Alkalinity Speciation

The actual measured pH of rainwater in equilibrium with atmospheric pC02 is somewhere around 5.6. Values of pH below 5.6 can either be due to an increased pC02 or to industrial emissions causing what is known as acid rain. As shown, the dissolution of C02 in water is an cause of acid water (Table 2.10). However, considering that soils and/or geologic systems are sources of HCOJ and COj , when water contacts soil or 

---

Therefore, the pH values of these solutions are between 11 and 12. The speciation model used by 8chwarzenbach and Fischer is certainly too simple but these authors have been the first to demonstrate the strong dependence of the polysulfide anion distribution on the alkalinity. According to Eqs. (26)-(28) higher pH values in dilute solutions will favor smaller anion sizes. 

Emmerich et al. (1982) reported that in soil solution of arid soils amended with sewage sludge, free Ni2+ ion accounted for 60-70% of the total Ni in soil solution. Sposito and Page (1984) indicated NiHC03+ and NiC03° were also important Ni speciations in alkaline and calcareous soils. 

Between pH values of ca. 6 and 12 aqueous solutions hold very little dissolved beryllium because of the low solubility of Be(OH)2. When the pH is raised above 12, the hydroxide begins to dissolve with the formation of, first, Be(OH)3 and then, at even higher pH values, Be(OH) (52). The presence of these species in strongly alkaline solutions was confirmed by means of solvent extraction experiments (90) and infrared spectroscopy (31). A speciation diagram is shown in Fig. 7, which was constructed using the values of log /33 = 18.8 and log /34 = 18.6 critically selected from Table III. The diagram illustrates clearly the precipitation and dissolution of Be(OH)2. 

Prause et al. 1985). At pH 6.5 and water alkalinity of 25 mg CaC03/L, elemental Pb+2 is soluble to 330 pg/L however, Pb+2 under the same conditions is soluble to 1000 pg/L (Demayo et al. 1982). In acidic waters, the common forms of dissolved lead are salts of PbS04 and PbCl4, ionic lead, cationic forms of lead hydroxide, and (to a lesser extent) the ordinary hydroxide Pb(OH)2. In alkaline waters, common species include the anionic forms of lead carbonate and hydroxide, and the hydroxide species present in acidic waters (NRCC 1973). Unfortunately, the little direct information available about the speciation of lead in natural aqueous solutions has seriously limited our understanding of lead transport and removal mechanisms (Nriagu 1978a). 

In natural waters, dissolved zinc speciates into the toxic aquo ion [Zn(H20)6]2+, other dissolved chemical species, and various inorganic and organic complexes zinc complexes are readily transported. Aquo ions and other toxic species are most harmful to aquatic life under conditions of low pH, low alkalinity, low dissolved oxygen, and elevated temperatures. Most of the zinc introduced into aquatic environments is eventually partitioned into the sediments. Zinc bioavailability from sediments is enhanced under conditions of high dissolved oxygen, low salinity, low pH, and high levels of inorganic oxides and humic substances. 

With these experimental surface reaction parameters and the speciation program MICROQL the species distribution in the river using metal and particle concentrations, pH and alkalinity measured in the river at the concerned sampling data was calculated. Various assumptions for the complexation in solution may be used, which affect ai and thus Kd-... 

Vertical concentration profiles of (a) temperature, (b) potential density, (c) salinity, (d) O2, (e) % saturation of O2, (f) bicarbonate and TDIC, (g) carbonate alkalinity and total alkalinity, (h) pH, (i) carbonate, ( ) carbon dioxide and carbonic acid concentrations, and (k) carbonate-to-bicarbonate ion concentration ratio. Curves labeled f,p have been corrected for the effects of in-situ temperature and pressure on equilibrium speciation. Curves labeled t, 1 atm have been corrected for the in-situ temperature effect, but not for that caused by pressure. Data from 50°27.5 N, 176°13.8 W in the North Pacific Ocean on June 1966. Source From Culberson, C., and R. M. Pytkowicz (1968). Limnology and Oceanography, 13, 403-417. 

Seby, F., Gautier, M.E, Lespes, G. and Astruc, M. (1997) Selenium speciation in soils after alkaline extraction. Sci. Total Environ., 207, 81-90. 

The active sites of these enzymes can have a nitrogen ligand, usually as histidine (acid phosphatases and some protein phosphatases), a nucleophilic serine residue (alkaline phosphatases), a cysteine residue in which the thiol group can form a covalent species with the phosphate ester (protein phosphatases), or an aspartate-linked phosphate (plasma membrane ion pumps). The inhibitory form of vanadium is usually anionic vanadate V(V), but cationic vanadyl V(IV) has also shown strong inhibition of some types of phosphorylase reactions. Above neutral pH, speciation of vanadyl ions produces anionic V(IV) species capable of inhibition of enzymes in the traditional transition-state analogue manner [5],... 

A rapid and simple MW-assisted digestion method with alkaline solution (TMAH or methanolic KOH solution) was developed for speciation analysis of inorganic Hg and methyl-Hg in biological tissues [41]. Extracts with quantitative recoveries of Hg species after the alkaline dissolution of the sample were directly analyzed by an automated on-line hyphenated system incorporating aqueous HG, cryogenic trapping, GC, and detection by A AS. The proposed method was validated by the analysis of biological CRMs (CRM 463, DORM-1, TORT-1) and one BCR sample from an interlaboratory study (Tuna Fish 2). 

Another area of growing interest is the chemical speciation of elements to provide more useful information than that afforded by the total concentrations. In this context, alkaline solubilization procedures are especially suitable for speciation work, because this approach allows samples to be directly analyzed without prior preparation (destruction of the matrix), thus preserving the chemical forms of the analytes of interest. 

---