![pH & Buffers
pH is a measure of the acidity or basicity of a solution. It indicates
the concentration of hydrogen ions [H ] or [H O ] in the solution
⁺ ₃ ⁺
and is expressed on a scale from 0 to 14
The pH is mathematically related to the concentration of hydrogen
ions as:
pH = −log[H ], where [H
⁺ ] is the concentration of hydrogen
⁺
ions in moles per liter
A Buffer is a solution that resists changes in pH when small
amounts of an acid or a base are added.
Buffers are typically made up of a weak acid and its conjugate base
(or a weak base and its conjugate acid)
Examples of buffer systems
• Acetic Acid / Acetate Buffer: Made from acetic acid
(CH COOH) and its conjugate base acetate (CH COO ).
₃ ₃ ⁻
• Carbonic Acid / Bicarbonate Buffer: Involves carbonic acid
(H CO ) and bicarbonate (HCO ). This is the primary buffering
₂ ₃ ₃⁻
system in human blood and helps regulate blood pH](https://image.slidesharecdn.com/basicsofforensicchemistry-250326101553-aaa87f20/85/Basics-of-forensic-chemical-science-pptx-14-320.jpg)
![Parameter/Property Petrol/Gasoline Diesel Kerosene
Density 710 – 770 kg/m³ at 15°C 820 – 870 kg/m³ at 15°C 0.78 – 0.82 g/cm³ at 15°C
Color (Varies from place
to place due to dye added)
Orange (dye - Phenyl azo
2- naphthol)
Yellow Colorless
Flash point < -21°C 35°C to 40°C 37°C to 65°C
Boiling point 25 to 75°C 250 to 350°C 190 to 250°C
TLC Solvent System
(Hexane: Toluene: Acetic
Acid [ 50 : 50 : 2])
Pink or Orange color (Rf
Value 0.49 & 0.51)
Violet Blue color spot (Rf
around 0.4)
Filter Paper Test (Place
two drops of fuel on a
filter paper)
Vanish without leaving
any trace behind
Leave patches Leave patches
Ultra Violet Lamp Chloranil spray reagent:
brick red
Green/Yellow Blue
Cetane number 5 – 20 40 – 55 NA
Octane number 90 – 92 15 – 25 NA](https://image.slidesharecdn.com/basicsofforensicchemistry-250326101553-aaa87f20/85/Basics-of-forensic-chemical-science-pptx-27-320.jpg)
Basics of forensic chemical science.pptx
- 1. UNIT 1: INTRODUCTION TO FORENSIC CHEMISTRY BY: KWASI KANTANKA SAFO
- 2. Forensic Chemistry Definition • Forensic chemistry involves the application of chemical principles and techniques to analyze evidence in criminal investigations. • It involves the identification, analysis, and interpretation of substances found at crime scenes, which can include drugs, toxins, explosives, biological materials, and other chemical compounds. • The main goal is to provide scientific evidence that can be used in court to support investigations and legal proceedings.
- 3. Cont’d Role • Collection and Preservation of Evidence: Forensic chemists are involved in ensuring that chemical evidence is properly collected, handled, and preserved to avoid contamination and maintain the integrity of the sample. • Analysis and Identification: Forensic chemists perform rigorous chemical analyses to identify substances and determine their significance in a criminal case. • Providing Expert Testimony: Forensic chemists may be called to testify in court as expert witnesses, explaining their findings and the methods used to arrive at their conclusions. • Maintaining Chain of Custody: A crucial aspect of forensic chemistry is maintaining the chain of custody, which ensures that evidence is tracked from collection through analysis to presentation in court, preventing tampering or contamination. nown Author is licensed under CC BY-SA
- 4. Basic Concepts of Chemistry
- 5. Periodic Trends Atomic Radius • Refers to the distance from the nucleus of an atom to the outermost electron shell. It is a measure of the size of an atom and is typically expressed in picometers (pm) or angstroms (Å), where 1 Å = 100 pm. Trends • Down a Group: Atomic radius increases • Across a Period: Atomic radius decreases Factors affecting Atomic radius • Number of electron shells: electron shells increases down a group, causing atomic size/radius to increase • Nuclear charge (Number of protons): Across a period (form left to right) the number of protons in the nucleus increases, which results in a greater positive charge causing a decrease in atomic radius because electrons are pulled closer to the nucleus • Shielding effect: repulsion of outer electrons by inner
