CHEMISTRY GLOSSARY

photomultiplier    →   fotomultiplikator

Photomultiplier (photomultiplier tube or PMT) is a very versatile and sensitive detector of radiant energy in the ultraviolet, visible, and near infrared regions of the electromagnetic spectrum. A typical photomultiplier tube consists of a photoemissive cathode (photocathode) followed by focusing electrodes, an electron multiplier (dynode) and an electron collector (anode) in a vacuum tube.

absorbance    →   apsorbancija

Absorbance (A) is a logarithm of the ratio of incident radiant power (P_o) to transmitted radiant power (P) through a sample (excluding the effects on cell walls).

The absorption of light by a substance in a solution can be described mathematically by the Beer-Lambert law

where A is the absorbance at a given wavelength of light, ε is the molar absorbtivity or extinction coefficient (L mol^-1 cm^-1), unique to each molecule and varying with wavelength, b is the length of light path through the sample (cm), and c is the concentration of the compound in solution (mol L^-1).

abundance of elements    →   rasprostranjenost elemenata

Elements in nature are mostly found in different compounds and, rarely, in the free (elementary) state. In Earth’s crust the most abundant of all elements is oxygen (with 49.5 %), then silicon (25 %), aluminium (7.5 %), iron (4.7 %), calcium (3.4 %), sodium (2.6 %), potassium (2.4 %), magnesium (1.9 %) and hydrogen (1.9 %). These nine elements make up almost 99 % of the Earth’s composition.

accumulator    →   akumulator

Accumulator (secondary cell, storage battery) is a type of voltaic cell or battery that can be recharged by passing current through it from an external D.C. supply. The charging current reverses the chemical reactions in the cell. The common types are the lead-acid accumulator and the nickel-cadmium cell.

acetal    →   acetal

Acetals are organic compounds having the structure R₂C(OR’)₂ (R’ ≠ H). They are organic compounds formed by addition of alcohol molecules to aldehyde or ketone molecules. Originally, the term was confined to derivatives of aldehydes (one R = H), but it now applies equally to derivatives of ketones (neither R = H ). Mixed acetals have different R’ groups. The formation of acetals is reversible; acetals can be hydrolysed back to aldehydes (ketone) in acidic solutions.

Acetal, 1,1-diethoxyethane (CH₃CH(OC₂H₅)₂), is an organic compound, pleasant smelling, formed by addition of ethyl alcohol to ethanal (acetaldehyde). It is used as a solvent and in synthetic organic chemistry.

acid-base titration    →   kiselo-bazna titracija

Acid-base titration is an analytical technique in volumetric analysis, where an acid of known concentration is used to neutralise a known volume of a base, and the observed volume of the acid required is used to determine the unknown concentration of the base. An acid-base indicator is used to determine the end-point of the titration.

activation energy    →   energija aktivacije

Activation energy (E_a) is the energy that must be added to a system in order for a process to occur, even though the process may already be thermodynamically possible. In chemical kinetics, the activation energy is the height of the potential barrier separating the products and reactants. It determines the temperature dependence on the reaction rate.

active site    →   aktivno mjesto

Active site is a pocket or crevice on an enzyme molecule that fits reactant molecules like a hand in a glove. The active site lowers the activation energy for reaction

adenosine triphosphate    →   adenozin trifosfat

Adenosine triphosphate (ATP) is nucleotide that is of fundamental importance as a carrier of chemical energy in all living organisms. It consists of adenin linked to D-ribose).

aglycon    →   aglikon

Aglycon (aglycone) is the non-sugar compound remaining after replacement of the glycosyl group from a glycoside by a hydrogen atom. The link between the glycon and aglycon is known as glycosidic bond.