solution, 0.0074 M hydrochloric acid solution, 0.0074 M acetic acid solution, a solution of hydrochloric acid with an unknown concentration, phenolphthalein solution, magnetic stir plate with magnetic stirrer, 50 mL burettes (2), 100 mL graduated cylinder
Discussion
When a base is added to an acid there will be a change in the pH of the solution. The pH depends on the amount of acid, base, and the nature of the acid and the base. Strong acids and bases have a greater effect on pH than weak acids and weak bases at the same concentration. Examples of acid-base reactions include:
HCl(aq) + NaOH(aq) strongacid strongbase → H 2 O(l) + NaCl(aq)
HC 2 weakacid H 3 O 2 (aq)+NaOH(aq) strongbase → NaC 2 H 3 O 2 (aq)+H 2 O(l)
HCl(aq)+NH strong acid 3 (aq) → NH + 4 (aq) + Cl - (aq) weak base
HC 2 H 3 O 2 (aq)+NH 3 (aq) weak acid weak base → NH + 4 (aq) + C 2 H 3 O - 2(aq)
In this experiment we will look at the titration curves that result from the four reactions written above. We will then take advantage of the point of inflection of a titration curve to determine the molarity of a sample of hydrochloric acid solution.
Procedure
SAFETY PRECAUTIONS Safety glasses are required for this experiment. Hydrochloric acid can burn skin and should be handled with care. Sodium hydroxide can damage skin and should be handled with care. The glass electrode is a fragile device and should be handled with care.
Students work in pairs throughout this experiment.
Part 1 Calibration of pH glass electrode
1. The calibration of the pH glass electrode requires the measurement of standard solutions at two different pH values. Pick two buffers to use for the calibration and obtain about 10 mL of each in small glass vials.
2. Connect the LoggerPro data interface and pH glass electrode.
3. Start the computer application Logger Pro 3.3 and check to see that a pH value is displayed on the computer monitor. If no value is displayed, check all connections and re-start the application.
4. The glass electrode can be taken from its storage solution and rinsed with distilled or deionized water. The glass electrode should never be exposed to air for more than a few seconds.
5. Place the electrode in the first buffer solution.
6. Open the Calibrate function of LoggerPro from the Experiment menu.
7. The window should have Live Calibration selected. Select Calibrate Now.
8. Gently stir the buffer solution with the pH probe for 5-10 seconds. Once the displayed voltage stabilizes record the pH of the buffer solution in the window and in the data sheet. Record the voltage reading displayed in the window in the data sheet and select Keep.
9. Repeat the same procedure for the second standard solution. The glass electrode should always be rinsed with distilled or deionized water when transferring it to a new solution.
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