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Analytical Procedures - Mercuric Ion
(Hg++)
February 24, 2002
Introduction Equipment Reagents Procedure Calculation |
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The analytical
method for the measurement of mercuric ion (Hg++) concentration in solution is a
double titration with magnesium.
An amount of
ethylene diamine tetra-acetic acid (EDTA) solution is added to the sample, and
the excess is back titrated with magnesium solution. Iodine ion is then
added to form a strong complex with the mercury ions, thereby liberating an
equivalent amount of EDTA. The EDTA is again titrated with magnesium
solution.
To 25 ml of sample (filtered through a folded filter if necessary) in a 400 ml beaker, add 10 ml EDTA solution and 10 ml buffer solution (pH must be 9.5 to 10). If there is any free chlorine in the sample boil briefly and add more buffer solution. Dilute with 150 ml distilled water and add four (4) drops of indicator. The colour should turn green. If the colour is red, add more EDTA solution. Titrate with magnesium solution until the colour changes from green to red. Fill the burette again with magnesium solution. Add approximately 3 g potassium iodide to the beaker. The colour will change to green again. Stir the sample for 1 minute, then titrate again with magnesium solution until the colour changes from green to red. The amount of magnesium solution (ml) is used to calculate the Hg++ content.
Hg++ (g/l) = (ml) x 0.05 x 200.6 / 25
For solutions of higher concentrations of Hg++ ions, reduce the sample volume accordingly, and adjust the actual sample volume in the calculation.