Chemicals, Apparatus, and Unit Operations

1

• Chapter 2
• Chemicals, Apparatus, and Unit Operations
• of Analytical Chemistry

2
Introduction

• At the heart of analytical chemistry is a core
  set of operations and equipment.
• The technology of analytical chemistry has
  improved with the advent of electronic analytical
  balances, automated titrators, and other
  computer-controlled instruments.
• The speed, convenience, accuracy, and precision
  of analytical methods have improved as well.

3

• 2A Selecting and handling reagents and other
  chemicals
• The purity of reagents influences the accuracy of
  analysis.
• Classifying Chemicals
• Regent grade conform to the minimum standards
  set forth by the Reagent Chemical Committee of
  the American Chemical Society (ACS).
• Primary-standard carefully analyzed by the
  supplier. The National Institute of Standards and
  Technology (NIST) is an excellent source.
• Special-purpose reagent chemicals are prepared
  for a specific application such as solvents for
  spectrophotometry and high-performance liquid
  chromatography.

4

• Rules for Handling Reagents and Solutions
• Select the best grade of chemical available. Pick
  the smallest bottle that is sufficient to do the
  job.
• Replace the top of every container immediately
  after removing reagent.
• Hold the stoppers of reagent bottles between your
  fingers. Never set a stopper on a desk top.
• Unless specifically directed otherwise, never
  return any excess reagent to a bottle.

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• Rules for Handling Reagents and Solutions (contd)
• 5. Never insert spatulas, spoons, or knives into
  a bottle that contains a solid chemical. Instead,
  shake the capped bottle vigorously or tap it
  gently against a wooden table to break up an
  encrustation. Then pour out the desired quantity.
  
• 6. Keep the reagent shelf and the laboratory
  balance clean and neat. Clean up any spills
  immediately.
• 7. Follow local regulations concerning the
  disposal of surplus reagents and solutions.

6

• 2B Cleaning and marking of laboratory ware
• Each vessel that holds a sample must be marked.
  Special marking inks are available for porcelain
  surfaces. A saturated solution of iron(III)
  chloride can also be used for marking.
• Every apparatus must be thoroughly washed with a
  hot detergent solution and then rinsed, initially
  with large amounts of tap water and finally with
  several small portions of deionized water.
• Properly cleaned glassware will be coated with a
  uniform and unbroken film of water. Do not dry
  the interior surfaces of glassware.
• An organic solvent, such as methyl ethyl ketone
  or acetone, may be effective in removing grease
  films.

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• 2C Evaporating liquids
• Evaporation is difficult to control because of
  the
• tendency of some solutions to overheat locally.
• Bumping can cause partial loss of the solution.
• Careful and gentle heating or use of glass beads
• will minimize such loss.
• Some unwanted substances can be eliminated
• during evaporation.
• Wet ashing is the oxidation of the organic
  constituents of a sample with oxidizing reagents
  such as nitric acid, sulfuric acid, hydrogen
  peroxide, aqueous bromine, or a combination of
  these reagents.

8

• 2D Measuring mass
• An analytical balance must be used to measure
  masses with high accuracy.
• An analytical balance is used for determining
  mass with a maximum capacity that ranges from 1 g
  to a few kgs with a precision of at least 1 part
  in 105
• A macrobalance has a maximum load of 160-200 g
  and a precision of 0.1 mg.
• A semimicroanalytical balance has a maximum load
  of 10-30 g and a precision of 0.01 mg.
• A microanalytical balance has a maximum load of
  1-3 g and a precision of 0.001 mg, or 1 µg.
• The traditional analytical balance had two pans
  and is considered an equal-arm balance.
• The single-pan analytical balance was far
  superior and replaced the traditional balance.
• The electronic analytical balance is the current
  balance that is widely used.

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• Fig. 2-2 Electronic analytical balance. (a) Block
  diagram. (b) Photo of electronic
• balance.
• Placing an object on the pan causes the pan and
  indicator arm to move downward, thus increasing
  the amount of light striking the photocell of the
  null detector.
• The increased current from the photocell is
  amplified and fed into the coil, creating a
  larger magnetic field, which returns the pan to
  its original null position.

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• Figure 2-3 Electronic analytical balances. (a)
  Classical configuration with pan beneath the
  cell. (b) A top-loading design.