- 6. Cont’d Ionization Energy • Refers to the amount of energy required to remove an electron from a neutral atom or molecule in its gas phase. it provides insight into how tightly an atom holds onto its electrons, which in turn influences the atom's reactivity and chemical behavior. Trends • Across a period: Ionization energy increases • Down a group: Ionization energy decreases Factors affecting Ionization Energy • Atomic radius: Smaller atoms have higher ionization energy whiles larger atoms have lower ionization energy • Nuclear charge: Higher nuclear charge/more protons increases ionization energy because the nucleus has a stronger attractive force on the electrons • Shielding effect: Atoms with more electron shells experience more shielding, hence lower ionization energy • Electron configuration: Atoms with a more stable electron configuration (such as a full outer shell, like noble gases) require more energy to remove an electron
- 7. Cont’d Electron Affinity • Refers to the amount of energy released when an electron is added to a neutral atom in its gas phase to form a negatively charged ion. It represents the atom's ability to accept an electron, and it provides insight into the atom's tendency to gain electrons and form anions. It is typically expressed in kilojoules per mole (kJ/mol). Trends • Across a period: Electron affinity increases (more negative) • Down a group: Electron affinity decreases (less negative) Factors influencing Electron Affinity • Atomic size: Smaller atoms generally have more negative electron affinities whiles larger atoms have less negative electron affinities • Nuclear charge: Atoms with a higher nuclear charge (more protons) tend to have more negative electron affinities because the increased positive charge attracts the added electron more strongly • Electron configuration: Atoms that are close to having a full valence shell (like halogens) generally have a high, negative electron affinity because gaining an electron brings them closer to a stable configuration
- 8. Bonding & Hybridization . Bonding • Refers to the interaction between atoms or ions that holds them together in a stable configuration • The type of bonding that occurs between atoms depends on their electronegativity, atomic size, and other factors. • There are three main types of bonding: Ionic - the bond formed by the complete transfer of valence electron to attain stability. OR the electrostatic interaction between a metal and a non metal Properties • High melting and boiling points • Generally soluble in water but insoluble in organic solvents • Conducts electricity in an electrolyte
- 9. Cont’d Covalent - Covalent bonds are formed when two atoms share one or more pairs of electrons. This usually occurs between non-metal atoms. Types • Single bond: One pair of electrons is shared between two atoms • Double bond: Two pairs of electrons are shared between two atoms • Triple bond: Three pairs of electrons are shared between two atoms Properties • Low melting and boiling points compared to ionic compounds • Do not conduct electricity in the solid or liquid phase • Often soluble in organic solvents and insoluble in water • Can exist as gases, liquids, or solids
- 10. Cont’d Hydrogen bonding – Occurs when a hydrogen atom is covalently bonded to a highly electronegative atom (Oxygen, Nitrogen, or Fluorine), and this hydrogen atom is attracted to a lone pair of electrons on another electronegative atom (usually Oxygen, Nitrogen, or Fluorine) in a neighboring molecule or the same molecule.. Properties • Hydrogen bonding contributes to the high boiling and melting points of substances like water • It plays a key role in biological molecules, such as the structure of DNA and proteins.
- 11. Cont’d Hybridization - is defined as the process of combining two atomic orbitals to create a new type of hybridized orbitals. The formation of hybrid orbitals with completely different energies, shapes, and so on is frequently the outcome of this intermixing. Hybridization Orbitals Involved Geometry Bond Angles Examples sp 1 s + 1 p Linear 180° BeCl₂ sp² 1 s + 2 p Trigonal planar 120° C₂H₄ (Ethene) sp³ 1 s + 3 p Tetrahedral 109.5° CH₄ (Methane) sp³d 1 s + 3 p + 1 d Trigonal bipyramidal 90° and 120° PCl₅ sp³d² 1 s + 3 p + 2 d Octahedral 90° SF₆
- 12. Electronic Transition Electronic transition refers to the movement of an electron between different energy levels or orbitals in an atom or molecule, typically caused by the absorption or emission of electromagnetic radiation. Types Absorption: In absorption, an electron in an atom or molecule absorbs energy from an external source (like light) and moves to a higher energy state or orbital. Emission: In emission, an electron transitions from a higher energy state to a lower energy state, releasing energy in the form of electromagnetic radiation (such as light).
