WEIGHING ON AN ANALYTICAL BALANCE AS PER USP (1251)

WEIGHTS AND BALANCES AND ANALYTICAL BALANCE

The requirements for weights into conformity with American National Standard ANSI/ASTM E617, “Laboratory Weights and Precision Mass Standards.”
Pharmacopeial tests and assays require balances that vary in capacity, sensitivity, and reproducibility. Unless otherwise specified, when substances are to be “accurately weighed” for Assay, the weighing is to be performed with a weighing device whose measurement uncertainty (random plus systematic error) does not exceed 0.1% of the reading.

Measurement uncertainty is satisfactory if three times the standard deviation of not less than ten replicate weighing’s divided by the amount weighed, does not exceed 0.001. Unless otherwise specified,

For titrimetric limits tests, the weighing shall be performed to provide the number of significant figures in the weight of the analyte that corresponds to the number of significant figures in the concentration of the titrant.

The class designations below are in order of increasing tolerances.
Class 1.1

Weights are used for calibration of low-capacity, high-sensitivity balances. They are available in various
denominations from 1 to 500 mg. The tolerance for any denomination in this class is 5 µg. They are recommended for calibration of balances using optical or electrical methods for accurately weighing quantities below 20 mg.

Class 1

Weights are designated as high-precision standards for calibration. They may be used for weighing accurately
quantities below 20 mg. (For weights of 10 g or less, the requirements of class 1 are met by USP XXI class M.)

Class 2

Weights are used as working standards for calibration, built-in weights for analytical balances, and laboratory weights for routine analytical work. (The requirements of class 2 are met by USP XXI class S.)

Class 3

Weights-requirements are met by USP XXI class S-1 and are used with moderate-precision laboratory balances

Class 4

Weights -requirements are met by USP XXI class P are used with moderate-precision laboratory balances.

A weight class is chosen so that the tolerance of the weights used does not exceed 0.1% of the amount weighed. Generally,

  • Class 2 may be used for quantities greater than 20 mg,
  • Class 3 for quantities of greater than 50 mg, and
  • Class 4 for quantities of greater than 100 mg.

Weights should be calibrated periodically, preferably against an absolute standard weight.

WEIGHING ON AN ANALYTICAL BALANCE AS PER USP (1251)

Weighing is a commonly used in laboratory, and the balance is an important piece of quality control instrument in  maximum analyses and weighing is a frequent source of inaccuracy that can be very difficult to Identify in the final analysis results.

The measurement procedure by weighing balance can be divided into three  steps:

  • Planning,
  • Checking the balance
  • Weighing the material

PLANNING

For correct weighing of material we need to take some initial step

  • To Installed and arrange the accessory equipment, Like – containers for weighing, receiving vessels, forceps, pipets, spatulas of proper size etc.
  • As per vendor recommended  operation & loading capacity shall be used with proper containers of size and be ensure loading capacity of the balance is not exceeded in any case.
  • Always clean and dry containers select and used  to receive the weighed material.
  • Keep the necessary chemicals near the balance if solutions or reagents are required.
  • Keep the material to be weighed near the balance before weighing.
  • If required grinding or drying of the material should be grinding or drying before used of balance
  • Some materials may have been heated or stored in a refrigerator.
  • If special temperature required for Materials must be keep to the temperature of the balance before they are weighed.
  • To prevent condensation of moisture of refrigerated materials must be keep at room temperature before the container is opened.

ANALYTICAL BALANCE

CHECKING THE BALANCE

  • The second step is that, always checked the balance before each weighing operation is performed, errors can easily take-place and due to this weighing error can be lead a faulty analytical data.
  • Before performing the weighing activity, It is necessary to check  by user for Balance Environment, Calibration, and Balance Uncertainties status.
  • Do not think and assume that the weighing balance has been left in the proper operating condition by the previous user.

Balance Environment

  • Weighing balance is installed and place in dedicated weighing balances area with sufficiently low levels of vibration and air flow and must have a constant electrical supply.
  • The balance and the surrounding work area of balance have to be kept neat and tidy. Balance area is good state of repair.
  • Use a camel’s hair brush or its equivalent to remove the dust the balance pan before any weighing and also to remove any materials that may have been left by the previous chemist.

NOTE—

  • Analyst must clean up debris, dispose of any spilled materials or paper, and remove the vessels and apparatus used in making the measurements after used of balance.
  • When a balance is relocate, it must be keep at recommended storage condition like temperature of its new environment and balance shall be be re-calibrated.

Calibration

  • Balance calibration and verification is necessary activity of balance for correct measurements.
  • When turn on the power of balance and allow the balance to equilibrate for at least 1 hour before proceeding with the calibration. (In case of Microbalances may require up to 24 hours to reach equilibrium.)
  • If the weighing 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.
  • But 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.

