An analytical balance, also known as a laboratory balance, is a precision instrument used for measuring the mass of substances with high accuracy and precision. It is commonly found in laboratories, particularly in fields such as chemistry, biology, and pharmacology, where precise measurements are crucial.
The principle of operation for an analytical balance is based on the fundamental concept of balancing mass using a force-measuring mechanism. Analytical balances are designed to measure the mass of an object with high precision and accuracy. Here’s an overview of the principle of operation for an analytical balance:
Counterbalance Force: An analytical balance is equipped with a measuring pan or platform where the sample to be measured is placed. This pan is attached to a balance beam or lever. When there’s nothing on the pan, the beam is in balance, meaning the forces acting on either side of the pivot point are equal.
Gravitational Force: The fundamental principle behind analytical balances is the comparison of the gravitational force acting on the sample with the force provided by a calibrated mass, typically in the form of a counterweight or a set of calibrated weights.
Calibrated Weights: In the case of traditional mechanical analytical balances, calibrated weights are placed on one side of the balance beam, and the sample is placed on the other side. The operator adjusts the position of the weights until the balance beam returns to a horizontal or “balanced” position. This indicates that the gravitational force acting on the sample is equal to the gravitational force acting on the calibrated weights.
Load Cell or Strain Gauge: In modern electronic analytical balances, the balance operates on the principle of a load cell or strain gauge. A load cell is a sensor that deforms when subjected to a force (in this case, the mass of the sample). This deformation is proportional to the applied force.
Measurement of Deformation: As the sample is placed on the balance pan, it causes a deformation in the load cell or strain gauge. This deformation is measured electronically and converted into a mass reading. The balance’s internal circuitry amplifies and converts this signal into a digital display of the sample’s mass.
Taring: Analytical balances also include a taring feature. Taring allows the operator to zero out the balance when an empty container (e.g., a weighing boat or beaker) is placed on the pan. The balance considers the container’s weight as zero, and subsequent measurements only account for the mass of the substance added to the container.
Readout: The measured mass is displayed digitally on the balance’s screen. The display typically shows the mass with high readability, often in milligrams (mg) or micrograms (μg).
Environmental Factors: Analytical balances are sensitive to environmental conditions such as temperature, humidity, and air currents. These factors can affect the accuracy of measurements, so it’s important to use analytical balances in controlled laboratory conditions.
High Precision: Analytical balances are designed to provide extremely accurate measurements. They typically have a higher level of precision compared to other types of balances. They can measure the mass of substances down to a fraction of a milligram (e.g., 0.1 mg or even 0.01 mg).
Enclosed Design: Analytical balances are enclosed in a glass or plastic case to protect them from environmental factors like air currents and dust, which can affect measurements. The enclosure also helps maintain a stable temperature.
Anti-Vibration Features: Analytical balances are typically equipped with features to minimize the impact of vibrations, which can cause fluctuations in measurements. They may have vibration-dampening platforms or leveling systems to ensure stability.
Calibration: Regular calibration is crucial to maintain the accuracy of an analytical balance. This involves adjusting the balance to ensure it measures accurately. Calibration is typically performed using standardized weights.
Readability: Analytical balances have a high readability, which is the smallest increment in mass that the balance can display. This is usually expressed in milligrams (mg) or micrograms (μg).
Taring: Analytical balances allow for taring, which means setting the balance to zero with an empty container on the pan. This enables you to measure only the mass of the substance you’re interested in without the container’s weight.
Advanced Display: The display on an analytical balance is digital and often features a touchscreen or keypad for input. It may show the measured mass in various units and allow for data output to a computer or printer.
Draft Shield: The balance typically has a draft shield or draft chamber to further protect the sample from air currents. It’s usually made of glass and can be opened for placing or removing the sample.
Weighing Pans: Analytical balances have a weighing pan or platform where the sample is placed. The material and design of the pan are carefully chosen to minimize contamination and static electricity effects.
External Factors: Environmental factors such as temperature, humidity, and air pressure can affect the accuracy of measurements. Therefore, analytical balances are often used in controlled laboratory conditions.
Maintenance: Analytical balances require regular maintenance, including cleaning and calibration, to ensure their accuracy. Users should follow manufacturer guidelines and best practices for maintenance.
Two types of balances are used in the laboratory:
1. Mechanical Balance:
These are subdivided into various types depending upon the number of pans, reading mechanism and precision.
a. Trip balance:
This type of balance consists of two pans of equal size suspended with a beam that is supported in the centre of gravity by the edge of a sharp fulcrum. Substance to be weighed is placed in the right-hand pan and is counter balanced with known weights placed in the left-hand pan. Aligning the position of beam arm-bridge indicates correct weight. It is not precise and is used for weighing heavy things.
b. Spring balance:
This is a single pan balance used to weigh heavier things. The pan is attached with a spring, which stretches with weight. The weight is indicated on a scale by a pointer attached to the spring. It is not precise.
c. Analytical Balance:
These can be of two-pan type or one-pan type. Two-pan type mechanically operates on the same principle as trip balance. However, its beam is provided with side screws for fine adjustment of beam to zero weight and a pointer in the centre, which moves on a scale. It is relatively more precise. Single pan type has a beam of unequal arms. One arm is provided with a pan to place article for weighing, whereas it is counterbalanced by a single weight located at the opposite end of the beam. It is most precise among mechanical balances.
2. Electrical/Electronic Balance:
It is a single pan balance and employs magnetic field to counteract the weight placed on the pan. The pan is attached to a coil, which is placed in a magnetic field generated by electric current. When more weight is placed on the pan more current is required to produce the magnetic field. This increase in current flow is converted by a microprocessor into numerical value for weight, which is displayed on a panel. These are of two types. One hanging pan type, usually protected in a glass case for weighing very small amounts. The other is top loading type commonly used for weighing larger quantities.
- Place the paper or plastic weighing boat in the centre of the pan.
- Perform tarring by pressing Tar button on the panel.
- Place the substance to be weighed on the weighing boat with the help of a scoop or spetulum. The amount should roughly be slightly more than the required weight.
- Reduce the substance with spetulum gradually until desired weight is indicated on the display panel.
Balance needs to be periodically checked for accuracy. This is done by weighing a 100 g (or an appropriate) standard weight. The variation should be <0.5%. If not, the balance should be adjusted accordingly.
Precautions and Maintenance:
- These should be protected from rusting and tripping mechanism & should be periodically cleaned.
- Analytical balances should be kept in a glass box to keep these dust-free.
- Balances must be placed on an absolute level surface for correct weighing. Analytical balances are usually provided with a spirit level and adjustable legs for levelling.
- The surface on which the balance is placed should not be vibrating.
- The pans should be absolutely clean.
- Place the weighing object in the centre of the pan.
- Always use standard weights. All weights should be placed in a dust-proof box. Small weights should be handled with forceps.
- Material to be weighed should be placed in a pre-weighed plastic tray, boat or glazed paper that could be easily washed.
- Close the door of the cabinet while weighing.
- Do not weigh a substance when it is hot or cold.
- Do not weigh a quantity that is beyond the permissible limits for the balance.
- Keep the balance with door of the cabinet closed and switched off when not in use.
- Use dust cover.
- Clean the pan after each use.
Overall, analytical balances are essential tools in scientific research and quality control, enabling precise and reliable measurements of mass for a wide range of substances and applications