lecture 2

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CHEMISTRY 59-320
ANALYTICAL CHEMISTRY
Fall - 2010
Lecture 3
Principle of operation (3)
• The QCM-Z, based on impedance
analysis, provides unique information on
changes in the mass properties of
adsorbed or deposited layers on a quartz
crystal. The mass change is proportional
to the change in resonance frequency.
Tolerances (allowable
deviations)
Weighting in a Vacuum
is the Most Accurate
• Weights of objects in air can be
corrected to the weight in vacuum by
• Wvac= Wair + Wair((0.0012/Do)(0.0012/Dw))
• Wvac = weight in vacuum, g
• Wair = weight in air, g
• Do = density of object
• Dw = density of weights
• 0.0012 = density of air
Buoyancy Correction
Buoyancy: Upward force exerted on an object in a liquid or gaseous fluid. An object
weighed in air appears lighter than its actual mass by an amount equal to the mass of
air that it displaces.
Where da is the density of
air (0.001 2 g/mL near 1 bar
and 25°C),11 dw is the
density of the calibration
weights (typically 8.0
g/mL),
and d is the density of the
object being weighed.
2.4 Burets
Parallax
Operating a buret
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Wash buret with new solution.
Eliminate air bubble before use.
Drain liquid slowly.
Deliver fraction of a drop near end
point.
Read bottom of concave meniscus.
Estimate reading to 1/10 of a division.
Avoid parallax.
Account for graduation thickness in
readings.
Position the black field just below the meniscus.
Avoid parallax error by reading at eye level.
Place the flask on a white background.
Place the buret tip in the neck of the flask while your swirl.
Proper technique for titration.
2.5 Volumetric Flasks
A volumetric flask is calibrated to contain a
particular volume of solution at 20°C when
the bottom of the meniscus is adjusted to the center
of the mark on the neck of the flask
The proper usage of volumetric
flask
• Glass is notorious for
adsorbing traces of
chemicals—especially cations.
Adsorption is the process in
which a substance sticks to a
surface. (In contrast,
absorption is the process in
which a substance is taken
inside another, as water is
taken into a sponge.)
• For critical work, you should
acid wash glassware to
replace low concentrations of
cations on the surface with H+.
2.6 Pipets and Syringes
.
Pipets deliver known volumes of liquid.
1. Transfer pipet
2. Measuring pipet
2.7 Filtration
Liquid from which a substance precipitates
crystallizes is called the mother liquor.
Liquid that passes through the filter
is called filtrate.
2.8 Drying
desiccator
2.9 Calibration of volumetric glassware
Calibration: the process
of measuring the actual
quantity of mass, volume,
and so on, that
corresponds to an
indicated quantity on the
scale of the instrument.
Correction for thermal
expansion
c' c

d' d
Temperature Dependence of
Molarity
• The Molarity of a solution is temperature
dependent, therefore when preparing or
standardizing solutions you have to record
the temperature of solutions.
• Conversion Formula:
• Mnew temp = Mold tempx(Dnew temp/Dold temp)
• See examples in Text
2.10 Introduction to Microsoft Excel
This is a problem from s2-3.

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