Download Environmental Analysis - Evergreen State College Archives

1
Environmental Analysis
Analytical Chemistry Lecture 1
Sept 30, 2002
Welcome to Environmental Analysis! This is the third time this program has been
offered. The first two versions was very successful and we hope this year will be even
better.
XXXXXX [Fall Schedule]
My name is Jeff Kelly. I will be teaching the lecture and workshop portions the analytical
chemistry of the program. Clyde Barlow will be teaching the chemistry lab portion of the
program and Jim Stroh will teach the geology/hydrology/geochemistry sections.
You should all have a copy of the program schedule by now. We have this room on
Monday's from 8 to 3 in the afternoon and Wednesday's from 8 to 12.
These are for lecture/workshops in the morning and the Program Meetings on Monday
afternoons. Monday afternoon sessions will include:
Lab lectures
Guest lectures
Program planning such as field trip prep
Misc
On Tuesday mornings we will have lecture/workshops in the CAL. Jim, Clyde and Rip
Himinway will teach computer skills needed for program activities. These will included
spreadsheet use, mapping, GIS programming, and GPS use.
There are two lab sessions. Tuesday afternoon 1- 5 and Thursday morning 8-12. You will
attend both sessions. During one session you will have an analytical chemistry lab and
during the other a geology/field work lab. Note geology lab starts ½ hour later than the
chemistry lab.
Thursday afternoon is not a program scheduled time but the lab is reserved for project
work.
There will be frequent instrument workshops that will be scheduled for groups of 4,2 and
1 in the few spare hours left in the week.
In addition there will be several one-day field trips this quarter. They will be on Fridays.
Finally there is the week-long field trip the 3rd week which will be discussed at this
afternoon’s session.
Now about program enrollment status. As of Friday the program was completely full with
no waiting list. The number of people who withdrew or did not pay tuition exactly
matched the number on the wait list. So if you were on the wait list last week you are now
in program.
Although there will be some small adjustments, things are looking good right now.
This program is designed for students with a year of college chemistry and college math
and who are ready to start analytical chemistry. Previous academic experience with
geology is also a plus.
There are a number of you with weak science backgrounds who will do fine in this
program. We know this is the case from our experience with previous versions of the
program. However it will take extra effort and a “can do” attitude because you will have
to continually look up terms and concepts you haven’t encountered before.
This extra effort is going to be particularly hard if you also have to work to go to school.
So realize that this program involves a lot of learning and a lot of work and gear up your
commitment level for the year ahead.
Are there any questions about the schedule or enrollment?
In addition to lecture and lab space, we have the Chem Cave as a home room. It has a
coffee pot, microwave oven and small refrigerator. It also has a networked computer and
tables and chairs for working homework problems or preparing reports. You will find the
Chem Cave also a good place for small group and project meetings.
Let me say a few words about the chemistry textbook:
The Text book by Tom Harris is very readable. Previous students have found it especially
clear and have forgiven him his corny humor.
I have already made some changes to the Reading schedule so make note of the following.
XXXXXX
The reading assignment for this week is to skim Chapters 0,1,2 and the Appendices A and
B. These should virtually all be review on very basic material. Be sure you have
refreshed yourself about logs and exponents as they will we used extensively in all parts
of the program.
In particular, you should learn where topics are in these chapters so that you can go back
to them when they come up in lecture, workshop or lab. For example, there are sections in
Chapter 2 on how to use a burette and the analytical balance. When you get to a lab that
uses a burette or analytical balance you should go back and read or reread these sections
carefully.
Read the first 6 sections of Chapter 6 which reviews general, solubility and complex-ion
equilibrium. Your welcome to read the rest of the chapter now or review it later for Week
6.
2
Jim will talk to you about the hydrology and geochemistry books at 10:30 this morning. 3
However you can see that we are skipping around a bit in the analytical chemistry book to
provide background for topics that come up in other parts of the program.
I think you are going to find that the integration of chemistry and geology in this program
is very extensive. Chemistry provides the basis for the models and procedures used to
describe and test geological and environmental systems.
Now let me talk about the chemistry in the program. I do not tend to regurgitate the
material in the book. Therefore it is important that you read the assigned sections of the
book before lecture time.
During lectures I will go over those topics I think you need to hear about – either very
important topics, ones that are more difficult to understand, or ones I think make good
examples.
I will make my lecture notes available in two different ways. They will be available in a
folder in the Chem Cave. The notes will also be available as a Word document in the
Environmental Analysis directory in the CAL. I usually get the notes there the day of the
lecture. Copies of the overheads will also be available in the Chem Cave folder.
