WEIGHING IN THE LABORATORY
WEIGHING IN THE LABORATORY
Introduction
2.0 Objectives
3.0 Main Content
3.1 Types of Weighing Balances
3.2 Weighing
3.3 Weighing on an Analytical Balance
3.3.1 Planning
3.3.2 Checking the Balance
3.3.3 Balance Environment
3.3.4 Calibration
3.3.5 Balance Uncertainties
3.3.6 Rules for Weighing
3.3.7 Weighing by Difference to Overcome the Problem
of Balance Calibration Errors
3.4 Sample Weighing Experiment
3.4.1 Using the Top-Loading Balance
3.4.2 Using the Analytical Balance
3.4.3 Weighing by Difference using the Tare Function of
the Analytical Balance
3.5 Report Sheet
4.0 Summary
5.0 Conclusion
6.0 Tutor-Marked Assignment
7.0 References/Further Reading
1.0 INTRODUCTION
Weighing scale is very fascinating and interesting. These did not come
to existence overnight, they evolved from a barter system where units
were absent compared to the modern day standards. When the standard
units were accepted by businesses and governments across different
regions, an industrial revolution was set to begin and everyone benefited
from that. Today the old age classical weighing scales have become
obsolete but they served the humanity for a long time. In the modern
times, one can see the weighing scales ruling the industrial and domestic
space.
160
2.0 OBJECTIVES
At the end of this unit, you should be able to:
• identify different equipment used in laboratory weighing
• differentiate between a to top-loading balance and an analytical
balance
• carry out simple weighing experiments in the laboratory.
3.0 MAIN CONTENT
3.1 Types of Weighing Balances
Here is a list of different products that you should know about.
Fig. 1.1: Iron Scale
1. The conventional wooden or iron scale was the oldest weighing
machine invented by man. It consisted of two containers attached
to each other through a pulley system. These weighing scales
were meant for domestic purposes and they often reported wrong
measurements. It was more of a weighing balance than a
weighing scale.
2. The next stage was the spring balance. A spring connected to a
fixed and rigid system that was used to weigh mass of objects by
hanging them with the spring. The spring will develop tension in
it and it will reflect a standard value on the rigid body attached to
it. The value will reflect the weight of the object in standard unit
of mass. Precision and accuracy were still missing in this case.
Fig.1.2: Spring Balance
3. The modern digital weighing scales are the third kind of
weighing machines invented so far. These are the products that
are electronically operated and display results very precisely.
They have many types, styles and weigh ranges. Broadly, they
can be categorised into two categories namely; Industrial Scales
and Domestic Scales.
Fig. 1.3: Digital Weighing Scales
4. Hydraulic Scales are used mainly in the laboratories and
industrial units. It made use of liquid displacement technique to
weigh the mass of heavy objects. The object placed on the top of
weighing panel displaces the liquid and the volume change
indicates the weight of the object.
Today, the weighing scales have changed a lot over a course of time.
Most of the modern invented equipments are precise, accurate and
minimize the errors while weighing. These weighing machines
constitute of electronic circuits, digital chips, electronic sensors and
motion sensors. They are portable, lightweight and easy to operate and
even an unskilled person can operate these devices.
3.2 Weighing
When mass amounts are specified in chemical procedures the following
terms are commonly used:
a. "Weigh out about 2g of ...... ". This statement means that you are
required to weigh an amount of approximately two grams. The
accuracy to which this mass amount needs to be known is not
high and the top-loading balance will suffice.
b. "Accurately weigh out about 0.2g of ....". This statement means
that you should, with the aid of the analytical balance, weigh out
an amount that is close to 0.2 g, but you must know the exact
amount to an accuracy of ± 0.1 mg. Note that this does not mean
that you must weigh out exactly 0.2000 g. An amount between
0.1900 g and 0.2100 g is perfectly acceptable. However, you
must know the exact amount to the nearest tenth of a milligram.
When weighing out triplicate samples, it is not necessary that all
three weights be exactly the same, indeed, it is poor procedure to
attempt to do so.
163
In this unit, you will be asked to make a variety of weighing. It is
important for you to realise with what sort of accuracy these weighing
should be made. Depending on the desired accuracy you should use the
proper balance to make your weighing. There are two types of balances
available to you in this course:
A. The Top-Loading Balance
The top-loading balance digitally displays a mass reading, in grams, to 2
decimal places. The uncertainty in a single reading on the top -loading
balance is 0.05g.
