4️⃣PAG 6: Titration Flashcards
How to spell the indicator used p…
phenolphthalein
Safety
Dilute sodium hydroxide solution Causes skin and serious eye irritation Wear gloves and eye protection, and use a pipette filler
Spilling hydrochloric acid while filling the burette Causes eye irritation Fill the burette slowly below eye level and use a funnel
Do not fill the burette above head height and use a pipette filler to draw the solution into the pipette. When the procedure is complete, wash hands thoroughly.
Equipment
- 50 cm3 burette plus stand and burette holder
- 3 250 cm3 beaker
- 250 cm3 conical flask
- 25 cm3 bulb pipette plus pipette filler
- white tile
- funnel
- phenolphthalein indicator with dropper
- 0.100 mol/dm3 hydrochloric acid
- sodium hydroxide of unknown concentration
- wash bottle of distilled water
Method - full, if actually doing it
1 Collect some hydrochloric acid in a beaker and label it.
2 Rinse your burette first with tap water, then distilled water and finally with some of the hydrochloric acid. Discard the waste into a beaker labelled ‘waste beaker’.
3 Clamp the burette in the stand, so that the tip can comfortably fit into a conical flask.
4 Using a funnel, fill the burette with the hydrochloric acid beyond the zero mark.
5 Put the waste beaker below the tap. Open the tap to let the solution run out until the jet is filled – aim to get the bottom of the meniscus exactly on the zero mark.
There should be no bubbles in the burette. If there are any bubbles, open the tap fully and gently tap the outside of the burette near the bubble. Once all the bubbles have gone, shut the tap. You may need to top up the burette with hydrochloric acid.
6 Collect some sodium hydroxide in another beaker and label it.
7 Rinse the 25.0 cm3 pipette with tap water, then distilled water and then with the sodium hydroxide.
8 Use the pipette and the pipette filler to transfer exactly 25.0 cm3 of the sodium hydroxide into a clean dry conical flask.
9 Add three drops of phenolphthalein indicator into the flask and swirl. Place the conical flask on the white tile directly below the burette.
10 Record the initial burette reading in the results table below (this should be around 0.00 cm3).
11 Carry out a rough titration by adding the acid to the alkali in small amounts at a time. Swirl the flask after every addition, and continue until the indicator changes from pink to colourless. Note the final burette reading and record it in the table. Your teacher will show you how to read the burette.
12 Repeat the titration accurately by adding the rough titre minus 5 cm3 quickly. Then when you get near the end point, add the acid drop-wise. Use your wash bottle to fire water into the neck of the conical flask to ensure that all of the acid is transferred into the conical flask.
13 Record the initial and final burette readings in the appropriate column in the table.
14 Repeat the accurate titrations until you have two concordant results (within 0.10 cm3 of each other).
Method simpler - in 6 points
Use a pipette and pipette filler to add 25 cm3 of dilute sodium hydroxide solution to a clean conical flask.
Add a few drops of phenolphthalein indicator and put the conical flask on a white tile.
Fill the burette with dilute hydrochloric acid and note the starting volume.
Slowly add the acid from the burette to the conical flask, swirling to mix.
Stop adding the acid when the end-point is reached (when the colour first permanently changes from pink to colourless). Note the final volume reading.
Repeat steps 1 to 5 until you get concordant titres (see the Analysis).
Results + analysis
Results
Record the results in a suitable table. The one here also shows some sample readings.
Run Rough 1 2 3
End reading (cm3) 25.45 24.80 47.90 23.70
Start reading (cm3) 0.00 1.00 23.80 0.00
Titre (cm3) 25.45 23.80 ✓ 24.10 23.70 ✓
Readings should be recorded to two decimal places, ending in 0 or 5 (where the liquid level is between two graduations on the burette). The titre is the volume added (the difference between the end and start readings).
Tick (✓) at least two concordant titres. These are titres within 0.10 cm3 of each other.
Titres - calculate the mean titre
25.45 23.80 24.10 23.70
23.80+23.70
/ 2
= 23.75 cm3
Describe four steps needed to obtain accurate results.
Make sure the burette is vertical, and take the readings from the bottom of the meniscus. Near to the end-point, rinse the inside of the flask with distilled water and add the acid drop by drop.
Explain why a pipette is used to measure the acid, rather than a measuring cylinder.
The pipette allows the same volume of acid to be added each time, helping to make the results repeatable.
Key points
● The equation for the reaction taking place is: ?
● The rough titre ? when calculating the mean as ?
The mean must be calculated using ? results.
● The white tile allows the ? change to be seen much ?.
● Phenolphthalein is ? in acid and turns ? in an alkali.
● To improve the accuracy, ? water can be used to wash the pipette into the conical flask to ensure all the NaOH reacts.
● When reading the volume of acid in the burette, it should be read at ? level from the
bottom of the ? (the curve of the liquid).
Key points
● The equation for the reaction taking place is: HCl + NaOH → NaCl + H2 O
● The rough titre must not be used when calculating the mean as it is not accurate enough.
The mean must be calculated using concordant results.
● The white tile allows the colour change to be seen much easier.
● Phenolphthalein is colourless in acid and turns pink in an alkali.
● To improve the accuracy, deionised water can be used to wash the pipette into the conical
flask to ensure all the NaOH reacts.
● When reading the volume of acid in the burette, it should be read at eye level from the
bottom of the meniscus (the curve of the liquid).
