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- To: Seaint(--nospam--at)seaint.org
- Subject: pH
- From: GSKWY(--nospam--at)aol.com
- Date: Wed, 18 Jun 2003 10:31:06 EDT
Although nobody asked, here is an explanation of what pH means. Any comments or corrections?
pH: What do the numbers mean?
For most chemical solutions, the concentration of ions in solution can be expressed on a linear scale in units of molarity (moles per liter), where a mole is the gram equivalent weight of the substance. However hydrogen (H+) and hydroxide (OH-) ion concentrations vary over such a wide range that it is normally more convenient to express them on a log scale, as pH. The pH of a solution is defined as the negative logarithm to base 10 of the H+ ion concentration, or in equation form:
pH = -log[H+]
In water solutions, there is a fixed relationship between the concentration of H+ ions and OH- ions. The product of the two concentrations must equal Kw, the "ion product constant" of water, or:
Kw = [H+][OH-]
Kw increases slightly with temperature, but for most practical applications, it can be taken as 10^-14. Since pH is the negative log of the H+ ion concentration:
pH = 14 - log [OH-]
The pH value of a solution thus indicates the ratio of H+ to OH- ions. In pure water, a small number of water molecules dissociate into according to the reaction:
H2O -> H+ + OH-
The concentration of each ion will be equal, resulting in a neutral pH of 7. A decrease of one pH unit is therefore equivalent to an order of magnitude (factor of ten) increase in the hydrogen ion concentration. In acidic solutions, the concentration of H+ ions is greater than the concentration of OH- ions so the pH is less than seven. In alkaline (basic) solutions, the concentration of OH- ions is greater than the concentration of H+ ions so the pH is greater than seven. Although it is possible to produce solutions with a negative pH or a pH greater than 14, for practical purposes, the range of pH is 1 (very acidic) to 14 (very alkaline).
The pH of an acid becomes lower as the pH increases; that is, as the acid becomes stronger. The pH of a base becomes higher as the concentration increases.
Hydrochloric, sulfuric, and nitric acid are strong acids and have low pH values even in relatively dilute solutions. Boric, citric, and phosphoric acids are weaker acids (they have higher pH values.) Tap water and seawater typically have neutral pH values. Sodium bicarbonate and ammonium hydroxide are mildly alkaline solutions; both have a pH of approximately 10. Sodium hydroxide (NaOH) is a strongly alkaline solution, with a pH or approximately 13.
The effect of the acidity or alkalinity of the solution is different for different metals. For example, iron and nickel are quite resistant to highly alkaline environments, whereas aluminum can be severely corroded by solutions with a pH value greater than 9.
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