Symbols, their Names, and SI Units

All other units in SI are derived from the nine base and supplementary units. They will often have their own unit name and symbol, but all can be reduced through appropriate definitions to the nine primary units. Table 6D-2 lists the approved derived units with special names and their formulas and symbols. Table 6D-3 is a representative list of derived SI units which are widely used in chemical engineering but which do not have approved names. 

The international unit itself may eventually be replaced by the SI unit termed the katal, the SI derived unit for catalytic activity (see Chapter 1). It is defined as moles per second. The name katal had been used for this unit for decades, but did not become an official SI derived unit until 1999 with Resolution 12 of the 21st CGPM, on the recommendation of the International Federation of Cfinical chemistry and Laboratory Medicine. Both the International Union of Pure and Applied Chemistry and the lUB now recommend that enzyme activity be expressed in moles per second and that the enzyme concentration be expressed in terms of katals per liter (kat/L). Thus, lU = lO mol/fiOs = 16.7 X 10 mol/s, or l.Onkat/L - 0.06U/L. The formal adoption of the katal is hoped to discourage the use of a non-SI unit called unit, symbol U, defined as micromoles per minute. Units are more commonly used than the katal in practice at present, but their definition lacks coherence with the SI system. 

In the SI system the seven basic units fisted in Table 1.2-1 are identified and their values are assigned. From these seven basic well-defined units, the units of other quan-dties can be derived. Also, certain quantities appear so frequently that they have been given special names and symbols in the SI system. Those of interest here are listed in Table 1.2-2. Some other derived units acceptable in the SI system are given in Table 1.2-3, and Table 1.2-4 lists the acceptable scaling prefi.xes. [It should be pointed out... 

Table A. 1 gives the seven base quantities, assumed to be mutually independent, on which the SI is founded, and the names and symbols of their respective units, called SI base units. ...

Several SI derived units have special names and symbols. They are listed in Table 11.3 v/ith their definitions in terms of other SI units. Examples of SI derived units without special names can be found in Table 11.4. 

It is sometimes necessary, or convenient, to form names and symbols that are decimal multiples or submultiples of SI units. Only one prefix is used when a multiple of an SI unit is formed, which should be in the numerator. These prefixes and their approved symbols are given in Table A.3. Symbols for all units used in this book, SI or otherwise, are given inside the front cover. 

Further steps toward universality are taken by the replacement of element and compound names wherever possible by symbols and formulas, and by adding to data in older units their recalculated SI equivalents. The usefulness of the reference sections has been increased by giving journal-title abbreviations according to the Chemical Abstracts Service Source Index, by listing in each reference all of its authors and by accompanying references to patents and journals that may be difficult to access by their Chemical... 

Figure 2.2 Some examples of polyhedral composite building units with their corresponding pore symbols and common names. The nodes are tetrahedrally coordinated atoms such as Si or Al. Bridging oxygen atoms have been left out for clarity.

The SI consists of seven base quantities from which all the other quantities (secondary or derived quantities) can be derived. The table in the slide presents the name of the quantities, their unit and the symbol of the unit. 

By convention, physical quantities are organized in the International System (SI) of quantities and units, which is built upon seven base quantities (Table 1.1), each of which is regarded as having its own dimension. The current definitions of the corresponding base units are given in the IUPAC Green Book, Quantities, Units and Symbols in Physical Chemistry.10 A clear distinction should be drawn between the names of units and their symbols, e.g. mole and mol, respectively. 

As discussed previously the pressure is a scalar physical quantity with the following dimensional equation [P] = [ML T % In the Systhne International d unites (SI), the pressure unit is a derived unit having a special name pascal , with the symbol Pa, hence 1 Pa = IN.m" = 1 kg.m .s l However, there also exist several obsolete units of pressure relative to different systems or used in particular scientific, and technical fields. Although these obsolete units should be discontinued their remanence exists for practical uses and they are listed in Table 19.1. 

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