B. The Analytical Balance
The analytical balance is more accurate than the top-loading balance. Its
digital display gives mass, in grams, to 4 places after the decimal. The
uncertainty in a single reading on the analytical balance is ~ 0.0002 g.
You will use this balance if the experiment calls for accurate
measurement of mass. Never use the analytical balance if the toploading
balance will do. The analytical balances are usually located in
the “Balance room” of a laboratory.
3.3 Weighing on an Analytical Balance
Weighing is a frequent step in analytical procedures, and the balance is
an essential piece of laboratory equipment in most analyses. In spite of
this, weighing is a common source of error that can be difficult to detect
in the final analytical results. The procedure described here applies
directly to electronic balances; therefore, certain portions of the
procedure are not applicable to other types of balance. The weighing
procedure can be separated into three basic steps: planning, checking the
balance, and weighing the material.
3.3.1 Planning
The initial step is the proper to assembling of the equipment, such as
containers for weighing, receiving vessels, forceps, pipettes, spatulas of
proper size, and so forth. Use containers of size such that the loading
capacity of the balance is not exceeded. Make sure that the containers
selected to receive the weighed material are clean and dry. Assemble the
necessary chemicals if solutions or reagents are required. Preparation of
the material to be weighed is often necessary. The material may require
grinding or drying. Some materials may have been heated or stored in a
refrigerator. Materials must be brought to the temperature of the balance
before they are weighed. To avoid condensation of moisture,
164
refrigerated materials must be allowed to come to room temperature
before the container is opened.
3.3.2 Checking the Balance
In the next step it is important to remember that, unless the balance is
checked before each weighing operation is performed, errors can easily
occur, resulting in faulty analytical data. The balance user should check
the balance environment, calibration, and balance uncertainties. Do
not assume that the balance has been left in the proper operating
condition by the previous user.
3.3.3 Balance Environment
The balance is placed in a suitable location with sufficiently low levels
of vibration and air current. It must have a constant electrical supply.
The balance and the surrounding work area have to be kept neat and
tidy. It is good practice to use a camel's hair brush or its equivalent to
dust the balance pan before any weighing so as to remove any materials
that may have been left by the previous operator.
NOTE: Individuals must clean up debris, dispose of any spilled
materials or paper, and remove the vessels and apparatus used in making
the measurements.
When a balance is moved, it must be allowed to adjust to the
temperature of its new environment and be recalibrated.
3.3.4 Calibration
If necessary, turn on the power, and allow the balance to equilibrate for
at least 1 hour before proceeding with the calibration. (Microbalances
may require up to 24 hours to reach equilibrium). If the balance power
has gone off and then has come back on, as in a power outage, certain
types of balance may display a message indicating that the balance must
be calibrated before a weighing is made. If the operator touches the
balance bar, the message may be cleared and the balance may display
zeros; however, the balance will not give the correct weighing until it
has been calibrated. Electronic analytical balances have an internal
calibration system based on an applied load. The calibration applies for
the current ambient temperature.
3.3.5 Balance Uncertainties
Drift Reduction: Drift is one of the most common errors, and it is also
one of the easiest to reduce or eliminate. Balance drift can be present
CHM 192 MODULE 4
165
without the operators being aware of the problem. Check the sample, the
balance, and the laboratory environment for the following causes of
errors, and eliminate them when:
• a balance door is open
• temperatures of the balance and the material to be weighed are
not the same
• the sample is losing or gaining weight
• the balance has been recently moved but has not been allowed to
equilibrate to its surroundings or has not been recalibrated
• air currents are present in the laboratory
• temperatures in the laboratory vary
• the balance is not properly leveled
• laboratory operations are causing vibration
• hysteresis of the mechanical parts occurs during weighing.
3.3.6 Rules for Weighing
• Do not handle objects to be weighed with bare hands. Use tongs
or paper towels if no appropriate tongs are available.
• Never weigh chemicals directly on the balance pan; use a glass
container or weighing paper or filter paper.
• If you spill a chemical on the top -loading balance, clean it
immediately. Never spill chemicals inside the analytical balance
enclosure. Keep the weighing chamber and weighing pan clean.
• Before using the balance, be sure that the pan is clean. If it is
dirty report it to your instructor, then brush it off with the brush
provided.
