Day 1 lecture questions Flashcards
What is Ka and pKa
Ka = dissociation constant = ionisation constant
- how much product will be formed relative to reactants
Dissociation of protons from acids and association of protons with bases measured using a dissociation constant (Ka).
[products]/[reactants]
[H+][A-]/[HA]
incr. Ka = decr. in pKa = incr. acidity
pKa is the pH at which 50% of the protons have dissociation (or associated)
for acids: [HA] → [H+] + [A-]
pKa = pH where [H+] = [A-]
incr. pKa = decr. acidity
pH (environment) > pKa = ionised
pKa = -log(base 10)(Ka)
what is Kow
Partition Coefficient between water and “organic phase” (Kow). A measure of hydrophobicity.
Determines distribution of toxins in animal bodies.
The partitioning tendency of a chemical between water and an organic phase
Kow = conc. in octanol phase / conc. in water phase
• Octanol is an organic solvent that is used as a
surrogate for natural organic matter and biological
membranes
incr. Kow = decr. solubility
= incr. bioconcentration factor (lipid solubility)
e. g.
incr. Cl = incr. Kow = incr. bioaccumulation likelihood
incr. aroma (benzine rings) = incr. Kow = decr. water solubility
What is Ksp
the solubility product
decr ksp = decr. solubility
What is the Henderson Hasslebalch equation?
Henderson Hasslebalch equation relates
ionised and non-ionised concentrations to
pH for acids: [HA] → [H+] + [A-]
What is the Bronsted-Lowry definition of acids and bases?
• Acids are compounds which “give up
protons easily
• Bases are proton acceptors
acid: HA → A+ H+
ie HA gives up the proton H+
base: R + H+→ RH+
ie R accepts the proton H+
What are intrinsic properties of toxicants?
Physical properties: • molecular weight/atomic weight, • melting / boiling point, • vapour pressure and • phase at ambient conditions (liquid, solid, gas)
Chemical Properties
• pKa
(for weak acids and bases),
• solubility in water and organic solvents
• octanol/water partition co-efficient Kow
• Half life
What are the properties of the receiving environment?
pH Water Hardness OM (soil, sediments) Clay content Kd (XXXX) Koc (XXX)
Sources/examples of toxicants
“Heavy” Metals – e.g. Cd, Cu, Ni, Pb, Zn
• Metalloids – e.g. As, Se
• Organophosphate pesticides
• Organochlorine pesticides
• Industrial organic compounds – e.g. petrol
• Pharmaceuticals and health-care products
• Organic contaminants:
Hydrocarbons
Pesticides (herbicides and insecticides)
Polychlorinated biphenyls (PCBs)
Polychlorinated dibenzodioxins (PCDDs)
Detergents
Pharmaceuticals
Endocrine disrupting compounds
Geologic weathering
Mining effluent
Industrial effluent
Domestic effluent and stormwater runoff
- lots of zink in tyres - can end up in runoff
Inputs from rural areasA subject swallows benzoic acid (benzine ring with COOH). Stomach environment pH is 2, what form is benzoic acid in and what does that mean for uptake?
Uncharged - still has proton attached, meaning it has hydrophobic properties and can be easily taken up
Explain some features/aspects of soil and water environments that might influence toxicity of a compound.
• Soils – predominantly negatively charged particle surfaces, but some +ve charge surfaces also
• High concentrations of ions in soil solution
• Variable redox status
• Some tropical soils have net +ve charge
• Soil pH varies widely from but usually 4-9
• Organic matter in soils may be uncharged and
therefore strongly attract non-polar contaminants
(e.g. PAHs)
• Rivers, lakes – low concentrations of ions
• Water pH predominantly 6-8
• Normally aerated in surface layers, may be anoxic at depth
• May have significant amounts of suspended
particulate materials (OM, clays)
what governs fate of an element ksp kd kow ph in relation to pka cation exchange capacity/sorption
Nox no observed effect concentration
Ecx
answer
What is the effect of pH on CEC
incr. pH = incr. CEC
What is Kd?
The soil-water partitioning coefficient
> How strongly a chemical interacts with the solid phase
> How likely it is to sorb to soil
- dependent on soil type (CEC)
What does a high Kd indicate?
That a contaminant/chemical/whatever has a high likelihood of being adsorbed
Being sorbed means less mobile, and less bioavailable
What does a low Kd indicate?
That a contaminant/chemical/whatever has a low sorption capacity and is likely to partition into water
Whether a contaminant is more likely to stay in soil or move into water
= incr. mobility
= increased bioavailability
What is the effect of pH on metal solubility
Most metals are soluble at low pH, and not high pH because of the low solubility of hydroxide salts
high pH can also dissolve OM, making metals like Cu more available
Which is toxic, Cr(III) (chromium) or Cr(VI) (chromate)
Chromate
What effect does speciation have on contaminants?
Effects mobility and toxicity.
What does oxidation mean?
Loss of electrons
OIL (oxidisation is loss)
LEORA (loses electrons, oxidised, reducing agent)
What are the acronyms for redox reactions?
OIL RIG (oxidation is loss, reduction is gain (electrons))
GEROA (Gain Electrons, Reduced, Oxidising Agent)
LEORA (Loss Electrons, Oxidised, Reducing Agent)
A change from Fe2+ to Fe3+ - oxidisation or reduction?
Being oxidised
What is the major determinator of the ionisation of chemicals?
pH
