Information AboutAlkalinity |
| CATEGORIES ABOUT ALKALINITY | |
| chemical oceanography | |
| acid-base chemistry | |
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This neutralizing capacity is equal to the Stoichiometric sum of the Base s in solution. In the natural environment Carbonate Alkalinity tends to make up most of the total alkalinity due to the common occurrence and dissolution of Carbonate rocks and presence of Carbon Dioxide in the atmosphere. Other common natural components that make up alkalinity include Borate , Hydroxide , Phosphate , Silicate , Nitrate , and Sulphide . Solutions produced in a laboratory may contain a virtually limitless number of bases that contribute to alkalinity. Alkalinity is usualy given in the unit mEq/l. THEORETICAL TREATMENT OF ALKALINITY In typical Seawater : AT = + 2[CO3−2 T + + [OH− T + 2 + [HPO4−2 T + − [H+ sws − [HSO4−] (subscript T indicates the total concentration of the species in the solution as measured. This is opposed to the free concentration, which takes into account the significant amount of Ion Pair interactions that occur in seawater) Alkalinity can be measured by Titrating a sample with a strong acid until all the buffering capacity of the aforementioned ions is consumed. This point is functionally set to pH 4.5. At this point, all the bases of interest have been protonated to the zero level species, hence they no longer cause alkalinity. For example, the following reactions take place during the addition of acid to a typical seawater solution: HCO3− + H+ → CO2 + H2O CO3−2 + 2H+ → CO2 + H2O B(OH)4− + H+ → B(OH)3 + H2O OH− + H+ → H2O PO4−3 + 2H+ → H2PO4− HPO4−2 + H+ → H2PO4− + H+ → [Si(OH)40 It can be seen from the above protonation reactions that most bases consume one proton (H+) to become a zero level species, thus increasing alkalinity by one per mole of base. CO3−2 however, will consume two protons before becoming a zero level species (CO2), thus it increases alkalinity by two per mole of CO3−2. and [HSO4− decrease alkalintiy, as they act as sources of protons. They are often represented collectively as [H+]T. EXAMPLE PROBLEMS Sum of contributing species The following equations demonstrates the relative contributions of each component to the alkalinity of a typical seawater sample. Contributions are in μmol/kg-H2O and are obtained from ''A Handbook of Methods for the analysis of carbon dioxide parameters in seawater ''" {Link without Title} ,"(Salinity = 35, pH = 8.1, Temp = 25°C). AT = + 2[CO3−2 T + + [OH− T + 2 + [HPO4−2 T + − [H+ − − [HF Phosphates and Silicate, being nutrients are typically negligible. At pH = 8.1 and [HF are also negligible. So, AT = + 2[CO3−2 T + + [OH− T − [H+]
AT = 2480 μmol/kg−H2O Addition of CO2 The addition (or removal) of CO2 does not contribute to alkalinity. This is unexpected as it is likely (depending on pH) dissociation into bicarbonate or carbonate - both contributing species to alkalinty. However the net reaction produces the same number of positively contributing species as negative contributing species. ie At neutral pH's: CO2 + H2O → HCO3− + H+ At high pH's: CO2 + H2O → CO3−2 + 2H+ Dissolution of carbonate rock The dissolution (or precipitation) of carbonate rock has a strong influnce on the alkalinity. This is because carbonate rock is composed of CaCO3 and its dissociation will add Ca+2 and CO3−2 into solution. Ca+2 will not influence alkalinty, but CO3−2 will increase alkalinity by 2 units. CaCO3(s) → Ca+2 + CO3−2 EXTERNAL LINKS
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