Experiment 5a
Standardization of NaOH by Titration

Preparation of Diluted Base Solution
Equipment

1. Calculate the volume of 7 M NaOH to the nearest 0.1 mL required to prepare 500 mL of a 0.3 M sodium hydroxide solution from the 7 M NaOH shelf reagent. Sample Calculations: Preparation of Diluted Base.

2. Using a 50 mL graduated cylinder, obtain this volume of 7 M base solution.

3. Add the required volume of 7 M NaOH to a 1 L Florence flask or designated container. Dilute to about 500 mL total volume with distilled water.

4. Stopper the Florence flask, mix the solution thoroughly, and set aside for titrations.

Preparation of Burette

NOTE: If there is any sign of leakage from the burette, ask your instructor for assistance.

1. Drain any liquid from the burette and discard. Rinse the burette with three 5-mL portions of distilled water.

2. Using a clean, dry 100-mL beaker, rinse the burette with three 5-mL portions of the diluted base. Always use a funnel to add liquids to the burette but be sure to remove it BEFORE taking any volume readings. Acid and base wastes may be kept in a labeled beaker.

3. Using the 100-mL beaker, fill the burette with the diluted base solution.

4. Drain some base through the tip and be sure no air bubbles remain in the burette tip. Ask your instructor for assistance if you are unable to remove air bubbles.

5. Again fill the burette, if necessary, to near the top of the graduated scale.

Preparation of Standardized Acid Solution
Equipment

1. Obtain 40 mL of the 0.5000 M H₂SO₄ in a clean, dry, labeled 50 mL beaker.

2. Use a 10-mL volumetric pipette and rubber bulb to pipette exactly 10.00 mL of the standardized acid into a clean 250-mL Erlenmeyer flask.

3. Add three drops of the phenolphthalein indicator and mix by swirling.

4. Repeat steps 2 and 3, preparing three acid solutions for titration. Set these aside.

The Titration
Equipment

1. After removing the funnel, record the initial burette reading to two decimal places. The base should be near the top of the graduated scale. Remove any drop adhering to the burette tip by touching off in a waste container.

2. Position the tip of the burette slightly below the rim of the titration flask. This will prevent the loss of titrant.

3. Place a piece of white paper under the titration flask; it will make the faint pink end point more readily visible.

4. The first titration is often performed quickly to determine the approximate volume of base required for each of the subsequent titrations.

5. Begin titrating the acid by adding a stream of the base. Swirl the flask and occasionally rinse its sides with distilled water from your wash bottle; it is also good to rinse the tip of the burette. The pink coloration quickly dissipates as the base is mixed with the acid. As the titration progresses, the pink coloration will persist for longer periods when the flask is swirled.

6. As the endpoint is approached, "partial drops" of base may be added by carefully dispensing a small drop onto the burette tip and washing it into the flask with the wash bottle.

7. Titrate to the faintest pink end point, splitting drops, as the endpoint is approached. When the pink coloration persists for at least 30 seconds the endpoint has been reached. Addition of too much base leads to a deep pink coloration.

8. Record the final burette reading to two decimal places.

9. Subtract the initial reading from the final reading to determine the volume (mL) of base used in the titration. In principle, each titration should require the same volume of base.

10. Discard the neutralized (pink) solution down the drain and rinse the flask with distilled water before further use.

11. Refill the burette and repeat the experiment (Steps 1-8) in triplicate. Titrate to the same faint pink endpoint in the replicate experiments.

NOTE: It may take more than three titrations to achieve the precision required (maximum range = 0.0020 M). Record data for all of your trials. Do not clean-up until you have completed sufficient tirations to achieve the required precision.

12. Calculate the molarity of the base from each of the three titrations and the range.

13. Repeat the titrations until the three closest calculated molarities agree within a range of 0.0020 M or less. Points will be awarded for meeting or exceeding the required precision.

14. Report the molarity of the NaOH as the average of your best (closest) three titrations. Mark the three best titrations, that is, those used in your final calculations, on your data page.

15. Before leaving the laboratory, drain the NaOH from the burette and dispose of it in the indicated waste bucket. Rinse the burette with five 10-mL portions of distilled water and then fill the burette with distilled water.

16. You MUST have your laboratory instructor check your lab station before leaving. The instructor will check to ensure that your burette has been properly cleaned.

NOTE: 10 points will be deducted from your grade for this experiment if the burette is not properly cleaned and filled with distilled water at the end of the lab period!