🔆Environmental Chemistry I Unit 4 – Aquatic Chemistry: Water Properties

Key Concepts

• Water is a unique and essential compound with special properties that make it crucial for life on Earth
• Aquatic systems involve complex chemical equilibria influenced by factors such as pH, temperature, and dissolved gases
• pH is a measure of the acidity or basicity of a solution determined by the concentration of hydrogen ions (H+)
• Alkalinity refers to the capacity of water to neutralize acids and maintain a stable pH
• Dissolved gases, such as oxygen and carbon dioxide, play important roles in aquatic ecosystems and redox reactions
• Aquatic pollutants and contaminants can have significant environmental impacts on water quality and aquatic life
• Understanding the chemical properties and interactions in aquatic systems is essential for addressing environmental issues and developing solutions
• Lab techniques and analysis methods are used to measure and monitor water quality parameters and contaminants

Water's Unique Properties

• Water has a high specific heat capacity, meaning it can absorb a large amount of heat energy without a significant change in temperature
  • This property helps regulate Earth's climate and maintain relatively stable temperatures in aquatic environments
• Water has a high surface tension due to strong cohesive forces between water molecules
  • This allows water to form droplets, capillary action in plants, and enables insects to walk on water's surface
• Water is an excellent solvent, able to dissolve a wide range of substances (polar and ionic compounds)
  • This property is crucial for the transport of nutrients and waste products in living organisms and aquatic ecosystems
• Water has a high boiling point and low freezing point compared to other similar-sized molecules
  • These properties are due to the strong hydrogen bonds between water molecules, which require more energy to overcome
• Water's density is highest at 4°C, and ice is less dense than liquid water
  • This unique density behavior allows aquatic life to survive in frozen water bodies, as ice floats on top while liquid water remains below

Chemical Equilibria in Aquatic Systems

• Chemical equilibria in aquatic systems involve dynamic processes where forward and reverse reactions occur at equal rates
• The equilibrium constant (K) describes the ratio of products to reactants at equilibrium and depends on temperature
• Le Chatelier's principle states that a system in equilibrium will shift to counteract any disturbance and re-establish equilibrium
  • For example, adding a reactant or product, changing pressure, or altering temperature can shift the equilibrium position
• Solubility equilibria govern the dissolution and precipitation of substances in water
  • Solubility product constant (Ksp) determines the maximum concentration of ions that can remain in solution before precipitation occurs
• Acid-base equilibria involve the transfer of protons (H+) between species in water
  • The dissociation constants (Ka and Kb) describe the strength of acids and bases, respectively
• Complexation equilibria occur when metal ions form stable complexes with ligands (ions or molecules that bind to metal ions)
  • The formation constants (Kf) indicate the stability of the complexes formed

pH and Alkalinity

• pH is a logarithmic scale that measures the concentration of hydrogen ions (H+) in a solution
  • pH values range from 0 to 14, with 7 being neutral, below 7 acidic, and above 7 basic
• The pH scale is defined as: $pH = -log[H+]$, where $[H+]$ is the concentration of hydrogen ions in moles per liter (M)
• Alkalinity is a measure of the capacity of water to neutralize acids without significant changes in pH
  • It is primarily determined by the concentration of bicarbonate (HCO3-), carbonate (CO32-), and hydroxide (OH-) ions
• Alkalinity acts as a buffer system, resisting changes in pH when acids or bases are added to the water
  • Carbonate and bicarbonate ions can react with added H+ to form carbonic acid (H2CO3), which helps maintain a stable pH
• Total alkalinity (TA) is the sum of all titratable bases in water and is expressed in milligrams per liter (mg/L) of calcium carbonate (CaCO3)
• Measuring alkalinity involves titrating a water sample with a strong acid until a specific pH endpoint is reached (usually pH 4.5)

