Measuring Volume in analytical Laboratory
The precise
measurement of volume is as important to many analytical methods as the precise
measurement of mass.
(1) 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 is used when the
liter represents an inconveniently large volume unit.
** The
microliter (µL) is 10-6 L or 10-3 mL.
(2) The Effect of Temperature on Volume Measurements
** The volume
occupied by a given mass of liquid varies with temperature, as does the device
that holds the liquid during measurement. Most volumetric measuring devices are
made of glass, which fortunately has a small coefficient of expansion. Thus,
variations in the volume of a glass container with temperature need not be
considered in ordinary analytical work.
** The
coefficient of expansion for dilute aqueous solutions (approximately 0.025%/°C)
is such that a 5°C change has a measurable effect on the reliability of
ordinary volumetric measurements.
** Volumetric
measurements must be referred to a standard temperature, often 20°C. The ambient
temperature of most laboratories is usually close enough to 20°C so that there
is no need for temperature corrections in volume measurements for aqueous
solutions. In contrast, the coefficient of expansion for organic liquids may be
large enough to require corrections for temperature differences of 1°C or less.
(3) Apparatus for Precisely Measuring Volume
** Volume may
be measured reliably with a pipet, a buret, or a volumetric flask.
** Volumetric
equipment is marked by the manufacturer to indicate not only the manner of calibration
(usually TD for “to deliver” or TC for “to contain”) but also the temperature
at which the calibration strictly applies. Pipets and burets are usually calibrated
to deliver specified volumes. Volumetric flasks, on the other hand, are calibrated
to contain a specific volume.
** Glassware types include Class A
and Class B. Class A glassware is manufactured to the highest tolerances from
Pyrex, borosilicate, or Kimax glass (see tables). Class B (economy ware)
tolerances are about twice those of Class A.
Pipets
** Pipets
permit the transfer of accurately known volumes from one container to another.
** Common types
are shown in Figure (1), and information concerning their use is given in Table
(1).
 |
| Table (1) |
** A volumetric, or transfer, pipet (Figure 1a) delivers a single, fixed
volume between 0.5 and 200 mL. Many such pipets are color coded by volume for
convenience in identification and sorting. Measuring pipets (Figure 1b and
c) are calibrated in convenient units to permit delivery of any volume up to a
maximum capacity ranging from 0.1 to 25 mL.
** All
volumetric and measuring pipets are first filled to a calibration mark, but the
manner in which the transfer is completed depends on the particular type.
Because most liquids are attracted to glass, 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, but it is blown
out of other types of pipets (see Table 1).
** Handheld
Eppendorf micropipets (see Figure 1d and Figure 2a) deliver adjustable
microliter volumes of liquid. With these pipets, a known and adjustable volume
of air is displaced from the plastic disposable tip by depressing the pushbutton
on the top of the pipet to a first stop. This button operates a springloaded piston
that forces air out of the pipet. The volume of displaced air can be varied by
a locking digital micrometer adjustment located on the front or top of the device.
The plastic tip is then inserted into the liquid, and the pressure on the
button released, causing liquid to be drawn into the tip. The tip is then
placed against the walls of the receiving vessel, and the pushbutton is again
depressed to the first stop. After 1 second, the pushbutton is depressed
further to a second stop, which completely empties the tip. The range of
volumes and precision of typical pipets of this type are shown in the margin.
The accuracy and precision of automatic pipets depend somewhat on the skill and
experience of the operators and thus should be calibrated for critical work.
** Numerous
automatic pipets are available for situations that call for the repeated
delivery of a particular volume. In addition, motorized, computer-controlled
microliter pipets are now available (see Figure 2b). These devices are
programmed to function as pipets, dispensers of multiple volumes, burets, and
sample dilutors. The volume desired is entered using a joystick and buttons and
is displayed on an LCD panel. A motor-driven piston dispenses the liquid. Maximum
volumes range from 10 µL to 20 mL.
Burets
** Burets, like
measuring pipets, make it possible to deliver any volume up to the maximum capacity
of the device. 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. This valve is the principal source of
difference among burets. The simplest pinchcock valve consists of a
close-fitting glass bead inside a short length of rubber tubing that connects
the buret and its tip (see Figure 3a). Only when the tubing is deformed does
liquid flow past the bead.
** A buret
equipped with a glass stopcock for a valve relies on a lubricant between the
ground glass surfaces of stopcock and barrel for a liquid-tight seal. Some
solutions, notably bases, cause glass stopcocks to freeze when they are in
contact with ground glass for long periods. Therefore, glass stopcocks must be
thoroughly cleaned after each use. Most burets made in the last several of
decades have Teflon® valves, which are unaffected by most common reagents and
require no lubricant (see Figure 3b).
Volumetric Flasks
** Volumetric
flasks (see Figure 4) are manufactured with capacities ranging from 5 mL to
5 L and are usually calibrated to contain (TC) a specified volume when filled to
a line etched on the neck.
