Chem. 250 – 12/02 Lecture

Announcements - IA. Solutions to Furlough Questions and

Homework will be posted soonB. New Homework Set (Ch. 13: 1, 2, 5,

12, 19, 23, 27, 33, 34)C. Topics

1. Water Composition (finish up)2. Complexation3. Water Pollution Problems4. Toxic Metals

Water ChemistryA Left-Over Problem

1. A water sample has a measured alkalinity of 0.4 mM and a pH of 6.7. Determine the concentration of [OH-], [HCO3

-], [CO32-], and [CO2].

Water Chemistry Chemical Reactions

Solubility of other metals:- Na and K are soluble but usually not present in soils at high concentrations.- Many other metals that are present in soils in reasonable concentrations (e.g. Al, Fe) are not very soluble at “normal” pH values.Example: Al(OH)3(s) ↔ Al3+ + 3OH-

Ksp = [Al3+][OH-]3 = 10-33

At pH = 8, [Al3+] = 10-15 M, at pH = 6, [Al3+] = 10-9 M


• Complexation reactions:– Many metals exist in water with total

dissolved concentrations greater than equilibrium. This can be cause by formation of metal-ligand complexes.

– Examples (for Al3+)Al3+ + 4OH- ↔ Al(OH)4

- K = 2.0 x 1033

Al3+ + 3C2O42- ↔ Al(C2O4)3

3- K = 4.0 x 1015


Aluminum Species

1.00E-20

1.00E-18

1.00E-16

1.00E-14

1.00E-12

1.00E-10

1.00E-08

1.00E-06

1.00E-04

1.00E-02

1.00E+00

1.00E+02

3 5 7 9 11 13 15

pH

log

conc

.

OH-Al3+[Al(OH)4]-AlOH2+[Al(OH)2]+Al(OH)3(aq)Total Al


• Redox Reactions:– Under conditions with plentiful oxygen, most

elements are present in oxidized formsexamples: SO4

2-, CO32-, colloidal Fe(OH)3, NO3

- (most oxidized forms)

– Under conditions with depletion of oxygen, decomposition products tend to be in reduced forms:examples: FeS, CH4, NH3

Water Chemistry Water Pollution Issues

Acid Rain- Man is a source of both acidic precursor gases (SO2 and NOX) and basic precursors (NH3 and soil dust)- In regions downwind of large SO2 and NOX sources (industrial areas), acidic rain is expected from oxidation products (H2SO4 and HNO3)- Besides acid rain, harm can come from acidic snow and fog


Acid Rain – continuedDamage from acid rain depends on:

- amount of excess acid (over natural)- regional extent of pollutions- ability of soils to buffer acid

Carbonate containing soils tend to reduce damage from acid rain while granite or basaltic soils are carbonate poor and suffer more damage from acidification


Acid Rain – continuedIn poorly buffered soils, pH drop is also accompanied by increased solubility of metals.In pH = 5, [Al3+] = 10-6 M => toxic effects start (e.g. precipitation of Al(OH)3 in fish gills)


Acidic Mine RunoffIn many mining regions, rock is rich in sulfides (e.g. FeS2)As this rock is exposed, the sulfides can be oxidized (e.g. 2FeS2 + 7O2 + 2H2O ↔ 2Fe2+ + 4SO4

2- + 4H+)Where sulfides are high enough in concentration, water runoff can have a pH less than zeroBoth the low pH and dissolution of heavier metals lead to increased toxicity


Oxygen DepletionDepletion of oxygen in the ocean and lakes (usually seasonally) is a natural processPollution is a larger problem in bodies of water that are thermally stratified (it is harder to restore oxygen levels)Water depleted of oxygen will support fewer fish species, and the chemistry changes in anoxic waters


Oxygen Depletion – cont.Pollution can deplete oxygen content in the

following ways1) Thermal pollution (e.g. power plant outflow)2) Organic compounds that consume oxygen3) Nutrients (e.g. N or P containing fertilizers) which promote algae growth (but lead to oxygen consumption when algae peak)


Oxygen Depletion – cont.The ability of polluted water to consume

oxygen can be measured through two tests (biological oxygen demand or BOD and chemical oxygen demand or COD).

