Aqueous Solutions: Electrolytes and Nonelectrolytes

Solutions Overview

• Many chemical reactions and biological processes occur in aqueous environments
• Understanding substance properties in water solutions is important
• A solution is a homogenous mixture of two or more substances
• The substance in smaller amount is called the solute
• The substance in larger amount is called the solvent
• Solutions can be gaseous (air), solid (alloy), or liquid (seawater)
• Aqueous solutions are where the solute is liquid or solid, and the solvent is water

Electrolytes vs. Nonelectrolytes

• Solutes in water are categorized as electrolytes or nonelectrolytes
• Electrolytes conduct electricity once dissolved in water
• Nonelectrolytes do not conduct electricity when dissolved in water

Strong Electrolytes and Hydration

• Strong electrolytes fully dissociate into ions in solution
• Dissociation is the breaking of a compound into cations and anions
• Hydration is the process where water molecules surround an ion in solution
• Hydration stabilizes ions and prevents cation/anion recombination

Solution Formation Steps

• First, if the solute is solid or liquid, its molecular units must be dispersed, requiring energy
• Introducing the solute into the solvent can be favorable or unfavorable
• It depends on the solute/solvent nature and intermolecular forces
• More energy released in step 2 than consumed in step 1 favors solution formation and solubility
• Even endothermic dissolution can be favored due to increased entropy

Two-Step Dissolving Process

• Dissolving solids in liquids is a two-step process
• The solid crystal lattice breaks down in an endothermic process
• Individual particles are surrounded by solvent molecules in an exothermic process
• The overall process is endothermic or exothermic based on the net balance

Colligative Properties of Solutions

• Colligative properties depend on solute molecule/ion concentration, not solute identity
• Colligative properties include: vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure
• Adding salt to water lowers the freezing temperature, increases the boiling temperature, lowers vapor pressure and changes osmotic pressure
• Adding alcohol to water lowers the freezing point below that of pure water or pure alcohol

Lowering of Vapor Pressure

• In pure solvent, the surface is occupied by solvent molecules
• Adding non-volatile solute adds solute/solvent molecules, reducing solvent molecule surface fraction
• The vapor pressure of the solution is lower at the same temperature
• P0 is pure solvent vapor pressure
• Ps is solution vapor pressure
• P0 - Ps is the lowering in vapor pressure
• (P0 - Ps) / P0 is the relative lowering of vapor pressure
• Raoult in 1886 related relative lowering of vapor pressure to mole fraction
• Raoult's law states that relative lowering in vapor pressure of a dilute solution equals the solute's mole fraction
• For n moles of solute and N moles of solvent: P0 - Ps / P0 = n / (n + N)

Elevation in Boiling Point

• Boiling point is the temperature at which vapor pressure equals atmospheric pressure
• Adding non-volatile liquid to pure solvent lowers the vapor pressure
• Increasing the temperature of the solution required to equalize vapor pressure to atmospheric pressure
• The difference between the boiling point of the solution and the pure solvent is elevation in boiling point

Depression in Freezing Point

• Freezing point is the temperature at which the vapor pressure of a liquid equals the vapor pressure of corresponding solid
• Raoult's law states that adding non-volatile solid to solvent lowers its vapor pressure
• Solution's vapor pressure equals solid solvent’s at lower temperature
• The difference between the freezing point of the pure solvent and its solution is depression in freezing point.

