Analytical Chemistry Study Notes

Analytical Chemistry - General Perspective

1.1 Introduction

• Analytical chemistry is a sub-discipline of chemistry with interdisciplinary character.
• Chemicals are essential in our lives, found in food, clothing, and medicines.
• Analytical chemistry involves developing methods for chemical analysis.
• This unit covers analytical chemistry, chemical methods like gravimetry and volumetry, electrical methods (potentiometry, amperometry, voltammetry, coulometry), optical methods (emission, absorption, UV, IR, Raman spectroscopy), nuclear and thermal methods, and separation methods.
• A separate course (MCH-002) covers separation methods in detail.
• The unit concludes with emerging needs and recent trends in analysis.

Objectives:

• Broaden perception of analytical chemistry.
• Distinguish between various analytical techniques.
• Classify chemical, electrical, optical, nuclear, and thermal methods of analysis.
• Classify separation methods.
• Understand the utility of analytical techniques.
• Highlight the development of methods of chemical analysis.

1.2 Analytical Chemistry - An Introduction

• Chemical analysis answers "What is it?" and "How much?" questions about materials.
• Analytical chemists develop methods of analysis using principles from various fields like chemistry, physics, biology, biochemistry, geology, engineering, and computer science.
• The constituents analyzed can be elements, ions, radicals, functional groups, or compounds.
• Analytical chemistry involves the theory and practice of methods to determine the composition of substances.
• Analytical data is crucial for characterizing the quality and suitability of materials before production or release.
• Analysis results form the basis for processing calculations and cost estimations in chemical industries.
• Over the past decades, there has been significant development in analytical chemistry, with new techniques for separation, detection, structure elucidation, and quantification at lower levels, along with multi-component analysis and shorter analysis times.
• Analytical methods can be divided into chemical (classical), physical (instrumental), and physicochemical (instrumental) methods.
• An analytical technique is based on measuring a property related to the nature or amount of the substance under examination.
• Qualitative analysis identifies the nature of the substance, while quantitative analysis determines the amount.
• Quantitative analysis is often preceded by qualitative analysis.
• The sample is a small part of the material examined, and its analysis results represent the whole material.
• When choosing an analytical technique, consider the sample type, information needed, purpose, experience, equipment, sample preparation, time, and cost.
• Accuracy, sensitivity, and selectivity are crucial factors in selecting an analytical method.
• Instrumental methods, being relative, require calibration with standards using an analytical calibration curve of instrument response versus concentration.

1.3 Classification of Different Analytical Techniques

• Analytical chemistry involves the development of methods for chemical analysis based on physical property measurements.
• Analytical techniques can be classified based on the type of properties measured:
  • Chemical methods
  • Electrical methods
  • Optical methods
  • Nuclear methods
  • Thermal methods
  • Separation methods

1.3.1 Classification of Chemical Methods of Analysis

• Chemical methods rely on chemical reactions, with direct mass measurement through weighing (gravimetry) or volume measurement (volumetry).
• Gravimetry:
  • Analysis by weight using an analytical balance.
  • Accurate macro-analysis procedure based on precipitation of an ionic or molecular substance.
  • The precipitate is separated, dried, and weighed to determine the analyte amount using calculations.
  • Example: Estimating barium as barium sulfate ($BaSO_4$).
• Volumetry:
  • Analyte amount determined by measuring the volume of a reagent solution with accurately known concentration.
  • Saves time compared to gravimetry.
  • Also known as titrimetry, characterized by titration.
  • A known volume of reactant is titrated with a titrant until the equivalence point, indicated by an indicator.
  • The volume of titrant at the equivalence point is used to calculate the analyte amount.
  • Volumetry is simpler than gravimetry, but requires rapid reactions to reach equilibrium quickly after each titrant addition.

1.3.2 Classification of Electrical Methods of Analysis

• Electrical methods measure electrochemical properties of a solution, such as potential, current, quantity of current, resistance, and dielectric constant.

