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Systematic and Random Errors

Chemistry (Year 12) - Volumetric Analysis

Siobhan O'Halloran

Systematic Errors

Being able to identify systematic and random errors will reduce the chances of making them while conducting a titration. This will help increase the accuracy and validity of titration results. Systematic errors produce constant bias in the results of a titration. Each of the following examples of systematic errors will result in consistently incorrect titre volumes:

  • Inappropriate indicator – if the pH range of the indicator chosen is not consistent with the approximate equivalence point for the titration, this will result in the end point continuously falling above or below the actual equivalence point, and the wrong amount of titre volume is consistently added.

  • Misinterpretation of colour change – if the addition of titre volume is stopped too far before or after the colour change (which indicates the end point) has occurred, this will affect the results, as the wrong amount of titre volume is being consistently added.

  • Incorrect rinsing of glassware – rinsing the pipette or burette with water will dilute the concentration of the solution stored and rinsing the conical or volumetric flask with the solution they will store will increase the concentration of the solution stored.

  • Inappropriate primary standard – hygroscopic substances absorb moisture in the air, affecting the weight of the primary standard, as well as the concentration of the standard solution, which will be diluted. This may lead to an incorrect amount of titre volume added to the analyte. The weight and concentration reacting in the titration will also differ from the weight and concentration which is calculated.

Random Errors

Random errors are irregular and inconsistent, affecting each trial differently. Several trials should be taken and averaged to minimize the effects of these errors. Examples of random errors include:

  • Glassware errors – every piece of glassware for each type of equipment differs slightly in volume, therefore using different glassware for the same procedures may affect titre volume results. Not rinsing glassware between trials may result in old chemicals affecting the results inconsistently. Bubbles left in glassware can cause space and hence affect the volume measurements taken.

  • Inconsistent interpretation of meniscus – randomly viewing the meniscus from different angles for each trial or reading the top and bottom of the meniscus can result in the variation of titre measurements between trials. The meniscus should be consistently read from either the top or the bottom to ensure consistent titre volume measurements.

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