** 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 pipet.
** Some are also calibrated on a to-deliver (TD) basis, and
they are distinguished by two reference lines on the neck. If delivery of the stated
volume is desired, the flask is filled to the upper line.
(4) Using Volumetric Equipment
** Volume
markings are blazed on clean volumetric equipment by the manufacturer.
** An equal degree
of cleanliness is needed in the laboratory if these markings are to have their
stated meanings.
** Only clean
glass surfaces support a uniform film of liquid. Dirt or oil causes breaks in
this film, so if breaks are present, the surface is almost certainly dirty.
Cleaning
** A brief
soaking in a warm detergent solution is usually sufficient to remove the grease
and dirt responsible for water breaks.
** Prolonged
soaking should be avoided because a rough area or ring is likely to develop at
a detergent/air interface. This ring cannot be removed and causes a film break
that destroys the usefulness of the equipment.
** After being
cleaned, 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.
Avoiding Parallax
** The top
surface of a liquid confined in a narrow tube exhibits a marked curvature, or meniscus.
It is common practice to use the bottom of the meniscus as the point of
reference in calibrating and using volumetric equipment. This minimum can be
established more exactly by holding an opaque card or piece of paper behind the
graduations.
** In reading
volumes, the eye must be at the level of the liquid surface to avoid an error
due to parallax. 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 blow shows Reading a buret:
(a) The student
reads the buret from a position above a line perpendicular to the buret and
makes a reading (b) of 12.58 mL.
(c) The student
reads the buret from a position along a line perpendicular to the buret and
makes a reading (d) of 12.62 mL.
(e) The student
reads the buret from a position below a line perpendicular to the buret and
makes a reading (f ) of 12.67 mL.
To avoid the
problem of parallax, buret readings should be made consistently along a line
perpendicular to the buret, as shown in (c) and (d).
(5) Directions for Using a Pipet
** The
following directions are appropriate specifically for volumetric pipets but can
be modified for the use of other types as well.
** 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 (such as the one shown at the top of the
next page) or one of a number of similar, commercially available devices.
** Many devices
are commercially available for filling pipets and dispensing liquids from them.The
device shown here is offered by many suppliers and manufacturers. Originally
called the Propipette®, it is a very handy device for the task.
** The
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 your thumb and
forefinger. The bottom of the device fits snugly on the top of a pipet.
Operation begins by opening valve A an squeezing bulb B to expel the air in the
bulb. Valve A is then closed, and valve C is opened to draw liquid into the
pipet to the desired level, after which C is closed. The liquid level is then
adjusted in the pipet by carefully opening valve D, and finally, the liquid in
the pipet is delivered by opening valve D completely.
Cleaning
(1) Draw
detergent solution to a level 2 to 3 cm above the calibration mark of the
pipet.
(2) Drain
this solution and then rinse the pipet with several portions of tap water.
(3) Inspect
for film breaks, and repeat this portion of the cleaning cycle if necessary.
(4) 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.
(5) Repeat
this rinsing step at least twice.
Measuring an Aliquot
 |
| Figure (5) |
(2) thoroughly
wet the entire interior surface (Figure 5b).
(3) Repeat
with at least two additional portions.
(4) Then
carefully fill the pipet to a level somewhat above the graduation mark. Be sure
that there are no bubbles in the bulk of the liquid or foam at the surface.
(5) Touch
the tip of the pipet to the wall of a glass vessel as shown in Figure 5c (not
the container into which the aliquot is to be transferred),
(6)
slowly allow the liquid level to drop. As the bottom of the meniscus coincides
exactly with the graduation mark (Figure 5d), stop the flow.
(7) 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 as shown in Figure 5e.
(8) Then
place the pipet tip well within the receiving vessel, and allow the liquid to
drain (Figure 5f ).
(9) When
free flow ceases, rest the tip against the inner wall of the receiver for a
full 10 seconds (Figure 5g, h).
(10) Finally,
withdraw the pipet with a rotating motion to remove any liquid adhering to the
tip. The small volume remaining inside the tip of a volumetric pipet should not
be blown or rinsed into the receiving vessel.
(11) Rinse
the pipet thoroughly after use.
(6) Directions for Using a Buret
A buret must be
scrupulously clean before it is used, and its valve must be liquid-tight.
Cleaning
(1) Thoroughly
clean the tube of the buret with detergent and a long brush.
(2) Rinse
thoroughly with tap water and then with distilled water.
(3) Inspect
for water breaks.
(4) Repeat
the treatment 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, taking care to avoid the area adjacent to the hole.
** Insert the
stopcock into the barrel and rotate it vigorously with slight inward pressure.
** A proper
amount of lubricant has been used when:
(a) the area of
contact between stopcock and barrel appears nearly transparent.
(b) the seal is
liquid-tight.
