Acidity is produced by substances that yield hydrogen ions on hydrolysis or a substance is considered acid if it will neutralize hydroxyl ions. The definition for acidity is the reciprocal of that used for alkalinity with the substitution of the word hydrogen for hydroxyl. In other words, a substance is acidic if it will neutralize hydroxyl ions.
The determination of acidity is made at the end point pH of the indicator used in a titration.
The same requirements apply to determinations of acidity as for alkalinity. The acidity titration, however, measures a property that is somewhat difficult to describe. The usual acidity titration cannot be interpreted in terms of any single ion because some solutes react with hydroxide ions at slow rates and some of the products are unstable. Often
interpretation of the results is very uncertain. In any event, the solutes contributing to acidity are usually separately determined by other procedures.
In contrast to the alkalinity determination which can be defined for most all waters as the determination of carbonate and bicarbonate ion concentrations, the titrated acidity of a water is expressed in terms of milliequivalents per liter (meq/L) of hydrogen ions or as equivalent concentration of calcium carbonate or of sulfuric acid. The determination has no particular geochemical usefulness. It may, however, provide some indication of the behavior of water in certain treatment processes.
Among dissolved species which may contribute to acidity on titration are undisassociated or partly disassociated acids along with carbonic acid. One of the most common species of this type is the bisulfate ion, HS04- which may constitute about half the total dissolved sulfate species at a pH of 2. Also, undisassociated hydrofluoric acid could be present in significant proportions at this pH. Metal ions that combine with hydroxide and precipitate can affect the acidity titration. Metal ion species may also undergo oxidation during the titration and produce [H+] ions that combine with the titrating base.
The importance of hydroxide ions in the attack of natural waters on rock minerals has been indicated previously by the presence of hydrogen ions in most of the reaction equations that have been shown. The three sources of most of the [H+] that participate in such reactions are
1) hydrolysis, as in [H2O] = [H+] + [OH-],
2) the disassociation of acidic solutes H2CO3 = [H+] + [HCO3-], and
3) oxidation reactions such as [H2S] + 4[H20] = [S04-2] + 10[H+] + 8 electrons. ,
Once formed, the [H+] may be stored and redistributed by cationic exchange, but this is not a process by which the hydrogen ions are produced.
In natural systems where the production of [H+] is more rapid than the equilibrium rate with the minerals available, the pH may remain more acid than the equilibrium in carbonate systems. Most naturally occurring water that is strongly acid is in thermal areas where a low pH is maintained by a solution of acidic compounds, gases, and by oxidation of sulfur species. Acidity in surface or ground waters may be attributable to natural causes such as humic acids extracted from swamps or peat beds.
Usually, acidity is not a direct or specific pollutant. It is merely a measure of the effects of a combination of substances and conditions in the water.
The synergism and antagonism of toxic substances are markedly affected by acidity. Strong acids may cause the formation of carbonic acid in quantities that are adverse to the well being of the organisms present.
Addition of acids to waters may be harmful not only in producing adverse acid conditions, but also by increasing the toxicity of various components in the water. For instance, a reduction of about 1.5 pH units can cause a thousand fold increase in the acute toxicity of a metal-cyanide complex.
Also, the availability of many nutrient substances varies with the acidity. With higher pH values, for instance, iron tends to become unavailable to some plants.
In general, moderate acidity in irrigation water is beneficial to alkali soils for it helps to neutralize carbonates and prevents the precipitation of calcium. In fresh waters, the pH values and acidities of natural streams vary widely depending upon the soil and vegetation of the watershed.
Brook trout were found to live comfortably in different streams with pH values ranging from 4.1 to 9.5, but in all cases the streams were substantially unpolluted and acidity if any was due to organic acids of natural origin. Mineral acid pollution, however, can be quite detrimental to fish at pH values considerably higher than those that fish can tolerate in unpolluted waters containing only acids of natural origin.