CHEMISTRY GLOSSARY

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)

referent electrode    →   referentna elektroda

Referent electrode is an electrode whose potential is known and completely independent of analyte concentration. Mostly used referent electrodes are calomel and silver/silver chloride electrode.

Table: Dependence of referent electrodes potentials on KCl concentration

reversible reaction    →   reverzibilna reakcija

Reversible reaction is a chemical reaction that can proceed in both the forward and backward directions. When reversible reactions reach equilibrium the forward and reverse reactions are still happening but at the same rate, so the concentrations of reactants and products do not change. A reversible reaction is denoted by a double arrow pointing both directions in a chemical equation.

rhenium    →   renij

Rhenium was discovered by Walter Noddack, Ida Tacke and Otto Berg (Germany) in 1925. The origin of the name comes from the Latin word Rhenus meaning river Rhine. It is rare and costly, dense, silvery-white metal. Tarnishes in moist air. Resists corrosion and oxidation. Dissolves in nitric and sulfuric acids. Has a very high melting point. Rhenium is found in small amounts in gadolinite and molybdenite. Mixed with tungsten or platinum to make filaments for mass spectrographs. Its main value is as a trace alloying agent for hardening metal components that are subjected to continuous frictional forces.

saturated solution    →   zasićena otopina

Saturated solution is a solution that holds the maximum possible amount of dissolved material. When saturated, the rate of dissolving solid and that of recrystallisation solid are the same, and a condition of equilibrium is reached. The amount of material in solution varies with temperature; cold solutions can hold less dissolved solid material than hot solutions. Gases are more soluble in cold liquids than in hot liquids.

seawater    →   more

Seawater is a complex mixture of 96.5 % water, 3.5 % salts, and smaller amounts of other substances, including dissolved inorganic and organic materials, particulates, and a few atmospheric gases. The world's oceans cover nearly 71 % (361 840 000 km²) of the Earth's surface (510 100 000 km²), with an average depth of 3 682.2 m.

The density of seawater is higher than that of fresh water because of its higher salinity. Seawater's freezing point is lower than that of pure water and its boiling point is higher. The average salinity of the ocean is 35 ‰, which means that for every kilograms of water, there are 35 g of salt. The relative abundance of the major salts in seawater are constant regardless of the ocean. Only six elements and compounds comprise about 99 % of sea salts: chlorine (Cl^-), sodium (Na⁺), sulfur (SO₄^2-), magnesium (Mg²⁺), calcium (Ca²⁺), and potassium (K⁺).

second-order reactions    →   reakcije drugog reda

Second-order reaction is a reaction with a rate law that is proportional to either the concentration of a reactant squared, or the product of concentrations of two reactants.

For a general unimolecular reaction,

The reaction rate expression for a second order reaction is

If assumed that the concentration of reactant A is [A]_o at t=0 and [A] at time T, the variables in the rate equation and integrate can be separated. The integrated rate law for a second-order reaction can be easily shown to be

silver/silver-chloride electrode    →   srebro/srebrov klorid elektroda

Silver/silver-chloride electrode is by far the most common reference type used today because it is simple, inexpensive, very stable and non-toxic. It is mainly used with saturated potassium chloride electrolyte, but can be used with lower concentrations such as 3.5 mol dm^-3 or 1 mol dm^-3 potassium chloride. Silver/silver-chloride electrode is a referent electrode based on the following halfreaction

solubility product constant    →   konstanta produkta topljivosti

Solubility product constant (K_sp) (or the solubility product) is the product of the molar concentrations of the constituent ions, each raised to the power of its stoichiometric coefficient in the equilibrium equation. For instance, if a compound A_aB_b is in equilibrium with its solution

the solubility product is given by

spectrophotometer    →   spektrofotometar

Spectrophotometer is an instrument for measuring the amount of light absorbed by a sample.

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).