CHEMISTRY GLOSSARY

glass electrode    →   staklena elektroda

Glass electrode is a hydrogen-ion responsive electrode usually consisting of a bulb, or other suitable form, of special glass attached to a stem of high resistance glass complete with internal reference electrode and internal filling solution system. Glass electrode is also available for the measurement of sodium ions.

The glass electrode, which consists of a thin wall glass bulb, has an extremely high electrical resistance. The membrane of a typical glass electrode (with a thickness of 0.03 mm to 0.1 mm) has an electrical resistance of 30 MΩ to 600 MΩ. The surface of a glass membrane must be hydrated before it will function as a pH electrode. When a glass surface is immersed in an aqueous solution then a thin solvated layer (gel layer) is formed on the glass surface in which the glass structure is softer. This applies to both the outside and inside of the glass membrane.

The simplest explanation for the working of the thin glass electrode is that the glass acts as a weak acid (Glass-H).

The hydrogen ion activity of the internal solution is held constant. When a solution of different pH from the inside comes in contact with the outside of the glass membrane, the glass is either deprotonated or protonated relative to the inside of the glass. The difference in pH between solutions inside and outside the thin glass membrane creates electromotive force in proportion to this difference in pH.

gravimetry    →   gravimetrija

Gravimetry is the quantitative measurement of an analyte by weighing a pure, solid form of the analyte. Since gravimetric analysis is an absolute measurement, it is the principal method for analysing and preparing primary standards.

A typical experimental procedure to determine an unknown concentration of an analyte in a solution is as follows:

- quantitatively precipitate the analyte from the solution

- collect the precipitate by filtering and wash it to remove impurities

- dry the solid in an oven to remove the solvent

- weigh the solid on an analytical balance

- calculate the analyte concentration in the original solution based on the weight of the precipitate.

hafnium    →   hafnij

Hafnium was discovered by Dirk Coster (Denmark) and Georg Karl von Hevesy (Hungary) in 1923. The origin of the name comes from the Latin name Hafnia meaning Copenhagen. It is silvery, ductile metal. Exposed surfaces form oxide film. Resists alkalis and acids (except HF). Toxic. Metal ignites and burns readily. Hafnium is obtained from mineral zircon or baddeleyite. Used in reactor control rods because of its ability to absorb neutrons.

Knudsen burette    →   Knudsenova bireta

Knudsen's automatic bulb-burette, developed by the Danish physicist Martin Knudsen (1871-1949), is designed in a way that even routine field analysis in a boat laboratory would provide highly accurate measurements. The burette is filled with a mixture of silver nitrate from reservoir R, located above the burette, by opening the A valve. When the solution crosses the three-way C valve the A valve is closed preventing further solution flow in to the burette. Any extra solution is caught in the W bowl. Turn the C valve, which marks the zero on the scale, in order to allow atmospheric air to enter the burette. Since most open-ocean samples lie in a relatively small chlorinity range, the burette is designed so that much of its capacity is in the bulb (B). This allows the titration to be quick (by quickly releasing contents from the B area) and reduces the error that occurs from the slow drainage along the inner wall of the burette.

Each millimeter is divided in to twenty parts (double millimeter division of the Knudsen burette) which allows for highly accurate measurements (the scale is read up to a precision of 0.005 mL). From 0 to 16 the burette isn't divided, that usually starts from 16 and goes until 20.5 or 21.5. A single double millimeter on a Knudsen burette scale corresponds to one permille of chloride in the seawater sample. This burette can be used for titration of water from all of the oceans and seas, with the exemptions being areas with very low salinity (e.g. the Baltic Sea) and river estuaries which require the use of normal burettes.

Knudsen’s pipette    →   Knudsenova pipeta

Kudsen's automatic pipette, developed by the Danish physicist Martin Knudsen (1871-1949), allows quick and accurate transfer of a constant volume of liquid (sea water), usually around 15 mL. On the top of pipette is a double sided C vent that can establish flow between the body of the pipette and one of the branches (A or B), or isolate the body of the pipette from both of the branches. Sucking through the B branch the pipette is filled with liquid, it is closed with a twist of the C valve and the liquid is released by rotating the valve towards the A branch (so atmospheric air can enter the pipette). Emptying the pipette takes around 30 seconds. Before it's first use, the pipette must be calibrated with distilled water.

krypton    →   kripton

Krypton was discovered by Sir William Ramsay and Morris W. Travers (England) in 1898. The origin of the name comes from the Greek word kryptos meaning hidden. It is colourless, odourless rare noble gas. Reacts only with fluorine. Krypton is obtained from production of liquid air. Used in lighting products. Some is used as inert filler-gas in incandescent bulbs. Some is mixed with argon in fluorescent lamps. The most important use is in flashing stroboscopic lamps that outline airport runways.

low-density polyethylene    →   polietilen niske gustoće

Low-density polyethylene (LDPE) is a thermoplastic polyolefin with a density of 0.910-0.940 g cm^-3. It is a recyclable plastic used predominantly in film applications due to its toughness, flexibility and relative transparency. LDPE has a low melting point, making it popular for use in applications where heat sealing is necessary. Typically, LDPE is used to manufacture flexible films such as those used for plastic retail bags and garment dry cleaning and grocery bags.

metal    →   metal

Metals are materials in which the highest occupied energy band (conduction band) is only partially filled with electrons.

Their physical properties generally include:

- They are good conductors of heat and electricity. The electrical conductivity of metals generally decreases with temperature.

- They are malleable and ductile in their solid state.

- They show metallic lustre.

- They are opaque.

- They have high density.

- They are solids (except mercury)

- They have a crystal structure in which each atom is surrounded by eight to twelve near neighbours

Their chemical properties generally are:

- They have one to four valence electrons.

- They have low ionisation potentials; they readily lose electrons.

- They are good reducing agents.

- They have hydroxides which are bases or amphoteric.

- They are electropositive.

Metallic characteristics of the elements decrease and non-metallic characteristics increase with the increase of valence electrons. Also metallic characteristics increase with the number of electron shells. Therefore, there is no sharp dividing line between the metals and non-metals.

Of the 114 elements now known, only 17 show primarily non-metallic characteristics, 7 others are metalloids, and 89 may be classed as metals.

neodymium    →   neodimij

Neodymium was discovered by Carl F. Auer von Welsbach (Austria) in 1885. The origin of the name comes from the Greek words neos didymos meaning new twin. It is silvery-white, rare-earth metal that oxidizes easily in air. Reacts slowly in cold water, more rapidly as heated. Metal ignites and burns readily. Neodymium is made from electrolysis of its halide salts, which are made from monazite sand. Used in making artificial ruby for lasers. Also in ceramics and for a special lens with praseodymium. Also to produce bright purple glass and special glass that filters infrared radiation. Misch metal, used in the manufacture of pyrophoric alloys for cigarette lighters, contains about 18 % neodymium metal. (Typically composition of misch metal are Ce:Nd:Pr:La:Other rare earth=50:18:6:22:4). Neodymium is used to create some of the most powerful permanent magnets on Earth, known as NIB magnets they consist of neodymium, iron, and boron.

niobium    →   niobij

Niobium was discovered by Charles Hatchett (England) in 1801. The origin of the name comes from the Greek word Niobe meaning daughter of Tantalus in Greek mythology (tantalum is closely related to niobium in the periodic table). It is shiny white, soft, ductile metal. Exposed surfaces form oxide film. Niobium occurs in a mineral columbite. It is used in stainless steel alloys for nuclear reactors, jets and missiles. Used as an alloy with iron and nickel. It can be used in nuclear reactors and is known to be superconductive when alloyed with tin, aluminium or zirconium.