Analytical Balance

Principle

One of the essential laboratory procedures in analytical chemistry is measuring the mass of an object. An analytical balance is used to determine a mass with high accuracy. Its maximum capacity ranges from 1 g to a few kilograms with a precision of at least 1 part in 105.

Different apparatus techniques are used to weigh the object accurately. It is convenient to use a weighing bottle for drying and storing solids. Detectable amounts of water or oil from skin may be transferred to the sample. Thus, touching dried samples with fingers should be avoided. Tongs, chamois finger cots, clean cotton gloves, or strips of paper may be used to handle sample for weighing. In determining a series of sample masses, weighing by difference is applicable. One sample is then transferred from the bottle to a container. The amount of sample removed is controlled by the gentle tapping of the bottle with its top and slight rotation of the bottle. The difference between the two masses is the mass of the sample. Moisture in solids are generally removed by oven drying. However, this is not appropriate to use for samples that decompose or when water is not removed at the temperature of the oven. It is better to use desiccators to minimize the moisture uptake of dried materials as they cool. Moisture from the atmosphere is rapidly absorb by hygroscopic substances, so proper treatment of these substances is needed. Weighing bottle for each sample to be weighed is needed, then place approximate amount in the individual bottles and heat until the required time is reached. Quickly cap the bottles and cool in a desiccator when the heating is complete. After opening it briefly to remove any vacuum, weigh one of the bottles. Quickly empty the contents of the bottle into its receiving vessel, cap immediately, and weigh the bottle along with any solid that did not get transferred. Sample masses by difference may be obtain by repeating the procedure for each sample. Difference is also used to determine the mass of a liquid. Liquids that are relatively nonvolatile and noncorrosive can be transferred to previously weighed containers with tightly fitting cover. The mass of the container is subtracted from the total mass.

Different types of balances are used in the laboratory for weighing. The capacity of the most common balances (macrobalances) ranges between 160 to 200 g. Measurements with a standard deviation of +- 0.1 mg. Balances with a maximum capacity of 10 to 30 g with a precision of +- 0.011 mg are called semimicroanalytical balance. The common microanalytical balance has capacity of 1 to 3 g with a precision of 0.001. traditional analytical balance had two pans attached to either end of a lightweight beam pivoted about a knife edge located in the center of the beam. It is designed for weighing by direct comparison. single-pan analytic balance was faster and more convenient to use in weighing samples.  it is designed for weighing by substitution.A light weight beam supported by a prism shaped knife edge is a fundamental part of thesingle-pan balance. A pan for holding the object to be weighed is attached to the left of the beam and the hangers held the full set of masses. A set of knobs on the external of the balance case controls the lifting of the masses from beam one at a time. To balance the pan and masses on the left end of the beam, a counterweight was placed on the right end of the beam. A second knife edge is located near the left of the beam that supports the second planar surface. Electronic analytical balance also appeared and is now replacing the single-pan analytic balance. It has a coil that fits over the inner pole of the cylindrical permanent magnet that surrounds the pan above a metal cylinder. The cylinder, the pan, and indicator arm, and anything loaded on the pan are supported or levitated by an electric current in the coil that produces a magnetic field.  The amount of light striking the photocell of the null detector increases as the indicator arm moves downward as an object is placed on the pan. It produces a larger magnetic field that returns the pan to its original position. Devices, such as the electronic analytical balance, is called a servo system where in a small current causes a mechanical system to uphold a null position. Electronic balances are calibrated by weighing a standard mass and adjusting the current so that the exact mass of the standard appears on the display.

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