Summary
Highlights
This course focuses on physical analysis methods for chemical systems, meaning methods that do not involve chemical reactions. Examples include pH measurement, conductivity, and using electromagnetic waves like UV-visible and infrared.
pH can be measured using pH paper, color indicators, or a pH meter. pH values typically range from 0 to 14, with 0-7 being acidic, 7 neutral, and 7-14 basic. pH (potential of hydrogen) is defined by the concentration of hydrogen ions (H+) or hydronium ions (H3O+) in water. Mathematically, pH = -log([H3O+]/C0), where C0 is a reference concentration of 1 mol/L. This formula can also be rearranged to [H3O+] = C0 * 10^(-pH).
Conductivity is measured using a conductimeter and is relevant for solutions containing ions. A calibration curve of conductivity versus concentration typically shows a linear relationship, indicating proportionality. Conductivity quantifies how easily electricity passes through a solution, which depends on the presence and mobility of ions. It is the inverse of resistance and is expressed in Siemens.
The conductivity of a solution is determined by the sum of the molar ionic conductivities of each ion multiplied by its concentration. This is known as Kohlrausch's Law. It's important to use concentration in mol/m³ for physical calculations. For NaCl, the conductivity is proportional to the concentration of Na+ and Cl- ions, which are typically equal.
UV-visible spectroscopy involves sending UV or visible light through a solution and measuring the light absorbed. This energy excites electrons to higher electronic levels. The instrument, a spectrophotometer, measures either transmittance (in percentage) or absorbance (unitless). Absorbance is typically preferred for analysis, where peaks indicate absorbed wavelengths. This method can identify the color of a solution by identifying the absorbed color and its complementary color, and can also determine concentration using Beer-Lambert's Law (Absorbance = k * Concentration).
IR spectroscopy uses infrared light, which has less energy than UV-visible light. Instead of exciting electrons, it causes molecular bonds to vibrate. IR spectra are often presented in transmittance and use wavenumber (inverse of wavelength) on the x-axis. Different regions of the spectrum correspond to different types of molecular vibrations. A 'fingerprint' region (below 1500 cm⁻¹) is unique to each molecule, while higher wavenumbers can identify specific functional groups and bond types. A database of known bond vibrations is used to interpret IR spectra.