Equations for Boiling Point Elevation and Freezing Point Depression

• The equation ∆T = Kbb, where Kb is the ebullioscopic constant gives the elevation of boiling point
• The equation ∆T = Kfb, where Kf is the cryoscopic constant gives the depression of freezing point

Osmosis and Osmotic Pressure

• Semipermeable membrane allows solvent molecules to enter solution, increasing volume
• It does not allow bigger molecules like solute to pass
• Osmosis is the spontaneous flow of solvent molecules through a semipermeable membrane
• It can go from pure solvent into a solution or from a dilute to a concentrated solution
• Applying extra pressure from the solution side can stop solvent molecule flow
• Osmotic pressure of the solution is the pressure that stops the solvent flow
• Osmotic pressure is colligative, depending on the amount of solute, not the nature of the solute
• Osmotic pressure (TT) is proportional to molarity (C) and temperature (T), experimentally

Isotonic, Hypotonic, and Hypertonic Solutions

• Isotonic solutions are two solutions with equal osmotic pressure at a given temperature; no osmosis occurs
• Hypotonic solution is that which has lower osmotic pressure, solute concentration is less than surrounding
• If separated by a semipermeable membrane, a hypotonic solution causes water to move out
• Hypertonic solution is that which has higher osmotic pressure, solute concentration is more than surrounding
• If separated by semipermeable membrane, a hypertonic solution causes water to move into hypertonic

Examples and Properties of Isotonic Solutions

• Common examples of isotonic solutions include: 0.9% normal saline and lactated ringers, useful for fluid loss
• Isotonic solutions have 250 to 375 mOsm/L osmolality
• They remain in extracellular compartment and distribute between intravascular and interstitial spaces
• Blood is isotonic
• Tap water and pure water are hypotonic

Gas Solubility in Liquids

• Gas solubility in liquids decreases with increasing temperature
• Heating provides thermal energy to overcomes attraction between gas and solvent decreasing gas solubility gas
• Four major factors affect gas solubility in liquids: pressure, temperature, size, and chemical reactivity
• Increasing pressure increases the gas's solubility in a liquid

Impacts on Biology: Gas Solubility and Breathing

• We inhale ≈500 cm3 of air per breath, which changes lung volume
• Diaphragm depression and chest expansion decreases pressure by ≈100 Pa versus atmosphere
• Expiration occurs as diaphragm rises and chest contracts, increasing pressure by ≈100 Pa above atmosphere
• Total air in lungs ≈ 6 dm3
• Additional air exhaled forcefully after normal expiration ≈ 1.5 dm3
• The lungs have some air at all times to prevent alveoli collapse.
• Gas exchange affects air composition in lungs during breathing
• Alveolar gas is the mixture of newly inhaled and air exhaled
• Oxygen concentration in arterial blood ≈ 40 Torr (5.3 kPa)
• Freshly inhaled air concentration ≈ 104 Torr (13.9 kPa)
• Capillary arterial blood is in an alveolus for ≈ 0.75 s where high pressure gradient fully saturates blood in ≈ 0.25 s
• Fluid collection in the lungs (pneumonia) thickens the respiratory membrane, slowing diffusion
• The body tissues then suffer oxygen starvation
• Carbon dioxide travels in the opposite direction across the respiratory

More impacts on Biology: Gas Solubility, Breathing and Hyperbaric Oxygen

• The tissue partial pressure gradient is less, ≈ 5 Torr (0.7 kPa) in blood and 40 Torr (5.3 kPa) in air atequilibrium
• Carbon dioxide is more soluble in alveolar fluid than oxygen, therefore equal amounts of both are exchanged per breath
• Hyperbaric oxygen chambers with elevated oxygen pressures are used in certain types of disease
• Carbon monoxide poisoning treatments, shock consequences, anaerobic bacteria disease treatment (gas gangrene, tetanus)
• Scuba diving supplies air at higher pressure matching surrounding water pressure
• Pressure increases by ≈ 1 atm per 10 m descent
• Breathing air at high pressure causes nitrogen becomes very soluble in fatty tissue
• It also dissolves dissolves into the central nervous system, bone marrow, and fat reserves producing nitrogen narcosis
• Rapid ascent from scuba diving results in nitrogen leaving lipid solution as bubbles causing: the bends (painful/fatal), arterial embolisms (obstructions) and loss of consciousness

Colloids Defined

• Colloid mixtures have properties between solutions/suspensions
• Mixtures contain particles 1-1000 nm in diameter evenly distributed throughout the solution
• These are known as colloidal dispersions
• The substance being dispersed is in the dispersed phase
• The substance in which it is dispersed is in the continuous phase.
• Colloid classification determined by substance dimensions of more than a molecule but still seen with naked eye
• Quantified at 1-1000 nm and smaller substances are identified as solutions, and larger ones are suspensions.