This will depend on the concentrations of organic carbon and the forms present.


Sewage Problems:Raw sewage is a big source of oxygen demand

Sewage Treatment:- Primary Treatment: removal of most suspended organic material (physically based)- Secondary Treatment: decomposition of remaining organic compounds (biologically based)- Tertiary Treatment: removal of other chemicals (nitrates and phosphates) plus disinfection (most common with chlorine gas)


Addition of chlorine for disinfection results in other problems – creation of disinfectant byproducts (DBPs)

DBPs include trihalomethanes which are toxic

Water Chemistry Second Set of Questions

1. The Ksp for Fe(OH)2 and Fe(OH)3 are 7.9 x 10-16 and 1.6 x 10-39. What are saturated concentrations of Fe2+ and Fe3+ at a pH of 5 and 7? What will happen as hot spring water containing high concentrations of Fe2+ is introduced to a river with pH of 8 and with oxygen present? If Fe is measured and known to be in the form Fe3+ with a conc. of 1.3 μM at pH of 7.0, what might this suggest?

2. Why do the concentrations of some elements in water reflect concentrations in soils while not for other elements?

Water Chemistry Second Set II

1. Acidic rain will cause greater or lesser damage when a water basin has the following types of rocks?

a) graniteb) limestone c) basalt d) marble2. Statues made from the above types of rock will

be affected most from acidic precipitation?3. As run-off from a mine is introduced into a river,

what transformations occur to dissolved metals?4. Lakes under which of the following conditions

are more susceptible to oxygen depletiona) Summer (vs. winter) b) cold climatec) High altitude d) lakes (vs. rivers)

Toxic Metals (Chapter 13)• Four metals covered in detail:

Pb, Hg, Cd, and As• Behavior is somewhat similar (can

bioaccumulate, difficult to remove or “destroy”, can form different species, density is high)

• These metals all have industrial sources (although natural As is very important also)

Toxic MetalsBioaccumulation

• Bioaccumulation– When humans are exposed to constant

concentrations (e.g. Pb in air or Hg in fish), and elimination rates are slow, concentrations can become much higher than in the environment

– Example:• A 0.50 kg rat injests 0.20 mg of Pb per day• Half-life in rat is 7 days (originally Lifetime)• What is steady-state amount/concentration?

Toxic Metals Bioaccumulation

• Bioaccumulation– If other animals have slow excretion

rates, animals at the top of the food chain typically will have higher metal concentrations (e.g. tuna fish, swordfish, sharks in ocean; polar bears)

Toxic Metals Analysis Methods

• Atomic Spectroscopy (and Mass Spectrometry) Most Commonly Used

• Variety of Possible Methods:– Atomic Absorption Spectroscopy (AAS),– Atomic Emission Spectroscopy (AES)– Mass Spectrometry (MS)– X-Ray Fluorescence (XRF)

• X-Ray Fluorescence has the advantage of often needing no sample preparation (can analyze solid or liquid samples)

• For other methods, the first step involves atomization (conversion to “free” atoms in gas phase or simple ions for atomic mass spectrometers)


• Atomic Spectroscopy tends to have very good sensitivity (ability to detect small quantities) due to the narrow spectral lines

• The first step (and often difficult part) involves atomization (conversion from ions in liquid to atoms in gas phase)

• Three atomization methods are common:– flame (mostly with AAS)– graphite furnace (only with AAS)– inductively coupled plasma (ICP) – with AES or

MS


• Flame Atomization– used for liquid samples– liquid pulled by action of

nebulizer– nebulizer produces spray

of sample liquid– droplets evaporate in

spray chamber leaving particles

– fuel added and ignited in flame

– atomization of remaining particles and spray droplets occurs in flame

– optical beam through region of best atomization sample in

fuel (HCCH)

oxidant (air or N2O)

burner head

spray chamber

nebulizer

light beam


• Electrothermal Atomization (Process)– Sample is placed

through hole onto L’vov platform

– Graphite tube is heated by resistive heating

– This occurs in steps (dry, char, atomize, clean)

Graphite Tube in Chamber (not shown)

