Jessica Sproul
Chapter 2 Summary
Dr. Rahni
Analytical Chemistry
Chemicals and Apparatus: Putting the Tools to
Work
2A) Selecting and Handling Reagents and Other
Chemicals
2A-1) Classifying Chemicals
1) Reagent Grade
i) Reagent grade chemicals meet minimum requirements regarding
impurities put forth by the Reagent Chemical Committee of the
American Chemical Society.
ii) These are used whenever possible in analytical chemistry.
2) Primary-Standard Grade
i) Primary-Standard grade chemicals have exceptional purity and are
carefully analyzed by the supplier.
3) Special-Purpose Reagent Chemicals
i) These are chemicals prepared for a specific use.
ii) Information regarding their properties related to their specific use is
placed on the label.
2A-2) Rules for Handling Reagents and Solutions
1) Select the best grade chemical available at all times.
2) Replace cap to every container immediately after use of chemical.
3) Never set a stopper on a desktop.
4) Never return excess reagent to a bottle.
5) Never insert a spatula, spoon, knife, or other instrument into a solid
reagent container.
6) Keep the reagent shelf and lab balance neat and clean.
7) Observe local regulations regarding disposal or surplus chemicals.
2B) Cleaning and Marking Laboratory Ware
A Chemistry Student approached the Master and asked, “Master, how do I
achieve enlightenment in Chemistry?” The Master replied, “Wash your
glassware.”
(http://www2.truman.edu/~jmccormi/LabManual/Cleaning.htm)
1) There are two broad degrees of clean in chemistry; quantitative and
normal. Quantitatively clean glassware is required for the most
demanding applications where a quantity is being measured at high
precision, such in analytical or physical chemistry. Glassware at this
level of cleanliness has no residues (e. g., grease) or other impurities
on it. Normal clean glassware is free of large amounts of impurities,
but some grease may be tolerated. Glassware that has been cleaned
normally is used where high degrees of precision are not required,
such as in a synthesis.
(http://www2.truman.edu/~jmccormi/LabManual/Cleaning.htm)
i) Mark separate trials individually with pencil, wax pencil, or other
marking ink.
ii) Clean glassware thoroughly with hot detergent solution, rinsed
with tap water, and then rinsed with deionized water.
iii) Don’t clean interior of glassware with paper towel as it is a
waste of time and could potential contamination.
2C) Evaporating Liquids
1) Evaporation is a good way to decrease the volume of a volatile
solution.
2) It is also a good way to eliminate unwanted species, such chloride and
the nitrate ion.
2D) Measuring Mass
2D-1) Types of Analytical Balances
1) Semimicroanalytical Balance: maximum load of 10-30 grams; precision
of ±0.01 mg
2) Microanalytical Balance: capacity of 1-3 grams; precision of ±0.001
mg.
2D-2) The Electronic Analytical Balance
1) This type of balance has somewhat complex internal technology, but
allows the user to weigh samples and tare objects with great precision
and ease.
(http://sartorius.balances.com/sartorius/imgs/weighpan.jpg)
2D-3) The Single-Pan Mechanical Analytical Balance
1) This type of balance has a complex internal mechanical structure,
which allows the user to weigh samples with relative ease and
precision.
2D-4) Precautions in Using an Analytical Balance
1) Center the load.
2) Protect balance from corrosion.
3) Observe special precautions for weighing liquids.
4) Consult instructor if balance needs adjustment.
5) Keep balance and its case extremely clean.
6) Always allow a heated object to return to room temperature before
weighing.
7) Use tongs or gloves to prevent the contamination of a dried sample by
the uptake of moisture.
2D-5) Sources of Error in Weighing
1) Correction for Buoyancy
i) This is an error that comes about if the density of the object is
significantly different than that of the standard weights.
ii) The correction term is: W1 = W 2 + W 2(dair/dobject – dair/dwts)
2) Temperature Effects
i) Considerable errors arise when the temperature of the object is
vastly different than room temperature. Allow time for the object
to reach equilibrium with the atmosphere.
3) Other Sources of Error
i) Porcelain or glass objects may acquire a static charge that will
disrupt balances. A faintly damp chamois cloth can be used to
prevent the release of static charges into the balance.
