Chapter 7 & 8 Chemistry - Introduction to Concept in Chemistry & Error in Analytical Chemistry
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Transcript of Chapter 7 & 8 Chemistry - Introduction to Concept in Chemistry & Error in Analytical Chemistry
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Lecture 1 & 2
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Introduction to chemical analysis
Basic step in an analysis
Stoichiometry
Error in chemical analysis
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Chemical analysis includes any aspect of the chemicalcharacterization of a sample material.
Analytical Chemistry - Science of Chemical
Measurements
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Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)
Qualitative analysis is what.
Quantitative analysis is how much.
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Classical and Modern Chemical
Analysis of samples
Classical analysis
Based on the use of chemical reactivity and stoichiometry to
directly or indirectly measure amounts of analyte (the target
of the analysisa compound or element) The detection limit of classical analysis is limited by the need
to establish and maintain equilibrium in the measurement
reaction
Generally used to measure mg or larger amounts of simplecompounds
Classical methods of analysis often have very high precision
and good accuracy
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Modern (Instrumental) Analysis
Based on the use of instrument transducers to relate a
physical property (e.g., absorption of light) to the amount of
analyte Transducer must be calibrated by measurement of
standards (with known amounts of analyte and matrix)
The detection limit of instrumental analysis depends on the
slope of the transducer-amount relationship for the analyteas well as any interferences.
Often instrumental methods can detect very small amounts
of analytes but with poorer precision than classical analysis
Classical and Modern Chemical
Analysis of samples
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Different methods provide a range of precision,
sensitivity, selectivity, and speed capabilities.
Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)
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The sample size dictates what measurement techniques
can be used.
Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)
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Quantitation:
How much of substance X is in the sample?
Detection: Does the sample contain substance X?
Identification:
What is the identity of the substance in the sample?
Separation:
How can the species of interest be separated from thesample matrix for better quantitation and
identification?
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Fig. 1.1. Steps in an
analysis
An analysis involves
several steps and
operations which depend
on:
the particular problem
your expertise
the apparatus or
equipment available.
The analyst should beinvolved in every step.
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WHAT DO CHEMICAL ANALYST DO?
Research Analytical Chemist
Applies known measurement techniques to welldefined compositional or characterization questions.
Creates and /or investigates novel techniques orprinciples for chemical measurements.
Conducts fundamental studies of chemical/physicalphenomena underlying chemical measurements.
Senior Analyst: Develops new measurement methods onexisting principles to solve new analysis problems.
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CHEMICAL ANALYSIS AFFECTS MANY
FIELDS
Physical-, Organic-, , Chemistry:
Theory guides but Experiment decides
Biotechnology:
Distinguishing isomers with differingbioactivities.
Biosensors
Materials Science:
High-temperature superconductors
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Manufacturing:
Quality control of packaged foods specifications
Forensics:
Chemical features for criminal evidence
CHEMICAL ANALYSIS AFFECTS MANY
FIELDS
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Laboratory safety is a must!
Learn the rules.
Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)
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Definition The word Stoichiometry comes from the Greek stoicheion,
which means to measure the elements
A good definition of the terms meaning in the study of
chemistry is the quantitative study of reactants andproducts in a chemical reaction.
Stoichiometry allows one to calculate how much of a given
product a reaction is expected to produce based on how
much of the reactants are available Given the mass, volume and density, or the number of
moles of reactants, one can calculate the mass, volume (if
the density is known) or moles of product
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Review of Fundamentals
Atomic, Molecular, and Formula Weights
Moles:
1mole = 6.022 x 1023
(atoms, molecules or formula units)
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How Do We Express Concentrations of
Solutions?
Molarity (M)= moles/liter or mmoles/mL
Normality (N) = equivalence/liter or meq/mL
Molality (m) = moles/1000g solvent
In normality calculations, the number of equivalents
is the number of moles times the number of reacting
units per molecule or atom.
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Example 1 1 M sulfuric acid (H2SO4) is 2 N for acid-base reactions because
each mole of sulfuric acid provides 2 moles of H+ ions.
1 M sulfuric acid is 1 N for sulfate precipitation, since 1 mole
of sulfuric acid provides 1 mole of sulfate ions.
Example 2
36.5 grams of hydrochloric acid (HCl) is a 1 N (one normal)
solution of HCl.
