Chem titration prac (1 Viewer)


Mar 11, 2004
Here is the question, with a few words highlighted:

Design a detailed procedure for all aspects of a titrimetric analysis that you could perform to accurately determine the amount of ascorbic acid in the Vitamin C tablets.
Stating the obvious, you need to
  • provide a procedure for analysis by titration​
  • ensure it is within the competence of a high school chemistry student​
  • taking account of analysing provided tablets​
  • titration is clearly going to use an acid / base reaction​
  • the question notes accuracy so steps to ensure accuracy will be needed​
The strange part of the question, for me, is the directing word being "design". That usually goes with planning an experiment that you will subsequently undertake. A more usual question would be to "outline" or "describe" a procedure, or even the more challenging "justify". The former two terms focus on what is done, the latter focuses on why steps are undertaken. The high mark value suggests a long response is required that demonstrates knowledge and depth of understanding, but the wording does not point to the need to explain why you chose particular steps... you need to judge whether any such details are required.

I would hope to see:
  • description of a titration using a strong base like NaOH with the weak acid
  • steps include washing and measurement in appropriate glassware to ensure accuracy
  • perhaps also steps that should not need to be sated, but if omitted / done incorrectly will reduce accuracy... for example, removing the filter funnel from the burette after filling it and before titrating, or ensuring no air pockets / bubbles in or below the burette's tap, or washing down walls of conical flask to ensure no splashes leave unreacted materials on the walls of the flask
  • repetition for check on reliability and some idea of precision of final concentration (ie, to check if results are consistent with 500 mg / tablet)
  • use of an appropriate indicator including the sought colour change to identify the end point (for a "justify" question, explaining the choice of indicator is important... for a "describe" question, stating which indicator to use is needed but explaining how / why it was selected is not... for a "design" question, you need to judge)
  • noting that NaOH is a secondary standard and thus covering standardised it with a primary standard or that procedure anticipates a suitable stock solution of known concentration for the procedure
  • consideration of the vitamin C being in a solid tablet - will you pre-dissolve it? crush it? titrate onto tablet? (last one is a bad idea)
  • recognition that accurate results will require that the tablet contain no other acid that might react and cause an inflated conclusion of ascorbic acid content; otherwise, an additional acid needs to be accounted for

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