• Types of electrical methods:

  • Potentiometry
  • Amperometry
  • Voltammetry
  • Coulometry
  • Conductometry and High-Frequency Methods

• Potentiometry:

  • Based on measuring the potential difference across an electrochemical cell.
  • The term "potentiometry" comes from "potential," referring to the half-cell potential (electrode potential).
  • Analysis results are computed directly from the cell voltage or used to determine the equivalence point in potentiometric titrations.
  • Potentiometric titrations involve redox titration curves based on half-cell potentials.

• pH-metry:

  • A special class of potentiometry measures the potential of an indicator electrode as a function of hydrogen ion concentration.
  • Modified voltmeters with high impedance mV meters and glass electrodes are used to measure pH instantaneously.

• Amperometry:

  • Involves current measurements.
  • The term "amperometry" comes from "ampere," the unit of current.
  • Used to detect the equivalence point in titrations (amperometric titration), using a microelectrode as an indicator electrode and a reference electrode.
  • Current is measured at a fixed potential as a function of titrant volume.
  • Plotting the data yields two straight lines with different slopes on either side of the equivalence point.

• Biamperometry (Dead Stop End Point Titration):

  • A modification of amperometric titration using two polarized microelectrodes.
  • A small potential is applied between the electrodes in a well-stirred solution, and the current is measured as titrant is added.

• Voltammetry:

  • An electroactive species is consumed at the surface layer of the indicator electrode in an electrolytic cell.
  • The current resulting from electron transfer is measured as a function of applied potential.
  • Current versus potential curves (I-E curves) are plotted.
  • The shape of the I-E curves depends on the polarization of the indicator electrode.
  • Polarography:
    • A special type of voltammetry using a dropping mercury electrode as the indicator electrode.
    • A continuously changing potential is applied, and the resulting current is monitored.
    • Current-potential curves are known as polarograms or polarographic waves.
    • The half-wave potential ($E_{1/2}$) is used for qualitative identification, and the wave height is used for quantitative estimation.
    • Recent developments include using three electrodes to obtain sharply defined polarographic waves in non-aqueous solvents.

• Coulometry:

  • Analytical methods based on measuring the quantity of electricity.
  • The term comes from "coulomb," a unit of current quantity.
  • Requires that the species determined interacts with 100% current efficiency.
  • Two techniques:
    • Coulometry at constant potential (potentiostatic coulometry)
    • Coulometry at constant current (amperostatic coulometry), often called coulometric titrations.

• Conductometry and High-Frequency Methods:

  • The measurement of conductance (reciprocal of resistance) can be useful in chemical analysis.
    • Computed directly from conductance measurements.
    • Applied to determine the equivalence point of titrations (conductometric titrations).
  • Conductometric titrations relate changes in conductance to concentration changes of ionic species in the titration reaction.
  • Conductances are measured using alternating current of 3-6 volts with a frequency of 50-1000 Hz.
  • High-Frequency Methods:
    • Modified technique using much higher frequencies (several mega Hz).
    • Applied to measure dielectric constant and for titrations where electrodes do not contact the solution.