(c) no grease
has worked its way into the tip.
Notes
[1] Grease
films that are unaffected by cleaning solution may yield to such organic
solvents as acetone or alcohols. Thorough washing with detergent should follow
such treatment. Silicone lubricants are not recommended because contamination by
such preparations is difficult—if not impossible—to remove.
[2] So
long as the flow of liquid is not impeded, fouling of a buret tip with stopcock
grease is not a serious matter. Removal is best accomplished with organic
solvents. A stoppage during a titration can be freed by gentle warming of the
tip with a lighted match.
[3] Before
a buret is returned to service after reassembly, it is advisable to test for
leakage. Simply fill the buret with water and establish that the volume reading
does not change with time.
Filling
(1) Make
certain the stopcock is closed.
(2) Add 5 to 10
mL of the titrant, and carefully rotate the buret to wet the interior
completely.
(3) Allow the
liquid to drain through the tip.
(4) Repeat this
procedure at least two more times.
(5) fill the
buret well above the zero mark.
(6) Free the
tip of air bubbles by rapidly rotating the stopcock and permitting small
quantities of the titrant to pass. Finally, lower the level of the liquid just
to or somewhat below the zero mark.
(7) Allow for
drainage (≈ 1 min), and then record the initial volume reading, estimating to
the nearest 0.01 mL.
Titration
** Figure (6) illustrates
the preferred method for manipulating a stopcock.
** When you position
your hand as shown, your grip on the stopcock tends to keep the stopcock firmly
seated.
** 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 to ensure thorough mixing.
** Decrease the
volume of the increments as the titration progresses, and add titrant drop by
drop as you reach the immediate vicinity of the end point (Note 2).
** When it
appears that only a few more drops are needed to reach the end point, rinse the
walls of the container (Note 3).
** 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.
Notes
[1] When
unfamiliar with a particular titration, many workers prepare an extra sample. No
care is taken with its titration since its functions are to reveal the nature
of the end point and to provide a rough estimate of titrant requirements. This deliberate
sacrifice of one sample frequently results in an overall saving of time.
[2] Increments
smaller than one drop can be taken by allowing a small volume of titrant to
form on the tip of the buret and then touching the tip to the wall of the flask.
This partial drop is then combined with the bulk of the liquid as in Note 3.
[3] Instead
of being rinsed toward the end of a titration, the flask can be tilted and
rotated so that the bulk of the liquid picks up any drops that adhere to the
inner surface.
(7) Directions for Using a Volumetric Flask
** Before being
put into use, volumetric flasks should be washed with detergent and thoroughly
rinsed.
** Only rarely
do they need to be dried. If required, however, drying is best accomplished by clamping
the flask in an inverted position.
** Insertion of
a glass tube connected to a vacuum line hastens the process.
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.
** The
foregoing procedure may be inappropriate if heating is needed to dissolve the
solute.
(a) Instead,
weigh the solid into a beaker or flask, add solvent.
(b) Heat to
dissolve the solute.
(c) Allow the
solution to cool to room temperature.
(d) Transfer
this solution quantitatively to the volumetric flask, as described in the next
section.
Quantitative Transfer of Liquid to a Volumetric Flask
(1) Insert a
funnel into the neck of the volumetric flask.
(2) use a
stirring rod to direct the flow of liquid from the beaker into the funnel.
(3) With the
stirring rod, tip off the last drop of liquid on the spout of the beaker.
(4) Rinse both
the stirring rod and the interior of the beaker with distilled water and
transfer the washings to the volumetric flask as before.
(5) Repeat the
rinsing process at least two more times.
Diluting to the Mark
(1) After the
solute has been transferred.
(2) fill the
flask about half full and swirl the contents to hasten solution.
(3) Add more
solvent and again mix well.
(4) Bring the
liquid level almost to the mark, and allow time for drainage (≈1 min).
(5) use a
medicine dropper to make any necessary final additions of solvent (see Note
below).
(6) Firmly stopper
the flask, and invert it repeatedly to ensure thorough mixing.
(7) 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.
Notes
** If, as
sometimes happens, the liquid level accidentally exceeds the calibration mark, the
solution can be saved by correcting for the excess volume.
** Use a
selfstick label to mark the location of the meniscus.
** After the
flask has been emptied, carefully refill to the manufacturer’s etched mark with
water.
** Use a buret
to determine the additional volume needed to fill the flask so that the
meniscus is at the gummed-label mark. This volume must be added to the nominal
volume of the flask when calculating the concentration of the solution.
Reference: Fundamentals of analytical chemistry / Douglas A. Skoog, Donald M. West, F. James Holler, Stanley R. Crouch. (ninth edition) , 2014 . USA
Reference: Fundamentals of analytical chemistry / Douglas A. Skoog, Donald M. West, F. James Holler, Stanley R. Crouch. (ninth edition) , 2014 . USA
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