1. 1. Definition- Errors, Accuracy, Precision
2. Sources of errors
3. Types of errors
4. Significant figures
5. Precision and accuracy
6. Minimization of errors
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2. Errors
All measurements are subject to error, which contributes to
the uncertainty of the result
it is impossible to perform a chemical analysis that is totally
free of errors or uncertainties
Error is the difference between a true value and a
measured/ observed value
Error= True value- observed value
Eg: If a tablet contains 500mg PCM and an analyst
observed 490mg of PCM after analysis, then error is 10mg
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3. Sources of Errors
Improper sampling: occur due to improper sampling. Eg:
taking 15mg instead of 20mg
Errors during preparation: occur during sample
preparation. Eg: taking sample A instead of B
Error by analyst: occur by analyst. Eg: due to ignorance or
accident by the analyst. Also known as manual errors
Error by equipment: occur due to improper functioning
of the instrument.
Error due to calibration: Occur due to improper
calibration. Eg: pipette measuring only 19.5ml instead of
20ml
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4. Sources of Errors (continued)
Reporting error: Occurs due to wrong data observation or
collection. Eg: burette reading 58ml instead of 60ml
Calculation error: Miscalculation of data
Error by method selection: selection of wrong method
by the analyst. Eg: Usage of Mohr’s method instead of
Volhard’s method for determination of chloride in sample
Error due to transport and storage: occur due to
improper handling of materials during transport and
storage. Eg: storage of insulin at room temperature and not
at 40
Error due to laboratory environment: Occur due to
unsuitable laboratory environment for analysis. Eg: pipette
measuring only 19.5ml instead of 20ml
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5. Types of Error in Experimental Data
There are three types of errors in experimental data:
Determinate (systematic) errors: determinable and
can be avoided or corrected.
Indeterminate (Random) errors: may be accidental
Gross errors: obvious and easily identified
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6. 1. Determinate / Systematic Errors
Determinable and can be avoided/corrected
May be constant, in case of an uncalibrated weight
being used in all weighings
Classified as:
Instrumental errors
Operative errors
Errors of the method
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7. a. Instrumental Errors
Common to all instruments as each one has a different
accuracy
Manufacturer usually provides necessary tables
factoring the reliability of results
Calibration of one instrument is not applicable for all
instruments
In volumetric analysis, burette, pipette and flask are
calibrated
If temperature is different, then volume measured may
be incorrect
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8. b. Operative errors
Personal errors which can be reduced by experience
Occur during transfer of solutions, incomplete drying
of samples etc
Difficult to correct
May also be introduced due to physical disability of the
analyst. For example, color blindness
Also include mathematical errors in calculations
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9. c. Errors of the Method
Most serious errors
Example: Usage of Mohr’s method in place of
Volhard’s method (for low pH chloride containing
sample analysis)
Other methodical errors include: co-precipitation of
impurities, side reactions, impurities in reactions
In some cases, correction may be simple- as running a
reagent blank
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10. 2. Indeterminate/ Random Errors
Accidental in nature
Revealed by small difference in successive
measurements taken by the same analyst at virtually
identical conditions
Cannot be predicted/ determined
Follow random distribution, hence, mathematical law
of probability can be applied to arrive at a conclusion
regarding most probable results
Eg: an analyst reads a result incorrectly and notes
down the same reading. Error is random and unique
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11. Significant Figures
In chemistry, Significant figures are the digits of value
which carry meaning towards the resolution of the
measurement
The number of digits in a value, also a ratio, that
contribute to the degree of accuracy of the value are
significant figures.
Significant figures (also known as significant
numbers) are an integral aspect of statistical and
mathematical calculations, which deal with numerical
accuracy and precision
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13. Accuracy
Accuracy indicates the closeness of the measured
value to the true/ accepted value
Difficult to measure because mostly true value is
unknown, therefore, an accepted value is used
Expressed as absolute or relative error
Absolute error= True value- observed value
Relative error= Observed value- true value/ true value
(mostly reported as %)
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14. Precision
Precision is a measure of how close a series of
measurements are to one another.
Precise measurements are highly reproducible, even if
the measurements are not near the correct value.
High precision does not always mean the results are
accurate
Describes reproducibility of the results
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17. Minimization of errors
Calibration of instruments and equipment-
determinate/ systematic errors can be eliminated as they
are the most common reason for errors
Periodic calibration of instruments is necessary for accurate
results
Running a blank determination- using a blank,
impurities present in the reagents and solvents can be
determined. Ultimately, errors can be reduced.
Normal determination Analyte A + Reagent B + X
Blank determination Reagent B + X
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18. Minimization of errors (contd)
Control determination- standard substance (known
conc) is analysed and then compared with normal
determination
Normal determination Analyte A + reagent B + X
Control determination Standard Z + Reagent B +X
Standard addition / recovery studies- Known standard
added in analyte solution and estimate value and done
separately with only analyte
Mostly performed to validate the method of analysis
Recovery studies are also performed- known amount of
analyte A added to sample solution A. Additive amount of
both should be obtained. If not, error in method
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19. Minimization of errors (contd)
Internal standard addition- standard substance
(different than analyte known conc) added to sample
and is analysed in same / identical conditions
Independent method of analysis-Two methods are
used and compared. Eg: Determination of HCl with
NaOH (neutralization) and AgNO3 (precipitation)
Parallel determination- Duplicate / triplicate
determination of analytes reduce accidental/ random
errors
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20. Minimization of errors (cntd)
Amplification method- when small amount of
impurity is present, this method is used
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