Since hydrochloric acid is a strong acid that dissociates
completely in water, a 1 N solution of HCl would also be 1 N
for H+ or Cl- ions for acid-base reactions.
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Analytical Molarity: gives the total number of moles of a
solute in one liter of the solution.
Example: a sulfuric acid solution that has an analytical
concentration of 1.0M can be prepared by dissolving 1.0 mol or98 g of pure H2SO4 in water and diluting to exactly 1.0L.
Equilibrium Molarity: expresses the molar concentration of a
particular species in a solution at equilibrium.
Example: The species molarity of H2SO4 in a solution with an
analytical concentration of 1M is 0.0M because the sulfuric
acid is entirely dissociated into a mixture H3O+, HSO4
- and SO42-
ion.
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Describe the preparation of 2.00L of 0.108M BaCl2
from BaCl2.2H2O (244.3 g/mol)
The mass ofBaCl2.2H
2O is then
= 0.216 mol BaCl2.2H2O
= 52.8 g BaCl2.2H2O
244.3 g BaCl2.2H2O
mol BaCl2.2H2O
X0.216 mol BaCl2.2H2O
0.108 mol BaCl2
L
2.00L X1mol BaCl2.2H2O
1 mol BaCl2X
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Solid Samples:
% (wt/wt) = (wt analyte/wt sample) x 100%
pt (wt/wt) = (wt analyte/wt sample) x 103 ppt
ppm (wt/wt) = (wt analyte/wt sample) x 106 ppm
ppb (wt/wt) = (wt analyte/wt sample) x 109 ppb
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Liquid Samples
% (wt/vol) = (wt analyte/vol sample mL) x 100%
pt (wt/vol) = (wt analyte/vol sample mL) x 103 ppt
ppm (wt/vol) = (wt analyte/vol sample mL) x 106
ppm ppb (wt/vol) = (wt analyte/vol sample mL) x 109 ppb
Liquid Analyte
% (vol/vol) = (vol analyte/vol sample mL) x 100% pt (vol/vol) = (vol analyte/vol sample mL) x 103 ppt
ppm (vol/vol) = (vol analyte/vol sample mL) x 106 ppm
ppb (vol/vol) = (vol analyte/vol sample mL) x 109 ppb
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The units ppm or ppb are used to express trace concentrations.
These are weigh or volume based, rather than mole based.
Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)
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The equivalents (based on charge) of cations and anions
are equal.
Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)
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Definitions:
The difference between a measured value and the
true or known value.
The estimated uncertainty in a measurement orexperiment.
Errors are caused by :
Faulty calibrations
Faulty standardization
Random variations
Uncertainties in results
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Measurement is influenced by many uncertainties.
Example:
Results for the quantitative determination of iron.
Reliability of the data can be assessed in several ways:
Design experiments reveal the presence of errors can be
performed Standard of known composition analyze and the results
compared with the known composition
Calibrating equipment enhances the quality of data
Statistical test
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Replicates
are samples of about the same size that are carried through
an analysis in exactly the same way.
2 to 5 replicates carry out in an experiment.
Results are seldom the same.
What should u do?
Find the central value from the set of results.
Central value should be more reliable than any of the
individual results.
Mean or median is usually used as the central value for a set
of replicate measurements.
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Mean,
also called the arithmetic mean, or the average.
dividing the sum of replicate measurements by the
number of measurement in the set.
wherexi represents the individual values ofxmaking up
the set ofN replicate measurement.
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Median
The middle result when replicate data are arrangedaccording to increasing or decreasing value.
For an odd number of results, the median can be
evaluated directly
For an even number, the mean of the middle pair is used
Example:
Calculate the mean and median for the data shown below:
19.4, 19.8, 19.5, 20.1, 19.6, 20.3
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Describes the reproducibility of measurements
The closeness of results that have been obtained in
exactly the same way
Determine by simply repeating the measurements onreplicate samples
Three terms are widely used to describe the replicate
data:
Standard deviation
Variance
Coefficient of variation
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Standard deviations, describes the spread of individual
measurements about the mean
wherexi is one ofNindividual measurements, and is the
mean
Variance, is the square of the standard deviation
Example :
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Example :
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The closeness of the measurement to the true or acceptedvalue and is expressed by the error.