• Do not overload the balance. The maximum capacity of the top -
loading balance is 620g. The maximum capacity for the
analytical balance is 110g.
• Do not weigh hot or cold objects on the balance. Hot objects will
give erroneously low readings due to buoyancy of hot air, while
cold objects will give high readings.
• Check to be sure that the balance is leveled. It is level if the
bubble in the Level Indicator is in the centre while the balance is
“OFF”. Your instructor may need to adjust the leveling Feet.
3.3.7 Weighing by Difference to Overcome the Problem of
Balance Calibration Errors
How accurate are your balance readings? There is no way for you to
know. In order to overcome the problem of inaccurate readings due to
lack of calibration or miscalibration, chemists designed a method called
weighing by difference. It does not matter how far off each reading of
your balance is if you weigh your sample by difference.
166
For example: Weighing a solid sample by difference from a beaker: To
find the mass of the sample in the beaker, first the empty beaker is
placed on the balance and the mass is read. Then the solid is added to
the beaker and the mass of beaker with solid is read. The mass of the
solid sample is the difference between the two readings.
Notice that the weighing pan of the analytical balance is enclosed in
glass. This glass case is designed to protect the balance from
temperature fluctuations and air currents that cause the balance to drift -
that is, the digital display continues to change in one direction (up or
down). To make sure that the temperature of the air in the balance does
not change, keep your hands (which are warmer than the air in the
balance) out of the enclosure as much as possible, and keep the balance
doors closed.
Using Balance 1, a calibrated balance, the following masses are
recorded:
Mass of Empty Beaker: 24.7423 g
Mass of Beaker + Solid: 26.7587 g
Mass of sample: 26.7587g – 24.7423 g = 2.0164 g
Using Balance 2, an uncalibrated balance (all readings are low by
0.5000 g) the following masses are recorded:
Mass of Empty Beaker: 24.2423 g
Mass of Beaker + Solid: 26.2587 g
Mass of sample: 26.2587g – 24.2423 g = 2.0164 g
Does it matter which balance, 1 or 2, you choose to use?
_______________________________________________________
Explain:
__________________________________________________________
Throughout the weighing process you should protect the object you are
weighing from coming in contact with your hands by handling it with a
paper towel.
Introduction
2.0 Objectives
3.0 Main Content
3.1 Types of Weighing Balances
3.2 Weighing
3.3 Weighing on an Analytical Balance
3.3.1 Planning
3.3.2 Checking the Balance
3.3.3 Balance Environment
3.3.4 Calibration
3.3.5 Balance Uncertainties
3.3.6 Rules for Weighing
3.3.7 Weighing by Difference to Overcome the Problem
of Balance Calibration Errors
3.4 Sample Weighing Experiment
3.4.1 Using the Top-Loading Balance
3.4.2 Using the Analytical Balance
3.4.3 Weighing by Difference using the Tare Function of
the Analytical Balance
3.5 Report Sheet
4.0 Summary
5.0 Conclusion
6.0 Tutor-Marked Assignment
7.0 References/Further Reading
1.0 INTRODUCTION
Weighing scale is very fascinating and interesting. These did not come
to existence overnight, they evolved from a barter system where units
were absent compared to the modern day standards. When the standard
units were accepted by businesses and governments across different
regions, an industrial revolution was set to begin and everyone benefited
from that. Today the old age classical weighing scales have become
obsolete but they served the humanity for a long time. In the modern
times, one can see the weighing scales ruling the industrial and domestic
space.
160
2.0 OBJECTIVES
At the end of this unit, you should be able to:
• identify different equipment used in laboratory weighing
• differentiate between a to top-loading balance and an analytical
balance
• carry out simple weighing experiments in the laboratory.
3.0 MAIN CONTENT
3.1 Types of Weighing Balances
Here is a list of different products that you should know about.
Fig. 1.1: Iron Scale
1. The conventional wooden or iron scale was the oldest weighing
machine invented by man. It consisted of two containers attached
to each other through a pulley system. These weighing scales
were meant for domestic purposes and they often reported wrong
measurements. It was more of a weighing balance than a
weighing scale.
2. The next stage was the spring balance. A spring connected to a
fixed and rigid system that was used to weigh mass of objects by
hanging them with the spring. The spring will develop tension in
it and it will reflect a standard value on the rigid body attached to
it. The value will reflect the weight of the object in standard unit
of mass. Precision and accuracy were still missing in this case.
