Pro-oxidant activity of vitamin C in drinking water ... - Ã bo Akademi

Review of the literature3.2. The role of bicarbonate and pH in drinking waterTo prepare safe drinking water, the water plants have to analyse which ions and chemicalcompounds are present in the water. The most important parameters for drinking water arehardness, pH (alkalinity and acidity) and salinity. The presence and combination of anumber of ions will determine these features. Calcium and magnesium ions give thehardness, while bicarbonate, carbonate and hydroxyl ions give the alkalinity in water.Chloride and sulphate ions (as HCl and H 2SO 4) contribute to the acidity and salinity of thewater (Table 3, guideline values). Thus, calcium carbonate (CaCO 3) contributes both tohardness and alkalinity of water, while calcium chloride (CaCl 2) gives hardness and salinity.The pH of drinking water usually originates from dissolved carbon dioxide (CO 2), whichforms carbonic acid (H 2CO 3). Humic acids and other organic acids, which originate fromdecayed plants, can also make the water acidic. Acidification of the ground water incombination with a bedrock that cannot counteract or buffer acidic particles will lead toacidic water. When the acidity originates from natural sources, the pH of the water isaround 3-4. It is not harmful to drink such mildly acidic (pH~ 3-4) drinking water.However, indirectly, such acidic water will cause problems as it can extract metals from thesoil and water pipe systems. Acidic water can extract iron, manganese, copper, aluminiumand heavy metals such as cadmium and lead. Therefore, water plants adjust the pH to 6.5-9.5 (European guideline values, Council Directive 98/83/EC). Contributing factors inincreased copper corrosion from copper pipes are the concentration of sulphate andchloride ions. 156Acidic water is usually adjusted to more alkaline pH values with calcium carbonate(CaCO 3). 156-158 Alkalinity is a measure of susceptibility for acidification, in other wordsbuffer capacity for acid addition. The higher the alkalinity in the water, the better the watercan resist acidification. Bicarbonate, carbonate and hydroxyl ions give different alkalinity tothe water. If the pH in drinking water is below 8.3, bicarbonate alkalinity is prevailing. Inmost drinking water that has a pH between pH 5 and 8, the alkalinity and pH come frombicarbonate ions (Figure 4). Bicarbonate and carbonate alkalinity will exist in drinking waterwith pH values between 8.3 and 9.4. Corrosive alkalinity (hydroxyl alkalinity from sodiumhydroxide, NaOH) can only exist at pH values above 9.4. Furthermore, when water istreated for adjustment in pH the content of other components has to be quantified.23