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• In each electronic balance, the pan is tethered
  to a system of constraints known collectively as
  a cell.
• The cell has several flexures that permit limited
  movement of the pan.
• At null, the beam is parallel to the
  gravitational horizon.
• Electronic balances feature an automatic taring
  control that causes the display to read zero with
  a container (such as a boat or weighing bottle)
  on the pan.
• Some electronic balances have dual capacities and
  dual precisions.
• These features permit the capacity to be
  decreased from that of a macrobalance to that of
  a semimicrobalance (30 g) with a corresponding
  gain in precision to 0.01 mg.

12

• Figure 2-4 Single-pan mechanical analytical
  balance.
• Components
• A light-weight beam is supported on
• a planar surface by a prism-shaped
• knife edge (A).
• A second knife edge (B) is located
• near the left end of the beam and
• support as a second planar surface.
• The two knife edges are prism-shaped
• agate or sapphire devices that form
• low-friction bearings with two planar surfaces
• contained in stirrups also of agate or sapphire.

13

• Weighing with a Single-Pan Balance
• The beam of an adjusted balance is in horizontal
  position.
• Placing an object on the pan causes the left end
  of the beam to move downward.
• Masses are then removed systematically until the
  imbalance is less than 100 mg.
• The angle of deflection of the beam is directly
  proportional to the additional mass that must be
  removed to restore the beam to its horizontal
  position.
• The optical system measures this angle of
  deflection.
• A reticle is scribed with a scale that reads 0 to
  100 mg.
• A vernier makes it possible to read this scale to
  the nearest 0.1 mg.

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• Precautions in using an analytical balance
• 1. Center the load on the pan as well as
  possible.
• 2. Protect the balance from corrosion.
• 3. Observe special precautions for the weighing
  of liquids.
• 4. Consult your instructor if the balance appears
  to need adjustment.
• 5. Keep the balance and its case scrupulously
  clean. A camels hair brush is useful for
  removing spilled material or dust.
• 6. Always allow an object that has been heated to
  return to room temperature before weighing it.
• 7. Use tongs, finger pads, or a glassine paper
  strip to handle dried objects to prevent
  transferring moisture to them.

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• Sources of error in weighing
• Correction for Buoyancy
• A buoyancy error is an error that develops when
  the object being weighed has a significantly
  different density than the masses.
• Buoyancy corrections may be accomplished with
  the equation
• W1 is the corrected mass of the object,
• W2 is the mass of the standard masses,
• dobj is the density of the object,
• dwts is the density of the masses, and
• dair is the density of the air displaced by
  masses and object.
• The value of dair is 0.0012 g/cm3.

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• Figure 2-5. Effect of buoyancy on weighing data
  (density of weights 8 g/cm3). Plot of relative
  error as a function of the density of the object
  weighed.

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18

• Temperature Effects
• Allow heated objects to return to room
  temperature before you attempt to weigh them.
• Convection currents within the balance case exert
  a buoyant effect.
• Warm air trapped in a closed container weighs
  less than the same volume at a lower temperature.
  Both effects cause the apparent mass of the
  object to be low.
• Figure 2-6. Absolute error as a function of time
  after an
• object was removed from a 110C drying
• oven.
• A porcelain filtering crucible.
• B weighing bottle containing about
• 7.5 g of KCl.

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• Other Sources of Error
• A porcelain or glass object will occasionally
  acquire a static charge causing a balance to
  perform erratically, especially when the relative
  humidity is low.
• Spontaneous discharge frequently occurs after a
  short period.
• A low level source of radioactivity in the
  balance case will ionize enough ions to
  neutralize the charge.
• The optical scale of a single-pan mechanical
  balance should be checked regularly for accuracy,
  particularly under loading conditions that
  require the full-scale range.
• A standard 100-mg mass is used for this check.

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• Auxiliary Balances
• Less precise than analytical balances.
• Offer the advantages of speed, ruggedness, large
  capacity, and convenience.
• A sensitive top-loading balance will accommodate
  150-200 g with a precision of about 1 mg.
• Most are equipped with a taring device.
• Some are fully automatic, require no manual
  dialing or mass handling, and provide a digital
  readout of the mass.
• Modern top-loading balances are electronic.
• A triple-beam balance that is less sensitive than
  a typical top-loading auxiliary balance is also
  useful.
• This is a single-pan balance with three decades
  of masses that slide along individual calibrated
  scales.
• The precision may be one or two orders of
  magnitude less.