Thus it is possible for you to listen to lectures and not have to take detailed notes because
you can read them or copy them in the Chem Cave or the CAL. So you can listen to what
I am saying without having to try to write things down. If you are a compulsive notetaker
– fine, but some people get so wrapped up in their notes they miss concepts presented in
the lecture.
Because I get ideas and sometimes even text from other sources, I can not make the notes
directly available such as through a webpage. However this approach has worked in the
past.
You will notice that we call these sessions lecture/workshop. During some of these
sessions we will break into small groups to work on solving a few problems as examples.
There will be weekly homework assignments that you are to turn in. Many times you will
start on these assignments in workshop.
You will need a scientific calculator. If you need to buy one get one that includes simple
statistics functions. Complex statistics will be done on spreadsheets.
We strongly urge you to form study groups to learn this material. Our experience has
been that students working on their own are about 4 times more likely to not make it
through the program.
There will be exams in analytical chemistry about every 4-5 weeks. These will generally
be in-class exams and closed-book, so you really need to understand what is going on in
the program and how to solve problems like those from the homework and workshop.
You also really need to keep caught up.
The Environmental Analysis program is an integration of chemistry and geology with
water as the integrating focus or theme. We will be particularly examining surface water
in lakes during the coming year.
The chemistry we will be covering in these sessions comes under the title of analytical
chemistry.
Analytical chemistry is the branch of chemistry dealing with the separation and analysis of
chemical substances. Analytical chemistry includes both qualitative and quantitative
analysis. Qualitative analysis is concerned with what is present, quantitative analysis with
how much is present.
The qualitative identification of the substances present in an analytical sample is of great
importance. Often the answer to a vexing environmental problem lies in the identification
of the substance that is altering an ecosystem or inhibiting a chemical or biological
process. But in this program we will concentrate primarily on quantitative analysis.
Quantitative analysis deals with the analysis of both inorganic and organic material. Thus
clinical chemistry in medicine, analysis of organic substance in industrial and
environmental samples and the determination of the composition of a mineral are all part
of analytical chemistry.
Analytical chemistry involves the study of the principles of general chemistry,
the methods and procedures necessary to the actual determination of the composition of a
sample and the theory and operation of the instruments that are employed in chemical
analysis.
It is especially meaningless to study analytical chemistry without learning to do the
associated lab procedures.
One of the interesting features of analytical chemistry is that the topics are the same topics
of general chemistry – equilibrium, thermodynamics, acids and bases, oxidation/reduction,
quantum mechanics, etc.
However the focus of the material is on the use of these concepts in the accurate
determination of sample composition.
Thus, if you have studied chemistry, you have already learned most of what we will cover
in analytical chemistry and major sections of the text will be review.
As I have told many of you, analytical chemistry is just the topics of general chemistry all
over again. The focus is not on the concepts which, it is assumed, you have already
learned, but on determining the amounts of materials present in a given sample.
4
5
The goal of this portion of the program is to give you an understanding the principles of
analysis and a knowledge of the methods and instruments used in analysis, so that you can
apply and, if necessary, modify analytical methods to solve a particular problem.
In addition we hope to give you the background and skills to evaluate and interpret the
results of a quantitative analysis.
Above all, analytical chemistry involves problem solving. By asking questions and
gathering information, the analytical chemist determines what the actual problem is, then
uses experience and knowledge to map out a scheme for solving the problem.
Thus, there is a sharp contrast between a well-trained analytical chemist and the lab
technicians who simply twist the dials of an instrument or follow "cookbook" analytical
procedures.
Over ten years ago, Clyde and I worked as Visiting Analytical Chemists at the Center for
Process Analytical Chemistry at the University of Washington developing methods to
analysis the content and properties of industrial materials. We continue to develop
methods for the analysis of biological and biomedical samples here at Evergreen.
Clyde and I each teach half time and do research half time and so combined we fill one of
the two faculty positions in the program.
Some Definitions
XXXXXX
In order to discuss work in analytical chemistry you need to know some of the basic terms
that are pervasive in the practice and literature.
In qualitative analysis, elements, ions, and compounds are identified in a sample. For
example, we may say that an analyst has identified penicillin in an antibiotic. This is
equivalent to saying that penicillin is present at or above some level. If the amount of
penicillin in the sample is found, the word quantitation or quantified is often used to
describe the process.
Only samples are analyzed. In the situation above, we would say that the sample is
analyzed for penicillin.
It would be incorrect to state that penicillin is analyzed unless we are referring to a sample
of "pure" penicillin that is analyzed to determine whether other compounds are present.
