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
What affects availability of a chemical to organisms?
pH Presence of other compounds/solutes Kow Ksp Kd
What affects Kd
Soil type
> clay content
> OM content
> Buffering capacity (CEC and pH)
Give an example of a negatively charged metal
Molybdenum
What happens to pH with increasing CO2 concentration?
Decreases (becomes more acidic)
Explain the difference between accuracy and precision
Precision: tight grouping of results
Accuracy: results closely reflect TRUE value
Want accuracy AND precision
Explain chromatography
The separation of a compound into its constituent parts
Think of the incursion experiment with black textas
> Sample is dissolved in a mobile phase (gas or liquid)
Mobile phase then forced through a stationary phase
What is liquid chromatography used for?
Determination of non-volatile samples that are soluble (in polar or non-polar mobile phases)
What is the accuracy range of liquid chromatography (units)
microg/L —> mg/L
What are the two components of liquid chromatography
> Column separation
> Detection
What is the difference between Quality Assurance and Quality Control?
QA: Makes sure you’re doing the right things the right way (i.e. getting the same results) - precision
QC: Makes sure the results are accurate and representative
What is NOEC
No Observed Effect Concentration
> Highest concentration at which no statistically significant difference to the control
What is LOEC
Lowest Observed Effect Concentration
> Lowest concentration that causes an effect that is statistically significant to the control
What are the 3 main methods of measuring toxicity
> Graphical estimation
> Curve fitting and regression analysis
> Hypothesis testing
- compare means
- requires replicatesWhat is the difference between LC50 and EC50?
LC50 is the concentration at which 50% of a population dies
EC50 is the concentration at which 50% of the population if affected (i.e. doesn’t die, but could have reduced growth or reproduction, etc)
True or false: soils are easier, and generally cost less to remediate than water
False
What are some things that need to be considered before remediating a contaminated site?
> The nature of the contaminant
The concentration of it
- background concentration (in the surrounding environment)
- contamination concentration
T/F: Bacteria have detoxifying organs
False
What is a biomarker?
Biological response to environmental chemicals which give a measure of exposure and sometimes toxicity
Early warning of an adverse effect
e.g. UV exposure to humans - early warning (biomarker) = sunburn
What does someone mean when they refer to PBT? Give an example.
How PERSISTENT, BIOACCUMULATIVE, and TOXIC a chemical is
PBT chemicals are often regulated first
Organochlorines
DDT
What are PCBs, what properties make them particularly bad, and where are they used?
- Polychloronated biphenyls
- very aromatic structures (benzene rings)
- Used in electrical transformers, so if they explode (which happens sometimes), PCBs go everywhere
T/F: polyflurinated substances are persistent in the environment
(explain your response)
TRUE - very, very persistent and mobile in the environment
C-F bond is very strong
e.g. of uses: Teflon, scotchguard, firefighting foam
What are the properties of a toxicant and the receiving environment that determine the toxicant’s fate?
TOXICANT
- Physical properties
- Molecular weight
- Melting/boiling point
- Vapour pressure
- Phase at ambient conditions
- Chemical properties
- Ka
- Ksp
- Kow
- Half life
- Concentration (?)
- Species (?)
RECEIVING ENVIRONMENT
- pH
- Kd
- Koc
- Clay content (soil, sediment)
- OM content (soil, sediment)
- Water hardness
- Proximity to other environment types (?)
- e.g. being close to a waterway might pose a greater risk than being further away
T/F: Things with a charge tend to dissolve in water
True
T/F: Things with no charge tend to be hydrophilic
False. Uncharged chemicals are usually hydrophilic
T/F: Metals often dissolve in water
True.
Able to form cations, and the charge makes them more soluble
T/F: Pesticides generally have a low Kow and are hydrophobic
True and False.
- Pesticides ARE generally hydrophobic, but this means they have a HIGH Kow (i.e. they partition to the octanol phase)
- Helps them get through biological membranes
T/F: Chemicals with a charge are able to pass through biological membranes easier
False.
- Think of water (charged) and how it can’t pass through biological membranes (with the hydrophobic tails) and needs special channels (aquaporins).
- Charges are repelled by the hydrophobic regions of membranes
T/F: Cr and Pb are commonly taken up by plants and are problematic soil contaminants
False.
Cr and Pb generally form highly insoluble hydroxides and phosphates in soil, making their uptake (and therefore toxicity) minimal.
Can use phosphorus to precipitate Pb
What is a method for removing dissolved metals from an acid water
Precipitate them by adding hydroxide (lime) (i.e. incr. pH)
hydroxides are very insoluble (low Ksp
What is the effect of having more Cl atoms in a compound?
incr. Kow (hydrophobicity)
incr. bioaccumulation
T/F: B and Cl are highly insoluble
False. B and Cl have highly soluble salts (e.g. NaCl)
What is the effect of having more benzene rings
Incr. benzene rings = incr. aroma
= incr. Kow (won’t find in water phase)
= incr. bioaccumulation
= incr. potential toxicity
T/F: pH affects the permanent negative charge on soil particles
False. The negative charge gained by isomorphous substitution is unaffected by changes in pH. Soils have both pH dependent and pH independent charge.
T/F: OM has permanent negative charge that is unaffected by pH
False.
OM doesn’t have permanent negative charge, only pH-dependent charge
