Analytical balances are precise weighing devices used to measure small masses in clinical and laboratory sciences. There are two main types - mechanical balances which use lever systems to balance unknown masses against calibrated weights, and electronic balances which use electromagnetic forces controlled by a null detector to restore the balance beam to equilibrium. Both must be accurate, sensitive, and calibrated regularly to ensure proper performance.

1. Weighing is a required part of almost any analysis,
both for measuring the sample and for preparing
standard solutions.
In clinical laboratory sciences we deal with rather
small weights, in the order of grams to a few
milligrams or less. Standard laboratory weighing is
typically made to three or four significant figures,
and so the weighing device must be both accurate
and sensitive.
There are various sophisticated ways of achieving
this, but the most useful and versatile device is the
analytical balance

2. Analytical Balances
Most analytical balances used today are
electronic balances .
The mechanical single-pan balance is still used,
though, and so we will describe its operation.
Both types are based on comparison of one
weight against another (the electronic one for
calibration ) and have in common factor such
as zero-point drift and air buoyancy.

3. Analytical Balances
The weight of an object is the force exert on it by
the gravitational attraction. This force will differ at
different locations on Earth.
Mass, on the other hand is the quantity of matter of
which the object is composed and invariant.
We really deal with masses rather than weights

4. Analytical Balances
They are two types
1- Mechanical.
2- Electrical.
The mechanical analytical balances are also
two types
1- The equal arm balances.
2- The single pan substitution balance.

5. Analytical Balances
The equal arm balance:
Consist of a knife edge supporting the exact
center of the beam when
L1 = L2
Set of weights (in grams) for use
with an open two-pan balance

6. The equal arm balance
The length of the two lever arms are equal,
therefore the mass supported by each arm will
equal for the system to be in equilibrium.
If an unknown mass is added to one side the
beam is deflected, the deflection is offset by
adding standard weight mass until the beam
return to the horizontal position so that
mu L1 =ms L2

7. The single pan substitution balance
It is situated asymmetrically on the central knife.
It has one pan and a counter weight on the other side.
Series of calibrated weights are supported. When
unknown weight is placed on the weighing pan, the
beam deflects in the direction of the heavier side.
To return the system to equilibrium, a correspondent
mass of calibrated weights is removed from the front of
the beam. This process is a substitution process.

8. Quality of mechanical balances:
The condition of the knife edge is critical
to the performance. It is made of extremely
hard material. So the area of contact must be
with reduced fiction. There is considerable at
the point of contact with the beam. Therefore
shock can easily damage these parts.
Mechanical balance are also equipped with
dampers oppose the movement of the beam,
allowing it to come to equilibrium rapidly.
Damping is either magnetic plate or air damper
(piston).

9. Advantage of substitution balance over the
equal arm balance:
Equal arm balance is time consuming.
The sensitivity in the equal arm balance
depends on the load placed on the balance,
while the load is always on the substitution
balance.
Error in substitution balance is eliminated
because comparison of unknown mass with
known mass takes place on the same arm.
Two knife edges can reduce friction.

10. Electrical Balances
In electrical balance, an electromagnetic force is
used to counter balance the object′s weight and
restore the beam to it′s zero position.
The magnetic of the force is proportional to the
load.
Some sort of electronic ′s (digital) is used to
converts the current to a digital display of the
weight.

11. Electrical Balances
The null detector: The position of the balance
beam and is used to determine the balance point
at which the system is in equilibrium.
To return the system to equilibrium,
electromagnetic restoring forces are used in place
of standard weight.
The signal from the null detector is applied directly
to control the current through an electromagnetic
to return the balance to equilibrium.

12. Electrical Balances
When the null meter read zero, the
compensating force required to bring the
balance to equilibrium is proportional to the
weight of pan.
The current required produce the compensating
force is converted to a digital display of the
weight.

13. Electrical Balances
Advantages of electronic balance over
mechanical balance:
High precision “the agree between repeated
weights of the same mass”.
No hand.
Time to make weighing is short.

14. Electrical Balances
Quality assurance and maintenance:
The performance of an analytical balance should judged
by:-
1. Sensitivity, “the ratio of the charge in scale division to
specific weight change.
2. Precision.
3. Accuracy “ The agreement between the measured weight
and the calibrated weight of an object”.
4. Readability “The smallest fraction of the scale division
that can be read.
The capacity of the balance must also be consider in
selecting the balance.

15. Maintenance
1. The balance should be placed away from detector
vibration.
2. Object that will over load the balance and exceed its
capacity should not be placed on the pan.
3. Plastic container could generate electrostatic forces
4. For optimum balance performance, preventive
maintenance and calibration should be done according
to the manufacture instructions at least one a year.
5. Calibration should be checked every time the balance
has been moved.
6. The warm up time for electronic balance should be
verified.
7. A calibration weight at least 60% of the capacity
should be used in calibration and at different pan
position.

16. The Triple Beam Balance
The Triple Beam Balance is a typical mechanical balance.
It has a beam which is supported by a fulcrum. On one side is
a pan on which the object is placed. On the other side, the
beam is split into three parallel beams , each supporting one
weight. In measuring the weight of an object, rather than
adding additional weights, each of the three weights can be
slid along the beam to increase their lever arm.
It works just like a tetter-totter. If you have two people of
unequal weight, the heavier person sits closer to the fulcrum to
decrease their lever arm.

17. The Triple Beam Balance
The triple beam balance is a rugged, inexpensive
weighing instrument. It is named for the three
beams which carry weights.
A. The middle beam reads only in 100 g
increments.
B. The far beam reads only in 10 g increments.
The weights in each of these must always sit in a
"notch". They cannot be placed at arbitrary points
on the beam.
C. The weight on the front beam can be placed to
read continuously from 0 to 10 grams.