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Logarithms can be used to make calculations easier. For example, two numbers can be multiplied just by using a logarithm table and adding. These are often known as logarithmic properties, which are documented in the table below. [2] The first three operations below assume that x = b c and/or y = b d, so that log b (x) = c and log b (y) = d.
Positive numbers less than 1 have negative logarithms. For example, = = + + = To avoid the need for separate tables to convert positive and negative logarithms back to their original numbers, one can express a negative logarithm as a negative integer characteristic plus a positive mantissa.
The natural logarithm can be defined for any positive real number a as the area under the curve y = 1/x from 1 to a [4] (with the area being negative when 0 < a < 1). The simplicity of this definition, which is matched in many other formulas involving the natural logarithm, leads to the term "natural".
In chemistry the negative of the decimal logarithm, the decimal cologarithm, is indicated by the letter p. [63] For instance, pH is the decimal cologarithm of the activity of hydronium ions (the form hydrogen ions H + take in water). [64] The activity of hydronium ions in neutral water is 10 −7 mol·L −1, hence a pH of 7. Vinegar typically ...
The distribution coefficient, log D, is the ratio of the sum of the concentrations of all forms of the compound (ionized plus un-ionized) in each of the two phases, one essentially always aqueous; as such, it depends on the pH of the aqueous phase, and log D = log P for non-ionizable compounds at any pH.
Since the common logarithm of a power of 10 is exactly the exponent, the characteristic is an integer number, which makes the common logarithm exceptionally useful in dealing with decimal numbers. For positive numbers less than 1, the characteristic makes the resulting logarithm negative, as required. [38]
= = Log P is positive for lipophilic and negative for hydrophilic substances or species. Definition of the D-value The P-value only correctly refers to the concentration ratio of a single substance distributed between the octanol and water phases.
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