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Analytical Procedures - Mercuric Ion (Hg++)
February 24, 2002

Introduction

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.

Equipment

• 25 ml pipette
• 10 ml pipette
• 400 ml beaker
• 25 ml burette
• 50 ml drop flask

Reagents

• 0.1 M EDTA solution (37.22 g disodium salt of EDTA dissolved in one (1) litre of water)
• 0.05 M magnesium solution (1.2156 g Mg is dissolved in nitric acid (minimum amount required) and diluted with water to one (1) litre)
• Buffer solution (100 g NH₄NO₃ is added to 570 ml of NH₃ and diluted with water to one (1) litre)
• Indicator (0.75 g methyl yellow (4-dimethylaminoazobenzol), 1.0 g Erichrome Black T in 200 ml triethanolamine (0.1% methylthymoleblue can also be used), KJ crystals)
• Potassium iodide

Procedure

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.

Calculation

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.

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