Acidity, of natural waters

Acidity of natural waters refers to the total acid content that can be titrated to pH 8.3 with NaOH. This pH is the second equivalence point for titration of carbonic acid (H2C03) with OH-. Almost all weak acids in the water also will be titrated in this procedure. Acidity is expressed as millimoles of OH- needed to bring 1 L of water to pH 8.3. 

Ghassemi,M., Christman, R.F., 1968. Properties of the yellow organic acids of natural waters. Limnol. Oceanogr., 13 583-597. 

Pershina (Perminova), I. V., Vermool, V. M., Polenova,T. V., and Ivanova, E. K. (1989). Study of molecular weight distribution and spectral parameters of the fulvic acids of natural water origin. I. Gel-permeation chromatography fractionation of fulvic acids. Bulletin of Moscow University [Vestnik MGU], Series 2 (Chemistry) 30,176-182. 

We will focus first on immediate contributions to acidity and alkalinity. For the acidity of natural waters, the contributors are strong and weak acids, salts of strong acids and weak bases, hydrolysis of Fe and AP, and oxidation and hydrolysis of Fe " and Mn +. 

This is deadly to most forms of aquatic life. Water downstream from a mine may be contaminated by acid mine drainage, the result of microbial oxidation of discarded waste material at the mine site. Acid mine water principally contains sulfuric acid produced by the oxidation of iron pyrites (FeS2). Industrial wastes and acid rain may also contribute to the acidity of natural waters. 

Silica. The siUca content of natural waters is usually 10 to (5 x lO " ) M. Its presence is considered undesirable for some industrial purposes because of the formation of siUca and siUcate scales. The heteropoly-blue method is used for the measurement of siUca. The sample reacts with ammonium molybdate at pH 1.2, and oxaUc acid is added to reduce any molybdophosphoric acid produced. The yellow molybdosiUcic acid is then reduced with l-amino-2-naphthol-4-sulfoiiic acid and sodium sulfite to heteropoly blue. Color, turbidity, sulfide, and large amounts of iron are possible interferences. A digestion step involving NaHCO can be used to convert any molybdate-unreactive siUca to the reactive form. SiUca can also be deterrnined by atomic... 

Sometimes we need to know how the concentrations of the ions present in a solution of a polyprotic acid vary with pH. This information is particularly important in the study of natural waters, such as rivers and lakes (Box 10.1). For example, if we were examining carbonic acid in rainwater, then, at low pH (when hydronium ions are abundant), we would expect the fully protonated species (H2C03) to be dominant at high pH (when hydroxide ions are abundant), we expect the fully deprotonated species (C032 ) to be dominant at intermediate pH, we expect the intermediate species (HC03, in this case) to be dominant (Fig. 10.20). We can verify these expectations quantitatively. 

Landrum, R.F., Nihart, S.R., Edie, B J., Gardner, W.S. (1984) Reverse-phase separation method for determining pollutant binding to Aldrich humic acid and dissolved organic carbon of natural waters. Environ. Sci. Technol. 18, 187-192. 

Preston et al. [698,699] have described novel approaches to 15N-isotope dilution determination of ammonium and of free amino acids in natural waters, incorporating chemical derivatization and conventional GC-MS analysis. 

Skurlatov, Y.I., Zepp, R.G., and Baughman, G.E. Photolysis rates of (2,4,5-trichlorophenoxy)acetic acid and 4-amino-3,5,6-trichloropicolinic acid in natural waters, J. Agric. Food Chem., 31(5) 1065-1071, 1983. 

Bichsel Y, Von Gunten U (1999) Oxidation of iodide and hypoiodous acid in the disinfection of natural waters. Environ Sci Technol 33(22) 4040 045... 

One specific application in the first category is to estimate the time required for a reaction to reach equilibrium in nature. If equilibrium is assumed in modeling a geochemical process, it is important to know the limitations (e.g., the timescale for the assumption to be valid). For example, in acid-base reactions, the reaction is rapid and the timescale to reach equilibrium is much less than one second. Hence, pH measurement of natural waters is usually meaningful and can be used to estimate species concentrations of various pH-related reactions. However, in redox reactions, the reaction is often slow and it may take days or years to reach equilibrium. Therefore, pe (or Eh) measurement of natural waters may not mean... 

Mg(II) forms a complex with 8-hydroxyquinoline-5-sulfonic acid (37) at pH 9.0 with Tris-HCl buffer, which can be determined by ELD (X x = 388 nm, ka = 495 nm) with micellar enhancement by cetyltrimethylammonium chloride (38). Masking of Ca(II) is achieved by EGTA (19). The method was applied in a SIA system for analysis of natural waters . After elution of the Mg(II) ions adsorbed on an alkali-activated PTFE tube with 0.1 M HCl and addition of A,A -bis(salicylidene)-2,3-diaminobenzofuran (39), the end analysis was by fluorometric determination of the Mg(II) complex (kex =475 nm, kfl = 545 nm). Possible interference of Ca(II) is masked on addition of the chelating agent... 

Faust, B. C., and J. Hoigne, Sensitized photo-oxidation of alkylphenols by fulvic acid and natural water , Environ. Sci. Technol., 21,957-964(1987). 

Adsorption chromatography is an efficient way to isolate organic acids from large volumes of water. The nonionic, macroporous, Amberlite XAD-8 and the weak-base anion-exchange resin Duolite A-7 are two resins well suited for this purpose. These resins have been successfully used to extract organic acids from natural waters at sites where it was necessary to process thousands of gallons of sample. 

Mercury nitrate, corrosive sublimate, white arsenic, and lead acetate are occasionally used as reagents, and soap is described as an indication of hard waters, as when added to them a decomposition takes place, the acid unites with alkali, and the oil is disengaged such waters as these are generally called hard waters, and are unfit for washing cloaths, as also for boiling pulse and the harder kinds of flesh. 18 The list of qualitative reagents for the many constituents of natural waters comprises the principal reagents in use at present. 

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