If we intend to determine what fraction of a sample is some named material, the word
assay or determination should be used.
The named material is called the analyte. For instance, suppose we have a sample or iron 6
and want to determine how much carbon is present in the sample. We can say that an
assay for carbon is to be run. Carbon is the analyte.
To say that "an analysis of iron will be made" means that the sample is iron and that an
identification and quantitation of the impurities will be carried out on a sample of iron.
However, if the iron content itself is to be determined, the correct word for the procedure
is assay. Thus an analysis of iron conveys a different meaning from an assay for iron.
Most chemical analyses are developed through much trial and error. For each analytical
procedure, it is extremely important to demonstrate that the procedure measures what the
analyst says it measures on a specific type of sample.
This is done by comparing the new method with prior methods that are known to give
accurate determinations. This process is called validation of the analytical method.
For instance, an analyst might develop a new procedure to measure the amount of cobalt
in samples of animal livers. He or she must then validate the new method by showing that
its results agree with those obtained using older, accepted procedures.
However, if a procedure is used with a different type of sample, unexpected problems
might become obvious. Perhaps the sample contains some component that causes an
incorrect result. When that happens, we say there is interference from that component.
When developing chemical analyses, you will almost always find that problems arise from
interfering species, or interferents. These interfering species cause the result to be greater
or less than what it would be if they were absent.
For example, when you use the new cobalt method for liver on a specimen of bone, high
levels of calcium may interfere with the assay. Calcium would, therefore, be an
interferent. As a result, the new procedure that is validated for use with animal liver is
not validated for bone.
A method, or protocol, is a fixed sequence of actions to be carried out in the analysis of a
sample. Often you will see references in the literature to standard methods approved by
regulatory agencies such as the U.S. Environmental Protection Agency (EPA) or the
American Society for Testing and Materials (ASTM).
These methods are specific for use with certain analytes in certain types of samples. They
commonly define both the sample handling requirements and the analytical techniques
that are to be used.
Some methods require calibration. This involves making standard samples containing
known quantities of analyte and running them through the method. This established the
relationship between quantity of analyte and analytical signal. This step is used
particularly with instrumental methods where the signal may vary from instrument to
instrument.
What kind of signals are you aware of??????
Mass, volume, voltage, current, light emission, light absorption, temperature change, etc
Some of you may know the term calibration is also used to describe the process whereby
the volume of analytical glassware is determined, usually by weighing aliquots of water
that fill the glassware.
XXXXX
When approaching an analytical problem, there is a general sequence of steps that are
followed in developing the protocols and deciding which analytical techniques to use. A
general outline of these steps is shown in this transparency.
Once the problem is defined the first step is to decide on an appropriate method for the
analyte of interest.
XXXXX
For example, say there are 120 tons of ore sitting outside the laboratory window to be
analyzed. Such a pile is sometimes called a lot. Different piles may have come from
different locations in the same ore body and would have different lot numbers.
Before any method can be used on a single pile, a sample that represents the contents of
the pile must be taken. This sample is sometimes called a bulk sample. How can such a
sample be obtained? One of the topics we will look at are the statistics and methodology
of obtaining bulk samples that represent a lot. Suppose we prepare a 10 kg bulk sample
from the 120 tons of ore.
Next, a decision must be made about what method can be used for sample preparation.
The sample preparation method must be chosen with the characteristics of the assay
method in mind.
XXXXX
For instance, assume that the method we prefer can only measure some property of an
aqueous solution, but we have 10 kg of solid chunks of ore in a bucket in the lab. We
want to convert our bulk sample into a laboratory sample.
Furthermore, if some interferents present in the prepared sample would be especially
detrimental to performing the preferred assay, these interferents should be removed before
running the assay, if possible. If some interferents cannot be removed, their effects must
be accounted for by a correction to the results.
Both these last two steps require some knowledge of the descriptive chemistry of the
analyte and interferents such as solubilities, volatilies, or ease of oxidation.
7
An understanding of chemical properties is on of major tools used in chemical analysis.
Some of this knowledge you know as rules. "All inorganic nitrates are soluble." While
others are specfic exceptions to rules. "All inorganic halides are soluble, expect the
halides of silver, mercury(I) and lead."
In addition, the fundamentals of competitive equilibria, must be mastered in order to
understand chemical analysis. Not only do the problems of preparing a sample and
eliminating interferents depend on it, but understanding the assay method itself may rely
on your knowledge of the equilibria involved.
After we perform the assays, the next step must be to decide how to treat the data
obtained.
Data reduction is part of the assay. The response of an analytical instrument or method
does not produce a direct measure of the amount of a given material in a sample.
