Analytical Balance: Parts, Principle, and Applications

An analytical balance is the most common laboratory balance that determines the mass of chemicals or objects with high accuracy. As a highly sensitive instrument, it can measure small masses from sub-milligram (i.e., 0.00001 gm or 0.01 mg) to a few kilograms. It is also known as chemical balance or laboratory balance.

The accurate and precision measurement capability of tiny masses has increased the need for this instrument in laboratories. Therefore, the analytical balance is applicable in various fields such as pharmaceuticals, food, plastic, chemical manufacturing, quality assurance lab, calibration laboratories, etc.

Different types of Analytical Balance
Different types of Analytical Balance

Parts of Analytical Balance

The analytical balance consists of various parts that are described below;

Parts of Analytical Balance
Parts of Analytical Balance
  1. Balance plate (pan): It is placed inside a draft shield, usually stainless steel. It provides a surface to hold the objects to be measured.
  2. Weights: It is an object whose exact weight is known and fixed. It helps to calibrate the balance and also helps to make sure that the analytical balance is functioning accurately.
  3. Draft shield: It is a see-through enclosure that is rectangular. It helps to protect samples and containers from environmental influences such as airflow or dust. It also plays a significant role in precise and accurate measurement.
  4. Door handle: It helps to open the draft shield to load the object in a balanced plate.
  5. Level indicator: It helps to check balance is at the level.
  6. Power button: It is used to switch on or off balance.
  7. Tare button: It helps to get the balance back to zero even after putting a container in which mass is to be placed on a balanced plate. It is also known as the Re-zero button.
  8. Mode button: It sets the system needed to measure either mg, or gm mode.
  9. Display panel: It indicates various functions such as results, errors, information for function settings, and function on progress.
  10. Level adjustment feet: These are movable legs that can be adjusted to bring balance to the reference position.

Principle of Analytical Balance

Primarily analytical balance works on the principle of ‘electromagnetic compensation.’ Therefore, weight is measured with the help of an electromagnet. This instrument does not measure the weight of an object directly; instead, it measures the force that acts downward on a balanced plate.

When electrical current is supplied to the electromagnetic servomotor fitted beneath the balance plate, it generates force against the mass of an object. Thus, the current required to create this force is directly proportional to the mass of an object if the device is correctly calibrated before measurement. As a result, the weight measured is displayed on the screen. Similarly, the null detector employs a light source and detector in many balances to indicate when the weight and electromagnetic forces are equal.

Types of Analytical Balance

Two pan-analytical balance

  • Also known as equal arm analytical balance, which works on the principle of the first-class lever.
  • It consists of a symmetrical beam and three knife edges.
  • Two terminal knives support the pans, and the central knife-edge acts as a pivot about which a beam swings.
  • Two pans are balanced against each other. Here an object to be measured is placed on one pan, whereas the known weight is placed in another.
  • The most common sources of error in two pan balances are; 
  • Two pans must be equidistant from the center. If one arm is slightly longer than another one, an error occurs in the result. 
  • As the weight increases, the beam might gently flex over the knife edge, resulting in measurement inaccuracies.

Single-pan mechanical balance

  • Also known as unequal arm mechanical balance, Erhard Mettler introduced it in 1946.
  • It is faster and more convenient than two pan balances.
  • It consists of a beam with two knife edges; one supports the weighing pan and the other acts as a pivot.
  • A fixed counterweight balances the load on the pan.

Electronic single-pan balance

  • Also known as electromagnetic force balance which has replaced mechanical or single-pan balance.
  • The current passing generates a magnetic force that balances the load of an object placed in a balance pan.
  • Therefore, the mass of the load is proportional to the current needed to balance it.
  • This type of balance gives a direct reading of the mass applied. In contrast, the other two types of analytical balance depend on comparing two forces (unknown weight or either external or internal weight).

Microbalance

  • This analytical balance can measure samples to at least 1 million parts of a gram.
  • Used to accurately measure small amounts of sample.
  • Quartz crystal microbalance is a more sensitive form of microbalance that determines a tiny amount of mass (i.e., viruses or any other tiny object that is needed to be measured).

Procedure for Operating Analytical Balance

Planning 

  • Gather all proper equipment and necessary reagents, vessels, forceps, etc.
  • Use clean and dry containers that are lighter in weight.
  • Bring samples back to the temperature of the balance.

Selection of proper location

  • Choose a place that is firm, flat, and free of vibration.
  • Avoid places that have direct sunlight, airflow, and temperature change.

Leveling the analytical balance

  • Check that the balance does not shake and that the air bubble in the indicator is centered. If not, center it by turning the leveling feet of the balance.

Calibrating analytical balance

  • Calibration is required to ensure the balance is weighing correctly. Calibration of analytical balance can be done in two ways that are; external and internal calibration. Automatic calibration is an internal type of calibration. The basic steps to perform external and internal calibration is given below;

External calibration: 

  • Turn on the balance and wait for an hour to warm up.
  • Press a button to enter calibration mode.
  • Then load the calibration weight on the weighing pan and also press the button to perform calibration.
  • Once the calibration is completed signal will display on the screen.

Internal calibration

  • Turn on the balance and wait for an hour to warm it up.
  • For internal calibration, the balance is usually provided with a button.
  • If internal calibration is needed, press the provided button for auto-calibration.
  • Then, the balance will automatically perform the calibration.

