CHEMISTRY GLOSSARY

agar    →   agar

Agar, also called agar-agar, is an extract of certain species of red algae (of the Gelidium and Gracilaria genera) that is used as a gelling agent in microbiological culture media, foodstuffs, medicine, and cosmetic. The predominant component of agar is agarose, a polysaccharide made up of subunits of the sugar galactose.

agarose    →   agaroza

Agarose is a polysaccharide obtained from agar that is used for a variety of life science applications especially in gel electrophoresis. The structure of agarose consists of alternating (1→3)-linked β-D-galactopyranose and (1→4)-linked 3,6-anhydro-α-L-galactopyranose.

analytical balance    →   analitička vaga

Analytical balances are instruments used for precise determining mass of matter. Analytical balances are sensitive and expensive instruments, and upon their accuracy and precision the accuracy of analysis result depends. The most widely used type of analytical balances are balances with a capacity of 100 g and a sensitivity of 0.1 mg. Not one quantitative chemical analysis is possible without usage of balances, because, regardless of which analytical method is being used, there is always a need for weighing a sample for analysis and the necessary quantity of reagents for solution preparation.

The working part of the balance is enclosed in a glass-fitted case. The baseplate is usually of black glass or black slate. The beam has agate knife-edges at its extremes, supporting stirrups from which balance pans are suspended. Another agate or steel knife-edge is fixed exactly in the middle of the beam on its bottom side. This knife-edge faces downwards and supports the beam. When not in use and during loading or unloading of the pans, the balance should be arrested.

The principle of operation of a modern laboratory balance bears some resemblance to its predecessor - the equal arm balance. The older instrument opposed the torque exerted by an unknown mass on one side of a pivot to that of an adjustable known weight on the other side. When the pointer returned to the center position, the torques must be equal, and the weight was determined by the position of the moving weights.

Modern electronic laboratory balances work on the principle of magnetic force restoration. In this system, the force exerted by the object being weighed is lifted by an electromagnet. A detector measures the current required to oppose the downward motion of the weight in the magnetic field.

equal-arm balance    →   vaga s jednakim krakovima

The simplest type of balance, the equal-arm balance, is an application of a first class lever. The beam of the balance is supported on a central knife-edge, usually of agate, which rests upon a plane agate plate. The point of support is called the fulcrum. Two pans of equal weight are suspended from the beam, one at each end, at points equidistant from the fulcrum. A long pointer attached at right angles to the beam at the fulcrum indicates zero on a scale when the beam is at rest parallel to a level surface.

To prevent the knife-edge from becoming dull under the weight of the beam and pans the balance is equipped with a special device called an arrest. The arrest is operated by means of milled knob underneath the base plate in the middle and in front of the balance (sometimes the arrest knob is at one side of the balance).

The object to be weighed is placed on one pan, and standard weights are added to the other until the balance of the beam is established again. When not in use and during loading or unloading of the pans, the balance should be arrested.