CHEMISTRY GLOSSARY

precision balance    →   tehnička vaga

Precision balances typically display results from three to one decimal places (0.001 g up to 0.1 g). The readability precision balances are reduced when compared to analytical balances but, precision balances accommodate higher capacities (up to several kilograms). In its traditional form, it consists of a pivoted horizontal lever of equal length arms, called the beam, with a weighing pan, also called scale, suspended from each arm.

In electronic top pan, or toploader balances, mass is determined not by mechanical deflection but by electronically controlled compensation of an electric force. The signal generated enables the mass to be read from a digital display. The mass of the empty container can be stored in the balance’s computer memory and automatically deducted from the mass of the container plus its contents.

balance    →   vaga

Balance is an instrument to measure the mass (or weight) of a body. Balance beam type scales are the oldest type and measure weight using a fulcrum or pivot and a lever with the unknown weight placed on one end of the lever, and a counterweight applied to the other end. When the lever is balanced, the unknown weight and the counterweight are equal. The equal-arm balance consists of two identical pans hung from either end of a centrally suspended beam. The unequal-arm balance is made with one arm of the balance much longer than the other.

More modern substitution balances use the substitution principle. In this calibrated weights are removed from the single lever arm to bring the single pan suspended from it into equilibrium with a fixed counter weight. The substitution balance is more accurate than the two-pan device and enables weighing to be carried out more rapidly.

Electromagnetic force restoration balances also use a lever system but a magnetic field is used to generate the force on the opposite end of the lever and balance out the unknown mass. The current used to drive the magnetic coil is proportional to the mass of the object placed on the platform.

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.

unequal-arm balance    →   vaga s različitim krakovima

The lever principle on which these scales are constructed is based on the law of physics that at equilibrium the force applied at one end of the lever multiplied by the length of the arm (distance from the fulcrum to the point where the force is applied) must be equal to the product of the force acting at the opposite end of the lever and the length of the other arm.

The unequal-arm balance is preferred for work when large amounts are to be weighed.

technical atmosphere    →   tehnička atmosfera

See Atmosphere

atmosphere    →   atmosfera

1. Atmosphere is the column of air which is extending several hundred kilometers above the surface the Earth's surface. The density of this air decreases as you proceed up from the surface. The air in the atmosphere consists of 78 % nitrogen, 21 % oxygen, and 0.9 % argon. The remaining 0.1 % of the atmosphere consists of ozone, water vapor, carbon dioxide, methane, helium, and neon. The atmosphere is divided into different regions. The lowest two layers are the troposphere (the layer closest to the earth) and the stratosphere respectively. These two layers contain more than 99 % of the atmospheric molecules.

2. Standard atmosphere (atm) is an obsolete pressure and stress unit which should be discontinued. It is unit of pressure equal to the air pressure measured at mean sea level.

1 atm = 101 325 Pa

Technical atmosphere (at) is an obsolete MKpS pressure and sttress derived unit.

1 at = 98 066.5 Pa

1 atm = 1.033 227 453 at

auxochrome    →   auksokromna grupa

Auxochrome is a group or substructure in a molecule that influences the intensity of absorption of the molecule.

chemical technology    →   kemijska tehnologija

Chemical technology is a branch of applied chemistry that concerns technical methods and devices in order to manufacture a chemical product.

Curie    →   Curie

Maria Sklodowska-Curie (1867-1934) Polish-born French chemist who went to Paris in 1891. She married the physicist Pierre Curie (1859-1906) in 1985 and soon began work on seeking radioactive elements other than uranium in pitchblende (to account for its unexpectedly high radioactivity). By 1898 she had discovered radium and polonium although it took her years to purify them. In 1903 the Curies shared the Nobel Prize for physics with Henri Becquerel, who had discovered radioactivity.