Dissolved Gases and Redox Reactions

• Dissolved gases, such as oxygen (O2), carbon dioxide (CO2), and nitrogen (N2), are important components of aquatic systems
• The solubility of gases in water depends on factors such as temperature, pressure, and salinity (Henry's law)
  • Higher temperatures and salinity decrease gas solubility, while higher pressure increases solubility
• Dissolved oxygen is crucial for aquatic life and is influenced by photosynthesis, respiration, and decomposition processes
  • Oxygen saturation levels vary with temperature and can be measured using a dissolved oxygen meter or Winkler titration method
• Redox (reduction-oxidation) reactions involve the transfer of electrons between species, changing their oxidation states
  • Oxidation is the loss of electrons, while reduction is the gain of electrons
• Redox potential (Eh) measures the tendency of a species to acquire or lose electrons, with higher Eh values indicating a more oxidizing environment
• Dissolved gases participate in redox reactions, such as the oxidation of organic matter by oxygen or the reduction of nitrate (NO3-) to nitrogen gas (N2) in denitrification

Aquatic Pollutants and Contaminants

• Aquatic pollutants and contaminants can have detrimental effects on water quality and aquatic ecosystems
• Nutrients, such as nitrogen and phosphorus, can lead to eutrophication when present in excess
  • Eutrophication causes algal blooms, oxygen depletion, and fish kills due to the rapid growth and decomposition of algae
• Heavy metals (lead, mercury, cadmium) can accumulate in aquatic organisms and cause toxicity
  • Bioaccumulation occurs when contaminants are transferred up the food chain, with higher concentrations in top predators
• Organic pollutants, such as pesticides, pharmaceuticals, and personal care products, can persist in the environment and have endocrine-disrupting effects
• Microplastics, tiny plastic particles less than 5 mm in size, can be ingested by aquatic organisms and cause physical and chemical harm
• Thermal pollution, caused by the release of heated water from power plants or industrial processes, can alter aquatic ecosystems and affect species distribution

Environmental Impact and Applications

• Understanding the chemical properties and interactions in aquatic systems is essential for assessing and mitigating environmental impacts
• Water quality monitoring programs use chemical, physical, and biological indicators to evaluate the health of aquatic ecosystems
  • Parameters such as pH, dissolved oxygen, nutrients, and contaminant levels are regularly measured and compared to established standards
• Wastewater treatment processes rely on chemical principles to remove pollutants and pathogens before discharging treated water into the environment
  • Primary treatment involves physical separation of solids, while secondary treatment uses biological processes to degrade organic matter
• Drinking water treatment employs chemical methods to ensure the safety and quality of water for human consumption
  • Coagulation, flocculation, sedimentation, filtration, and disinfection steps are used to remove contaminants and inactivate pathogens
• Remediation techniques, such as bioremediation and phytoremediation, use chemical and biological processes to clean up contaminated sites
  • Microorganisms or plants are used to degrade, immobilize, or extract pollutants from soil and water
• Environmental regulations and policies, such as the Clean Water Act and the Safe Drinking Water Act, set standards and guidelines to protect and restore aquatic resources

Lab Techniques and Analysis

• Various lab techniques and analysis methods are used to measure and monitor water quality parameters and contaminants
• Titration is a common technique used to determine the concentration of a substance in solution by reacting it with a known concentration of another substance
  • Acid-base titrations (pH and alkalinity) and redox titrations (dissolved oxygen) are examples of titration methods used in aquatic chemistry
• Spectrophotometry involves measuring the absorption or emission of light by a sample at specific wavelengths to determine the concentration of a substance
  • UV-Vis spectrophotometry is used to measure nutrients (nitrate, phosphate) and organic compounds in water samples
• Chromatography techniques, such as gas chromatography (GC) and high-performance liquid chromatography (HPLC), separate and identify individual components in a mixture
  • These methods are used to detect and quantify organic pollutants, such as pesticides and pharmaceuticals, in water samples
• Atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectrometry (ICP-MS) are used to measure trace metals in water samples
  • These techniques provide high sensitivity and selectivity for detecting and quantifying metal contaminants
• Microbiological methods, such as membrane filtration and multiple-tube fermentation, are used to detect and enumerate indicator bacteria (coliforms) in water samples
  • These tests assess the microbiological quality of water and indicate the potential presence of pathogens