Basic Analytical Tools (CHM112)

8/12/2019 Basic Analytical Tools (CHM112)

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Basic Analytical

Tools

Dr. Kathlia A. De Castro-Cruz

Analytical Chemistry 1

Basic Analytical Toolskadecastro-cruz 1


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Basic Analytical Tools

What is Analytical Chemistry?

! It is what analytical chemist do

" Analytical chemists work to improve the abilityof all chemists to make meaningful

measurements.

! Often described as the area of chemistryresponsible for characterizing the compositionof matter, both qualitatively (what is present)and quantitatively (how much is present) misleading definition

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! The craft of analytical chemistry is not inperforming a routine analysis on a routinesample (chemical analysis), but in improving

established methods, extending existingmethods to new types of samples, anddeveloping new methods for measuringchemical phenomena

!

The science of inventing and applying theconcepts, principles, and strategies formeasuring the characteristics of chemicalsystems and species

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! Typical problems on which analytical chemistswork include

" qualitative analyses (what is present?),

" quantitative analyses (how much is present?),

" characterization analyses (what are the

materials chemical and physical properties?),

"

and fundamental analyses (how does thismethod work and how can it be improved?)

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Classification of Analysis

! Based on goal of analysis

" Qualitative analysis an analysis carriedout to determine only the identity of a pure

analyte, the identity of an analyte in amatrix, or the identity of several or allcomponents of a mixture

"Quantitative analysis the analysis of amaterial for how much of one or morecomponents is present

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! Based on nature of method used

" Physical property analysis involves no

chemical reactions and at times relatively

simple devices (although possiblycomputerized) to facilitate the measurement.Physical properties are especially useful for

identification, but may also be useful for

quantitative analysis in cases where the valueof a property, such as specific gravity orrefractive index, varies with the quantity of an

analyte in a mixture.

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! Based on nature of method used

" Wet chemical analysis - involves chemical

reactions or classical reaction stoichiometry,

but no electronic instrumentation beyond aweighing device

" Instrumental analysis can also involve

chemical reactions, but it always involves

modern sophisticated electronicinstrumentation. Instrumental analysistechniques are high-tech techniques, oftenutilizing complex hardware and software

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Calculateand report

the results(5)

ObtainSample

(1) Preparethe sample

(2)

Workthe data

(4)

Carry out

the analysisMethod

(3)

General Analytical

Strategy

Analytical Strategy



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Calculateand report

the results(5)

ObtainSample

(1)Prepare

the sample

(2)

Workthe data

(4)

Carry out

the analysisMethod

(3)

General Analytical

Strategy

Analytical Strategy

The sample mustbe representative

of the bulk

system; its

integrity must be

maintained; andthe chain of

custody must be

documented.

A portion of the sampleis prepared for the

analysis by weighing it

(or measuring its

volume) and carrying

out certain physicaland/or chemical

processes, such asdrying, dissolving, etc.

Obtain weight or volume data on the prepared samplePrepare reference standards of the analyte or substances

with which the analyte will react.

Standardize solutions or calibrate equipment

Obtain the required data for the sample

A final calculation may benecessary to obtain the

desired results.

This requirescalculations and/

or the plotting of a

calibration curve

from which the

desired resultscan be derived.

Statistics are

usually involved



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10 Basic Analytical Toolskadecastro-cruz

Sampling

! The process of obtaining samples for analyses

" Samples must possess all the characteristicsof the entire bulk system with respect to theanalyte and the analyte concentration in thesystem (representative sample)

! Plays a crucial role in the quality and success

of an analysis


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Sampling

! Samples maybe

" Bulk sample

"

Primary sample" Secondary sample

" Sub sample

"

Laboratory sample

" Test sample


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Sampling Solid

! Sample preparations

" Particle size reduction

#Maybe crushing, milling, grinding orpulverizing

" Sample homogenization and division

" Solid-liquid extraction

" Total dissolution

# May use acids


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Sampling Solid

!

