This document discusses different types of errors that can occur in analytical chemistry measurements and methods. It describes determinate errors, which include instrumental errors from faulty tools, methodic errors from defective experimental methods, operational errors from improper technique, and personal errors from the analyst. It also discusses indeterminate errors, which are random errors that cannot be attributed to a known cause. The document explains how errors can propagate in calculations and discusses accuracy and precision in measurements.
2. Causes of Determinate Errors
Instrumental errors
• These errors arise due to faults in the tools used by the analyst.
Glasswares such as pipettes, burettes, measuring flasks, etc. used in
volumetric analysis, instruments using electronic circuits, such as pH
meter are all calibrated at a certain temperature.
• If these are used in some other temperature, the calibration is
disturbed and the measurements become unreliable. So instrumental
errors are due to the use of uncalibrated weights, ungraduated
glassware and other instruments such as pH meter.
• Errors may also be introduced due to voltage fluctuation in the source
of current supply.
3. Methodic errors
• Adoption of defective experimental methods causes these errors.
• These may arise due to incompleteness of a reaction and incorrect sampling.
• For example, Kjeldahl’s method used for the determination of nitrogen may not
give consistent results in certain cases as in case of some organic compounds,
containing ring nitrogen.
• The digestion with concentrated H2SO4 may not completely convert the ring
nitrogen to ammonium sulphate.
• This is particularly true for pyridine compounds in which the results of nitrogen
determination are low.
• Some sources of methodic errors include coprecipitation, postprecipitation, of
impurities, side reaction, slight solubility of precipitate, impurities in reagent, etc.
• These are most serious errors. These are inherent in the method and can not be
minimized or corrected unless the conditions of the determination are changed.
4. Operational errors
These errors arise due to lack of knowledge or total ignorance of handling equipment
and not taking the necessary precautions in measurements as exemplified below.
• Lack of experience of the analyst resulting error in weighing and volume readings.
• Introduction of foreign materials in the experimental sample due to carelessness
such as not covering the sample container.
• Errors may be due to defective operations such as during transfer of solution,
incomplete drying of samples and bumping during sample dissolution, etc.
• Weighing a crucible when it is hot and cooling in a desiccator with a poor desiccant.
• The use of indicators in quantities is more than necessary. This leads to erroneous
results, since the indicator may also get titrated.
• Ignition of precipitate in incorrect temperature.
• Allowing hygroscopic materials to absorb moisture before or during weighing.
• Failure to apply buoyancy correction when required.
5. Personal error or human errors
These errors are due to factors for which the individual analyst is
responsible and are not connected with the method or procedure.
• Mechanical loss of material in various steps of analysis.
• Errors in reading in burette.
• Improper washing of a precipitate.
• Insufficient cooling of crucible.
• Using impure reagent.
• Mathematical error in calculation.
6. Indeterminate Errors
• These are random or accidental errors which arise from uncertainties
in a measurement that are unknown and not controlled by the
analysts.
• These are revealed by small differences in values of successive
measurements made under identical condition by the same analyst.
• These errors follow a random distribution.
8. Indeterminate Errors
• Small errors occur more frequently than large one.
• Positive and negative errors of the same magnitude are likely to occur
equally.
• Narrow peaked curves with steep slopes indicate a relatively high
precision.
• Broad curve indicates a relatively low degree of precision.
• These errors can not be attributed to any known cause. They are random
in nature and lead to both high and low results with equal probability.
• They cannot be eliminated or corrected and are the ultimate limitation on
the measurement.
13. Accuracy
• Accuracy is a measure of how closely the result of an experiment
agrees with the true or accepted value.
• In other words, it expresses the correctness of a measurement.
• However, accuracy is never known.
14. Absolute method
• In this method to determine the accuracy the experiments are
repeated several times. This is known as replicate analysis.
• The consistency of the result in replicate analysis is very often taken
as a test of accuracy.
• However, this is not always so since for some unidentifiable reasons
the magnitude of error in every measurement might have been the
same and the result consequently might have been consistent
although necessarily accurate.
• The difference between the mean of an adequate number of results
and the actual result may be taken as the measure of the accuracy of
the method.
15. Comparative method
• In this method the accuracy is judged by using two or more
independent techniques such as gravimetric, titrimetric,
spectrophotometric, etc. to solve the same problem.
• Independent techniques are methods based on different physical and
chemical principles.
• If two independent techniques give the same result the result is
thought to be free from error, hence accurate.
20. Precision
• When a sample is analysed several times, the individual results are
rarely the same.
• Instead, the results are randomly scattered.
• Precision is a measure of how closely the result of an experiment
agrees with those of other measurements made in the same ways.
• In other words, it expresses the reproducibility of the results.
• This can be achieved unlike accuracy.
• The same mistake may be made over and over again.
• An experimental procedure may be precise but due to some constant and
unknown errors the results may be inaccurate.
