CHEMISTRY GLOSSARY

melting point    →   talište

Melting point is the temperature at which a solid becomes a liquid at normal atmospheric pressure.

A more specific definition of melting point (or freezing point) is the temperature at which the solid and liquid phases of a substance are in equilibrium at a specified pressure (normally taken to be atmospheric unless stated otherwise). A pure substance under standard condition of pressure has a single reproducible melting point. The terms melting point and freezing point are often used interchangeably, depending on whether the substance is being heated or cooled.

normal conditions    →   normalni uvjeti

Gas is under normal (or standard) conditions when: p₀ = 10⁵ Pa and T₀ = 273.15 K (0 °C). IUPAC recommends that the former use of the pressure of 1 atm as standard pressure (equivalent to 101 325 Pa) should be discontinued. At these conditions, the molar volume of gas V_m0 is 0.022 711 m³ (22.711 L).

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.

platinum    →   platina

Platinum was discovered by Antonio de Ulloa (South America) in 1735. The origin of the name comes from the Spanish word platina meaning silver. It is rare, very heavy, soft, silvery-white metal. Resists oxygen and water. Platinum is produced from deposits of native, or elemental. Used in jewellery, to make crucible and special containers and as a catalyst. Used with cobalt to produce very strong magnets. Also to make standard weights and measures. Resists corrosion and acid attacks except aqua regia.

polarography    →   polarografija

Polarography is a volumetric technique which is based on a diffusion controlled analyte travel to the surface of dropping mercury electrode (DME). The surface of the working electrode (dropping mercury electrode) is constantly renewed under dropping conditions and, thus, the conditions under which reaction takes place are readily reproducible. Depolarisation potential enables identification of ions present in the solution, and by measuring the diffusion current their concentration is calculated. Polarography was discovered in 1922 by the Czech chemist Jaroslav Heyrovský (1890-1967).

potentiometric titration    →   potenciometrijska titracija

Potentiometric titration is a volumetric method in which the potential between two electrodes is measured (referent and indicator electrode) as a function of the added reagent volume. Types of potentiometric titrations for the determination of analytes in photoprocessing solutions include acid-base, redox, precipitation, and complexometric.

Potentiometric titrations are preferred to manual titrations, since they are more accurate and precise. They are also more easily adapted to automation, where automated titration systems can process larger volumes of samples with minimal analyst involvement.

A titration curve has a characteristic sigmoid curve. The part of the curve that has the maximum change marks the equivalence point of the titration. The first derivative, ΔE/ΔV, is the slope of the curve, and the endpoint occurs at the volume, V', where ΔE/ΔV has the maximum value.

practical salinity    →   praktični salinitet

Practical salinity S_P is defined on the Practical Salinity Scale of 1978 (PSS-78) in terms of the conductivity ratio K₁₅ which is the electrical conductivity of the sample at temperature t₆₈ = 15 °C and pressure equal to one standard atmosphere, divided by the conductivity of a standard potassium chloride (KCl) solution at the same temperature and pressure. The mass fraction of KCl in the standard solution is 0.0324356 (32.4356 g of KCl in 1 kg of solution). When K₁₅ = 1, the Practical Salinity P S is by definition 35. The conductivity of that reference solution is C(35,1568,0) = 42.914 mS/cm = 4.2914 S/m (Siemens per meter). Note that Practical Salinity is a unit-less quantity. Though sometimes convenient, it is technically incorrect to quote Practical Salinity in "psu". When K₁₅ is not unity, S_P and K₁₅ are related by the PSS-78 equation

At a temperature of t₆₈ = 15 °C, R_t is simply K₁₅ and Practical Salinity S_P can be determined from the above equation. For temperatures other than t₆₈ = 15 °C, Practical Salinity S_P is given by the following function of R_t (k = 0.0162)

salinity    →   salinitet

Salinity (S) is a measure of the quantity of dissolved salts in seawater. It is formally defined as the total amount of dissolved solids in seawater in parts per thousand (‰) by weight when all the carbonate has been converted to oxide, the bromide and iodide to chloride, and all organic matter is completely oxidized.