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• 2E Equipment and manipulations associated with
  weighing
• Drying or ignition to constant mass is a process
  in which a solid is cycled through heating,
  cooling, and weighing steps until its mass
  becomes constant to within 0.2 to 0.3 mg.
• Constant mass ensures that the chemical or
  physical processes that occur during the heating
  (or ignition) are complete.
• Figure 2-7. Weighing Bottles are convenient for
• drying and storing solids.
• Plastic bottles are rugged but abrade easily
• and not easily cleaned
• as compared to glass.

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• Desiccators and Desiccants
• Oven drying is the most common way of removing
  moisture from solids.
• Dried materials are stored in desiccators while
  they cool.
• Figure 2-8. A typical dessciator. The base
  section of a dessicator contains a chemical
  drying agent, such as anhydrous calcium chloride
  or calcium sulfate (Drierite).
• Very hygroscopic materials should be
• stored in containers equipped with
• snug covers, such as weighing bottles.
• The bottles remain covered while in
• the desiccator.

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• Manipulating Weighing Bottles
• Heating at 105C to 110C for 1 hour is
  sufficient to
• remove the moisture.
• Figure 2-9. The recommended way to dry a sample.
• Figure 2-10. Avoid touching dried objects with
  your
• fingers. Instead, use tongs, chamois finger cots,
  
• clean cotton gloves, or strips of paper to handle
  dried
• objects for weighing.

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• Weighing by Difference
• The bottle and its contents are weighed.
• One sample is then transferred from the bottle to
  a container.
• The bottle and its residual contents are then
  weighed.
• The mass of the sample is the difference between
  the two masses.
• Weighing Hygroscopic Solids
• Place the approximate amount of sample needed in
  the individual bottle and heat for an appropriate
  time.
• Then, quickly cap the bottles and cool in a
  desiccator.
• Weigh one of the bottles after opening it
  momentarily to relieve any vacuum.
• Quickly empty the contents of the bottle into its
  receiving vessel, cap immediately, and weigh the
  bottle again along with any solid that did not
  get transferred.
• Repeat for each sample and determine the sample
  masses by difference.

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• Weighing Liquids
• The mass of a liquid is always obtained by
  difference.
• Liquids that are noncorrosive and relatively
  nonvolatile can be transferred to previously
  weighed containers.
• The mass of the container is subtracted from the
  total mass.
• A volatile or corrosive liquid should be sealed
  in a weighed glass ampoule. The ampoule is
  heated, and the neck is then immersed in the
  sample.
• As cooling occurs, the liquid is drawn into the
  bulb.
• The ampoule is then inverted and the neck sealed
  off with a small flame. The ampoule and its
  contents, along with any glass removed during
  sealing, are cooled to room temperature and
  weighed.
• The ampoule is then transferred to an appropriate
  container and broken.
• A volume correction for the glass of the ampoule
  may be needed if the receiving vessel is a
  volumetric flask.

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• 2F Filtration and ignition of solids
• Apparatus
• Simple crucibles serve only as containers.
  Porcelain, aluminum oxide, silica, and platinum
  crucibles maintain constant mass.
• The solid is first collected on a filter paper.
  The filter and contents are then transferred to a
  weighed crucible, and the paper is ignited.
• Filtering crucibles serve not only as containers
• but also as filters. A vacuum is used to hasten
  the
• filtration.
• Figure 2-11 Adaptors for filtering crucibles.

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• Sintered-glass crucibles are manufactured in
  fine, medium, and coarse porosities.
• The upper temperature limit is usually 200C.
• Made of quartz and can tolerate substantially
  higher temperatures without damage.
• A Gooch crucible has a perforated bottom that
  supports a fibrous mat.
• Small circles of glass matting are used in pairs
  to protect against breaking during the
  filtration.
• Glass mats can tolerate temperatures in excess of
  500C and are substantially less hygroscopic.

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• Filter paper
• Paper is an important filtering medium.
• Ashless paper is manufactured from cellulose
  fibers that have been treated with hydrochloric
  and hydrofluoric acids.
• 9- or 11-cm circles of ashless paper leave a
  residue that weighs less than 0.1 mg, which is
  negligible under most circumstances.
• Ashless paper can be obtained in several
  porosities.
• A coarse-porosity ashless paper is most effective
  for filtering solids, but even with such paper,
  clogging occurs. This problem can be minimized by
  mixing a dispersion of ashless filter paper with
  the precipitate prior to filtration.