Total Dissolution

Name Formula Description Example Uses

Water H2O clear, colorless liquid with lowvapor pressure, highly polar

Dissolving polarand ionic

compoundsHydrochloric

acidHCl Commercially available

concentrated solution is 38% (12

M) HCl; evolves pungent fumesand must be handled in fume

hood

Dissolving somemetals and metal

ores

Sulfuric acid H2SO4 Commercially availableconcentrated solution is 96% (18

M) H2SO4; a dense, syrupy liquid;reacts on contact with skin and

clothing; evolves much heatwhen mixed with water

Organic samples,such as for

Kjeldahl analysis;also oxides of Al

and Ti


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Sampling Solid

!

Total Dissolution


Nitric acid HNO3 Commercially availableconcentrated solution is 70%

(16 M) HNO3; reacts withclothing and skin (turns skin

yellow); evolves thick brownand white fumes when in

contact with most metals

Dissolving noblemetals (e.g.,

copper and silver)and also some

organic samples

Hydrofluoric

acid

HF Commercially available

concentrated acid is 50% (26M) HF; must be stored inplastic containers, since it

attacks glass; very damagingto skin

Dissolving silica-

based materialsand stainless steel


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Sampling Solid

!

Total Dissolution


Perchloric acid HClO4 Commerciallyavailable

concentratedsolution is 72% (12

M) HClO4

Dissolving difficultorganic samples and

stable metal alloys

Aqua regia A mixture ofconcentrated HNO3

and HCl in the ratio

of 1:3 HNO3:HCl

Dissolving highlyunreactive metals,

such as gold


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Sampling Solid

! Sample preparations

" Fusion

#dissolving of a sample using a molteninorganic salt, generally called a flux, as thesolvent.

# This flux dissolves the sample and, upon

cooling, results in a solid mass that is then

soluble in a liquid reagent.


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Sampling Liquids and Extracts

! Extraction from Liquid solutions

" Maybe liquid-liquid, solid phase or purgeand trap

! Dilution and Concentration

! Derivatization

"To preserve the sample that are unstable


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Reagents for Sample Preparation

!

A reagent is a substance used in a chemicalreaction in an analytical laboratory because of itsspecific applicability to a given system orprocedure.

" Primary standard: A specially manufactured

analytical reagent of exceptional purity forstandardizing solutions and preparing referencestandards.

" ACS certified: A reagent that meets or exceeds thespecifications of purity put forth by the AmericanChemical Society. The certificate of analysis is on

the label.


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" Certified reagent: A reagent that meets the

standards of purity established by themanufacturer. The certificate of analysis is onthe label.

" USP/NF: Reagents that meet the purityrequirements of the U.S. Pharmacopeia (USP)and the National Formulary (NF). Generally of

interest to the pharmaceutical profession,these specifications may not be adequate forreagent use.


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" Spectro grade or spectranalyzed: Solvents ofsuitable purity for use in spectrophotometricprocedures. A certificate of analysis is on thelabel.

" High-performance liquid chromatography(HPLC) grade: Solvents of suitable purity foruse in liquid chromatography procedures.

"Practical: Chemicals of sufficiently high qualityto be suitable for use in some syntheses.

Organic chemicals of practical grade maycontain small amounts of intermediates,isomers, or homologs.


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" Technical: Chemicals of reasonable purity forapplications that have no official standard forpurity.


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Interferences

! Substance that affects the analytical signal orthe background

! Foreign species that either attenuate the signalfrom the analyte or produce a signal that isindistinguishable from that of the analyte

! Also known as interferent


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Interferences

!

Method of eliminating interferences in chemicalanalysis

Method Basis of Method

Masking Immobilization of interferent as anonreactive complex

Mechanical Separation

Precipitation

and filtration

Difference in solubility of compounds

formedDistillation Difference in volatility of compounds

Extraction Difference in solubility in twoimmiscible liquids


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Interferences

!