Chlorinity is the oldest of the salinity measures considered and is still a corner-stone in the study of dissolved material in seawater. Based on the principle of constant relative proportions it provides a measure of the total amount of dissolved material in seawater in terms of the concentration of halides. The relationship between chlorinity (Cl) and salinity as set forth in Knudsen’s tables is

In 1962, however, a better expression for the relationship between total dissolved salts and chlorinity was found to be

Practical Salinity (S_P) was introduced as a replacement for Chlorinity. Practical Salinity is is relatively easy to measure using standard conductometers, measurements are more precise and less time consuming than measurements of Chlorinity and accurate measurements can even be made in situ. Practical salinity S_P is defined on the Practical Salinity Scale of 1978 (PSS-78) in terms of the conductivity ratio K₁₅ which is the electrical conductivity of the sample at temperature t₆₈ = 15 °C and pressure equal to one standard atmosphere, divided by the conductivity of a standard potassium chloride (KCl) solution at the same temperature and pressure. The mass fraction of KCl in the standard solution is 0.0324356 (32.4356 g of KCl in 1 kg of solution).

Note that Practical Salinity is a unit-less quantity. Though sometimes convenient, it is technically incorrect to quote Practical Salinity in "psu". For most purposes one can assume that the psu and the ‰, are synonymous.

The global average salinity of ocean waters is about 35 ‰, that is, about 35 g of solid substances are dissolved in 1 kg of seawater.

solar cell    →   sunčeva ćelija

Solar cell, or photovoltaic cell, is a device that captures sunlight and transforms it directly to electricity. All solar cells make use of photovoltaic effect, so often they are called photovoltaic cells. Almost all solar cells are built from solid-state semiconducting materials, and in the vast majority of these the semiconductor is silicon.

The photovoltaic effect involves the generation of mobile charge carriers-electrons and positively charged holes-by the absorption of a photon of light. This pair of charge carriers is produced when an electron in the highest filled electronic band of a semiconductor (the valence band) absorbs a photon of sufficient energy to promote it into the empty energy band (the conduction band). The excitation process can be induced only by a photon with an energy corresponding to the width of the energy gap that separates the valence and the conduction band. The creation of an electron-hole pair can be converted into the generation of an electrical current in a semiconductor junction device, wherein a layer of semiconducting material lies back to back with a layer of either a different semiconductor or a metal. In most photovoltaic cells, the junction is p-n junction, in which p-doped and n-doped semiconductors are married together. At the interface of the two, the predominance of positively charged carriers (holes) in the p-doped material and of negatively charged carriers (electrons) in the n-doped material sets up an electric field, which falls off to either side of the junction across a space-charge region. When absorption of a photon in this region generates an electron-hole pair, these charge carriers are driven in opposite directions by the electric field, i.e. away from the interface and toward the top and bottom of the two-layer structure, where metal electrodes on these faces collect the current. The electrode on the top layer (through which light is absorbed) is divided into strips so as not to obscure the semiconducting layers below. In most widely used commercial solar cells, the p-doped and n-doped semiconductive layers are formed within a monolithic piece of crystalline silicon. Silicon is able to absorb sunlight at those wavelengths at which it is most intense-from the near-infrared region (wavelengths of around 1200 nm) to the violet (around 350 nm).

supercritical fluid    →   superkritični fluid

Supercritical fluid is any substance above its critical temperature and critical pressure (see phase diagram). It shows unique properties that are different from those of either gases or liquids under standard conditions. A supercritical fluid has both the gaseous property of being able to penetrate anything, and the liquid property of being able to dissolve materials into their components. Solublity increases with increasing density (i.e. with increasing pressure). An example of this is naphthalene which is practically insoluble in low pressure carbon dioxide. At 100 bar the solubility is 10 g/L and at 200 bar it is 50 g/L. Rapid expansion of supercritical solutions leads to precipitation of a finely divided solid.