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• Heating Equipment
• Many precipitates can be weighed directly after
  being brought to constant mass in a
  low-temperature drying oven.
• The maximum attainable temperature ranges from
  140C to 260C.
• Microwave laboratory ovens greatly shorten drying
  cycles.
• An ordinary heat lamp can be used to dry a
  precipitate that has been collected on ashless
  paper and to char the paper as well.
• Burners are convenient sources of intense heat.
  The maximum attainable temperature depends on the
  design of the burner and the fuel combustion
  properties.
• The Meker burner provides the highest
  temperatures, followed by the Tirrill and Bunsen
  types.
• A heavy-duty electric furnace (muffle furnace) is
  capable of maintaining controlled temperatures of
  1100C or higher.

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• Filtering and Igniting Precipitates
• Preparation of Crucibles
• A crucible used to convert a precipitate to a
  form suitable for weighing must maintain a
  constant mass throughout drying or ignition.
• Backwashing a filtering crucible is done by
  turning the crucible upside down in the adaptor
  and sucking water through the inverted crucible.
• Figure 2-12
• (a) Washing by decantation.
• (b) Transferring the precipitate.
• The steps in filtering an analytical
• precipitate are decantation, washing,
• and transfer.

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• The last traces of precipitate on the inside of
  the beaker are dislodged with a rubber policeman.
• Many precipitates possess the property of
  creeping, or spreading over a wetted surface
  against the force of gravity.
• Filters are never filled to more than three
  quarters of capacity to prevent the possible loss
  of precipitate through creeping.
• The addition of a small amount of nonionic
  detergent, such as Triton X-100, to the
  supernatant liquid or wash liquid can help
  minimize creeping.
• A gelatinous precipitate must be completely
  washed before it is allowed to dry.

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• Figure 2-13. Preparation of a Filter Paper

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• Figure 2-14 Transferring Paper and Precipitate
  to a Crucible

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• Ashing Filter Papers
• If a heat lamp is used, the crucible is placed on
  a clean, nonreactive surface, such as a wire
  screen covered with aluminum foil.
• The lamp is positioned above the rim of the
  crucible.
• Charring is accelerated if the paper is moistened
  with a drop of concentrated ammonium nitrate
  solution.
• A burner produces much higher temperatures than a
  heat lamp.
• Partial reduction of some precipitates can occur
• through reaction with the hot carbon
• of the charring paper.
• Figure 2-15 Ignition of a precipitate.

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• Using Filtering Crucibles
• A vacuum filtration train is used when a
  filtering crucible can be used
• instead of paper.
• Figure 2-16 Train for vacuum filtration. The trap
  isolates the filter flask from the source of
  vacuum.

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• Rules for Manipulating Heated Objects
• Practice unfamiliar manipulations before putting
  them to use.
• 2. Never place a heated object on the benchtop.
  Instead, place it on a wire gauze or a
  heat-resistant ceramic plate.
• 3. Allow a crucible that has been subjected to
  the full flame of a burner or to a muffle furnace
  to cool momentarily.
• 4. Keep the tongs and forceps used to handle
  heated objects scrupulously clean. In particular,
  do not allow the tips to touch the benchtop.

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• 2G Measuring volume
• The precise measurement of volume is as important
  to many analytical methods as the precise
  measurement of mass.
• Units of Volume
• The unit of volume is the liter (L), defined as
  one cubic decimeter. The milliliter(mL) is one
  one-thousandth of a liter (0.001 L) and the
  microliter (µL) is 1026 L or 10-3 mL.
• The Effect of Temperature on Volume Measurements
• Most volumetric measuring devices are made of
  glass, which has a small coefficient of
  expansion.

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• The coefficient of expansion for dilute aqueous
  solutions (approximately 0.025/C) is such that
  a 5C change has a measurable effect on the
  reliability of ordinary volumetric measurements.

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• Apparatus for Precisely Measuring Volume
• Volume may be measured with a pipet, a buret, or
  a volumetric flask.
• Volumetric equipment is marked TD for to
  deliver or TC for to contain and also marked
  for the temperature at which the calibration
  strictly applies.
• Pipets and burets are usually calibrated to
  deliver specified volumes. Volumetric flasks are
  calibrated to contain a specific volume.
• Pipets
• Pipets permit the transfer of accurately known
  volumes.
• A volumetric pipet delivers a single fixed volume
  between 0.5 and 200 mL.
• Measuring pipets are calibrated in convenient
  units to permit delivery of any volume up to a
  maximum capacity ranging from 0.1 to 25 mL.

