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9. Why is it good practice to repeat titrations? e.g. accuracy of acid-alkali titrations

[Author © Dr Phil Brown GRIC, PhD: Doc Brown's chemistry exam revision notes on improving accuracy for acid-alkali titrations suitable for students of UK GCSE level and Advanced A-level chemistry courses, ~US grades 9-12 chemistry notes  [page updated RE-EDIT]

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Titration index  *  Full chemistry calculations index

Quiz 12 The basics of acid-alkali titration calculations Good exam practice questions for GCSE level
9. Extra note on accuracy - Why repeat titrations several times?
 
  • To be reasonably sure of your final result, you cannot base it on one titration, no matter how perfectly you think you have done it.
  • You need repeated AND consistent burette readings.
  • There can be many source of errors, some random, and, worst of all, some systematic error to do with your laboratory equipment or your titration technique.
  • Errors may arise from ...
    • dirty apparatus, grease in the burette can cause traces of liquid to remain, so giving a false larger than actual titration reading,
    • sometimes a result doesn't fit in at all with the other results (referred to as an 'outlier'), may be due to carelessness in observing the end-point or poor pipetting etc. - human error,
    • apparatus may be faulty,
  • If your results are close together, this is a good sign - consistent readings imply you have been accurate in your titrations.

    Your first titration is very likely to be a bit inaccurate, and is referred to as the initial rough titration.

    This gives you an approximate value to aim for much more accurately in subsequent titrations.

  • After repeating the titrations accurately e.g. three times, you then average your results and use the average in your calculations.

    You titration, at best, is likely to be measured to the nearest 0.05 cm3, but you can use the average to two decimal places because statistically, that is the most probable value.

    e.g. suppose you got burette reading titration values of 21.45, 21.40, 21.45 (which would be impressive!),

    the average is 21.43 cm3, and that goes into the calculation of ....

    However, if a titration reading seems way out, 'posh word', anomalous, it should be ignored and not included in you calculation e.g. if a titration value of 21.75 was obtained, its not close enough to the other three values to be considered of value.

All these ideas apply to the following investigations and analysis

4. Antacid indigestion tablet investigation (an introductory exercise)

5. Titrating a strong base-alkali (e.g, sodium hydroxide) with a standardised hydrochloric acid using phenolphthalein indicator and sulfuric acid determination

6. Titrating a weak acid with a standard sodium hydroxide solution using phenolphthalein indicator

7. How to titrate a weak base-alkali like ammonia with standardised hydrochloric acid using methyl orange indicator

8. Titrating calcium hydroxide solution with standard hydrochloric acid using phenolphthalein indicator to find out the solubility of calcium hydroxide in water

Titration index  *  Full chemistry calculations index

Quiz 12 The basics of acid-alkali titration calculations Good exam practice questions for GCSE level

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