- 13. Polarity Polarity refers to the distribution of electrical charge over the atoms in a molecule. It arises due to differences in the electronegativity of atoms that form a bond. Consequences of Polarity • Solubility - Polar molecules dissolve in polar solvents due to the attraction between their dipoles whiles nonpolar molecules dissolve in nonpolar solvents • Boiling & Melting points - Polar molecules generally have higher boiling and melting points than nonpolar molecules because the dipole-dipole interactions or hydrogen bonds between polar molecules require more energy to break. • Reactivity - polar molecules are more likely to participate in reactions with other polar molecules, while nonpolar molecules tend to react with nonpolar substances. • Separation – Separation techniques like TLC, Solvent extraction and Electrophoresis take advantage of differences in the polarity of compounds to achieve separation.
- 14. pH & Buffers pH is a measure of the acidity or basicity of a solution. It indicates the concentration of hydrogen ions [H ] or [H O ] in the solution ⁺ ₃ ⁺ and is expressed on a scale from 0 to 14 The pH is mathematically related to the concentration of hydrogen ions as: pH = −log[H ], where [H ⁺ ] is the concentration of hydrogen ⁺ ions in moles per liter A Buffer is a solution that resists changes in pH when small amounts of an acid or a base are added. Buffers are typically made up of a weak acid and its conjugate base (or a weak base and its conjugate acid) Examples of buffer systems • Acetic Acid / Acetate Buffer: Made from acetic acid (CH COOH) and its conjugate base acetate (CH COO ). ₃ ₃ ⁻ • Carbonic Acid / Bicarbonate Buffer: Involves carbonic acid (H CO ) and bicarbonate (HCO ). This is the primary buffering ₂ ₃ ₃⁻ system in human blood and helps regulate blood pH
- 15. Solutions & their Importance A solution is a homogeneous mixture of two or more substances, where one substance (the solute) dissolves in another (the solvent). Solutions in chemistry can be categorized in a number of ways: Based on the amount of solute • Saturated: A solution that cannot dissolve any more solute at a given temperature • Unsaturated: A solution that can dissolve more solute at a given temperature • Supersaturated: A solution that contains more solute than it can dissolve at a given temperature, and the excess solute will crystallize if the temperature is lowered Based on tonicity • Isotonic: A solution with the same concentration of dissolved solutes as the submerged object • Hypotonic: A solution with a lower concentration of dissolved solutes than the submerged object • Hypertonic: A solution with a higher concentration of dissolved solutes than the submerged object.
- 16. Cont’d Based on the state of matter of the solute and solvent • Solid – Solid: Examples include alloys like brass and bronze • Solid – liquid: Examples include solutions of sugar or salt in water • Solid – gas: Examples include substances like iodine or camphor sublimating into the air • Liquid – solid: Examples include hydrated salts or mercury in amalgamated zinc • Liquid – liquid: Examples include hand sanitizer • Gas – gas: Examples include air • Gas – solid: Examples include hydrogen gas and platinum • True solution - Is a homogeneous mixture in which the solute is completely dissolved in the solvent at the molecular or ionic level. • Colloidal solution - Is a type of heterogeneous mixture in which one substance is finely distributed within another substance. • A suspension - Is a type of heterogeneous mixture in which solid particles are dispersed throughout a liquid or gas but are large enough that they will
- 17. dd Basics of mole concept . Mole is a unit used to measure the amount of substance. One mole contains 6.022 × 10²³ (Avogadro’s number) elemental entities, such as atoms, molecules, ions, or other particles Avogadro’s Number is the number of particles in one mole of any substance: 6.022 × 10²³ particles/mol Molar mass is the mass of one mole of a substance. It is typically measured in grams per mole (g/mol). For elements, the molar mass is numerically equal to the atomic mass. For compounds, the molar mass is the sum of the atomic masses of all elements in the compound. Atomic mass/mass number is associated with the number of protons and neutrons in the nucleus of an atom Atomic number is the number of protons in the nucleus of an atom
- 18. Cont’d Example: The molar mass of water (H O) ₂ is: (2 × atomic mass of H) + (atomic mass of O) = 2(1.008 g/mol) + 15.999 g/mol = 18.015 g/mol To find the number of moles (n) in a given sample, you use the formula: n = m M Where; n = number of moles m = mass of the sample (in grams) M = molar mass of the substance (in grams per mole) The number of particles (atoms, molecules, ions) in a sample can be determined by multiplying the number of moles by Avogadro's number: Number of particles = n × NA Where; n = number of moles NA = 6.022 × 10²³ = Avogadro’s number
- 19. Cont’d Molarity, Molality and Normality .