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Figure 2-17 Typical pipets (a) volumetric pipet,
(b) Mohr pipet, (c) serological pipet, (d)
Eppendorf micropipet, (e) OstwaldFolin pipet,
(f ) lambda pipet.
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• All volumetric and measuring pipets are first
  filled to a calibration mark.
• A small amount of liquid tends to remain in the
  tip after the pipet is emptied.
• This residual liquid is never blown out of a
  volumetric pipet or from some measuring pipets.
• Handheld Eppendorf micropipets deliver adjustable
  microliter volumes of liquid.

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Figure 2-18 Variable-volume automatic pipet,
1001000 µL.
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• Burets
• The precision attainable with a buret is
  substantially greater than the precision with a
  pipet.
• A buret consists of a calibrated tube to hold
  titrant plus a valve arrangement by which the
  flow of titrant is controlled.
• Figure 2-19 Burets
• (a) glass-bead valve,
• (b) Teflon valve.

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• Volumetric Flasks
• Figure 2-20 Typical volumetric
• flasks are manufactured with
• capacities ranging from 5 mL to 5 L.
• They are used for the preparation of standard
• solutions and for the dilution of samples to a
• fixed volume prior to taking aliquots with a pi-
• pet.
• Some are also calibrated on a to-deliver
• (TD) basis, and they are distinguished by
• two reference lines on the neck.

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• Using Volumetric Equipment
• Volume markings are blazed on clean volumetric
  equipment by the manufacturer.
• Only clean glass surfaces support a uniform film
  of liquid.
• Dirt or oil causes breaks in this film.
• Cleaning
• Brief soaking in a warm detergent solution is
  usually sufficient.
• The apparatus must be thoroughly rinsed with tap
  water and then with three or four portions of
  distilled water.
• It is seldom necessary to dry volumetric ware.
• Prolonged soaking will cause the formation of a
  ring at a detergent/air interface.
• This ring cannot be removed and causes a film
  break.

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• Avoiding Parallax
• It is common practice to use the bottom of the
  meniscus as the point of reference in calibrating
  and using volumetric equipment.
• Parallax is the apparent displacement of a liquid
  level or of a pointer as an observer changes
  position.
• Parallax is a condition that causes the volume to
  appear smaller than its actual value if the
  meniscus is viewed from above and larger if the
  meniscus is viewed from below.
• Figure 2-21 Reading a buret.

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• Directions for Using a Pipet
• Liquid is drawn into a pipet through the
  application of a slight vacuum. Never pipet by
  mouth because there is risk of accidentally
  ingesting the liquid being pipetted instead, use
  a rubber suction bulb.

Propipette consists of a rubber bulb (B)
attached to three short sections of tubing. Each
section of tubing contains a small chemically
inert ball (A, C, and D) that functions as a
valve to permit air to flow normally in the
directions indicated by the arrows. The valves
are opened by pinching with thumb and forefinger.
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• Cleaning
• Draw detergent solution to a level 2 to 3 cm
  above the calibration mark of the pipet.
• Drain this solution and then rinse the pipet with
  several portions of tap water.
• Inspect for film breaks, and repeat this portion
  of the cleaning cycle if necessary.
• Finally, fill the pipet with distilled water to
  perhaps one third of its capacity and carefully
  rotate it so that the entire interior surface is
  wetted.
• Repeat this rinsing step at least twice.

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• Measuring an Aliquot
• Draw a small volume of the sample liquid into the
  pipet and thoroughly wet the entire interior
  surface. Repeat twice.
• Carefully fill the pipet to a level above the
  graduation mark. Be sure that there are no
  bubbles.
• Touch the pipet tip to the wall of a glass vessel
  and slowly allow the liquid level to drop.
• When the bottom of the meniscus coincides exactly
  with the graduation mark, stop the flow.
• Remove the pipet from the volumetric flask, tilt
  it until liquid is drawn slightly up into the
  pipet, and wipe the tip with a lintless tissue.
• Place the pipet tip well within the receiving
  vessel, and allow the liquid to drain.
• Finally, withdraw the pipet with a rotating
  motion to remove any liquid adhering to the tip.
• Rinse the pipet thoroughly after use.

