Titration Experiments

The following are optional titration experiments:

Optional Experiment: Preparation of a standard solution

Aim

To prepare a standard solution of sodium hydroxide.

Apparatus and materials

• one $$\text{250}$$ $$\text{cm^{3}}$$ volumetric flask, a beaker, a funnel, a plastic dropper

• A weighing scale, a spatula, a clean piece of paper

• solid sodium hydroxide

Method

Warning:

Concentrated bases can cause serious burns. We suggest using gloves and safety glasses whenever you work with a base. Handle all chemicals with care.

1. Place the paper on the scale and tare (zero) the scale.

2. Weigh $$\text{5.00}$$ $$\text{g}$$ of solid sodium hydroxide onto the paper.

3. Transfer the sodium hydroxide to the beaker and dissolve it in a minimal amount of water.

4. Place the funnel in the mouth of the volumetric flask and use it to transfer the solution from the beaker to the flask, be careful not to spill.

5. Pour another $$\text{5}$$ $$\text{cm^{3}}$$ of water into the beaker, swirl gently, and pour this into the flask through the funnel as well.

6. Repeat step $$\text{5}$$ another two times.

7. Pour $$\text{5}$$ $$\text{cm^{3}}$$ of water through the funnel into the flask.

8. Use the plastic dropper to pour water down the inside of the volumetric flask.

9. Fill the volumetric flask to the mark with water. Use the plastic dropper to fill the last few centimeters. Remember to lower yourself so that you are looking directly at the mark when adding the final drops.

10. Cap the flask and shake well. You now have a standard solution.

Questions

• What is the concentration of this standard solution?

• What is the purpose of rinsing the beaker and funnel with water?

• What is the purpose of rinsing the inside of the volumetric flask with water?

• Why do you need to make sure your eyes are level with the mark when adding the final few drops of water?

Optional Experiment: Titrations

Aim

To determine the concentration of acetic acid (ethanoic acid) in a sample of vinegar.

Apparatus and materials

• 4 conical flasks, one pipette, one burette, one small funnel, one beaker

• a retort stand, a white piece of paper or a white tile

• the standard $$\text{NaOH}$$ solution prepared in the previous experiment, white vinegar, phenolphthalein

Method

Warning:

Concentrated acids and bases can cause serious burns. We suggest using gloves and safety glasses whenever you work with an acid or a base. Handle all chemicals with care.

1. Clamp the burette to the retort stand and place the small funnel on top.

2. Making sure the burette is closed, carefully add the vinegar to the burette (lift the funnel slightly while doing this) until the burette is full.

3. Place a beaker below the burette and carefully let some of the vinegar run into it. This will ensure that there are no air bubbles in the burette. The top of the vinegar should now be between the $$\text{0}$$ and $$\text{1}$$ $$\text{cm^{3}}$$ marks. Record the value to the second decimal place.

4. Use your pipette to measure $$\text{20}$$ $$\text{cm^{3}}$$ of the sodium hydroxide standard solution into a conical flask.

5. Add $$\text{3}$$$$\text{4}$$ drops of phenolphthalein indicator to the conical flask. What colour is the solution?

6. Do a rough titration experiment by adding the vinegar to the conical flask quickly, and constantly swirling the conical flask. Stop as soon as the colour of the solution changes, and remains changed after swirling. Make a note of the reading on the burette at this point and determine the volume added:

(V(final) – V(initial)).

If the colour change does not remain when the flask is swirled, add more vinegar until the colour remains.

7. Repeat steps $$\text{2}$$$$\text{5}$$ with a second conical flask.

8. Quickly add vinegar to the conical flask until you have added $$\text{2}$$ $$\text{cm^{3}}$$ less than your rough titration volume. There should be no lasting colour change at this point.

9. Drop by drop (slowly and carefully), add vinegar. Swirl between each drop and, if necessary, rinse the sides of the flask with water. When the solution changes colour and remains that new colour, make a note of the volume on the burette.

Remember that the volume titrated is: (final volume reading) – (the initial volume reading).

10. For accuracy you should repeat steps $$\text{7}$$$$\text{9}$$ until you have three readings with a difference of no more than $$\text{0.1}$$ $$\text{cm^{3}}$$.

Observations and questions

• What colour was the sodium hydroxide solution when the phenolphthalein was added?

• What was the colour when enough acid was added?

Fill in the details of this experiment on a table like the one below:

 Titration Initial volume($$\text{cm^{3}}$$) Final volume($$\text{cm^{3}}$$) Volume($$\text{cm^{3}}$$) Rough 1 2 3
• From the table determine the average titration volume for this experiment and use that value in the rest of your calculations.