Balance Uncertainties

DRIFT REDUCTION

Drift is one of the most common errors, and it can be easy to reduce or eliminate. Balance drift can be come 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 as soon as possible before used of balance:

1. A weighing balance door is open.

2. Temperatures of the balance and the material to be weighed are not the same.

3. The sample is losing or gaining weight

4. The balance has been recently re-locate but has not been allowed to equilibrate to its surroundings or has not been re-calibrated.

5. Air flow are present in the laboratory and disturbed the function of balance

6. Temperatures in the laboratory constant vary.

7. The balance is not properly leveled.

8. Laboratory operations are causing vibration. Weighing platform is not anti-vibrating.

9. Hysteresis of the mechanical parts occurs during weighing.

MECHANICAL HYSTERESIS

Hysteresis in the balance is caused by

  • Excessive stretching of the springs due to overloading or to the accidental dropping of an object onto the pan.
  • Microbalances are very sensitive to overload and shock. When using a microbalance, set the lever to the rest position when adding or removing material. turn the lever to the weigh position to register the weight.

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.
  • In the case of electronic force restoration balances, springs are replaced by flextures

QUALITY ASSURANCE PROCEDURE FOR MEASUREMENT OF BALANCE DRIFT

With resect to extended period of time, balance drift and other day-to-day variations are monitored by weighing a fixed check-weight on a regular basis.

This balance drift check should be performed after the balance has been calibrated at the ambient laboratory temperature and made before the first weighing of the day or after any event that might disturb the balance’s calibration (power failure, re-location of  balance etc.).

Weighing of any object whose mass remains constant and does not exceed the load limit of the balance. A balance weight makes a reliable check-weight.

Perform the following procedures to reduce balance errors and the possibility of an incorrect reading because of drift:

1. To ensure the electrical power of the balance is on and the level bubble is in the center of the indicator.

2. Calibrate the analytical balance or the microbalance routine basis.

NOTE—

  • Some balances have a calibration lever, which must be returned fully to its original weighing position. Do not depend upon any prior calibration.

3. The first person to use the balance each day should be calibrate and record the weight in the log book for comparison with previous readings. If a deviation greater than those indicated below for Analytical Balances and Microbalances is observed, the balance should be reported for service.

NOTE—

  • Check weights tend to gain weight upon standing because of mishandling and exposure to contaminants in the atmosphere.
  • These weights can be cleaned by wiping with a lint-free cloth moistened with a small amount of an appropriate solvent such as diethyl ether.

Analytical Balances—

Select a Analytical balance to check the weight of an appropriate mass to examine an analytical balance. If possible, set the balance to read to 5 decimal places. Follow the manufacturer’s operating instructions.

Pick up the check-weight with a forceps, place weight carefully on the balance pan and weigh it.

NOTE —

Do not drop the weight on the balance pan, because damage to the balance could result.

Place the weight in the center of the pan to eliminate corner-weighing differences. The accuracy of the weight is not important: the only factor of interest is whether any drift has occurred. If no drift has taken place, the value should remain constant.

Periodic weighing of a fixed weight will determine whether the boards or knife edges in mechanical balances in the instrument are defective.

The check for drift at the most sensitive position will show whether a problem exists; the variation in the observed weight does not exceed ±0.2 mg.

For example, with a 20-g weight, if the mean value of the readings were 19.9984, the tolerance would be from 19.9982 to 19.9986 g. Thus, several readings must be taken before one can establish a tolerance.

NOTE—The check-weight need not be of high accuracy, but it is essential that its mass remain constant. In addition, the tolerance does not correspond to the value of 0.1%, specified under Weights and Balances 〈41〉, for weighing material accurately.

Rather, the tolerance is purposefully tight to reveal possible drift or calibration errors.

Microbalances—

Proceed as directed for Analytical Balances, but use a check the weight appropriate for the particular balance.

For example,

a 100-mg check-weight might be selected for a balance that has a load limit of 150 mg or a 10-mg check-weight might be used for an ultra-microbalance with a load limit of 15 mg.

The operator must know the maximum capacity of the balance to select the correct check-weight. The balance indicates the weight in milligrams. Record the weight as soon as the reading is stable for a few seconds.

The variation in weighing ought to be within a range commensurate with the specifications given by the balance manufacturer, but not greater than 0.1% of the amount of material typically weighed on the particular balance.

For example, if 10-mg samples are routinely weighed, the variation in the weighing of the check-weight cannot exceed 0.01 mg.

