A study of the chemical constitution and hydrochemical equilibria in a concentrated salt lake: lake Eyasi.

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University of Dar es Salaam
The chemical constitution of brines and inflows in the drainage basin of Lake Eyasi and variation patterns were examined. The evaporites, their distribution patterns and the interrelationship with the aqueous mufti-electrolyte system were examined as well as chemical equilibria parameters such as pKa and Ka, as well as pH, activity coefficient and buffer capacities. Vet methods were employed for the determination of all anions, except fluoride which was determined by ion selective techniques (fluoride selective electrode). The cationic constituents were assayed by Atomic Absorption Spectrophotometry (AAS). Phosphate equilibria parameters have been examined by potentiometric procedures. The apparent dissociation constant Ka for HF at different ionic strengths was determined through, measurement of fluoride electrode potential (Ep) by a fluoride selective electrode and pH monitored using a glass electrode. Lake Eyasi is a concentrated salt lake with both dilute and concentrated brines which have TDS values ranging from 1.00 g dm-3 to 385.00 g dm-3. The lower salinity values are associated with fresh water inflows which traverse the dried salt lake bed and solute concentrations increase as these inflows progress inward and dissolve more solute. Hence, these dilute brines do not qualify as representative salt lake samples (Lake Eyasi). Conductivity values correlate directly with salinity in dilute media. The re-defined pH scale approach yields values about 2 pH units above readings obtained without this provision. The upstream sources contribution is relatively low, therefore the accumulated salts in the brine are a result of repetitive annual evaporative concentration cycles stretching many thousands of years. However, the contribution of hot springs may be significant. The concentration of chemical constituents in the lake depends on location, depth and season. The ionic proportions of major ions remain unaltered unless a selective enrichment or depleting mechanism is in operation. The order of abundance of major ions is : Na+ >> K+>> Ca2+ = Mg2+ : Cl - > (C032- + HC03-) > SO42- > PO43- > F-. Lake Eyasi brine is dominated by sodium and chloride ions. This is reflected in the salt crusts, particularly those samples collected from the central area of the lake which possess a mean content of 92.00% NaCI. While C032- concentrations increase downstream, Cat+ and Mgt+ show a reverse decreasing trend upstream. Lake Eyasi brine contains high concentrations of both phosphate and fluoride with a mean value of 1.90 g dm-3 P04 3- and 0.273 g dm -3 F-, respectively. Trace metals in the lake salt crust are within permissible ranges for WHO and Tanzanian Standards quality for human consumption. The lake salt crusts have a mean value of 1.10% NaF. The phosphate pKa values evaluated by graphical methods, show a marked departure from those values obtained using the Debye Huckel limiting law equation which considers activity correction. Thus, the mean deviations for pKa1 and pKa2 are 1.86 and 2.16, respectively. However, apparent dissociation constants for phosphate have revealed both pKai and pKa2 to decrease with the increase in ionic strength of the solution. The phosphate buffer capacity was found to increase with ionic strength of the solution. The thermodynamic pKa noted for HF is 3.127 which is in good agreement with the literature values of 3.450. The analyses have given computed errors below ± 4.00% thus ascertaining good accuracy.
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Chemical structure, Tanzania
Ngambeki, W. W. (1996). A study of the chemical constitution and hydrochemical equilibria in a concentrated salt lake: lake Eyasi. Master dissertation, University of Dar es Salaam. Available at (