Homework
Fundamentals of
Analytical Chemistry
Chapter 5
Errors in Chemical Analysis
In some cases only certain parts of a
problem are marked with an asterisk – in
that case you are only responsible (for
purposes of any quiz) for those parts that
are marked.
Questions 1, 7 - 12
Error
EVERY MEASUREMENT CONTAINS
ERROR!!
Terms
Note that precision and accuracy are not
synonyms
Standard deviation, variance, coefficient of
variation
Accuracy
Accuracy
Absolute error
Difference between your value and the true or
accepted value
E = xi – xt
Types of Error
Will be positive or negative
For several measurements, xi will be the mean
Percent r.e.:
r.e.: Er = (E/x
(E/xt) * 100%
Parts per thousand: Er = (E/x
(E/xt) * 1000
Related to precision
Much more in Chap. 6
Systematic (determinate)
Relative Error
Random (indeterminate)
I don’
don’t care what Webster says!
Mean
Median
Precision
Error
Instrument
Method
Personal
Gross error
Usually ‘human error’
error’
1
Instrument Error
Errors occur because of an unreliable
value from a measuring device
Method Error
Can be any device
Most commonly associated with electronic
instruments
Instruments will have calibration method
Can also calibrate glassware and other
‘common’
common’ measuring devices!
Slow reactions, nonnon-specific reactions, side
reactions…
reactions…
Difficult to assess without known amount of
analyte
Cure is calibration
Most difficult to detect
End point determination
NIST
Lab certification
Fix is to modify method.
Personal Error
Measurements often require judgement
Buret volume
Pointer position for analog meter
Color change
Bias
Effect of Error
0 or 5
Even
Cure is awareness
Constant
Deviation from true value is always the same
absolute amount
Becomes more serious as sample size
becomes smaller.
AgCl precipitation – loss of analyte in rinse.
Proportional
Error increases as sample size increases
Cannot ‘fix’
fix’ by adjusting sample size
Detection of Error
Standard reference materials
Independent analysis
Blank determination
Sample is matrix + analyte
Blank is matrix only
‘Real world’
world’ samples may be impossible to
duplicate matrix
Variation in sample size
Constant error only!
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