• Using the previous worked examples, determine the concentration of acetic acid in the sample of vinegar. Remember that you have the following information:

• the volume of vinegar

• the volume of the sodium hydroxide solution

• the concentration of the sodium hydroxide solution

• The balanced chemical equation for this reaction is:

$$\text{CH}_{3}\text{COOH}(\text{aq}) + \text{NaOH}(\text{aq})$$ $$\to$$ $$\text{CH}_{3}\text{COONa}(\text{aq}) + \text{H}_{2}\text{O}(\text{l})$$

Titrations

Aim

To determine the concentration of a sodium hydroxide solution of unknown concentration.

Apparatus and materials

• one $$\text{250}$$ $$\text{cm^{3}}$$ volumetric flask, 4 conical flasks, one pipette, one burette, one small funnel, two beakers, one plastic dropper

• a retort stand, a weighing scale, a spatula, a clean piece of paper

• solid sodium hydroxide ($$\text{NaOH}$$), solid oxalic acid ($$\text{H}_{2}\text{C}_{2}\text{O}_{4}$$), phenolphthalein

Method

Warning:

Concentrated acids and bases can cause serious burns. We suggest using gloves and safety glasses whenever you work with an acid or a base. Handle all chemicals with care.

1. Prepare a standard solution using $$\text{11.00}$$ $$\text{g}$$ of oxalic acid in the $$\text{250}$$ $$\text{cm^{3}}$$ volumetric flask.

2. Label one of the beakers $$\text{NaOH}$$. Use your pipette to measure $$\text{100}$$ $$\text{cm^{3}}$$ of water into the beaker. Add approximately $$\text{4}$$ $$\text{g}$$ of $$\text{NaOH}$$ to the beaker and stir.

3. Clamp the burette to the retort stand and place the small funnel on top.

4. Making sure the burette is closed, carefully add the oxalic acid standard solution to the burette (lift the funnel slightly while doing this) until the burette is full.

5. Place the clean beaker below the burette and carefully let some of the oxalic acid solution run into it. This will ensure that there are no air bubbles in the burette. The top of the solution should now be between the $$\text{0}$$ and $$\text{1}$$ $$\text{cm^{3}}$$ marks. Record the value to the second decimal place.

6. Use your pipette to measure $$\text{20}$$ $$\text{cm^{3}}$$ of the $$\text{NaOH}$$ solution into a conical flask.

7. Add $$\text{3}$$$$\text{4}$$ drops of phenolphthalein indicator to the conical flask. What colour is the solution?

8. Do a rough titration experiment by adding the oxalic acid to the conical flask quickly, and constantly swirling the conical flask. Stop as soon as the colour of the solution changes, and remains changed after swirling. Make a note of the reading on the burette at this point and determine the volume added:

V(final) – V(initial).

If the colour change does not remain when the flask is swirled, add more oxalic acid until the colour remains.

9. Repeat steps $$\text{4}$$$$\text{7}$$ with a second conical flask.

10. Quickly add oxalic acid to the conical flask until you have added $$\text{2}$$ $$\text{cm^{3}}$$ less than your rough titration volume. There should be no lasting colour change at this point.

11. Drop by drop (slowly and carefully), add oxalic acid. Swirl between each drop and, if necessary, rinse the sides of the flask with water. When the solution changes colour, and remains that new colour, make a note of the volume on the burette.

Remember that the volume titrated is: (final volume reading) – (the initial volume reading).

12. For accuracy, you should repeat steps $$\text{9}$$$$\text{11}$$ until you have three readings with a difference of no more than $$\text{0.1}$$ $$\text{cm^{3}}$$.

Observations and questions

• What colour was the sodium hydroxide solution when the phenolphthalein was added?

• What was the colour when enough acid was added?

Fill in the details of this experiment on a table like the one below:

 Titration Initial volume($$\text{cm^{3}}$$) Final volume($$\text{cm^{3}}$$) Volume($$\text{cm^{3}}$$) Rough 1 2 3
• From the table, determine the average titration volume for this experiment and use that value in the rest of your calculations.

• Using the previous worked examples, determine the concentration of the sodium hydroxide solution. Remember that you have the following information:

• the volume of sodium hydroxide solution

• the volume of oxalic acid solution

• the concentration of the oxalic acid solution.

• The balanced chemical equation for this reaction is:

$$\text{H}_{2}\text{C}_{2}\text{O}_{4}(\text{aq}) + 2\text{NaOH}(\text{aq})$$ $$\to$$ $$\text{Na}_{2}\text{C}_{2}\text{O}_{4}(\text{aq}) + 2\text{H}_{2}\text{O}(\text{l})$$