Quantity of a Substance

An array ion collector (detector) consists of a large number of miniature electron multiplier elements arranged side by side along a plane. Point ion collectors gather and detect ions sequentially (all ions are focused at one point one after another), but array collectors gather and detect all ions simultaneously (all ions are focused onto the array elements at the same time). Array detectors are particularly useful for situations in which ionization occurs within a very short space of time, as with some ionization sources, or in which only trace quantities of a substance are available. For these very short time scales, only the array collector can measure a whole spectrum or part of a spectrum satisfactorily in the time available. 

After the analyzer of a mass spectrometer has dispersed a beam of ions in space or in time according to their various m/z values, they can be collected by a planar assembly of small electron multipliers. There are two types of multipoint planar collectors an array is used in the case of spatial separation, and a microchannel plate is used in the case of temporal separation. With both multipoint assemblies, all ions over a specified mass range are detected at the same time, or apparently at the same time, giving these assemblies distinct advantages over the single-point collector in the analysis of very small quantities of a substance or where ions are produced intermittently during short time intervals. 

Stoff-menge, /. quantity of a substance amount of material, -mischung, /. (Paper) pulp mixture, -miihle, /. (Paper) stuff engine, hoUander. -patent, n. patent on a substance. -rahmen, m. filter disk, -teilchen, n. particle of matter, -umsatz, m. change of substance, specif, metabohsm. -verbindung, /. (Patents) composition of matter, -ver-brauch, m. consumption of material, -ver-wandtschaft, /. chemical affinity. 

What this illustrates is that an MSDS describes the dangers of large industrial quantities of a substance, not the tiny amounts usually found in consumer products. Nonetheless, once you are experienced at reading an MSDS, it is a good place to get information on the safety of a chemical compound. In this book I cover safety issues only occasionally—and briefly. To do a more thorough job of that would require a much larger book, and there is already a wealth of information available. 

Dose—A general term denoting the quantity of a substance, radiation, or energy absorbed. For special purposes it must be appropriately qualified. If unqualified, it refers to radiation absorbed dose. 

Equivalents are measures of the quantity of a substance present, analogous to moles. Normality is a concentration unit, analogous to molarity. The similarities are so great that these two units should be very easy to learn to use, but many students have difficulty with them. Be sure that you understand the underlying definitions before you proceed. 

The equivalent is defined in terms of a chemical reaction. It is defined in one of two different ways, depending on whether an oxidation-reduction reaction or an acid-base reaction is under discussion. For an oxidation-reduction reaction, an equivalent is the quantity of a substance that will react with or yield 1 mol of electrons. For an acid-base reaction, an equivalent is the quantity of a substance that will react with or yield 1 mol of hydrogen ions or hydroxide ions. Note that the equivalent is defined in terms of a reaction, not merely in terms of a formula. Thus, the same mass of the same compound undergoing different reactions can correspond to different numbers of equivalents. The ability to determine the number of equivalents per mole is the key to calculations in this chapter. 

The quantity of a substance liberated at an electrode during electrolysis is proportional to the quantity of electricity passing through the solution. 

When large quantities of a substance are handled, sensitivity of the material to heating under confinement may need to be considered to demonstrate the effect on the stored/handled, and probably confined, substance in the event of an external heat load. Tests such as the steel sleeve test or Koenen test [24, 137], the Dutch pressure vessel test (DPVT) [143], and the United States pressure vessel test (US-PVT) [143] may be applicable. These tests are used mostly for transportation considerations. The tests generally subject the sample substances to very high energy inputs under confined conditions, and thus are more severe than the deflagration and autoclave tests previously discussed in Section 2.3.3.2. As an example, the Koenen test, used mainly in Europe, is illustrated in Figure 2.32. 

A spectrophotometer is a combination of spectrometer and photometer. A photometer is an instrument which is used for the production of light of selected colour or wavelength. The spectrometry involves the determination of the light absorptive capacity of a chemical. So spectrophotometric analysis provides a significant method of determining minute quantities of a substance. 

The relationship between mEq and mg quantities of a substance can be expressed in one of the following ways ... 

Generally, it has been found that the organic acids and bases do exist in aqueous solution as equilibrium mixtures of their respective neutral as well as ionic forms. Thus, these neutral and ionic forms may not have the same identical partition coefficients in a second solvent therefore, the quantity of a substance being extracted solely depends upon the position of the acid-base equilibrium and ultimately upon the pH of the resulting solution. Hence, extraction coefficient (E) may be defined as the ratio of the concentrations of the substance in all its forms in the two respective phases in the presence of equilibria and it can be expressed as follows ... 