For instance, we might find that a certain amount of yellow light is absorbed by a sample,
or that the sample solution conducts electricity, or that the sample's weight changes a
given amount when it is heated. Each assay method requires different calculations to
reduce the experimental data into the form required: the quantity of the analytes.
Usually, statistical tests then help us decide how well we really know the numerical
answer to the analytical question. A statistical analysis of the results is required since
each analysis and each sample analyzed is not exactly the same.
To evaluate such variations, three or more samples of the same material are usually treated
and assayed together. Then, quantitative measures can indicate how trustworthy the
results are.
A computer will make only some parts of the analysis easier: the data acquisition, data
reduction, statistical analysis, and record keeping. The rest still requires personal
judgment and skill.
It is important to remember that developing analyses is not always a "straight-path"
process. Often it is necessary to backtrack to an earlier step because of some unforeseen
difficulty.
On the brighter side, an analysis may be changed because some new technique makes the
task easier or cheaper, with the same or better quality of analysis.
This problem-solving process can be illustrated with a specific problem and its solution as
found in the literature of chemical analysis. It comes from the oil industry.
8
9
Solids were building up in a reactor of a petrochemical processing plant, and the
condition threatened to require a complete shutdown. This buildup was accompanied by
relatively high corrosion rates of the metal reactors.
The problem was defined in this manner: What is causing the solids built up and can the
solids be eliminated?
From previous experience, the analysts believed the problem might be due to the presence
of chlorine. The question was rephrase, how much chlorine was in the system during the
process.
The samples of the feed material (a liquid) that reacted in the synthetic process were
obtained for an assay of chlorine. In the company's laboratory, the chlorine content was
determined by two different methods. The total chlorine in a sample was quantitated by a
thermal method of analysis, in which changes in the temperature of a sample are measured
as it is heated.
In another method the chlorine content was determined using an electrochemical
method--one that depends on the oxidation or reduction of a substance. Samples were
prepared differently for the two assay methods.
After the measurements of chlorine were made, the numerical results from the two
methods were found to differ. The electrochemical method showed that there were no
chlorine in the sample while the thermal method showed apparently high levels of
chlorine.
By statistical analysis of results from a number of samples, the analysts determined that
the different results could not be due to random experimental errors. A check for
interferences was done.
The assay using the thermal method was found to be influenced by a high level of sulfur
that acted as an interferent in the assay, which gave a false positive result. Due to this
unanticipated problem, the original analysis was in error.
As a result, the analysts modified the analytical methodology by changing their technique
for the sample preparation. The sulfur present in the sample was removed before the
thermal-method assay was run. After that modification, the results from both methods
agreed that there was no chlorine in the samples. The chemists concluded that the
problem in the reactor was not excessive chlorine.
The results showed that the initial guess as to the cause of solid buildup was wrong. The
question was rephrase, how much sulfur was in the system during the process.
A subsequent analysis of sulfur in the feedstock samples demonstated that the answer to
the original question was that the problem of the sludge and corrosion lay in the high
sulfur levels of the feedstock.
10
You can see that solving a problem using chemical analyses is not always straight forward
and that there is much more to solving the problem than just picking a procedure and
getting a numerical answer.
Finally a short aside about numbers. In discussing chemical equilibria, we are often
dealing with numbers that are very small and can vary over several orders of magnitude.
For example, the hydrogen ion concentration in water can vary from about 10 M to 10-16
M.
When dealing with a wide range of numbers, such as these, chemists find it helpful to
express the numbers as p-functions where the p-function or p-value, is the negative
logarithm (to the base 10) of the number.
Some of you have encountered other mathematical functions such as δ/δt f(t) , means
taken the derivative with respect to time, or √, which means take the square root of.
Thus, the p-value of the number x is
px = -log x
p-values offer the advantage of allowing awkward numerical data to be expressed as
small, usually positive, numbers. The most common p-function is pH or the negative log
of the hydrogen ion concentration.
Consider these examples.
XXXXXX
What is the pH of an acid solution with [H+] = 4.2 x 10-3 M?
pH = - log [H+] =
= - log [4.2 x 10-3] = - ( -2.3768)
pH = 2.38
How many sig figs in the answer? The sig figs are in the matissa of the log. The
characteristic of the log give information about the exponent.
Calculate the molar concentration of Ag+ in a solution that has pAg of 6.372.
pAg = - log [Ag+] =
log [Ag+]
= - 6.372
[Ag+]
6.372
= antilog (-6.372) = 10-6.372
= 4.246 x 10-7
= 4.25 x 10-7
Why does the answer have 3 significant figures?