L’vov Platform

Sample in

T

time

dry char

atomize

Clean + cool down

Ar in chamber flow stops and optical measurements made


• Inductively Coupled Plasma (ICP)– A plasma is induced by radio

frequency currents in surrounding coil

– Once a spark occurs in Ar gas, some electrons leave Ar producing Ar+ + e-

– The accelerations of Ar+ and e- induce further production of ions and great heat production

– The sample is introduced by nebulization in the Ar stream

– Much higher temperatures are created (6,000 K to 10,000 K vs. <3500 K in flames)

ICP Torch

Quartz tube

Argon + Sample

RF Coil


• The lamp is a hollow cathode lamp containing the element(s) of interest in cathode. The lamp must be changed to analyze a different element.

• A very narrow band of light emitted from hollow cathode lamps are needed so that for absorption by atoms in flame mostly follows Beer’s law

• The monochromator serves as a coarse filter to remove other wavelength bands from light and light emitted from flames

Lamp source

Flame or graphite tube

monochromator Light detector

AA Spectrometers


Emission Spectrometers• In emission measurements, the plasma (or flame) is the light

source• A monochromator or polychromator is the means of wavelength

discrimination• ICP-AES is faster than AAS because switching monochromator

settings can be done faster than switching lamp plus flame conditions

• In ICP-MS, a mass spectrometer replaces the monochromatorPlasma (light source + sample)

Monochromator or Polychromator

Light detector or detector array

Liquid sample, nebulizer, Ar source


X-Ray Fluorescence• X-Rays interact with matter

by causing transitions with inner shell electrons

• An incoming X-ray will cause an inner shell electron to be removed

• An outer shell electron will replace the inner shell electron causing an X-ray emission

• The emitted X-ray will have a wavelength dependent upon the element

• X-Ray Fluorescence is not as sensitive or accurate as other methods

Pb

e-

e-

inner shell

outer shell

Toxic Metals Analysis Methods Comparison of Instruments

Instrument

Cost Speed Sensitivity

Flame-AA Low (~$10-15K)

Slow Moderate(~0.01 ppm)

GF-AA Moderate(~$40K)

Slowest Very Good

Sequential ICP-AES

Moderate Medium Moderate

Simultaneous ICP-AES

High Fast Good

ICP-MS Highest (~$200K)

Fast Excellent

Toxic Metals Lead

• Sources:– Ingestion from lead pipes, lead containing

glass or ceramics, lead paint– Inhalation (Was a problem in past with leaded

gasoline)• Toxicity:

– Affects normal development (more of a problem with children)

– Slow excretion rates• Analysis: Can use most of methods

Toxic Metals Mercury

• Sources:– Industrial Sources (in various products)– Coal Combustion– Food Chain Bioaccumulation

• Forms:– Reduced Hg (Hgo) (vapor or liquid)– Inorganic Hg (e.g. Hg2+)– Organic (CH3Hg+) – much more toxic because of slower

excretion rate and stability• Toxicity: Neurological Problems• Analysis: Cold vapor Atomization Methods (High T

methods are not efficient)

Toxic Metals Cadmium

• Sources:– Common industrial pollutant – released

in industrial processes– Also in cigarette smoke

• Toxicity:– Cd can replace Ca in bones

• Analysis: Can use most of methods

Toxic Metals Arsenic

• Sources:– Industrial sources (e.g. pesticides– Naturally present in water in certain

regions• Toxicity:

– Accute effects– Carcinogen

• Analysis: Difficult to atomize As (most common using hydride technique)

Toxic MetalsQuestions

1. Why do animals higher on the food chain have higher concentrations of Hg?

2. List 3 methods for analyzing metals3. Which method is best for a fast survey of soil

samples to find soils with high metal concentrations?

4. Which method is best for determining multiple elements at low concentrations?

5. Why is graphite-furnace AA a slow technique?

6. Which metal has a significant natural source?

Toxic MetalsQuestions

1. An organic form of Hg is found in river sediments. Bottom feeding fish have an uptake rate of 70 μg Hg per day. If the half-life of this form of Hg is 78 days, what would be the steady state mass of Hg in the fish?

Chem. 250 – 12/02 Lecture

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