Weighing

  • Preheat the balance for an hour before using it.
  • Press the ‘TARE’ button to set the balance at zero at the unload condition.
  • Now place the container or weighing paper at the center of the weighing pan.
  • Close the draft shield door and wait for 5 to 10 seconds to stabilize the LCD readout.
  • Again press the ‘TARE’ button to reset the balance to zero if you do not want to include the mass of the container.
  • Add the object to be weighed after eliminating the container from the balance.
  • Readjust the container’s balance, and wait for a few seconds (i.e., 5-10 seconds) for the reading to sustain.
  • Take a minimum of three readings, calculate the average to avoid errors, and record the reading in a datasheet.

Cleaning

  • Shut off the power chord while cleaning.
  • Use a piece of soap-wet, mild detergent-coated cloth to clean it.
  • While cleaning, ensure no liquid or dust goes inside the balance parts.

Factors affecting the Analytical Balance’s working

  1. Temperature: It is one of the crucial factors that can result in noticeable fluctuation in the measurement. An increase in room temperature can result in the expansion or evaporation of the sample. Whereas a decrease in room temperature can result in contraction or condensation of the sample. As a result, it causes fluctuation in measurement.
  2. Chemical reaction: The sample to be measured should remain chemically inert. It is because highly reactive samples are sensitive to atmospheric alteration.
  3. Vibrations: Vibration plays a significant role in fluctuation in measurement. When we measure the sample in a vibrating environment due to a refrigerator, ventilator, or other equipment, it alters the precision of balance due to rearrangement, dislodgement, or spilling of the sample. Therefore, vibration influences the number of samples available.
  4. Air currents: The sample should be measured in a well-ventilated room. Air movement across the pan will cause variations in the measurement. The air current results change in air pressure that results in fluctuation in mass measurement.
  5. Magnet: If the sample is magnetic or is magnetized, it may create a magnetic field that can influence the sensor coil of the balance and fluctuate the measurement. So, while weighing, the nature of the sample plays a significant role.
  6. Fingerprints: If the sample is in a tiny amount, the fingerprint might alter the result.

Applications of Analytical Balance

In the chemical and pharmaceutical industries

  1. Used in the determination and analysis of the density of solvent.
  2. Used in piece counting of pills in pharmaceutical industries.
  3. Used in percentage weighing that allows more efficient formulation in chemicals and pharmaceuticals.

In biotechnology and microbiology laboratories

  1. Used to prepare samples and reagents.
  2. Used in the calibration of pipettes.

In food industries

  1. Used in weighing ingredients to make food products
  2. Used in check-weighing packaged commodities to maintain 100% product weight and counts accuracy.
  3. Used to determine the gross weight of packaged goods.

In zoology laboratories

  • Used in weighing small animals like insects.

Advantages of Analytical Balance

  1. It is a simple and easy-to-use instrument.
  2. It can measure tiny masses up to four places to the right of the decimal point precisely and accurately.
  3. It assists laboratories in maintaining acceptable good laboratory practices.
  4. It is more sensitive and can detect small changes in masses.
  5. It ensures uniformity, consistency, reproducibility, and integrity in measurement.

Limitations of Analytical Balance

  1. It is not designed to measure large quantities.
  2. The accuracy and precision of measurement rely on environmental factors such as temperature, air current, etc.
  3. Liquid crystal displays (LCD) can be easily damaged due to heavy loads.

Precautions

  1. Always calibrate the analytical balance before determining the mass of an object.
  2. Weighing should be done in a proper environment, such as; free of vibration, with controlled temperature and humidity, no direct sunlight, and should have low air current.
  3. After turning on the power button, always preheat the balance for an hour to get a more accurate result.
  4. Make sure to press the ‘TARE’ button before weighing the sample to minimize the weight of the container.
  5. Never touch samples with bare hands. Oils from the skin can affect the accuracy of the weight. Therefore, always wear gloves while handling the sample.
  6. Do not place balance near the magnet to avoid the development of electrostatic charge in a sample that can fluctuate the reading.
  7. Always close the door of the draft shield while weighing the samples.
  8. Never use any corrosive chemicals to clean the balance pan. Instead, a camel brush can be used to clean it.
  9. Avoid loading things directly in the balance pan because doing so can contaminate the balance. Always use weighing paper or a vessel.

References

  1.  Standard operating procedure (SOPs) of analytical balance. Analytical Chemistry. Retrieved on 5th November 2022. Retrieved from https://www.chemwifi.com/2022/01/SOPs-Analytical-balance.html
  2. Why calibrate Analytical Balance and scales. Retrieved on  7th November 2022. Retrieved from https://www.depts.ttu.edu/meatscience/docs/WhyCalibrateAnalyticalBalancesandScales-6-30-20.pdf
  3. SOP for operation and calibration procedure for Analytical Balance. PHARMASKY. Retrieved on 7th November 2022. Retrieved from https://www.pharmaceuticalsky.com/2020/06/sop-for-operation-and-calibration.html

Samikshya Acharya

Hello, I am Samikshya Sharma. I have completed my post-graduate study in medical microbiology at the central department of microbiology, TU, Nepal. I hope my articles are helpful to you. Thank you!!

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