Bulb Pipette: used to deliver an accurate volume of a solution. This can be 10cm3 or 25 cm3.
Conical flask: used to hold chemicals and to make chemicals, substances can be stirred and swirled without the risk of spilling and reduces the loss of the evaporation due to the narrow neck.
Methyl Orange Indicator: a pH indicator frequently used in titrations. It is often chosen to be used in titrations because of its clear colour change.
Titration: The technique of titration is used to find out accurately how much of a chemical substance is dissolved in a given volume of a solution, that is, the concentration of the solution.
British Pharmacopoeia (BP): is the official collection of standards for UK medicinal products and pharmaceutical substances.
Introduction
The purpose of this experiment is to determine the exact concentration of sodium hydroxide. Sodium hydroxide is a volatile solution and therefore the concentration can vary due to evaporation. The titration will be repeated on 3 occasions, this is to ensure that a mean average volume of Hydrochloric acid can be calculated and this value will then be used to determine the concentration of sodium hydroxide through mathematical calculation.
Materials
- Burette & Stand
- Bulb pipette (10ml) and filler
- Conical Flask (100ml)
- Beaker (100ml)
- White Tile
- Funnel
-
Hydrochloric acid (0.5 mol dm-3 )
-
Sodium Hydroxide (approx 0.4mol dm-3 )
- Distilled water
- Methyl orange indicator
Method
Illustration showing how to set up equipment for titration, reading the meniscus and a sample results table.
- The equipment was set up as illustrated
- 20ml of sodium hydroxide was pipetted into the 100ml conical flask and approximately 3 drops of methyl orange indicator were added
- The flask was placed over the mouth of the burette and the burette was filled with 0.5 M HCl and the volume mark was noted to 2 d.p. (measurement was taken at the bottom of the meniscus)
- HCl was added to the conical flask containing the sodium hydroxide 0.5ml at a time whilst swirling to mix the reagents.
-
When a brief red colour was observed which quickly changes back to orange, the rate at which the acid was added was slowed to a dropwise rate. (Important not to rush at this stage)
- When the solution turned permanent red (longer than 2 minutes) the titration was stopped and the volume mark was noted to 2 d.p.
(Volume of acid used =Final volume – Initial volume)
- The titration was repeated on two more occasions ( 3 occasions in total)
Results
NaOH (aq) + HCl (aq) →NaCl (aq) + H2 O (l)
Average of titration results:
16.85 + 17.00 + 16.82 = 50.67/3 = 16.89ml
Percentage Difference:
17.00 – 16.82 = 0.18/16.89 =0.01065 x 100 = 1.066%
Concentration of NaOH from the average volume of HCl
0.01689 dm3 of HCl at 0.5 mol dm-3 concentration neutralises 0.02 dm3 of NaOH what is the concentration of NaOH?
Concentration = moles / volume
0.01689 dm3 x 0.5 mol dm-3 =8.445 x 10-3 mol
8.445 x 10-3 mol /0.02 dm3 = 0.422 mol dm-3
Discussion
The accuracy of the experiment was perceived to be quite good. The results were very close together, the percentage difference was only 1.066%. The values obtained in this experiment were very close to values obtained in the class. Although the 3 values obtained were very similar, if one was very different, it could be omitted as it would skew the average. The burette was read fairly accurately however it was difficult to read on occasion because the scale was only marked up with tenths and not hundreds of a millilitre.
Conclusion
The purpose of this experiment was to determine the exact concentration of sodium hydroxide through a titration experiment. The concentration of NaOH was determined to be 0.422 mol dm-3.
British Pharmacopoeia. Vol IV.2008. Appendix I B. A134. MHRA.London