Fig.1.2: Spring Balance
3. The modern digital weighing scales are the third kind of
weighing machines invented so far. These are the products that
are electronically operated and display results very precisely.
They have many types, styles and weigh ranges. Broadly, they
can be categorised into two categories namely; Industrial Scales
and Domestic Scales.
Fig. 1.3: Digital Weighing Scales
4. Hydraulic Scales are used mainly in the laboratories and
industrial units. It made use of liquid displacement technique to
weigh the mass of heavy objects. The object placed on the top of
weighing panel displaces the liquid and the volume change
indicates the weight of the object.
Today, the weighing scales have changed a lot over a course of time.
Most of the modern invented equipments are precise, accurate and
minimize the errors while weighing. These weighing machines
constitute of electronic circuits, digital chips, electronic sensors and
motion sensors. They are portable, lightweight and easy to operate and
even an unskilled person can operate these devices.
3.2 Weighing
When mass amounts are specified in chemical procedures the following
terms are commonly used:
a. "Weigh out about 2g of ...... ". This statement means that you are
required to weigh an amount of approximately two grams. The
accuracy to which this mass amount needs to be known is not
high and the top-loading balance will suffice.
b. "Accurately weigh out about 0.2g of ....". This statement means
that you should, with the aid of the analytical balance, weigh out
an amount that is close to 0.2 g, but you must know the exact
amount to an accuracy of ± 0.1 mg. Note that this does not mean
that you must weigh out exactly 0.2000 g. An amount between
0.1900 g and 0.2100 g is perfectly acceptable. However, you
must know the exact amount to the nearest tenth of a milligram.
When weighing out triplicate samples, it is not necessary that all
three weights be exactly the same, indeed, it is poor procedure to
attempt to do so.
163
In this unit, you will be asked to make a variety of weighing. It is
important for you to realise with what sort of accuracy these weighing
should be made. Depending on the desired accuracy you should use the
proper balance to make your weighing. There are two types of balances
available to you in this course:
A. The Top-Loading Balance
The top-loading balance digitally displays a mass reading, in grams, to 2
decimal places. The uncertainty in a single reading on the top -loading
balance is 0.05g.
B. The Analytical Balance
The analytical balance is more accurate than the top-loading balance. Its
digital display gives mass, in grams, to 4 places after the decimal. The
uncertainty in a single reading on the analytical balance is ~ 0.0002 g.
You will use this balance if the experiment calls for accurate
measurement of mass. Never use the analytical balance if the toploading
balance will do. The analytical balances are usually located in
the “Balance room” of a laboratory.
3.3 Weighing on an Analytical Balance
Weighing is a frequent step in analytical procedures, and the balance is
an essential piece of laboratory equipment in most analyses. In spite of
this, weighing is a common source of error that can be difficult to detect
in the final analytical results. The procedure described here applies
directly to electronic balances; therefore, certain portions of the
procedure are not applicable to other types of balance. The weighing
procedure can be separated into three basic steps: planning, checking the
balance, and weighing the material.
3.3.1 Planning
The initial step is the proper to assembling of the equipment, such as
containers for weighing, receiving vessels, forceps, pipettes, spatulas of
proper size, and so forth. Use containers of size such that the loading
capacity of the balance is not exceeded. Make sure that the containers
selected to receive the weighed material are clean and dry. Assemble the
necessary chemicals if solutions or reagents are required. Preparation of
the material to be weighed is often necessary. The material may require
grinding or drying. Some materials may have been heated or stored in a
refrigerator. Materials must be brought to the temperature of the balance
before they are weighed. To avoid condensation of moisture,
164
refrigerated materials must be allowed to come to room temperature
before the container is opened.
3.3.2 Checking the Balance
In the next step it is important to remember that, unless the balance is
checked before each weighing operation is performed, errors can easily
occur, resulting in faulty analytical data. The balance user should check
the balance environment, calibration, and balance uncertainties. Do
not assume that the balance has been left in the proper operating
condition by the previous user.
3.3.3 Balance Environment
The balance is placed in a suitable location with sufficiently low levels
of vibration and air current. It must have a constant electrical supply.
The balance and the surrounding work area have to be kept neat and
tidy. It is good practice to use a camel's hair brush or its equivalent to
dust the balance pan before any weighing so as to remove any materials
that may have been left by the previous operator.
