Question

Describe the nature of “random error” and “systematic error”. Provide three examples for each of these two categories of errors. Could they be eliminated and how?

Answer 1

Answer:-

Error:- An error is defined as the numerical difference between a measured value and the true or accepted value of an analytical determination.

The errors are broadly classified into two types:-

1) Systematic or Determinate Errors: Systematic errors have a definite source which can be identified. Systematic errors are further classified as:-

a) Instrument Errors :- These errors arise from imperfections in measuring devices. For example- measuring devices such as pipettes, burettes, measuring cylinders, measuring flasks, etc. deliver or contain volumes that are slightly different from those indicated by their graduations. As there are several reasons of these differences, but the important are:

• ​​​​​​​use of the glassware at a temperature which is significantly different from the temperature at which the glassware was calibrated.
• distortions in the walls of the container due to heating while drying the glassware.
• errors in the original calibration.
• contamination of the inner surfaces of the containers.
• ​​​​​​​errors due to fall in voltage , increased resistance in circuits due to unclean electrical contacts, etc. in instruments powered by electricity.

​​​​​​​b) Method Errors​​​​​​​ :- These errors generally arise from the non-ideal behaviour of reagents and reactions involved in a given analysis.

c) Personal Errors​​​​​​​:- These errors arise from erratic personal judgement, such as, estimation of the position of a pointer between two scale divisions, judgement of the colour of solution at the end point in a titration, judgement of the level of a liquid with respect to a graduation on a burette or a pipette, are sources of personal errors.

Correction of Systematic Errors:-

i) The systematic instrumental errors are corrected by periodic calibration of the instruments.

ii) The personal errors can be eliminated or reduced to a minimum by care and self-discipline.

iii) ​​​​​​ Method errors are rather difficult to detect. However, following procedures have been suggested for identification and elimination of the method errors.

• Analysis of standard samples:- Method errors can be detected by carrying out the analysis of a standard sample prepared in such a way that its composition is exactly the same as that of the material under test. For this purpose, standard materials containing carefully analysed constituents are available from the National Bureau of Standards.
• Independent analysis:- A dependable procedure for detecting method errors consists in carrying out parallel analysis of the sample by another independent method of established reliability.
• Blank determination:- ​​​​​ Blank determination in which all the steps involved in the analysis are carried out in the absence of the sample in exactly the same fashion, is quite useful for exposing method errors which are due to contaminations of the reagents and the glass vessels employed for analysis.

2) Random or Indeterminate Errors​​​​​​​:- Random or indeterminate errors arise from uncertainties which are inevitably associated with every physical or chemical measurement. These are random or accidental errors whose sources , though many, cannot be positively identified.

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