Method of eliminating interferences in chemicalanalysis

Method Basis of Method

Ion exchange difference in stability of reactantswith an ion-exchange resin

Chromatography Difference in rate of movement of asolute through a stationary phase

Electrophoresis Difference in migration rate in anelectrical field


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Interferences Elimination

!

Masking

" Addition of reagent to the solution of thesample to immobilize or chemically bind, the

interferent as a complex that no longercontributes to or attenuates the signal fromthe analyte

# Masking agent a reagent that chemically binds

an interferent as a complex & prevents it fromcausing errors in an analysis


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!

Masking

Masking AgentMetals that can be

masked

Metals thatcould not be

masked

F- Al+3, Fe3+, Ti4+, Be4+

CN- Cd2+, Zn2+, Hg2+, Co2+,Cu+, Ag+, Ni2+, Pd2+,Pt2+, Fe2+, Fe3+

Mg2+, Ca2+,Mn2+, Pb2+

Triethanolamine Al3+

, Fe3+

, Mn2+

2,3-dimercapto-1-propanol

Bi3+, Cd2+, Cu2+, Hg2+,Pb2+


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!

Demasking

" Releases metal ion from a masking agent

# Example: cyanide complexes can be

demasked using formaldehyde


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!

Separation by Precipitation

" Requires large solubility differences betweenthe analyte and the possible interferences

"

Needs to consider factors affectingprecipitation such as coprecipitationphenomena

"Could be# Separations Based on Control of Acidity

# Sulfide Separations


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!

Separation by Precipitation

" Could be

# Separations by other Inorganic precipitants

#

Separations by Organic Precipitants

# Separation of Species Present in Trace Amountsby Precipitation

#Separation by Electrolytic Precipitation

# Salt-induced precipitation of Proteins


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!

Separation based on control acidity

" Separations based on pH control

" Three categories

# Those made in relatively concentrated solutionsof strong acids

# Those made in buffered solutions at intermediatepH values

# Those made in concentrated solutions of sodiumor potassium hydroxide


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!

Separation based on control acidity


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!

Sulfide Separations


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!

Separation by other inorganic precipitants

" Phosphate, carbonate, and oxalate ions areoften employed as precipitants for cations

(nonselective)" Chloride and sulfate

# Highly selective


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!

Separation by organic precipitants

" Dimethylglyoxime (has remarkable selectivityin forming precipitates)

"

8-hydroxyquinoline


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!

Separation of Species Present in Trace Amountsby Precipitation

" Used in isolation species present in

microgram quantities" Collector

# Used to remove trace constituents from solution

#Example: Al2O3as collector of trace amounts oftitanium/CS for collection of traces of Zn and Pb


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!

Separation by Electrolytic Precipitation

" The more easily reduced species, either thewanted or the unwanted component of the

sample is isolated as a separate phase

! Salt-induced Precipitation of Proteins

" Addition of high concentration of salt (salting

out)


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!

Separation of species by distillation

" Distillation is widely used to separate volatileanalytes from nonvolatile interferents

"

Maybe

# Simple

# Fractional

# Steam


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! Separation by extraction

" The extent to which solutes both inorganicand organic, distribute themselves between

two immiscible liquids is used to accomplishseparation

" Governed by Distribution Law

"

Frequently used in separating inorganicspecies


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! Separation by extraction


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! Separation by Ion Exchange

" A process by which ions held in porous,essentially insoluble solid are exchanged for

ions in a solution that is brought into contactwith the solid

" Used to eliminate ions that would otherwise

interfere with an analysis


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Numbers in Analytical Chemistry

!

Fundamental Units of Measurements

" Measurements usually consist of a unit and a

number expressing the quantity of that unit.

#SI units after theSystme InternationaldUnits are the internationally agreed on unitsfor measurements

# Standards of length, mass, and time are themeter (m), kilogram (kg), and second (s),

respectively. Temperature is measured inkelvins (K), amount of substance in moles(mol), and electric current in amperes (A).

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!