1.3.3 Classification of Optical Methods of Analysis

• Optical methods of analysis, also known as spectroscopic methods, are based on the interaction of electromagnetic radiation (emr) with matter.
• Classification can be based on the type of effect (emission, absorption, or scattering) or the type of emr (x-ray, uv-vis, IR, etc.).
• Important spectroscopic methods:
  • Emission Spectroscopy
  • Absorption Spectroscopy
  • Ultraviolet and Visible Absorption Spectroscopy
  • Infrared Absorption Spectroscopy
  • Fluorophotometry
  • Turbidimetry and Nephelometry
  • Raman Spectroscopy
• Emission Spectroscopy:
  • Applies the characteristic spectrum produced by the excitation of elements to qualitative and quantitative analysis.
  • Depends on the electromagnetic radiation produced when the analyte is excited by thermal, electrical, or radiant energy.
  • Each element has a characteristic emission spectrum used for qualitative analysis.
  • Quantitative determinations are possible because the energy emitted for a given spectral line is proportional to the concentration of the element.
• Absorption Spectrometry:
  • Based on measuring the absorption of electromagnetic radiation by matter.
  • Absorption occurs when a chemical species selectively absorbs photons of certain electromagnetic radiation.
  • Measurements can be made at a single wavelength or over a wide range of wavelengths.
  • Filter Photometry: Uses filters to select a narrow wavelength range.
    Spectrophotometry: Uses a monochromator (prism or grating) to obtain approximately monochromatic radiation.
• Ultraviolet and Visible Absorption Spectroscopy:
  • Involves measuring the absorption of ultraviolet and visible radiation (180 to 780 nm) by atomic, ionic, or molecular species.
  • Involves transitions between electronic levels of absorbing chemical species.
  • Used for qualitative analysis (identifying unsaturated organic compounds) and quantitative analysis.
• Infrared Absorption Spectroscopy:
  • Involves the absorption of infrared radiation (0.78 to 1000 µm), increasing the energy of vibration or rotation associated with a covalent bond.
  • Requires a change in the dipole moment of the molecule.
  • Subdivided into near IR, middle IR, and far IR spectroscopy.
  • Used for qualitative and quantitative analyses, particularly for functional group identification of organic compounds.
• Fluorophotometry:
  • Measures fluorescence, the re-emission of radiation by certain chemical substances immediately after excitation.
  • The fluorescence intensity is proportional to the concentration of the fluorescent substance.
• Phosphorimetry:
  • Related to phosphorescence, where re-emission of radiation takes longer time (minutes, hours, or days).
• Turbidimetry:
  • Determines the concentrations of suspensions by measuring the opacity of the suspension.
  • Measures the intensity of transmitted light.
• Nephelometry:
  • Measures the intensity of light scattered by a suspension.
  • These methods are used when results need not be very accurate and other precise methods are unavailable.
• Raman Spectroscopy:
  • Involves the scattering of electromagnetic radiation by a liquid (solution) following the Raman effect (scattering with change of wavelength).
  • The shift in wavelength is caused by energy extraction from the incident radiation to raise molecules to higher vibrational states.
  • Raman and infrared techniques are complimentary, concerning vibrational energy change.

1.3.4 Classification of Nuclear Methods

• Nuclear methods provide analytical information based on nuclear properties.
• Types of nuclear methods:
  • Radiochemical Methods
  • Radiometric Methods
  • Isotopic Dilution Methods
  • Activation Analysis
  • Mossbauer Spectroscopy
  • Nuclear Magnetic Resonance Spectroscopy
  • Mass Spectrometry
• Radiochemical Methods:
  • Utilize the measurable activity of radioactive substances for sensitive analytical methods.
  • Classified into radiometric analysis, isotopic dilution methods, and activation analysis.
  • Offer high sensitivity, specificity, and good accuracy.
• Radiometric Methods:
  • Employ a radioactive reagent to separate the analyte completely from the sample.
  • The activity of the isolated material is then measured.
  • Radiometric Titration: a radioactive reagent is used to titrate the analyte, and the endpoint is established by activity measurements.
• Isotopic Dilution Methods:
  • A known amount of the same substance containing an active isotope is added to the unknown and thoroughly mixed.
  • A sample of the pure substance is isolated, and its activity is measured.
• Activation Analysis:
  • Based on measuring radioactivity induced in samples by irradiation with particles like neutrons, protons, deutrons, or helium-3 ions.
  • Neutron Activation Analysis: Uses thermal neutrons from a nuclear reactor.
• Mossbauer Spectroscopy:
  • Studies the phenomenon of the resonance fluorescence of gamma rays, involving intranuclear energy levels.
  • Characterized by the extreme sharpness of lines.
• Nuclear Magnetic Resonance Spectroscopy (NMR):
  • Based on measuring the absorption of electromagnetic radiation in the radiofrequency region by nuclei of atoms in strong magnetic fields.
  • The position of signals in NMR spectra is characterized in terms of chemical shift, which is useful in structure elucidation.
  • Electron Spin Resonance (ESR): An analogous method based on the absorption of microwave radiation by an unpaired electron when exposed to a magnetic field.
• Mass Spectrometry:
  • Converts molecules into charged particles (molecular ions), separates them based on mass-to-charge ratio, and measures the relative intensity of lines of the mass spectrum.
  • Capable of providing qualitative and quantitative information about the atomic or molecular composition of the sample.
  • An important tool for elucidating the structure of organic compounds.