4) *Note* Errors in weighing should fall well within the limits of the
experimental error due to the analytical operations. If, for example, an
error of 0.001 g. were made in weighing out a gram sample of fireclay
containing 0.25% Of MgO, the resulting error in the determination of
the magnesia could be no greater than 0.1% of its value, which is
negligibly small. A 1 mg. error, however, made in weighing the 0.0069
g. Of Mg2P207 would involve an error of over 14% in the magnesia
value, and this would be intolerable.
(http://humboldt.edu/~scimus/Instruments/AnalWgtSmith/AnalWgt.htm)
2D-6) Auxiliary Balances
1) These balances are less sensitive and less precise than analytical
balances, but they offer speed, ruggedness, large capacity, and
convenience.
2E) The Equipment and Manipulations Associated with
Weighing
2E-1) Weighing Bottles
1) Solids are often dried and stored in glass or plastic weighing bottles.
2E-2) Desiccators and Desiccants
1) These are used to dry solids that have absorbed moisture. They are
airtight receptacles that contain chemical drying agents.
2E-3) Manipulating Weighing Bottles
1) Clearly mark the container holding the weighing bottle.
2) Do not touch the weighing bottle with bear hands.
2E-4) Weighing by Difference
1) A measurement is taken with a large amount of the reagent in a
container, some reagent is removed and weighed, and the weight of
the reagent in the container is the difference of the two.
2E-5) Weighing Hygroscopic Solids
1) Special care must be taken with substances that rapidly absorb
moisture from the atmosphere, including very limited exposure to open
air.
2E-6) Weighing Liquids
1) The mass of a liquid is always found by difference.
2F) The Equipment and Manipulations for Filtrations and
Ignition
2F-1) Apparatus
1) Simple Crucibles
i) These serve as simple containers that are made of porcelain,
aluminum oxide, silica, or platinum.
2) Filtering Crucibles
i) These serve as both filters and containers, allowing liquids to
pass through, but not solids.
3) Filter Paper
i) This is a filtering medium that can be ignited to leave a residue
of less than 0.1 mg. This is useful for weighing the precipitate
collected on the paper.
4) Heating Equipment
i) An oven with the ability to maintain a constant temperature is
often used to bring a precipitate to a constant mass in order to
be weighed.
2F-2) Filtering and Igniting Precipitates
1) Preparation of Crucibles
i) The crucible is cleaned thoroughly and then subjected to the
same heating/drying method as that of the precipitate to be
weighed.
ii) The crucible is heated and weighed until a constant mass is
measured.
2) Filtering and Washing Precipitates
i) Precipitates can be transferred through decantation, washing,
and through the use of a rubber policeman.
2F-3) Directions for Filtering and Igniting Precipitates
1) Preparing a Filter Paper
i) The filter paper is folded into a cone in a manner demonstrated
by the following image and then wetted slightly so that suction is
formed between the paper and the filter itself.
(http://www.sfu.ca/chemistry/students/courses/chem126/images/techniqu
es/filter%20paper2%20.jpg)
2) Transferring Paper and Precipitate to a Crucible
i) Allow the paper to dry some and then carefully remove it from
the filter and place it in the crucible, being careful not to tear it.
3) Ashing Filter Paper
i) Be careful when ashing the filter paper; you do not want to lose
any of the precipitate due to carelessness.
2F-4) Rules for Manipulating Heated Objects
1) Practice unfamiliar manipulations before using them.
2) Never place a heated object on a benchtop. Place it on wire gauze
instead.
3) Allow heated crucibles to cool before transferring them to a desiccator.
4) Keep any tongs and forceps very clean so as to avoid contamination.
2G) Measuring Volume
2G-1) Units of Volume
1) The units of volume are the liter and the milliliter.
2G-2) The Effects of Temperature on Volume Measurements
1) Even as small a change as 5° C has a significant change on the
volume of a liquid as volume varies with temperature.
2G-3) Apparatus for the Precise Measurement of a Volume
1) Pipets
i) These permit accurate transfer of liquids from one container to
another.
(http://www.sonoma.edu/chemistry/chem115A/images/pipets.gif)
2) Burets
i) Burets allow for greater precision than pipets, and allow the
analyst to deliver any volume up to the maximum.
