Eh -pH diagram of Mercury (Quicksilver – Hg) – Mercury Geochemistry

Eh -pH diagram of Mercury (Quicksilver – Hg) – Simple but important.

In this article, my goal is to present you with some simple Eh-pH diagrams for Hg that you can use for your work (just refer to I though about providing some useful facts related to mercury in United States and other places of the world, including various news sources.

The Eh-pH diagrams I am presenting are prepared with “Geochemist’s Workbench”. Eh -pH diagrams depends on the concentration of main species (in our case dissolved mercury) and other dissolves species in association. There are three Eh-pH diagrams prepared using Geochemist’s Workbench. In all of them I have used Mercury activity = 10^-10. First diagram is prepared with only Hg, second one with 10 ppm of SO4, third one with another additional 50 ppm of chloride.

Figure 1: Eh-pH diagram of Mercury at 10^-10 activity – Diagram shows that Quicksilver (Liquid Hg) is the most stable, as we get reducing condition, mono and bi mercury irons becomes soluble.

Figure 2: Eh-pH diagram of Mercury (Hg) with 10 ppm of SO4 – Mercury has great affinity towards sulfur. It will compete with other element for sulfur in most cases. You can see the “Cinnabar (HgS)” phase in now one of the solid phase in the digram.

Figure 3: Eh-pH diagram of Mercury (Hg) with 10 ppm of SO4 and 50 ppm of Cl- : This diagram shows that Mercury can form soluble species with chloride. The Hg+ and Hg++ fields in the previous diagrams has been replaced by HgCl2 (still soluble).

So, In terms of mercury geochemistry, we should normally expect Hg as quicksilver phase or Cinnabar phase in oxidizing condition. Mercury only becomes soluble under extremely reducing conditions.

Note: The general shape of the various phases in the diagram remains same if we increase or decrease the concentrations of one or more elements, but the area and position of the boundary lines will shift.
If you have a particular water analysis, reply to this post and we will try to construct a Mercury Phase diagram for you.!!!

Hg – Mercury

at. no. 80
at. wt. 200.61
row 7
col. 2B
val. 1-2
orbits 2-8-18-32-18-2

There has been an increasing concern about Mercury (Hg) problems in ground water in USA and various other places of the world. Some recent news from various sources will support my statement.

Why is Mercury a problem?

  1. Hg is a Neurotoxic heavy metal.
  2. Readily adsorbed via respiratory tracts.

Where can we find Mercury (Hg)?

  1. In many products used in hospitals.
  2. Thermometers.
  3. Sphygmomanometers.
  4. Bleach.
  5. In some landfills.
  6. Sometimes naturally occurring in soil or rock type.

Increasing concern about Mercury problems – some news sources:

  1. Fish from florida mine site test toxic from Murcery.
  2. Longterm Dangers of Mercury
  3. Mercury in Mascara? Minn. Law Bans it!
  4. Mercury warnings to go up!

Mercury loves sulfur too much. It will compete with other molecules for sulfur and steal soufur out of hemoglobin. So, killing our blood cells!

A very detailed report on Mercury problem can be found :

  1.  Mercury (Hg)
  2. Mercury Sulfide speciation: pdf document


  1. agustina

    hello, i wanna know how temperature affects the Eh-Ph diagram for Mercury.
    Y really appreciate your answer.

    1. Editor (Post author)

      Tell me the temperature range you like to see the effect.
      It will all depend on the K value for all of the reactions. Equilibrium constant is dependent on temperature.

  2. Idongesit Ekpo

    Please I am a Nigerian doing a B.Sc. research on trace metal specification in sediments and I need to plot a pourbaix (Eh-pH)diagram for five different metals. I have been trying so hard to get it done by excel but to no avail. Please help me get this graphs and also assist to show me how to get it done. the data are as shown below:
    Location pH Eh(mV)
    CR1 4.57 46.00
    CR2 4.09 41.00
    CR3 5.50 20.00
    CR4 4.96 42.00
    CR5 5.00 38.00
    CR6 4.90 61.00
    CR7 5.31 29.00
    CR8 5.44 26.00
    CR9 5.67 16.00
    CR10 5.50 24.00

    Concentration of Trace metals in mg/kg across the ten Sampling Stations.
    Code Cr Co Mn Hg Ni V
    CR1 0.09 0.21 1.24 <0.01 0.42 0.52
    CR2 0.08 0.22 1.26 <0.01 0.46 0.54
    CR3 0.10 0.24 1.34 <0.01 0.47 0.69
    CR4 0.07 0.18 0.98 <0.01 0.38 0.45
    CR5 0.09 0.15 1.10 <0.01 0.32 0.47
    CR6 0.12 0.21 1.20 <0.01 0.40 0.52
    CR7 0.13 0.22 2.10 <0.01 0.62 0.49
    CR8 0.14 0.20 2.20 <0.01 0.59 0.60
    CR9 0.07 0.18 1.42 <0.01 0.49 0.52
    CR10 0.53 0.09 0.23 1.23 <0.01 0.52

    Thank you for your assistance.

  3. Nantaporn

    Hi, thanks for the useful info and website.
    However, I think you confuse between oxidizing and reducing conditions.
    Hg+ and Hg++ are formed in oxidizing conditions while cinnabar is in reducing condition, and the diagrams say that.


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