Chapter 10: : Precision and Accuracy of Chemical Analyses Flashcards
Measurement data can only give us an ____ of the “true” value
Estimate
In order to improve the reliability and to obtain information about the variability of results
Replicates
How many replicates are usually needed to carried out an entire analytical procedure
Several (often two to five) portions (replicates)
The best estimate of the true value
Central value for the set
- The most widely used measure of central value
- also called the arithmetic mean or the average
Mean
is the middle result when replicate data are arranged in increasing or decreasing order
Median
indicates the closeness of the measurement to the true or accepted value and is expressed by the error
Accuracy
is a measure of how close a series of measurements are to one another
Precision
Three terms widely used to describe the precision
- standard deviation
- variance
- coefficient of variation
the difference between the experimental value and the accepted value.
Error
is the correct value based on reliable references
Accepted value
is the value measured in the lab.
Experimental value
Error formula
Error= Experimental value - accepted value
where xi is a measurement of the quantity and xt is the true or accepted value of the quantity.
Absolute error
Absolute error formula
E= xi - xt
- often a more useful quantity than the absolute error
- also expressed in parts per thousand (ppt)
Relative error
Relative error formula:
Er = xi-xt/xt x 100
Types of errors in experimental data
- Random
- Systematic
- Gross error
also known as inderterminate error
- causes data to be scattered more or less symmetrically around a mean value
- usually small in values and not avoidable
Random error
- also known as determinate error
- causes the mean of a data set to differ from the accepted value
Systematic error
Examples of systematic error
- Miscalibration of pH meter
- Miscalibration of analytical balance
- Evaporation of liquids
- Spattering chemicals
differ from indeterminate and determinate errors
Gross error
are often the product of human errors
Gross error
Gross errors lead to ___
Outliers
results that appear to differ markedly from all other data in a set of replicate measurements.
Gross errors lead to outliers
- have a definite value
- Have an assignable cause
Systematic errors
Systematic errors may be either ____ or ____
Constant of proportional
- The absolute error is
constant with sample size
-The relative error varies when the sample size is changed.
Constant error
increase or decrease according to the size of the sample taken for analysis
Proportional errors
- the absolute error varies with sample size
- relative error stays constant when the sample size is changed
Proportional errors
Example of proportional error
Measured amount of water in a bucket by filling beakers. If the beaker is 5% smaller, you final volume will be off by 5%
Detection of systematic: Intrument and personal errors
- Periodic calibration of equipment
- Careful, discipline laboratory work
Detection of systematic: Method errors
- Analysis of Standard Samples (or SRMs)
- Blank determination
- Variation of sample size
are materials that contain one or more analytes at known concentration levels
SRMs (Standard Reference Materials)
contains the reagents and solvents used in a determination but no analyte
A blank
many of the sample constituents are added to simulate the analyte environment, which is called
Sample matrix
As the size of a measurement increases, the effect of a constant error decreases
Variation of Sample size
contains the solvent and all the reagents in an analysis
Blank solution
refers to the collection of all the constituents in the sample.
Matrix
