chapter 7 & 8 chemistry - introduction to concept in chemistry & error in analytical chemistry

7/29/2019 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.



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The sample size dictates what measurement techniques

can be used.



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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.



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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.



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The equivalents (based on charge) of cations and anions

are equal.



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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

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chapter 7 & 8 chemistry - introduction to concept in chemistry & error in analytical chemistry

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