Measuring Mass
In most analyses, an analytical balance must be used to measure masses with high accuracy. Less accurate laboratory balances are also used for mass measurements when the demands for reliability are not critical.
Types of Analytical Balances
What is An analytical balance ?
** An analytical balance is an instrument for
determining mass with a maximum capacity that ranges from 1 g to a few
kilograms with a precision of at least 1 part in 105 at maximum
capacity.
** The precision and accuracy of many modern analytical balances exceed
1 part in 106 at full capacity.
Types of Analytical Balances:
(1) An analytical balance: has a maximum
capacity that ranges from 1 g to several kilograms and a precision at maximum capacity
of at least 1 part in 105.
(2) A macrobalance: is the most common type of
analytical balance, and it has a maximum load of 160 to 200 g and a precision
of 0.1 mg.
(3) A semimicroanalytical balance: has a
maximum load of 10 to 30 g and a precision of 0.01 mg.
(4) A
microanalytical balance: has a maximum load of 1 to 3 g and a precision of
0.001 mg, or 1 μg.
** The
most common analytical balances (macrobalances) have a maximum capacity ranging
between 160 and 200 g. With these balances, measurements can be made with a
standard deviation of +0.1 mg. Semimicroanalytical balances have a maximum
loading of 10 to 30 g with a precision of +0.01 mg. A typical microanalytical
balance has a capacity of 1 to 3 g and a precision of +0.001 mg (1 μg).
** The
analytical balance has evolved dramatically over the past several decades. The
traditional analytical balance had two pans attached to either end of a
lightweight beam that pivoted about a knife edge located in the center of the
beam. The object to be weighed was placed on one pan. Standard masses were then
added to the other pan to restore the beam to its original position. Weighing
with such an equal-arm balance was tedious and time consuming.
** The
first single-pan analytical balance appeared on the market in 1946. The speed
and convenience of weighing with this balance were vastly superior to what
could be realized with the traditional equal-arm balance. As a result, this
balance rapidly replaced the latter in most laboratories. The single-pan
balance is currently being replaced by the electronic analytical balance, which
has neither a beam nor a knife edge. The single-pan balance is still used in
some laboratories, but the speed, ruggedness, convenience, accuracy, and
capability for computer control and data logging of electronic balances ensure
that the mechanical single-pan analytical balance will soon disappear from the
scene.
Precautions in Using an Analytical Balance
** An analytical balance is a delicate
instrument that you must handle with care. Consult with your instructor for
detailed instructions on weighing with your particular model of balance.
** Observe the following general rules for working with an analytical balance
regardless of make or model:
(1) Center the load on the pan as well as
possible.
(2)
Protect the balance from corrosion. Objects to be placed on the pan
should be limited to nonreactive metals, nonreactive plastics, and vitreous, or
glasslike, materials.
(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 camel’s-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.
Sources of Error in Weighing
(1) Correction for Buoyancy
** A
buoyancy error will affect data if the density of the object being weighed
differs significantly from that of the standard masses. This error has its
origin in the difference in buoyant force exerted by the medium (air) on the
object and on the masses.
** Buoyancy corrections for electronic balances may be
accomplished with the equation:
W1 = is the corrected mass of the object.
W2 = the mass of
the standard masses.
dobj = the density of the object.
dwts
= the density of the masses.
dair = the density of the air
displaced by masses and object. The value of dair is 0.0012 g/cm3.
** The
consequences of Equation are shown in Figure blow in which the relative error
due to buoyancy is plotted against the density of objects weighed in air
against stainless steel masses. Note that this error is less than 0.1% for
objects that have a density of 2 g/cm3 or greater. It is thus seldom
necessary to correct the masses of most solids. The same cannot be said for
low-density solids, liquids, or gases, however. For these, the effects of
buoyancy are significant, and a correction must be applied.
** The
density of masses used in single-pan balances (or to calibrate electronic
balances) ranges from 7.8 to 8.4 g/cm3, depending on the
manufacturer. Use of 8 g/cm3 is close enough for most purposes. If
greater accuracy is required, the manufacturer’s specifications for the balance
usually give the necessary density data.
Example : A bottle weighed
7.6500 g empty and 9.9700 g after introduction of an organic liquid with a
density of 0.92 g/cm3. The balance was equipped with stainless steel
masses (d 8.0 g/cm3). Correct the mass of the sample for the
effects of buoyancy.
Solution
The apparent mass of the liquid is 9.9700 -
7.6500 = 2.3200 g. The same buoyant force acts on the container during both
weighings. Thus, we need to consider only the force that acts on the 2.3200 g
of liquid. By substituting 0.0012 g/cm3 for dair , 0.92 g/cm3 for dobj , and 8.0 g/cm3 for dwts
in Equation above, we find that the corrected mass is:
(2) Temperature Effects
Attempts to weigh an object whose temperature
is different from that of its surroundings will result in a significant error.
Failure to allow sufficient time for a heated object to return to room
temperature is the most common source of this problem. Errors due to a
difference in temperature have two sources:
(a) convection currents within the balance
case exert a buoyant effect on the pan and object.
(b) 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. This error can amount to as much as 10 or 15 mg for typical
porcelain filtering crucibles or weighing bottles (see Figure ). Heated objects
must always be cooled to room temperature before being weighed.
(3) Other Sources of Error
(a) A porcelain or glass object will
occasionally acquire a static charge sufficient to cause a balance to perform
erratically. This problem is particularly serious when the relative humidity is
low. Spontaneous discharge frequently occurs after a short period.
(B) A low level source of radioactivity (such
as a Static-Master photographer’s brush containing a miniscule amount of
polonium) in the balance case will ionize enough ions to neutralize the charge.
Alternatively, the object can be wiped with a faintly damp chamois.
(C) 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.
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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