Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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hydrogen-ion concentration

The concentration of hydrogen ions (more properly hydronium ions—H 3 O + , although its true structure is probably (H,4H 2 O), existing in an aqueous solution. It is a measure of the active acidity or basicity and is expressed metrically as the number of moles or gram-formula weights (1.0078 g.) of hydrogen ions (H + ) per liter of solution. The hydrogen-ion concentration of a solution may also be expressed in terms of its pH, which is defined as the negative logarithm to the base ten of the hydrogen-ion concentration. In aqueous solutions, neutrality is the condition that exists when the concentration of hydrogen ions and hydroxyl ions are equal. At 25° C., neutrality occurs at a pH of 7.0, which is the pH value of pure (distilled) water when condensed hot and shielded from contact with atmospheric carbon dioxide.

Measurement of hydrogen-ion concentration, or pH, may be made colorimetrically, by means of the proper use of suitable neutralization indicators, or, more accurately, by potentiometric methods employing any of various electrodes, which exhibit the proper specificity for hydrogen ions.

Hydrogen-ion concentration is important in archival work because it has been adequately demonstrated that the presence of acid(s) in ink, leather, paper, etc., has or can have, a deleterious effect on such materials, the extent of the effect depending not so much on the volume of acid present, as on the type of acid and its concentration, i.e., a large volume of a relatively weak organic acid, such as formic acid, is less harmful than a smaller amount of a powerful, inorganic acid, such as sulfuric acid. As a decrease of pH means a logarithmic increase in acid concentration, levels of concentration below pH 5.0, or under certain circumstances, even 6.0, become important. Conversely, although not as serious a problem, a high concentration of hydroxyl ions, corresponding to a pH of 10.0 or above, can lead to serious oxydization of cellulosic materials.

The increase in hydrogen-ion concentration as pH declines is given be

        pH      moles/ liter
        7.0     0.0000001
        6.0      .000001
        5.0      .00001
        4.0      .0001
        3.0      .001
        2.0      .01
        1.0      .1
        0.0     1.0

See also:ACID ; ALKALI . (17 , 195 , 235 )

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