Chem 223 – Spectrophotometry, Beer's Law, and Precision
Spectrophotometry, Beer's Law, and Precision
Remember to bring AND WEAR your goggles.
"But we aren't doing any reactions this
week, and the chemicals are largely innocuous."
"They're hot and steam up."
"I look like I came from Mars."
We've heard 'em all.
You'll be in a lab.
the point that it's a reflex:
BRING AND WEAR YOUR SAFETY GOGGLES.
Simple way to tell if you've been careful this week: if you don't have blue or violet fingers, you did fine.
(Methylene blue is used to stain tissue during surgery, so it's safe, just ugly.)
Analytical Chemistry is the science that identifies the components of a portion of the physical world,
quantifies them, and characterizes their interactions.
Doing this requires:
A problem whose solution requires knowledge of chemical composition or dynamics.
One or more means of measurement
An understanding of the chemistry and physics that effects the measurement
A means to characterize the quality of the measurement (both precision and accuracy)
A theoretical or practical way to use the results of the measurement to solve a problem.
Our purpose here is to demonstrate a common means of measurement (spectrophotometry), to show
examples where the measurement tool works, to show examples where the measurement tool fails, and to
show examples where the chemistry interferes with the measurement.
All this will be done under
circumstances where the operator's manual dexterity is of minimal or no influence on the results.
In later labs, your skill will contribute to the precision of the experiment.
Many past students have felt
that imprecision was some mark of shame or that "skill is proportional to 1/standard deviation."
times when this is true, but in this experiment you will see differences in uncertainty caused by chemistry,
sample, and instrument.
What you should learn is:
Non-zero uncertainty is a fundamental property of nature.
Many factors contribute to uncertainty.
Understanding the sources of uncertainty can lead to their minimization.
Learning analytical chemistry means learning the "tools of the trade" PLUS developing "lab
technique" PLUS learning how to integrate many pieces of information into the solution of a
One of the hardest parts of doing analytical chemistry is that we must deal with the world as a SYSTEM
(many things interacting and happening at once), yet we have been taught to deal with learning and living
as COMPONENTS (single activities, single calculations, individual controls or pieces of apparatus).
Think of driving a car.
You don't think about each spark plug firing or turning the wheel 2.78º to the right
or pushing on the gas pedal by an additional 10
Newtons to accelerate.
You Just Drive.
You are taking
a systems approach, with a lot of activity implicit in what you do.
But when you learned to drive, you
DID pay attention to pressure on the gas or the angle at which you steered.