ment for foundation in science sudents

1. Lecture 1 & 2

2. • Introduction to chemical analysis
• Basic step in an analysis
• Stoichiometry
• Error in chemical analysis

3. • Chemical analysis includes any aspect of the chemical
characterization of a sample material.
• Analytical Chemistry - “Science of Chemical
Measurements”

5. Qualitative analysis is what.
Qualitative analysis is what.
Quantitative analysis is how much.
Quantitative analysis is how much.
©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

6. “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 analysis –a 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 simple
compounds
• Classical methods of analysis often have very high precision and
good accuracy

7. “Classical” and Modern Chemical
Analysis of samples
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 analyte
as well as any interferences.
• Often instrumental methods can detect very small amounts
of analytes but with poorer precision than classical analysis

10. Different methods provide a range of precision,
Different methods provide a range of precision,
sensitivity, selectivity, and speed capabilities.
sensitivity, selectivity, and speed capabilities.
©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

11. The sample size dictates what measurement techniques
The sample size dictates what measurement techniques
can be used.
can be used.
©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

12. • 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 the
sample matrix for better quantitation and
identification?

13. An analysis involves
An analysis involves
several steps and
several steps and
operations which depend
operations which depend
on:
on:
•the particular problem
•the particular problem
••your expertise
your expertise
••the apparatus or
the apparatus or
equipment available.
equipment available.
The analyst should be
The analyst should be
involved in every step.
involved in every step.
Fig. 1.1. Steps in an
analysis

15. WHAT DO CHEMICAL ANALYST DO?
• Research Analytical Chemist
 Applies known measurement techniques to well defined
compositional or characterization questions.
 Creates and /or investigates novel techniques or
principles for chemical measurements.
 Conducts fundamental studies of chemical/physical
phenomena underlying chemical measurements.
• Senior Analyst: Develops new measurement methods on
Analyst
existing principles to solve new analysis problems.

28. CHEMICAL ANALYSIS AFFECTS MANY
FIELDS
• Physical-, Organic-, …, Chemistry:
– “Theory guides but Experiment decides”
• Biotechnology:
– Distinguishing isomers with differing
bioactivities.
– Biosensors
• Materials Science:
– High-temperature superconductors

29. CHEMICAL ANALYSIS AFFECTS MANY
FIELDS
• Manufacturing:
– Quality control of packaged foods specifications
• Forensics:
– Chemical features for criminal evidence

30. Laboratory safety is a must!
Laboratory safety is a must!
Learn the rules.
Learn the rules.
©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

31. • Definition
– The word Stoichiometry comes from the Greek stoicheion,
which means to measure the elements
– A good definition of the term’s meaning in the study of
chemistry is the “quantitative study of reactants and
products in a chemical reaction”.
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

32. Review of Fundamentals
• Atomic, Molecular, and Formula Weights
• Moles:
1mole = 6.022 x 1023
(atoms, molecules or formula units)

33. 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
In normality calculations, the number of equivalents
is the number of moles times the number of reacting
is the number of moles times the number of reacting
units per molecule or atom.
units per molecule or atom.

34. 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.

39. • Analytical Molarity: gives the total number of moles of a
Molarity
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 or
98 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.

40. • Describe the preparation of 2.00L of 0.108M BaCl 2
from BaCl2.2H2O (244.3 g/mol)
0.108 mol BaCl2 1mol BaCl2.2H2O
2.00L X X
L 1 mol BaCl2
= 0.216 mol BaCl2.2H2O
• The mass of BaCl2.2H2O is then
244.3 g BaCl2.2H2O
0.216 mol BaCl2.2H2O X
mol BaCl2.2H2O
= 52.8 g BaCl2.2H2O

41. 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

42. 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

43. The units ppm or ppb are used to express trace concentrations.
The units ppm or ppb are used to express trace concentrations.
These are weigh or volume based, rather than mole based.
These are weigh or volume based, rather than mole based.
©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

44. The equivalents (based on charge) of cations and anions
The equivalents (based on charge) of cations and anions
are equal.
are equal.
©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

45. Definitions:
•The difference between a measured value and the
“true” or “known” value.
•The estimated uncertainty in a measurement or
experiment.
•Errors are caused by :
 Faulty calibrations
 Faulty standardization
 Random variations
 Uncertainties in results

46. • 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

47. 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.

48. Mean,
•also called the arithmetic mean, or the average.
•dividing the sum of replicate measurements by the
number of measurement in the set.
where xi represents the individual values of x making up
the set of N replicate measurement.

49. Median
• The middle result when replicate data are arranged
according 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

50. • Describes the reproducibility of measurements
• The closeness of results that have been obtained in
exactly the same way
• Determine by simply repeating the measurements on
replicate samples
• Three terms are widely used to describe the replicate
data:
 Standard deviation
 Variance
 Coefficient of variation

51. •Standard deviations, describes the spread of individual
measurements about the mean
•where xi is one of N individual measurements, and is the
mean
•Variance, is the square of the standard deviation

52. Example :

53. • The closeness of the measurement to the true or accepted
value and is expressed by the error.
• Accuracy 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 either absolute or relative
error.

54. Absolute Error, E
where xt is the true or accepted value of the quantity.
Relative Error, Er
Relative error is also expressed in parts of thousand (ppt).
Example:

55. Examples:

56. Analyst 1 - good precision, good accuracy
Analyst 2 - poor precision, good accuracy
Analyst 3 - good precision, poor accuracy
Analyst 4 - poor precision, poor accuracy

57. 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.

58. 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 while heating
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

59. Example:

60. 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 indicated by 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.

61. Method errors
• Arise from non-ideal chemical or physical behavior of the
reagent 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 occurrence of side reactions
 Example: small excess of reagent required to cause an
indicator 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.

62. 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 is prejudice, 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.

63. 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, notebook
entries, and calculations systematically.

64. 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.

65. b. Blank Determinations
• A blank contains 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.
c. 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 size.