Analytical Balance is the most important analytical instrument in the Pharmaceutical Industry. Based on the applications, weighing balances are utilized in different locations such as Quality Control & Manufacturing.
Laboratory Analytical Balance
Particularly, Analytical Balances are playing a key role in Analytical Chemistry (Quantitative and Qualitative analysis). Hence let us discuss one of the Analytical Technique – Good Weighing practices at the Analytical Laboratory (Quality Control and AR&D).
Based on the requirements different types of Balances are available such as Analytical Balance, Micro Balance Semi Micro Balance, Top Pan Balance, and so on.
Weighing Requirements in Laboratory
Ensure the availability of proper ancillaries such as weighing containers, suitable-size spatulas, receiving vessels, pipettes, and necessary reagents or solutions.
Use containers of a size such that the loading capacity of the Analytical balance is not exceeded. Make sure that the containers selected to receive the weighed material are clean and dry.
The material must be brought to room temperature before they are weighed. To avoid condensation of moisture, refrigerated materials must be allowed to come to room temperature before the container is opened and should be wiped with tissue paper.
User Responsibility (Analyst)
The user (analyst) should check the Analytical Balance Environment and cleanliness.
The analyst should verify the status of the balance being used for Calibration and performance checks.
The Source of balance uncertainties before each weighing has to be eliminated by the analyst; otherwise, errors can easily occur, resulting in faulty analytical data.
Do not assume that the balance has been left in the proper operating condition by the previous user.
Ensure Spirit Level, Temperature, Relative humidity, Sun Light, and Air Flow.
What is Balance Uncertainties
Drift: Drift is one of the most common errors, and it is also one of the easiest to reduce or eliminate. Check the sample, the balance, and the laboratory environment for the following causes of errors, and eliminate them.
A Balance door is open during sample weighing.
The 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.
Weighing Precautions of Laboratory Balance
Perform the following procedures to reduce balance errors and the possibility of an incorrect reading because of drift
Make certain that the electrical power to the balance is on and balance is warmed up and the level bubble is in the center of the indicator.
Make sure that performance verification of the Analytical balance or the microbalance is done as per the daily performance check procedure
Make sure that daily performance check using check-weight is done at an appropriate frequency and recorded weights in the log book are comparable with previous readings. If a deviation greater than those indicated by the procedure is observed, the balance should be reported for service.
Causes for Mechanical Hysteresis
The primary causes of hysteresis are as follows:
- Overloading or the accidental dropping of an object onto the pan.
- Microbalances are very sensitive to overload and shock.
Drift due to hysteresis can be eliminated by allowing the balance to stand without weighing long enough for it to recover.
- If stretching of the springs is excessive, an expensive balance overhaul may be needed
Best Weighing Procedure
Standard Operating Procedures (SOP) must include the best weighing procedures for Analytical Balance / Laboratory Balance.
Analytical Balance pre weighing precautions
Select the number of decimal places required for the analytical procedure.
If small quantities of material are used, requiring the balance reading to be set to the fifth/sixth decimal place to achieve the necessary accuracy.
Weighing (read) with four decimal places is preferred for weighing near-gram quantities.
Do not allow the material to remain in the balance for an extended period of time because changes, caused by interaction with atmospheric water or carbon dioxide, may take place.
Select the appropriate balance based on the derived operating range for the quantity and accuracy needed.
The operator must be familiar with precautions described in the Material Safety Data Sheet for the substance before weighing it.
Hazardous materials must be handled in an enclosure having appropriate air filtration. Many substances are extremely toxic, are possibly allergenic, and may be liquids or finely divided particles.
A mask that covers the nose and mouth should be used to prevent any inhalation of chemical dust while handling hazardous or toxic materials.
Fine powders have a tendency to pick up a static charge, which will cause the particles to fly around. This static charge must be eliminated before a suitable weighing can be made. An antistatic device may be used to minimize this problem.
Analytical Balance Receivers
The proper receiver for the material must be selected. The receiver’s weight plus the weight to be measured must not exceed the maximum load for the Analytical balance.
The size and shape of the receiver should permit it to fit into the space and on the balance pan without interfering with any operation.
It is important that the receiver be clean and dry.
Common receivers are weighing bottles, weighing funnels, flasks, beakers, and weighing paper, the weighing funnel is often the most satisfactory receiver because it functions as both a weighing dish and a transfer funnel, allowing easy transfer to volumetric flasks.
It is recommended that gloves, forceps, or another type of gripping device be used when handling receivers because oils from the hands will add weight.
Weighing paper may be used for solids. Paper receivers must be handled by hand, and great care must be used to prevent spills.
Types of Weighing Methods
Weighing is usually done by difference. The following methods are acceptable for good analytical results
Analytical Balance Weighing Method 1
Tare the empty receiver as follows, Place the receiver on the balance in the center of the pan and press the appropriate tare key on the balance.
This operation electrically sets the signal from the strain gauge to zero so that the weight of the receiver is no longer indicated.
Add the material to the receiver, and record the weight.
Transfer the weighed material to the final flask or receiver; then reweigh the original weighing receiver by placing it in the same position on the pan.
The second weight represents the untransferred material and is subtracted from the total material weight to determine the weight of the transferred material.
Analytical Balance Weighing Method 2
This method can be described as a quantitative transfer.
The material is added to the tared receiver, the amount is determined biff
The whole amount is transferred quantitatively (e.g., using a solvent) to the final receiver.
Weighing of Solids
Place the receiver on the balance pan, close the balance door, and tare the balance to nullify the weight of the receiver.
Carefully add the powdered material from a spatula until the desired amount is added.
Use care to avoid spilling. Close the balance door, and record the weight as soon as the balance shows a stable reading.
Weighing of Liquids
Liquids should always be weighed into a container that can be closed so that none of the material is lost.
It is best if the liquid can be added to its receiving container outside the balance because of the possibility of a spill.
Nonviscous liquids can be handled with a Pasteur capillary pipette equipped with a small rubber bulb such as a medicine dropper bulb.
The liquid is discharged into its receiver, the top is closed or stoppered, and the receiver and contents are weighed.
Small quantities of viscous liquids can be handled by touching a glass stirring rod to the surface of the liquid and then carefully touching the rod to the side of the receiving vessel, which allows some of the material to be transferred.
Viscous Liquid material can be transferred with a micro syringe into receiving container.
Handling of Spillages
If solids are spilled, remove the receiver, and sweep out all of the spilled material from the balance.
The spilled material must be properly disposed of and must not be swept out onto the balance table where other operators may come in contact with the chemical.
Then either start the process over or reweigh the remaining material.
Liquids spilled within the balance housing can cause serious damage to the balance, and they may be difficult to remove.
[Note—Never return any excess material to the original container. Any excess material must be disposed of in a proper manner.]Stay tuned here for never-before Pharma articles – Approved GLP SOP in Pharma Industry, Best Audit Agenda practices as per Guidelines
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