1.3.5 Classification of Thermal Methods of Analysis

• Thermal methods measure a property of the system as a function of temperature.
• Temperature can be an independent or dependent variable.
• Recorded curves help interpret the thermal behavior of the sample.
• Commonly used methods:
  • Thermogravimetric Analysis (TGA)
  • Derivative Thermogravimetry (DTG)
  • Differential Thermal Analysis (DTA)
  • Differential Scanning Calorimetry (DSC)
  • Thermometric Enthalpy Titrations (TET)
• Thermogravimetric Analysis (TGA):
  • Involves measuring the mass of a sample as its temperature is increased at a linear rate.
  • A solid sample undergoes a reaction that causes a loss of mass due to gas evolution:
    $Reactant(s) \rightarrow Product(s) + Gas \uparrow$
  • A plot of mass versus temperature (thermogram) determines thermal stabilities and sample compositions at different temperatures.
• Derivative Thermogravimetric Analysis (DTGA):
  • Plots the first derivative of the thermogram (dw/dT versus T).
  • Weight changes are observed as maxima or minima, making them clearer than in thermograms.
• Differential Thermal Analysis (DTA):
  • Measures the temperature difference between a sample and a reference as a function of temperature.
  • A plot of $\Delta T$ versus T determines the transition temperature and the nature of the change (exothermic or endothermic).
• Differential Scanning Calorimetry (DSC):
  • The sample and reference material are subjected to a precisely programmed temperature change.
  • When a thermal transition occurs, the temperature change is balanced by adding thermal energy to either the sample or the reference.
  • DSC directly measures both the temperature and enthalpy of a transition.
• Thermometric Enthalpy Titrations (TET):
  • Follow a titration process by measuring the enthalpy change during the course of a reaction carried out under controlled conditions.

1.3.6 Classification of Separation Methods

• Separation is necessary to isolate the analyte or remove interfering substances.
• Separations involve bringing the desired constituent into one phase and interfering elements into another, separating the phases by physical processes.
• Methods of separation:
  • Classical Methods:
    • Precipitation
    • Distillation
    • Sublimation
    • Formation of complexes
  • Modern Methods:
    • Chromatography
    • Solvent Extraction
    • Ion-Exchange
    • Electrophoresis
• Chromatography:
  • A multistage separation process where the sample is applied to a stationary phase and a mobile phase is percolated over it.
  • Solutes are separated based on differential migration.
  • Classified based on the nature of stationary and mobile phases and the mechanism of distribution:
    • Paper chromatography, thin-layer chromatography, liquid chromatography, high-performance liquid chromatography, gas chromatography, gel chromatography, partition chromatography, adsorption chromatography, ion exchange chromatography, electrochromatography, etc.
• Solvent Extraction:
  • A solute is isolated by distributing it between two immiscible liquids, exploiting the differential solubility in the solvents.
  • Can be applied as a single-stage or multistage procedure (counter-current extraction).
• Ion Exchange:
  • A stoichiometric process where a solid ion exchanger exchanges ions with an electrolyte solution.
  • Cation exchangers exchange cations, and anion exchangers exchange anions.
• Electrophoresis:
  • The movement of charged particles under the influence of an electric field.
  • Components of a mixture with different velocities can be separated.
  • Used for separating polysaccharides, nucleic acids, haemoglobins, and other high molecular weight compounds.

1.4 Criteria for Evaluating the Utility of Analytical Techniques

• Analytical chemistry is crucial in science and industry for detection, determination, separation, and structure elucidation.
• Analytical techniques provide valuable information across various branches of science and technology.
• Fundamental laws and mechanisms are developed based on analysis results.
• Kinetic studies employ quantitative measurements to understand reaction mechanisms.
• Quantitative analysis involves measurements related to the amount of sample and the amount of the desired constituent.
• Methods are named macro, meso, micro, and ultramicro based on the sample amount.
• Results are expressed in units such as percent, parts per million, or parts per billion.

Steps in Quantitative Analyses:

• Sampling
• Dissolution of the sample
• Separation of interfering substances
• Measurement
• Interpretation of the measurements

1.4.1 Sampling

• Sampling involves carefully handling a representative sample without losses or contamination.
• Conclusions about the bulk material are drawn from the analysis of a small portion.
• Knowledge of statistics aids in establishing sampling programs.
• Sampling techniques vary based on the material's specific characteristics and purpose.

