# C Use of Balances

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Technique C: Use of Balances

SECTION 1: PURPOSE OF TECHNIQUE

This technique describes how to use a laboratory balance to accurately measure the mass of a sample.

SECTION 2: OPERATIONS OF BALANCES

There are two general types of balances in the chemistry laboratory. There are precision balances, which usually measure to the nearest 0.01 grams (g), and the analytical balances, which usually measure to the nearest 0.1 milligrams (mg) (0.0001 g). The visibly distinguishing factor between the two is the presence of sliding doors on the analytical balance to protect from air currents. An example precision balance in the laboratory is model PB 602-S, or equivalent. The analytical balance is model AL 204, or equivalent. Both models are produced by Mettler-Toledo, LLC of Columbus, Ohio. Other manufacturers supply balances which have fundamentally similar basic operations.

Analytical Balance Precision Balance

Part A: Normal Weighing Procedure

1. Tare your scale before use, making sure the reading is 0. (See Part B for instructions on weighing material directly into a container).
1. Open one/two doors of the balance isolation cabinet (if using the analytical balance).
1. Place your sample on the balance pan.
1. Close side door(s) of the balance.
1. Wait until the stabilizing circle disappears.
1. Record the number as the mass of your sample.

When the number displayed no longer has a circle displayed next to it, the mass and balance has stabilized. Some balances have other ways to indicate that the balance is now stable. This means weighing analysis is complete and the number can be recorded.

Some balances are sufficiently sensitive to measure water as it is evaporating from aqueous solutions, or atmospheric water as it is hydrating the surface of reagents and equipment. In this situation, the balance may be stable and accurately displaying the correct mass, but the mass of the material may not be constant. In this situation, select the reading that will most closely represent the true value of the material being weighed.

A balance has a limit to how much weight it can read. There is normally a maximum capacity displayed on the front of the balance. Do not go over this range—most balance pans are balanced on a small point and exceeding the capacity of the balance may cause damage.

 AL 204 (analytical balance) PB 602-S (precision balance) Max: 210 g Min: 0.01 g Max: 610 g Min: 0.5 g

Part B: Weighing Procedure Using Tare Function

If you intend to use a container to carry your sample, you can tare (zero) the container on the balance. You will not have to subtract the initial weight of the container from the weight of the sample and the container together. However, review your procedure to see if you need the weight of the container and be sure to record the container weight if it will be needed in the future.

1. Set the empty container on the balance pan and close the doors if there is an isolation chamber. Press the tare button.
1. The balance should now display “0”. This allows the balance to ignore the weight of the container and accurately measure only the sample.
1. Add your sample to the container and record the weight that the balance shows. Remember to close the isolation chamber doors for an analytical balance.

SECTION 3: MAINTENANCE OF BALANCE ACCURACY

Modern balances are marvelous devices and are precision instruments. However, any balance will not give an accurate weight if it is mishandled. Depending on balance precision, a balance may need to be adjusted to match the gravitational acceleration of its location. Moving the balance and level may affect balance measurements. Avoid moving the balance in order to preserve its reading accuracy.

Balances should be left powered on in order to establish stable voltages and thermal gradients within electronic components. Some balances may enter a standby mode after a period of time, but should remain powered.

If a leveling bubble is present, the feet of the balance should be adjusted until the leveling bubble is centered as shown:

Centered Leveling Bubble

Precision

The precision of a balance can be monitored by recording the weight of a stable object over time. Most commercial and government laboratories monitor balances weekly, although some places require a check prior to each use. The calibration of the balance may often be adjusted to a standard mass. Usually this adjustment is done by an independent calibration company.

Static Charge

Contact between two materials and then separating them may generate a static charge. This can introduce unknown forces (either up or down) on the balance pan and cause an error. The static can be lessened by using a balance pan with doors to insulate it. This helps with the accuracy of an analytical balance.

Thermal Effects

Weighing an object that is not at constant temperature can affect the accuracy of a balance reading. If the object is either too hot or too cold, the weight could drift due to air currents and thermal expansion of the weighing pan. A constant temperature results with increased stability and accuracy. Laboratories are generally kept at a constant temperature to prevent measurement drifting due to thermal effects.

Buoyancy Error

When weighing is carried out in air, this can create buoyancy error, which may or may not be significant. The balance is subjected to buoyancy due to being submerged in air. Taring can prevent a drift due to buoyancy to help cancel this effect, or the buoyancy of air can be subtracted by calculation.

Refer to the following for more discussion and calculation:

https://chem.libretexts.org/Ancillary_Materials/Reference/Reference_Tables/Analytic_References/Appendix_09%3A_Correcting_Mass_for_the_Buoyancy_of_Air

SECTION 4: USER’S MANUAL FOR BALANCES

For further information on the balance in your laboratory, search online for the maker and model number. This manual can explain what features that specific balance can do which can vary between balance instruments.

https://www.labcompare.com/191234-Eliminating-Inaccuracy-in-Precision-Weighing-Caused-by-Static-Charge/

https://bitesizebio.com/33245/drift-measurements-analytical-balances/

https://www.mt.com/dam/mt_ext_files/Editorial/Generic/6/Weigh_Uncertain_Number5_0x0003d6750003db670009174c_files/measure_mass_force.pdf

https://www.mt.com/dam/product_organizations/laboratory_weighing/basic_weighing/Products/AL_and_PL/manuals/en/AL-PL-PL-S_OI_en_11780906C.pdf