NOTE: Individuals must clean up debris, dispose of any spilled
materials or paper, and remove the vessels and apparatus used in making
the measurements.
When a balance is moved, it must be allowed to adjust to the
temperature of its new environment and be recalibrated.
3.3.4 Calibration
If necessary, turn on the power, and allow the balance to equilibrate for
at least 1 hour before proceeding with the calibration. (Microbalances
may require up to 24 hours to reach equilibrium). If the balance power
has gone off and then has come back on, as in a power outage, certain
types of balance may display a message indicating that the balance must
be calibrated before a weighing is made. If the operator touches the
balance bar, the message may be cleared and the balance may display
zeros; however, the balance will not give the correct weighing until it
has been calibrated. Electronic analytical balances have an internal
calibration system based on an applied load. The calibration applies for
the current ambient temperature.
3.3.5 Balance Uncertainties
Drift Reduction: Drift is one of the most common errors, and it is also
one of the easiest to reduce or eliminate. Balance drift can be present
CHM 192 MODULE 4
165
without the operators being aware of the problem. Check the sample, the
balance, and the laboratory environment for the following causes of
errors, and eliminate them when:
• a balance door is open
• temperatures of the balance and the material to be weighed are
not the same
• the sample is losing or gaining weight
• the balance has been recently moved but has not been allowed to
equilibrate to its surroundings or has not been recalibrated
• air currents are present in the laboratory
• temperatures in the laboratory vary
• the balance is not properly leveled
• laboratory operations are causing vibration
• hysteresis of the mechanical parts occurs during weighing.
3.3.6 Rules for Weighing
• Do not handle objects to be weighed with bare hands. Use tongs
or paper towels if no appropriate tongs are available.
• Never weigh chemicals directly on the balance pan; use a glass
container or weighing paper or filter paper.
• If you spill a chemical on the top -loading balance, clean it
immediately. Never spill chemicals inside the analytical balance
enclosure. Keep the weighing chamber and weighing pan clean.
• Before using the balance, be sure that the pan is clean. If it is
dirty report it to your instructor, then brush it off with the brush
provided.
• Do not overload the balance. The maximum capacity of the top -
loading balance is 620g. The maximum capacity for the
analytical balance is 110g.
• Do not weigh hot or cold objects on the balance. Hot objects will
give erroneously low readings due to buoyancy of hot air, while
cold objects will give high readings.
• Check to be sure that the balance is leveled. It is level if the
bubble in the Level Indicator is in the centre while the balance is
“OFF”. Your instructor may need to adjust the leveling Feet.
3.3.7 Weighing by Difference to Overcome the Problem of
Balance Calibration Errors
How accurate are your balance readings? There is no way for you to
know. In order to overcome the problem of inaccurate readings due to
lack of calibration or miscalibration, chemists designed a method called
weighing by difference. It does not matter how far off each reading of
your balance is if you weigh your sample by difference.
166
For example: Weighing a solid sample by difference from a beaker: To
find the mass of the sample in the beaker, first the empty beaker is
placed on the balance and the mass is read. Then the solid is added to
the beaker and the mass of beaker with solid is read. The mass of the
solid sample is the difference between the two readings.
Notice that the weighing pan of the analytical balance is enclosed in
glass. This glass case is designed to protect the balance from
temperature fluctuations and air currents that cause the balance to drift -
that is, the digital display continues to change in one direction (up or
down). To make sure that the temperature of the air in the balance does
not change, keep your hands (which are warmer than the air in the
balance) out of the enclosure as much as possible, and keep the balance
doors closed.
Using Balance 1, a calibrated balance, the following masses are
recorded:
Mass of Empty Beaker: 24.7423 g
Mass of Beaker + Solid: 26.7587 g
Mass of sample: 26.7587g – 24.7423 g = 2.0164 g
Using Balance 2, an uncalibrated balance (all readings are low by
0.5000 g) the following masses are recorded:
Mass of Empty Beaker: 24.2423 g
Mass of Beaker + Solid: 26.2587 g
Mass of sample: 26.2587g – 24.2423 g = 2.0164 g
Does it matter which balance, 1 or 2, you choose to use?
_______________________________________________________
Explain:
__________________________________________________________
Throughout the weighing process you should protect the object you are
weighing from coming in contact with your hands by handling it with a
paper towel.
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