- 20. Cases/Exhibits .
- 21. Forensic chemical analysis Determining the composition of substances involves qualitative analysis (to identify the components) and quantitative analysis (to measure the amount of these components) Some common methods used in forensic chemistry for both qualitative and quantitative analysis are: Qualitative Analysis Methods These methods are used to identify the chemical nature, composition, or characteristics of substances. • Spectroscopic methods eg..; UV-Vis, IR, Raman • Chromatographic methods eg..; TLC, GC, HPLC • Microscopic and Elemental Analysis eg..; SEM, TEM, XRD, EDX • Wet Chemical Tests (Spot Tests) eg..; precipitation test for ions, color tests (Scott test for cocaine, Kastle-Meyer for blood)
- 22. Cont’d Quantitative Analysis Methods These methods measure the exact amounts or concentrations of substances in a sample. • Gravimetric Analysis - measures the mass of a substance after isolation • Titrimetric Analysis eg..; acid-base titration, redox titration • Spectroscopic Quantification eg..; UV-Vis, AAS, Flame photometry • Chromatographic Quantification eg..; GC, HPLC • Mass Spectrometry – measures concentration based on mass-to-charge ratio • Electrochemical methods eg..; Potentiometry, voltammetry
- 23. Bribe Trap Cases A bribe trap case involves setting up a covert operation to catch an individual in the act of accepting a bribe. Law enforcement agencies use scientifically validated techniques, including the application of phenolphthalein powder, to gather irrefutable evidence. Phenolphthalein acts as an indicator that reveals the presence of currency involved in a bribe through a visual chemical reaction. When treated currency is exposed to a basic medium (e.g., sodium carbonate solution), it produces a vivid bright pink color, confirming the transfer of money. Examination of phenolphthalein • Spot test - few drops of a basic solution, and a pink color confirms presence of phenolphthalein • TLC – develop plate in appropriate solvent, visualize by iodine fumes treatment of the developed plate and compare with standards. • UV-Vis Spectroscopy – In methanol, absorption peak of 277nm is observed and in aldehyde-free alcohol absorption peaks of 225 and 285nm were observed • HPLC – retention time is recorded and compared to standard.
- 24. Petroleum and Petroleum products Distillation and fractionation of Petroleum (Crude oil)
- 25. Commercial uses of petroleum fractions
- 26. Analysis of Petroleum Products Parameter Description Density & Specific Gravity Mass per unit volume Color Visual spectrum Flash point Lowest temperature at which vapors ignite Boiling point Determines fuel volatility Cetane number (Diesel) Measures combustion quality of diesel Octane number (Petrol) Anti-knock quality of gasoline Viscosity Resistance to flow
- 27. Parameter/Property Petrol/Gasoline Diesel Kerosene Density 710 – 770 kg/m³ at 15°C 820 – 870 kg/m³ at 15°C 0.78 – 0.82 g/cm³ at 15°C Color (Varies from place to place due to dye added) Orange (dye - Phenyl azo 2- naphthol) Yellow Colorless Flash point < -21°C 35°C to 40°C 37°C to 65°C Boiling point 25 to 75°C 250 to 350°C 190 to 250°C TLC Solvent System (Hexane: Toluene: Acetic Acid [ 50 : 50 : 2]) Pink or Orange color (Rf Value 0.49 & 0.51) Violet Blue color spot (Rf around 0.4) Filter Paper Test (Place two drops of fuel on a filter paper) Vanish without leaving any trace behind Leave patches Leave patches Ultra Violet Lamp Chloranil spray reagent: brick red Green/Yellow Blue Cetane number 5 – 20 40 – 55 NA Octane number 90 – 92 15 – 25 NA