Measures agreement between a result and the accepted
value, while precision describes the agreement among the
several results obtained in the same way. Precision can be determine by measuring replicate samples
Accuracy is more difficult to determine because the true
value is usually unknown.
Accuracy is expressed in terms of eitherabsolute orrelative
error.
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Examples:
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Analyst 1 - good precision, good accuracy
Analyst 2 - poor precision, good accuracy
Analyst 3 - good precision, poor accuracy
Analyst 4 - poor precision, poor accuracy
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Absolute Error, E
wherext is the true or accepted value of the quantity.
Relative Error, Er
Relative error is also expressed in parts of thousand (ppt).
Example:
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Chemical analyses are affected by at least two types of errors
which are:
i. Random error
ii. Systematic error
Random Error
Causes data to be scattered more or less symmetrically
around the mean value.
Is reflected by its precision.
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Systematic Error
Causes the mean of a data set to differ from the accepted
value.
Lead to bias in measurement results. Bias affects all of the
data in a set in the same way and that it bears a sign.
Example: unsuspected loss of a volatile analyte whileheating a sample.
Gross Error
Differ from the previous 2 errors.
Usually occur only occasionally, are often large, and may
cause a result to be either high or low.
Often the product of human errors.
Lead to outliers, results that appear to differ markedly from
all other data in a set of replicate measurement
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Three types of systematic error:
i. Instrumental errors
ii. Method errors
iii.Personal errors
Instrumental Errors Caused by non-ideal instrument behavior, by faulty
calibrations, or by use under inappropriate conditions.
Example : Pipets, burets, and volumetric flasks may hold
or deliver volumes slightly different from those indicatedby their graduations.
This measuring devices also maybe contaminated by
contaminants on the inner surfaces of the containers.
Calibration eliminates most systematic errors of this type.
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Method errors
Arise from non-ideal chemical or physical behavior of thereagent and reactions.
Some of the sources of non-ideality are:
Slowness of the reactions
Incompleteness of others
Instability of some species
Non-specificity of most reagents
Possible occurrance of side reactions
Example: small excess of reagent required to cause anindicator to undergo the color change that signals completion
of the reaction.
This type of error is often difficult to detect and thus the most
serious of the three types of systematic error.
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Personal errors
Result from carelessness, inattention, or personal limitations
of the experimenter.
Example: an analyst who is insensitive to color changes tends
to use excess reagent in volumetric analysis.
A universal source of personal error isprejudice, or bias.
Most of us, have a natural tendency to estimate scale readings
in a direction that improves the precision in a set of results.
As a result, digital and computer displays on pH meters,laboratory balances, and other electronic instruments to
eliminate number bias because no judgment is involved in
taking a reading.
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Systematic errors may be either constant or proportional
Constant error
The magnitude of a constant error stays essentially the same as
the size of the quantity measured is varied.
Absolute error is constant with sample size but relative errorvaries when sample size is changed.
Proportional error
Increase or decrease according to the size of the sample.
Absolute error varies with the sample size but relative error
stays constant with changing sample size.
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Example:
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Detection of Systematic Instrument and Personal Errors
Some instrument can be corrected by calibration.
Periodic calibration is desirable because the response of
most instrument changes with time as a result of wear,corrosion, or mistreatment.
Personal errors can be minimized by care and self-
discipline.
It is a good habit to check instrument readings, notebookentries, and calculations systematically.
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Detection of Systematic Method Errors
Bias is particularly difficult to detect.
One or more of these steps can be taken to recognize and
adjust for a systematic error in analytical method.
a. Analysis of standard samples
Standard reference materials are materials that contain
one or more analytes at known concentration levels.
Standard reference materials can be prepared by synthesis
or can be purchased from a number of governmental and
industrial sources.
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b. Independent Analysis
If standard sample are not available, independent
method can be used in parallel with the method being
evaluated. Independent method should differ as much as possible
from the one study.
This minimizes the possibility that some common factor
in the sample has the same effect on both methods.
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c. Blank Determinations
A blankcontains the reagents and solvents used in a
determination, but no analyte.
All steps of the analysis are performed on the blank
material.
Blank determinations reveal errors due to interfering
contaminants from reagents and vessels used in the
analysis.
d. Variations in Sample Size
As size of a measurement increases, the effect of a constant
error decreases.
Thus, constant errors can be detected by varying the sample
i