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Figure 2-22 Dispensing an aliquot.
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• Directions for Using a Buret
• Cleaning
• Clean the tube with detergent and a long brush.
  Rinse thoroughly with tap water and then with
  distilled water. Inspect for water breaks. Repeat
  if necessary.
• Lubricating a Glass Stopcock
• Carefully remove all old grease from a glass
  stopcock and its barrel with a paper towel and
  dry both parts completely. Lightly grease the
  stopcock. Insert the stopcock into the barrel and
  rotate it vigorously with slight inward pressure.
  
• A proper amount of lubricant has been used when
• (1) the area of contact between stopcock and
  barrel appears nearly transparent,
• (2) the seal is liquid-tight, and (3) no grease
  has worked its way into the tip.
• Buret readings should be estimated to the nearest
  0.01 mL.

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• Filling
• Ensure that the stopcock is closed.
• Add 5 to 10 mL of the titrant, and carefully
  rotate the buret to wet the interior completely.
• Allow the liquid to drain through the tip. Repeat
  twice.
• Then fill the buret well above the zero mark.
• Free the tip of air bubbles by rapidly rotating
  the stopcock and permitting small quantities of
  the titrant to pass.
• Lower the level of the liquid just to or somewhat
  below the zero mark.
• Allow for drainage (lt1 min), and then record the
  initial volume reading, estimating to the nearest
  0.01 mL.

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• Titration
• Figure 2-23 Recommended method
• for manipulating a buret stopcock.
• Be sure the tip of the buret is well within the
• titration flask, and introduce the titrant in
• increments of about 1 mL.
• Swirl (or stir) constantly.
• Decrease the volume of the
• increments progresseively toward the end point.
• When only a few more drops are needed to reach
  the end point, rinse
• the walls of the container.
• Allow the titrant to drain from the inner wall of
  the buret (at least 30 seconds) at the completion
  of the titration. Then, record the final volume,
  again to the nearest 0.01 mL.

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• Directions for Using a Volumetric Flask
• Volumetric flasks should be washed with detergent
  and thoroughly rinsed.
• Only rarely do they need to be dried.
• Drying is best accomplished by clamping the flask
  in an inverted position.
• Direct Weighing into a Volumetric Flask
• The direct preparation of a standard solution
  requires the introduction of a known mass of
  solute to a volumetric flask.
• Use of a powder funnel minimizes the possibility
  of losing solid during the transfer.
• Rinse the funnel thoroughly, and collect the
  washings in the flask.

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• Quantitative Transfer of Liquid to a Volumetric
  Flask
• The solute should be completely dissolved before
  diluting to the mark.
• Insert a funnel into the neck of the volumetric
  flask, and use a stirring rod to direct the flow
  of liquid from the beaker into the funnel.
• With the stirring rod, tip off the last drop of
  liquid on the spout of the beaker.
• Rinse both the stirring rod and the interior of
  the beaker with distilled water and transfer the
  washings to the volumetric flask as before.
  Repeat the rinsing process.

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• Diluting to the Mark
• After the solute has been transferred, fill the
  flask about half full and swirl the contents to
  hasten solution.
• Add more solvent and again mix well.
• Bring the liquid level almost to the mark, and
  allow time for drainage (1 min). Then, use a
  medicine dropper to make any necessary final
  additions of solvent.
• Firmly stopper the flask, and invert it
  repeatedly to ensure thorough mixing.
• Transfer the contents to a storage bottle that
  either is dry or has been thoroughly rinsed with
  several small portions of the solution from the
  flask.

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• 2H Calibrating volumetric glassware
• Measure the mass of a liquid of known density and
  temperature that is contained in (or delivered
  by) the volumetric ware.
• A buoyancy correction must be made since the
  density of water is quite different from that of
  the masses.
• The volume of the apparatus at the temperature of
  calibration (T) is obtained by dividing the
  density of the liquid at that temperature into
  the corrected mass.
• Finally, this volume is corrected to the standard
  temperature of 20C.

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• General Directions for Calibration
• All volumetric ware should be freed of water
  breaks before being calibrated.
• Burets and pipets need not be
• dry, but volumetric flasks should
• be thoroughly drained and dried
• at room temperature.