Solubility A measure of the maximum quantity of a substance that can dissolve in a given volume of solvent under a specified set of conditions. 

A chemist will often need to know the exact quantity of a substance present in a given sample of a chemical. This can be determined by carrying out a quantitative reaction and using the stoichiometry of the reaction to determine the amount of substance. 

Recovery factor is defined as the percentage (i %), or fraction R, of the total quantity of a substance extracted under specified conditions. (In our text we also use percentage extraction, E%1)... 

Injection of a known quantity of a substance in a real sample... 

Some values may be found in Lange s and Chemical Rubber Co Handbooks, Landolt Bornstein Tables, International Critical Tables, etc (Refs 15, 17, 22, 41, 42, 43 44) Heat of Combustion (Qc or He) (Verbrennungs warme in Ger). It may be defined as total heat evolved when a given quantity of a substance is completely oxidized by being caused to react in an excess of oxygen. 

The heats of formation for a reaction containing explosive chemicals can be described as the total heat evolved when a given quantity of a substance is completely oxidized in an excess amount of oxygen, result-... 

CA 27,4233(1933)1 obtained among the prnducts of reaction between di-p-nitrobenzo-hydrazide chloride in ale and Na azide, a small quantity of a substance which deflagrated violently ca 149°. It was suggested that this compd was the p-nitrobenzenyl azide of 1-amino, 5-p-nitropbenyltetrazole (See also under Di-p-nitro ben zohy dr azide Azide)... 

Catalysis, It may be defined as the effect produced on the rate of reaction either by a small quantity of a substance(called "catalyst ), which appeares to be unchanged in the reaction products, or by some physical energy, such as radiation, electricity or magnetism. Reactions accelerated by catalysts are called "positive catalytic reactions , while those retarded are known as "negative catalytic reactions . The Phenomenon of catalysis is essentially a branch of "surface chemistry ... 

Calorific value heat of combustion of a unit quantity of a substance usually expressed in British thermal units per pound (Btu/lb) can also be calories per gram (cal/g) or joules per gram (J/g), when required (ASTM D-2015 ASTM D-3286). 

Overview The English chemist Humphrey Davy wrote in 1812 If a piece of zinc and a piece of copper be brought in contact with each other, they will form a weak electrical combination, of which the zinc will be positive, and the copper negative. . . so initiating the history of the electrochemical cell. But it was Michael Faraday who, in 1834, laid the foundations of quantitative electrochemistry by relating the quantity of a substance electrolysed to the amount of electrical charge involved. 

The density of a substance is the ratio of its mass per unit volume. Density can be found mathematically by dividing the mass of a substance by its volume. The formula is d = , where d is density, m is mass, and V is volume. While mass and volume do depend on the quantity of a substance (these are extensive properties), the ratio is constant at a given temperature. The units of density, reported in standard references, is in terms of g/mL (or g/cc or g/cm3) at 20°C. The temperature is reported since the volume of a sample will change with temperature and, thus, so does the density. 

For grinding any quantity of a substance a large porcelain mortar (say 8 inches in diameter) with a heavy pestle is preferable to the small mortars usually supplied in the desks. One or more such mortars is placed in the laboratory for general use. 

The numerical value of the molecular weight expressed in grams (e.g. 16 g in the case of methane) is often used to represent the quantity of a substance. The gram molecular weight comprises the same number of molecules of any substance (18 g of water, 44 g of carbon dioxide or 68 g of isoprene contain the same number of molecules of the respective compounds). This number... 

A similar variation requires that you determine how long it will take to plate out a certain quantity of a substance. The process is very similar, except that it is worked in reverse. We ll examine this type of problem in the next sample. 

You would never express the mass of a lump of gold, like the one in Figure 5.11, in atomic mass units. You would express its mass in grams. How does the mole relate the number of atoms to measurable quantities of a substance The definition of the mole pertains to relative atomic mass, as you learned in section 5.1. One atom of carbon-12 has a mass of exactly 12 u. Also, by definition, one mole of carbon-12 atoms (6.02 x 1023 carbon-12 atoms) has a mass of exactly 12 g. 

The concentration of a very small quantity of a substance in the human body, or in the environment, can be expressed in parts per million (ppm) and parts per billion (ppb). Both parts per million and parts per billion are usually mass/mass relationships. They describe the amount of solute that is present in a solution. Notice that parts per million does not refer to the number of particles, but to the mass of the solute compared with the mass of the solution. 

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