1.4.2 Dissolution of the Sample

• Analyses are often performed on solutions of the sample.
• A suitable solvent is required to dissolve the sample rapidly without analyte loss.
• Inorganic samples are dissolved by:
  • Treatment with hydrochloric acid, nitric acid, mixtures of hydrochloric and nitric acids, sulphuric acid, or perchloric acid.
  • Fusion with an acidic or basic flux, followed by treatment with water or an acid.
• Organic solvents are used to dissolve organic samples.

1.4.3 Separation of Interfering Substances

• Interfering substances prevent the direct measurement of the species being determined.

• Separation can be achieved by:

  • Isolating the desired constituent in a measurable form.
  • Removing the interfering substances from the desired constituent.

• Separation techniques involve transferring the substance of interest from one phase to another:

  • Solid-liquid
  • Liquid-liquid
  • Solid-gas
  • Liquid-gas

1.4.4 Measurement

• Measurement depends on the analytical technique used.
• Gravimetric methods measure the weight of the analyte.
• Volumetric methods measure the volume of a solution of known concentration required to react with the analyte.
• Instrumental methods require calibration using a standard containing a known amount of the constituent.

Criteria for Method Selection:

• Complexity of the materials to be analyzed
• Probable concentration of the species of interest
• Accuracy
• Sensitivity and detection limit
• Selectivity
• Duration of an analysis
• Cost of equipment

1.4.5 Interpretation of the Measurement

• Results are reported in relative terms, such as percent, parts per million, or other suitable units.
• Statistical methods are commonly used.
• Analytical results are reliable only if all conditions are strictly obeyed.

1.5 Emerging Needs and Recent Trends

• Analytical chemistry is essential for identifying, determining, and separating substances.
• It is used in structure elucidation, reaction mechanisms, industrial testing, quality control, and diverse scientific fields.
• Analytical chemistry plays a fundamental and applied role in science and technology.

Recent Trends & Examples:

• Determination at parts per million or billion levels is increasingly required.
• Examples include determining impurities in semiconductor materials, pesticide residues in food, pollutants in the environment, analyzing blood samples, and analyzing protein molecules.

Historical Perspective:

• Antoine Lavoisier is considered the "father of analytical chemistry" for his quantitative analysis of mass conservation.
• Gravimetric analysis was developed in the seventeenth century.
• Volumetric analysis (titrimetry) allows saving time through the volumetric measurement of standardized solutions.
• Classical methods (gravimetry and volumetry) are simple and require no prior calibration or expensive equipment, and are still valuable for major constituents.

Modern Methods:

• Developed mainly in the last century.
• Classified into non-instrumental and instrumental methods.

Non-instrumental Methods:

• Primarily separation methods such as chromatography, ion exchange, electrophoresis, and solvent extraction.

Instrumental Methods:

• More sophisticated, based on physics and physical chemistry principles.
• Reduce analysis duration and can be applied to trace analysis.

Instrumental Method Drivers:

• The physical properties are on which these methods are based were not known earlier.
• Sophisticated instrumentation components are required and costly, such as vacuum-tube amplifiers, photoelectric tubes, photomultiplier tubes, transistors, and semiconductor devices.

Recent Trends:

• Focus on faster, more convenient, sensitive, and selective determination techniques.
• Automated analytical systems provide ready-made data with minimum operator effort.
• Computer techniques are applied for data processing and automated instrument control.

Three Major Contributions to Analytical Chemistry:

• The flow of theory from physical chemistry and physics into analytical chemistry.
• Application of electronics and development, by analytical chemists, for assembling new, faster and more sensitive instruments.
• Application of computers for both data processing and automated control of instruments.

1.6 Summary

• Analytical chemistry involves the detection, determination, and separation of substances.
• Methods developed early (gravimetry and volumetry) are classical methods, while those developed later are modern methods.
• Analytical methods are classified as chemical, electrical, optical, nuclear, thermal, and separation methods.
• Recent trends focus on faster, more convenient, and sensitive determination techniques, with automated systems and computer-aided analysis.