Water alkalinity

Various dissolved salts in natural water give it an alkaline pH. These are mainly carbonates, bicarbonates and phosphates (Scheme 8.7). The alkalinity of a water sample can be determined by titration with standardised sulphuric acid solution. With phenolphthalein as the indicator, the equivalence point occurs at about pH 8.3. At this point, all free hydroxide ion is neutralised, carbonate converted into bicarbonate, and phosphate (PO43–) into monohydrogen phosphate (HPO42–). The alkalinity to pH 8.3 is calculated in ppm CaCO3 based on Scheme 8.8. Therefore, each millimole of sulphuric acid required corresponds to 1.00 mmol CaCO3 in the water sample. If the titration of a 100.0 ml water sample with 0.01 M sulphuric acid consumes V ml of acid, the alkalinity is given by:

At the phenolphthalein end-point of pH 8.3, the water is still slightly alkaline and it is therefore common practice to titrate the sample to a methyl orange end-point, corresponding to about pH 4.5. This ensures that all carbonate is in the from of carbonic acid and all phosphate is present as dihydrogenphosphate (H2PO4–). The alkalinity to pH 4.5, often called the total alkalinity, is given by the same equation as above, but, of course, the volume of sulphuric acid solution may be greater.

Table 8.3 relates the total (T) alkalinity and phenolphthalein alkalinity (P) of various types of water to the dissolved salts that are present. In the absence of phosphate ions, there will be some correlation between the alkalinity caused by bicarbonate and the temporary hardness. For example, if the alkalinity to

phenolphthalein is zero (P = 0), the total alkalinity, assuming no phosphates or weakly acidic pollutants, will be caused by bicarbonate ion and the total alkalinity in ppm CaCO3 will equal the temporary hardness. The alkalinity of water fed to boilers must not be too high because of the risk of corrosion of non-ferrous fittings.