CHEMISTRY GLOSSARY

allotrope    →   alotrop

Allotropes are the elements which exist in two or more different forms in the same physical state. Allotropes generally differ in physical properties and may also differ in chemical activity.

Diamond, graphite and fullerenes are three allotropes of the element carbon. Graphite is a soft, black, slippery substance; by contrast, diamond is one of the hardest substances known. The different properties of the allotropes arise from their chemical structures. Diamonds typically crystallize in the cubic crystal system and consist of tetrahedrally bonded carbon atoms. Graphite crystallizes in the hexagonal system. In the fullerenes, the carbon atoms taking the form of a hollow sphere, ellipsoid, or tube.

In some cases, the allotropes are stable over a temperature range, with a definite transition point at which one changes into the other. For instance, tin has two allotropes: white (metallic) tin stable above 13.2 °C and grey (nonmetallic) tin stable below 13.2 °C.

The term allotropes may also be used to refer to the molecular forms of an element. Ozone is a chemically active triatomic allotrope of the element oxygen.

allotropic modification    →   alotropska modifikacija

Different substances of the same elementary system are called allotropes or allotropic modifications. In the case of oxygen, there are two allotropic modifications: "normal" dioxygen (O₂) and trioxygen (O₃) or ozone.

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.

nitride    →   nitrid

Nitrides are compounds of nitrogen with metal in which nitrogen has an oxidation number -3, are hard and resistant to high temperatures.

oil    →   ulje

Oils are triglycerides containing a high percentage of unsaturated fatty acids.

PE    →   PE

PE is an abbreviation for polyethylene, usually abbreviated as HDPE (High Density Polyethylene), MDPE (Medium Density Polyethylene), and LDPE (Low Density Polyethylene).

pectin    →   pektin

Pectins are complex highmolecular carbohydrates, formed mostly from galacturonic acid. They are present in many kinds of fruit and vegetable and are used in the production of fruit jellies.

refractory metal    →   vatrostalni metal

Refractory metal is a metal having an extremely high melting point, for example tungsten, molybdenum, niobium, tantalum, and rhenium

anomer    →   anomer

Anomers are diastereoisomers of cyclic forms of sugars or similar molecules differing in the configuration at the anomeric carbon (C-1 atom of an aldose or the C-2 atom of a 2-ketose). The cyclic forms of carbohydrates can exist in two forms, α- and β- based on the position of the substituent at the anomeric center. Anomer are designated α if the configuration at the anomeric carbon is the same as that at the reference asymmetric carbon in a Fischer projection. If the configuration differs the anomer is designated β. For example, α-D-glucopyranose and β-D-glucopyranose, the two cyclic forms of glucose, are anomers.

arginine    →   arginin

Arginine is an electrically charged amino acids with basic side chains. It is one of the least frequent amino acids. As a group the charged amino acids are important for making proteins soluble. These residues are generally located on the surface of the protein. Arginine is well designed to bind the phosphate anion, and is often found in the active centers of proteins that bind phosphorylated substrates. As a cation, arginine, as well as lysine, plays a role in maintaining the overall charge balance of a protein. Although arginine is considered an essential amino acid (it must be obtained through the diet), this is true only during the juvenile period in humans.

• Abbreviations: Arg, R
• IUPAC name: 2-amino-5-(diaminomethylideneamino)pentanoic acid
• Molecular formula: C₆H₁₄N₄O₂
• Molecular weight: 174.20 g/mol