Types of Colloids

• Colloids are classified by phase of dispersed substance and dispersed medium
• Types include sol, emulsion, foam, aerosol
• Sol is a colloidal suspension with solid particles in liquid
• Emulsion is colloids b/tween two liquid
• Foam is many gas particles trapped in liquid or solid
• Aerosol contains liquid or solid particles dispersed in gas

General Methods of Preparation of Colloids

• Lyophilic/Lyophobic colloidal solutions prepared differently
• Lyophilic colloids have dispersed phase particles with affinity for the dispersion medium and can be formed by mixing
• They are reversible and can be precipitated and directly converted back

Methods of Preparation of Lyophobic Colloids

• Lyophobic colloids can be prepared by two method types
• Condensation method: Smaller dispersed phase particles are condensed to colloid size through these methods:
  • Oxidation: colloidal sulphur solution through bubbling oxygen with hydrogen sulphide solution
  • Reduction: metals (silver, gold, platinum) obtained by treating aqueous salt solution with formaldehyde, phenyl hydrazine, hydrogen peroxide, or stannous chloride
  • Hydrolysis: boiling dilute salt solutions rapidly hydrolyze
  • Double decomposition: arsenic sulphide sol from hydrogen sulphide in cold arsenious oxide in water
• Excessive cooling: colloidal solution of ice in organic solvent by freezing water solution
• Exchange of solvent : colloidal solution of substance by pouring alcoholic solution in excess of water
• Change of physical state: Sols of mercury, sulphur by passing their vapor through a cold water and stabilizer

Purification of Colloidal Solutions

• Colloidal solutions prepared contain impurities
• Impurities can be separated through : dialysis and ultrafiltration
• Freshly prepared colloidal solutions contain electrolyte impurities
• Electrolytes stabilize sols in small concentrations but destabilize at a high concentration
• Removing electrolyte impurities is necessary
• Following methods are used to remove impurities:

Dialysis Process

• Animal membranes pass crystalloids and retain larger colloidal particles
• This property is used to purify sols
• In separating impurities, particles of true solution are diffused through a suitable membrane
• Dialysis does this with suitable membrane such as parchment paper or cellophane
• The dialyzer has a bag of parchment/cellophane where impure sol is purified in a tank with pure water
• Electrolyte impurities present in the sol diffuse out of the bag
• Electrolytes leave a pure sol behind

Electrodialysis

• Dialysis is a slow process, electrodialysis uses and electric field to quicken purification process
• The impurity ions move quickly to oppositely charged electrodes
• Electrolytes get quickened and process is hastened

Ultrafiltration

• Ordinary filter paper has large pores and removes impurities and colloidal particles
• Reducing the pore size of filter paper is possible so impurity of electrolyte from sol
• Treating ordinary filter paper with collodion/gelatin hardens it with formaldehyde
• Treatment reduces pore size and checks by colloidal particle passage through filter paper
• Filter papers obtained this way are called ultrafilters
• Ultrafiltration occurs with ultrafilter supported over wire mesh where sol is poured
• Impurity particles (electrolytes) pass through ultrafilter to retain by larger colloidal particle
• The process is slow, but applying pressure or a suction pump from the filtration quickens the process
• Removing impurities from different size allows separate colloidal particles from one another

Ultracentrifugation

• Ultracentrifugation separates colloidal particles from impurities by centrifugal force
• Impure sol is placed into a tube in an ultracentrifuge
• High rotation speeds causes colloidal particles to settle at tube bottom
• Impurities remain in centrifugate solution
• Settled colloidal particles are removed, mixed with the dispersing medium so pure solution solution is obtained