Titration of citric acid in juice: Technical Notes

Transcription

1 Page 25 TECHNICAL NOTES Titration of citric acid in juice: Technical Notes Introduction This is a fairly standard titration except for the following: commercial lemon juice is used. This juice is slightly coloured; and commercial lemon juice has a total acid concentration that is around 1 M. This needs dilution before use.

2 Page 26 Equipment and materials Safety glasses and gloves for each student Citric juice samples NaOH solution (approx. 0.1 mol L -1 ). Phenolphthalein indicator Burette Retort stand Burette clamp Pipette Pipette filler (bulb) Titration vessel (conical flask) Beakers Funnel White tile Risk management There is a possibility that incorrectly used glassware might break, causing cuts or other injury. With appropriate instruction on the use of burettes, pipettes and other glassware, there is low risk. Filling the burette while the burette is placed on a bench top poses the risk of eye injury since the solutions are being transferred above eye level. The burette stand (and the burette) should be placed on the floor while filling, so that the top of the burette is below eye-level. Phenolphthalein indicator is flammable: keep away from ignition sources. The amounts and concentrations of phenolphthalein indicator used in this exercise pose low risk.

3 Page 27 Sodium hydroxide is corrosive. The concentrations (0.1 M) of sodium hydroxide used in this exercise pose low risk. Students should wear gloves, and immediately clean up any spills. Students must never pipette by mouth: they must always use pipette fillers. Students must wear safety glasses/goggles. MSDS Sodium hydroxide solution would normally be made up by dissolving solid sodium hydroxide in deionised water to give a very dilute solution. This is a typical MSDS for a concentrated sodium hydroxide solution. <

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11 Page 35 Phenolphthalein Indicator solution is available in different concentrations and in different solvents. This is a typical MSDS. < dicators-powders/sip13_5- Phenolphthalein/SIP13_5_MSDS_2011_Phenolphthalein.pdf>.

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16 Page 40 Suppliers Lemon juice can be sources from any supermarket. For example, the following is available from Woolworths: Woolworths Select Lemon Juice 500 ml

17 Page 41 Sodium hydroxide (drain cleaner) can also be sourced from a supermarket. It will be of lower purity than sodium hydroxide from a specialist chemical supplier. Since sodium hydroxide is hydroscopic, any sodium hydroxide should be standardised before use. Preparation suggestions The amount of total acid in lemon juice is approximately 1 mol L-1. The lemon juice should be diluted by mixing 1 volume of lemon juice with 9 volumes of deionised water. 500 ml lemon juice will make up 5 L of diluted lemon juice. Storage suggestions Sodium hydroxide solution should be freshly made within a few days as the solution will react with carbon dioxide in the atmosphere. Any sodium hydroxide solution that has been stored for more a few days should be re-standardised before use. Copyright and Creative Commons The moral rights of the authors, Kieran Lim, Elise Meehan, Jessica Saw, Peta White, John Long, and Ian Bentley, have been asserted under the Australian Copyright Act 1968 (Cth). Excepting logos, trademarks or other third-party content as indicated, this resource is distributed under a Creative Commons Attribution-Non Commercial-Share Alike 4.0 International License.