Fundamental Units of Measurements

" Common prefixes used for exponential

notation

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!

Significant Figures

" The digits in a measured quantity, including all

digits known exactly and one digit (the last)whose quantity is uncertain.

" reflection of a measurements uncertainty

" Itis equal to the number of digits in themeasurement, with the exception that a zero

(0) used to fix the location of a decimal point isnot considered significant.

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!

Rules in Significant Figures

" For measurements using logarithms, such as

pH, the number of significant figures is equalto the number of digits to the right of the

decimal, including all zeros. Digits to the left ofthe decimal are not included as significant

figures since they only indicate the power of 10.

# Example: pH = 2.45 (has 2SF)

" Exact numbers, such as the stoichiometriccoefficients in a chemical formula or reaction,and unit conversion factors, have an infinitenumber of significant figures.

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!

Rules in Significant Figures

" Mathematical operations involving additionand subtraction are carried out to the lastdigit that is significant for all numbersincluded in the calculation.

# Example: the sum of 135.621, 0.33, and 21.2163

is 157.17

"

When multiplying and dividing, the generalrule is that the answer contains the samenumber of significant figures as that number in

the calculation having the fewest significantfigures.

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!

Units of expressing concentration

" Concentration is a general measurementunit stating the amount of solute present ina known amount of solution

" Molarity - The number of moles of solute per

liter of solution (M)

" Formality The number of moles of solute,

regardless of chemical form, per liter ofsolution (F)

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!


" Normality The number of equivalents of

solute per liter of solution (N).

#The number of equivalents, n, is based on areaction unit, which is that part of a chemicalspecies involved in a reaction

$ n forsalt = charge

$ nfor acid = no of replaceable H

$ nfor base = no of replaceable IH

$ nfor redox = no of electrons involved

# Relationship between Molarity and Normalityis: N = nx M

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!


" Molality The number of moles of solute per

kilogram of solvent (m).

"

Weight percent Grams of solute per 100 g ofsolution (% w/w).

" Volume percent Milliliters of solute per 100mL of solution (% v/v).

"

Weight-to-volume percent Grams of soluteper 100 mL of solution (% w/v).

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!


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!

Sample Problems

" Calculate the formula mass of CaSO4. Whatis the molarity of CaSO4in a solutioncontaining 1.2 g CaSO

4in a volume of 50

mL? How many grams of CaSO4are in 50mL of 0.086M CaSO4?

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Answers: 136.14 g/mol0.18 M, 0.59 g


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!

Sample Problems

" Calculate the molarity and molality of 49.0wt% HF. Density of HF = 1.30 g/mL

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Answers: 31.8 M, 48.0 m


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!

Sample Problems

" Normal alkanes are hydrocarbons with theformula CnH2n+2. Plants selectivelysynthesize alkanes with an odd number ofcarbons. The concentration of C29H60 insummer rain water collected is Hannove,Germany is 34 ppb. Find the molarity ofC29H60and express the answer using prefix.How many ppm of C29H60are in 23!MC29H60?

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Answers: 83 nM, 9.4 ppm


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!

Sample Problems

" The molarity of concentrated HClpurchased for laboratory used isapproximately 12.1M How many millilitersof this reagent should be diluted to prepare1.00oL of 0.100M HCl solution.

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Answers: 8.26 mL


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!

Sample Problems

" In a gravimetric analysis, we need enoughproduct to weigh accurately. Each tableprovide ~15 mg iron. How many tabletsshould we analyze to provide 0.25 g ofFe2O3product?

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Answers: 12 Tablets


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Errors in Analytical Measurements

!

Classification of errors

" Systematic errors (determinate errors)

generally arise from determinate or identifiablesources causing measured values to differ from

a true or accepted value. These errors can beavoided and the magnitude could be measured.

# Operational and personal errors these aredue to factors for which the individual analysts

is responsible and are not connected with themethod or procedure

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!