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• Calibrating a Volumetric Pipet
• Determine the empty mass of the stoppered
  receiver to the nearest milligram.
• Transfer a portion of temperature-equilibrated
  water to the receiver with the pipet, weigh the
  receiver and its contents (to the nearest
  milligram), and calculate the mass of water
  delivered from the difference in these masses.
• Calculate the volume delivered.
• Repeat the calibration several times, and
  calculate the mean volume delivered and its
  standard deviation.

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• Calibrating a Buret
• Fill the buret with temperature-equilibrated
  water.
• Lower the liquid level to bring the bottom of the
  meniscus to the 0.00-mL mark.
• Touch the tip to the wall of a beaker to remove
  any adhering drop. Wait 10 minutes and recheck
  the volume.
• Weigh (to the nearest milligram) a 125-mL
  conical flask fitted with a rubber stopper.
• Slowly transfer (at about 10 mL/min)
  approximately 10 mL of water to the flask. Touch
  the tip to the wall of the flask.
• Wait 1 minute, record the volume that was
  apparently delivered, and refill the buret.

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• Weigh the flask and its contents to the nearest
  milligram. The difference between this mass and
  the initial value is the mass of water delivered.
  Convert this mass to the true volume. Subtract
  the apparent volume from the true volume.
• This difference is the correction that should be
  applied to the apparent volume to give the true
  volume.
• Repeat the calibration until agreement within
  60.02 mL is achieved.
• Calibrating a Volumetric Flask
• Weigh the clean, dry flask to the nearest
  milligram.
• Then fill to the mark with equilibrated water and
  reweigh.
• With the aid of the table, calculate the volume
  contained.

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• 2I The laboratory notebook
• It is needed to record measurements and
  observations concerning an analysis. It should be
  permanently bound with consecutively numbered
  pages.
• Maintaining a Laboratory Notebook
• Record all data and observations directly into
  the notebook in ink.
• Supply each entry or series of entries with a
  heading or label.
• Date each page of the notebook as it is used.
• Never attempt to erase or obliterate an incorrect
  entry.
• Never remove a page from the notebook. Draw
  diagonal lines across any page that is to be
  disregarded. Provide a brief rationale for
  disregarding the page.

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• Notebook Format
• In one convention, data and observations are
  recorded on consecutive pages as they occur.
• The completed analysis is then summarized on the
  next available page spread.
• The first of these two facing pages should
  contain the following entries
• 1. The title of the experiment.
• 2. A brief statement of the principles on which
  the analysis is based.
• 3. A complete summary of the weighing,
  volumetric, and/or instrument response data
  needed to calculate the results.
• 4. A report of the best value for the set and a
  statement of its precision.
• The second page should contain the following
  items
• 1. Equations for the principal reactions in the
  analysis.
• 2. An equation showing how the results were
  calculated.
• 3. A summary of observations that appear to bear
  on the validity.

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Figure 2-24 Laboratory notebook data page.
67

• 2J Safety in the laboratory
• 1. Learn the location of the nearest eye
  fountain, fire blanket, shower, and fire
  extinguisher. Do not hesitate to use this
  equipment if the need arises.
• 2. Wear eye protection at all times.
• 3. Most of the chemicals in a laboratory are
  toxic. Avoid contact with these liquids. In the
  event of such contact, immediately flood the
  affected area with large quantities of water.
• 4. NEVER perform an unauthorized experiment.
  Unauthorized experiments are
• grounds for disqualification at many
  institutions.
• 5. Never work alone in the laboratory. Always be
  certain that someone is within earshot.
• 6. Never bring food or beverages into the
  laboratory. NEVER drink from laboratory
  glassware. NEVER smoke in the laboratory.

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• Always use a bulb or other device to draw liquids
  into a pipet. NEVER pipet by mouth.
• 8. Wear adequate foot covering (no sandals).
  Confine long hair with a net. A laboratory coat
  or apron will provide some protection and may be
  required.
• 9. Be extremely tentative in touching objects
  that have been heated because hot glass looks
  exactly like cold glass.
• Always fire-polish the ends of freshly cut glass
  tubing. NEVER attempt to force glass tubing
  through the hole of a stopper.
• ?11. Use fume hoods whenever toxic or noxious
  gases are likely to be evolved. Be cautious in
  testing for odors.
• ?12. Notify your instructor immediately in the
  event of an injury.
• ?13. Dispose off solutions and chemicals as
  instructed. It is illegal to flush solutions
• containing heavy metal ions or organic liquids
  down the drain in most localities.