WEIGHING THE MATERIAL

Properly choose the number of decimal places required for the analytical procedure. In the pharmaceutical company mostly small quantities of material are used, weighting requiring the balance reading to be set to the fifth decimal place to achieve the necessary accuracy.

Weighing read with four decimal places is preferred for weighing near-gram quantities.

Be ensure and do not allow the material to remain on the balance for an extended period of time because changes, caused by interaction with atmospheric water or carbon dioxide, may take place.

Load Limit

Properly choose the appropriate balance for the quantity and accuracy needed. Each balance has a load limit, which should not be exceeded. Each balance manufacturer supplies the maximum loading condition.

The weighing operator should aware load limit so that the balance will not be damaged.

NOTE—Electronic balances operate on a “load cell” principle that produces an electrical output proportional to the movement of the strain gauge and is linear over the range.

Receivers

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 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.

Common receivers are –

  • Weighing bottles, weighing funnels, flasks, and weighing paper.
  • The correct receiver depends upon the quantity and type of material (liquid, solid, or powder) to be weighed.
  • A vessel of low mass should be chosen when small amounts of material are to be weighed.
  • 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.
  • The weighing funnel is good receiver, because it can function as both a weighing dish and a transfer funnel, allowing easy transfer to volumetric flasks. Weighing funnels come in various sizes; the size suitable for the operation should be selected.
  • Weighing paper may be used for solids. Paper receivers must be handled by hand, and great care must be used to prevent spills.

Weighing by Difference

Weighing is usually done by difference. The following methods are acceptable for good analytical results.

METHOD – 1

  • Tare the empty receiver as follows.
  • Place the receiver on the balance in the center of the pan,
  • Press the appropriate tare key on the balance.
  • Balance electrically sets  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;
  • Re-weigh the original weighing receiver by placing it in the same position on the pan.

NOTE—

  • Do not change the set tare of the balance between these two weighing’s.
  • The second weight represents the un-transferred material and is subtracted from the total material weight to determine the weight of the transferred material.

METHOD – 2

If the empty receiver is not going to be tared,

  • Add the material to the receiver,
  • Place the receiver on the balance in the center of the pan. Record the weight,
  • Transfer the weighed material to the final flask or receiver;
  • Then re-weigh the original weighing receiver by returning it to the same position on the pan.
  • The second weight represents the sum of the weights of the receiver and the un transferred material; subtract this sum from the sum of the total material weight and the receiver weight to determine the weight of the transferred material.

METHOD – 3

This method is quantitative transfer. The material is added to the tared receiver, the amount is determined by difference, and then the whole amount is transferred quantitatively (e.g., by using a solvent) to the final receiver.

Materials-Handling Safety Procedures

  • 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.
  • Gloves should be used to prevent any contact with the skin.

NOTE—

  • The use of gloves is good practice for handling any chemical. If it is necessary to handle the container being weighed, the operator should put on gloves, not only for self-protection but also to prevent moisture and oils from being deposited on the weighed container.
  • During a weighing, the operator may be exposed to high concentrations of the pure substance; therefore, the operator must carefully consider these possibilities at all times.
  • Weighing’s are made on many different types of materials, such as large solids, finely divided powders, and liquids (viscous or non-viscous, volatile or nonvolatile). Each type of material requires its own special handling.

Weighing Solids

Solids come in two forms:

  • Large chunks, with or without powdery surface, and
  • Finely divided powders or small crystals.

If large chunks with a powdery surface are to be weighed, at least a piece of weighing paper must be placed on the balance pan to protect it from damage. Large non-reactive chunks that have no powdery surface may be placed directly on the pan (for example, a coated tablet).

NOTE—Solid pieces must be handled with forceps, never by hand.

STATIC CHARGE

  • Fine powders have a tendency to pick up 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.
  • The static charge depends upon the relative humidity of the laboratory, which in turn depends upon the atmospheric conditions.
  • Sometime, static charge is caused by the type of clothing worn by the operator; this charge causes large errors in the weighing when discharged.

WEIGHING PROCEDURE

  • Place the receiver on the balance pan
  • Close the balance door, and weigh as indicated for Weighing by Difference
  • 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.

SPILLS

  • 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.

NOTE—Never return any excess material to the original container. Any excess material must be disposed of in a proper manner.

Weighing Liquids

Liquids may be volatile or nonvolatile and viscous or non-viscous. Each type requires special attention.

WEIGHING PROCEDURE

  • Weigh as directed for Weighing by Difference.
  • 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.

NOTE—Liquids spilled within the balance housing can cause serious damage to the balance, and they may be difficult to remove.

  • Non-viscous liquids can be handled with a Pasteur capillary pipet 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.

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