Classification of errors

# Instrumental and reagent errors these arisefrom the faulty construction of balances, theuse of uncalibrated or improperly calibrated

weights, graduated glassware and otherinstruments

# Errors of method originate from incorrectsampling and from incompleteness of areaction

# Additive and proportional errors

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!

Measurement of errors

" Accumulated errors - Errors are associatedwith every measurement made in an analyticalprocedure, and these will be aggregated in the

final calculated result. The overall error fordeterminate error maybe calculated using

either of the following expression:

# where only a linear combination of individual

measurements is required to compute theresult, the overall absolute determinate error,

ET, is given by:ET=E1 +E2+E3+.......

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!

Measurement of errors

E1andE2etc., being the absolute determinateerrors in the individual measurements takingsign into account

#

where a multiplicative expression is requiredto compute the result, the overall relativedeterminate error,ETR, is given by

ETR=E1R+E2R+E3R+.....

E1RandE2Retc., being the relative determinateerrors in the individual measurements takingsign into account.

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Accuracy and Precision

!

Accuracy

" The closeness of an experimental measurement

or result to the true or accepted value.

"It is generally the more importantcharacteristic of quantitative data to beassessed, although consistency, as measured byprecision, is of particular concern in somecircumstances.

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!

Precision

" The closeness of agreement betweenreplicated measurements or resultsobtained under the same prescribedconditions.

" Can be assessed in several ways.

# Spread or range (i.e. the difference between

the highest and lowest value) is sometimes used

# Standard deviation of the data popularlyused.

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!

Precision

" Commonly divided into two categories:

# Repeatability - the precision obtained when

all measurements are made by the sameanalyst during a single period of laboratory

work, using the same solutions andequipment.

#

Reproducibility - the precision obtainedunder any other set of conditions, includingthat between analysts, or betweenlaboratory sessions for a single analyst.

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!

Precision vs Accuracy

" Commonly divided into two categories:

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Neither precisenor accurate

Precise but

not accurate

Accurate

and precise


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Statistics and Statistical Analysis

!

Mean, arithmetic mean or average

" obtained by dividing the sum of replicatemeasurements by the number ofmeasurements in the set:

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x =

xi

i=1

N

"

N

wherexirepresents individual values ofxmaking up a set ofN replicate measurements


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!

Median

" the middle result when replicate data are

arranged in order of size

!

Range" the difference between the largest and

smallest values in the data set.

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w = xl argest"xsmallest


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!

Deviation, d

" determines how far the measured value

deviates from the mean.

! Estimated Standard deviation, s

" measure of the dispersion of data around the

mean

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d = x" xi

s =(d1

2+ d2

2+ d3

2+ ...

n "1

where n is the number of measurements and n-1 is thedegrees of freedom


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!

Standard Deviation, s" the smaller it is numerically, the more precise the

data (the more the measurements are bunchedaround the mean)

"

For an infinite number of measurements (wherethe mean is m), the standard deviation issymbolized as s (Greek letter sigma) and is knownas the population standard deviation. 30 ormore measurements approximate an infinitenumber of measurements.

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"=

(xi# )2

i=1

N

$

N


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!

Relative Standard Deviation" Relates the standard deviation to the value of the

mean and represents a practical and popularexpression of data quality. It is calculated by

dividing the standard deviation by the mean andthen multiplying by 100 or 1000:

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RSD =s

x

relative % standard deviation = RSD x 100relative parts per thousand standard deviation = RSD x 1000


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!

Normal DistributionCurve

" For an infinite dataset (in which the

symbols and !apply), a plot offrequency ofoccurrence vs. themeasurement value

yields a smooth bell-shaped curve.

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Deviation from mean

Frequency

ofocc

urrence

ofeach

devia

tion

sd = s2

sd = s1

s1> s2

m +


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! Uncertainty - the range of possible values fora measurement

! Confidence Interval - the range of values

around an experimental result within whichthe true or accepted value is expected to lie

with a defined level of probability.