(http://www.startracksmedical.com/supplies/buret.jpg)
3) Volumetric Flasks
i) These are calibrated to a specific volume and are often used for
the preparation of standard solutions.
(http://www.osawindia.net/Purple_Water.jpg)
2G-4) Using Volumetric Equipment
1) Cleaning
i) To clean, briefly soak in warm detergent solution, then rinse with
tap water, followed by deionized water.
2) Avoiding Parallax
i) Avoid parallax errors by reading the volume of a liquid at its
meniscus.
2G-5) Directions for the Use of a Pipet
1) Cleaning
i) Use a rubber bulb to draw detergent solution into the pipet and
then rinse pipet with tap water, followed by deionized water.
2) Measuring an Aliquot
i) Use a rubber bulb to draw several small samples of the liquid
into the pipet. Wet the inner surfaces before taking your desired
measurement.
ii) Be careful when handling the liquid-filled pipet.
2G-6) Using a Buret
1) Cleaning
i) Thoroughly clean the buret with detergent and a long brush.
Rinse with tap water, followed by deionized water.
2) Lubrication of a Glass Stopcock
i) Remove all old grease and dry with paper towel before lightly
greasing the stopcock again.
3) Filling
i) Make certain that the stopcock is closed.
ii) Add 5-10 mL of liquid to be measured and allow them to pass
completely through the buret.
iii) Fill the buret above the zero mark and allow liquid to pass
through until the level is equal with the zero mark.
4) Titration
i) Be sure the tip of the buret is well within the titration vessel.
ii) Introduce titrant in increments of 1 mL, swirling flask constantly.
(http://www.dartmouth.edu/~chemlab/chem3-5/ionx2/overview/techniques.html)
2G-7) Using Volumetric Flasks
1) Direct Weighing into a Volumetric Flask
i) Place a known mass of solute within the flask with the aid of a
funnel.
2) Quantitative Transfer of Liquid to a Volumetric Flask
i) Insert a funnel into the neck of the flask and decant the solvent
into the flask.
3) Dilution to the Mark
i) After half of solute is added, swirl the flask. Add the remainder
of the solute and allow for drainage. Finally, using a medicine
dropper, make the final additions.
2H) Calibrating Volumetric Ware
2H-1) General Directions for Calibration Work
1) Calibrating a Volumetric Pipet
i) Weigh the dry, empty pipet to nearest milligram, then add liquid
and weigh again. Use density to determine the actual volume of
the liquid.
2) Calibrating a Buret
i) Weigh an empty 125-mL flask, then fill a buret to the 0.00 mL
mark. Empty the buret into the flask and use density to
determine the true volume.
3) Calibrating a Volumetric Flask
i) Weigh the clean, dry flask to the nearest milligram. Fill to mark
with water and reweigh. Calculate true volume using density.
2I) The Laboratory Notebook
2I-1) Maintaining the Laboratory Notebook
1) Record all data and observations directly in lab notebook.
2) Supply each entry with a heading or label.
3) Date each page.
4) Never erase or remove any entry.
5) Never remove a page from notebook.
2I-2) Format
1) Page One
i) Title of experiment
ii) Brief introduction
iii) Summary of weighing, volumetric, and/or instrument response
data needed for results
iv) Report of best value and statement of its precision
2) Page Two
i) Chemical properties
ii) Algebraic equation showing how results were calculated
iii) Summary of observations
2K) Safety in the Laboratory
1) Learn location of eye fountain, fire blanket, shower, and fire
extinguisher.
2) Wear eye protection at all times.
3) Don’t touch chemicals bare-handed.
4) Never perform unauthorized experiment.
5) Never work alone.
6) Never eat or drink in lab.
7) Always use a bulb to draw liquid into pipet.
8) Wear adequate foot covering.
9) Be careful with heated objects.
10) Always fire-polish ends of freshly cut glass. Never force glass tubing
through hole of a stopper.
11) Use fume hoods when evolving harmful gases.
12) Notify instructor of injuries.
13) Dispose of solutions and chemicals as instructed.
(http://epswww.unm.edu/facstaff/zsharp/lab%20stuff/safety.htm)