Chapter 3: Precision and Accuracy of Chemical Analyses Flashcards
The ______ of two or more
measurements is their average value
mean
mean is also called as
arithmetic mean or average
it is often caused by faulty calibrations or standardization or by random variations and uncertainties in results.
errors
TRUE OR FALSE
it is impossible to perform a
chemical analysis that is totally free of errors or uncertainties
TRUE
refers to the difference between a
measured value and the “true” or
“known” value
ERROR
often denotes the estimated uncertainty in
a measurement or experiment.
error
is the middle value in a set of data that has been arranged in numerical order. The ________ is used advantageously when a set of data contain an outlier, a result that differs significantly from others in the set.
median
can have a significant effect on the mean of the set but has no effect on the median.
outlier
describes the reproducibility of measurements. is the closeness of results to others obtained in the same
way.
precision
three terms widely used to describe the precision of a set or replicate data,
standard deviation
variance
coefficient of variation
These three (standard deviation, variance, and coefficient of variation) functions of how much an individual result differ from the mean, called
deviation from the mean
is the closeness of a
measured value to the true or
accepted value and is expressed by the error.
accuracy
describes the agreement among several results obtained in the same way. We can determine ______ just by measuring replicate samples
Precision
is often more difficult to determine because the true value is usually unknown. An accepted value must be used instead. It is expressed in
terms of either absolute or relative error.
Accuracy
TRUE or FALSE
The sign of absolute error tells you whether the value in question is high or low.
TRUE
the formula for absolute error
E= measured value - true value
The _______________ of a measurement
is the absolute error divided by
the true value. _________ may
be expressed in percent, parts per
thousand, or parts per million,
depending on the magnitude of the
result
relative error
Er is often a more useful quantity than the absolute error. Er stands as
relative error
causes data to be scattered more or less symmetrically around a mean value
random or indeterminate error
causes the mean of a data set to differ from the accepted value
systematic or determinate error
is a sample of about the
same size that is carried through an
analysis in exactly the same way.
replicate
TRUE or FALSE
the random error
in a measurement is reflected by its precision.
TRUE
One of the first questions to answer before beginning an analysis is
What maximum error can be tolerated in the result?
TRUE or FALSE
An example of a systematic error is the loss of a volatile analyte while heating a sample
TRUE
differ from indeterminate and
determinate errors. They usually occur only occasionally, are often large, and may
cause a result to be either high or low. They are often the product of human errors.
gross errors
Gross errors lead to
outliers
have a definite value, an assignable cause, and are of the same magnitude
for replicate measurements made in the same way
systematic errors
systematic errors leas to ______ in measurement results
bias
three types of systematic errors
instrumental errors
method errors
personal errors
are caused by nonideal instrument behavior, by faulty calibrations,
or by use under inappropriate conditions
instrumental errors
arise from nonideal chemical or physical behavior of analytical systems.
method errors
result from the carelessness, inattention, or personal limitations of the experimenter.
personal errors
affect measurement precision.
They arise from experimental
variables that cannot be controlled
or determined. Examples include
random fluctuations in electrical
noise, random inconsistencies in
measurment readings, or random
changes in laboratory temperature.
random or indeterminate errors
affect the accuracy of results. These
errors often occur when instruments
or measuring devices are uncalibrated
or are calibrated improperly. They
have the same effect on all samples.
Systematic errors can be very subtle
and difficult to detect, but finding
and eliminating them is an integral
part of the measurement process
systematic errors
occur infrequently and
often result from an experimental
blunder such as misreading a scale or
interpreting a number incorrectly. If
you mistakenly read a 9 as 4 or 3 as
gross error
is an occasional result in
replicate measurements that differs
significantly from the other results
outlier
measures the systematic error
associated with an analysis. It has a
negative sign if it causes the results to
be low and a positive sign otherwise
bias
determines the relationship between a measured quantity and the analyte
concentration. _________ is a very important part of any analytical procedure
calibration
eliminates most systematic errors of this type
calibration
The nonideal chemical or physical behavior of the reagents and reactions on which an analysis is based often introduce________________. Such sources of nonideality include the slowness of some reactions, the incompleteness of others, the instability of some species, the lack of specificity of most reagents, and the
possible occurrence of side reactions that interfere with the measurement process
systematic method errors
TRUE or FALSE
Errors inherent in a method are often difficult to detect and are thus the most
serious of the three types of systematic error
TRUE
Many measurements require personal judgments. Examples include estimating the position of a pointer between two scale divisions, the color of a solution at the endpoint in a titration, or the level of a liquid concerning a graduation in a
pipet or buret
Personal errors
universal source of error is
prejudice or bias
these are errors that are independent of the size of the sample being analyzed
constant errors
decrease or increase in proportion to the size of the sample.
proportional errors
systematic errors can be either _____ or ________
constant or proportional
TRUE or FALSE
To minimize the effects of
constant errors, keep sample size
as less as is feasible
FALSE
To minimize the effects of
constant errors, keep sample size
as large as is feasible
Detection of systematic method errors
Analysis of Standard Samples
Independent Analysis
Blank Determinations
Variation in Sample Size
The best way to estimate the bias of an analytical method is by analyzing ____________________________, materials that contain one or more analytes at known
concentration levels.
standard reference materials (SRMs)
A _________ solution contains the solvent
and all the reagents in an analysis and no analyte. Whenever feasible, _______ may
also contain added constituents to
simulate the sample matrix.
blank
TRUE or FALSE
as the size of a measurement increases,
the effect of a constant error decrease
TRUE
Constant errors can often be detected by varying the _________.
sample size
formula for relative error
initial weight- expected weight/ expected weight x 100
TRUE or FALSE
If the measurement result is low, the sign is negative; if the measurement result is
high, the sign is positive.
TRUE
TRUE or FALSE
method errors are
usually the most difficult to
identify and correct
TRUE
the formula for relative error
Er= measured value- true values/ true value x 100
TRUE or FALSE
effect of a constant error becomes more serious as the size of the quantity measured decreases.
TRUE
The term ______ refers to
the quantity measured by an
instrument.
response
TRUE or FALSE
Most personal errors can be minimized by careful, disciplined laboratory work.
TRUE
TRUE or FALSE
Errors due to limitations of the experimenter can usually be avoided
by carefully choosing the analytical method or using an automated procedure.
TRUE
are substances sold by the
National Institute of Standards and
Technology (NIST) and certified to
contain specified concentrations of
one or more analytes.
standard reference materials
The term ________ refers to the
collection of all the constituents in
the sample.
matrix
