There are a number of ways that you could carry out a titration in Chemistry. This is an outline of the required steps to undertake one of these methods.
It is important in this practical to use appropriate apparatus to make and record a range of volume measurements accurately.
To carry out an accurate titration using dilute hydrochloric acid, dilute sodium hydroxide solution, and phenolphthalein indicator.
Titration is a practical technique used to determine the amount or concentration of a substance in a sample. It is an example of quantitative analysis. An acid-alkali titration can be used to find out what volume of acid (or alkali) of known concentration exactly neutralises a known volume of alkali (or acid) of unknown concentration. This concentration can then be calculated.
Record the results in a suitable table. The one here also shows some sample readings.
|End reading (cm3)||25.45||24.80||47.90||23.70||23.90|
|Start reading (cm3)||0.00||1.00||23.80||0.00||0.00|
The titre is the volume added (the difference between the end and start readings). The burette is marked in 0.10 cm3 graduations. It is possible to record results to the nearest 0.05 cm3 by noting if the meniscus lies between markings.
Calculate the mean titre. In the calculation, ignore the rough run and any results that are not in close agreement (24.10 cm3 in the table above).
Ignoring the rough run, and run 2 (because it is not concordant):
= 23.80 cm3
Explain why a pipette is used to measure the acid, rather than a measuring cylinder.
A pipette is more precise than a measuring cylinder. Adding slightly different volumes of alkali to the flask will result in a systematic error. The pipette allows the same volume of acid to be added each time, helping to make the results repeatable.
Describe two steps needed to obtain accurate results.
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 the importance of a suitable indicator in obtaining accurate results.
The indicator must change colour sharply when the solution in the flask is neutralised. This means the volume of acid measured is very close to the true value. The white tile makes it easier to see the colour change. Misjudging the colour change could result in a random error.
It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances.
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
|Hazard||Possible harm||Possible precaution|
|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, using a funnel|