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! Confidence Interval

" Known standard deviation:

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CL() = x z"

Nwhere z is a statistical factor relatedto the probability level required

ConfidenceLevel%

zConfidene Level, %

z

50 0.67 95.4 2.00

68 1.00 99 2.58

80 1.29 99.7 3.00

90 1.64 99.9 3.29

95 1.96


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! Confidence Interval

" Using estimated standard deviation:

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where t is a statistical factor related to the probability levelbut in addition determined by the number of degrees of

freedom for the set of data, i.e. one less than the number ofresults.

CL() = x tsN


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Degrees ofFreedom

Factor for Confidence Interval

80% 90% 95% 99% 99.9%

1 3.08 6.31 12.7 63.7 637

2 1.89 2.92 4.30 9.92 31.6

3 1.64 2.35 3.18 5.84 12.9

4 1.53 2.13 2.78 4.60 8.60

5 1.48 2.02 2.57 4.03 6.86

6 1.44 1.94 2.45 3.71 5.96

7 1.42 1.90 2.36 3.50 5.40

8 1.40 1.86 2.31 3.36 5.04

9 1.38 1.83 2.26 3.25 4.78

10 1.37 1.81 2.23 3.17 4.59

11 1.36 1.80 2.20 3.11 4.44

12 1.36 1.78 2.18 3.06 4.32

13 1.35 1.77 2.16 3.01 4.22

14 1.34 1.76 2.14 2.98 4.14

! 1.29 1.64 1.96 2.58 3.29


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! Standard deviation of computed results

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!

t Test

" This is used to compare one set ofmeasurements with another to decide whetheror not they are the same.

" Three cases:

# Case 1. Comparing a Measured Result with aKnown Value

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tcalculated

=

x " known value

sn

If tcalculated> ttableat 95% confidence level, the two resultsare considered to be different.


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!

t Test

# Case 2. Comparing Replicate Measurements(Use to decide whether two sets of replicatemeasurements give the same or different

results )

$ For two sets of data consisting of n1 and n2measurements:

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tcalculated =x1

" x2

spooled

n1n2

n1 +n2

spooled =s1

2(n

1 "1)+ s22(n

2 "1)

n1+n

2 " 2

If tcalculated

> ttable

at95% confidence

level, the two resultsare considered to be

different.


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!

t Test

# Case 2. Comparing Replicate Measurements

$ If the population for both series of

measurement is different:

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tcalculated

=

x1

" x2

s1

2

n1

+

s2

2

n2

Degrees of freedom =

s1

2

n1

+s2

2

n2

"

#$

%

&'2

s1

2

n1

"

#$

%

&'2

n1+1

+

s2

2

n2

"

#$

%

&'2

n2+1

"

#

$

$$$

%

&

'

'''

(

)

*****

+

*

*

***

,

-

*****

.

*

*

***

/2


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!

t Test

# Case 3. Comparing Individual Differences (Iftwo different methods were used to makesingle measurements on several different

samples, are they different?)

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tcalculated

=

d

sd

n

sd =

di" d( )

#

2

n "1

If tcalculated> ttableat 95% confidence level, the two results areconsidered to be different.


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!

Q Test

# Use to help decide whether to retain or discardquestionable datum.

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Qcalculated =gap

range

range is the total spread of the data

gap is the difference between the questionable point andthe nearest value

If Qcalculated> Qtable, the questionablepoint should be discarded


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!

Q Test

# Values of Q for rejecting data

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Q (90% confidence)Number of

Observations0.76 4

0.64 5

0.56 6

0.51 7

0.47 8

0.44 9

0.41 10


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References

"

Kenkel, J. (2003). Analytical Chemistry forTechnicians, 3rded. CRC Press, US.

" Harvey, D. (2000). Modern AnalyticalChemistry, McGraw-Hill, US, 104-134.

" Harris, D. (2007). Quantitative ChemicalAnalysis, 8thed., Freeman, NY.

Documents

Basic Analytical Tools (CHM112)