The first synthesis of mercury thiocyanate was probably completed in 1821 by Jöns Jacob Berzelius: Evidence for the first pure sample was presented in 1866 prepared by a chemist named Otto Hermes. It is prepared by treating solutions containing mercury and thiocyanate ions. The low solubility product of mercury thiocyanate causes it to precipitate from solution. Most syntheses are achieved by precipitation: The compound adopts a polymeric structure with Hg2+ centers linearly coordinated to two S atoms with a distance of 2.381 Å. Four weak Hg2+--N interactions are indicated with distances of 2.81 Å.
Uses
Mercury thiocyanate has a few uses in chemical synthesis. It is the precursor to potassium trismercurate and caesium trismercurate. The Hg3− ion can also exist independently and is easily generated from the compounds above, amongst others. Its reactions with organic halides yield two products, one with the sulfurbound to the organic compound and one with the nitrogen bound to the organic compound.
Use in chloride analysis
It was discovered that mercury thiocyanate can improve detection limits in the determination of chloride ions in water by UV-visible spectroscopy. This technique was first suggested in 1952 and has been a common method for determination of chloride ions in laboratories worldwideever since. An automated system was invented in 1964 and then a commercial chloroanalyzer was made available in 1974 by Technicon. The basic mechanism involves the addition of mercury thiocyanate to a solution with unknown concentration of chloride ions and iron as a reagent. The chloride ions cause the mercury thiocyanate salt to dissociate and the thiocyanate ion to bind Fe, which absorbs intensely at 450 nm. This absorption allows for the measurement of concentration of the iron complex. This value allows one to calculate the concentration of chloride. It can be used for determining the concentration of chloride ions in aqueous solution. Mercury thiocyanate without iron is added to a solution with an unknown concentration of chloride ions, forming a complex of the mercury thiocyanate and chloride ion that absorbs light at a 254 nm, allowing more accurate measurements of concentration than the aforementioned technique using iron.
Pharaoh's serpent
Mercury thiocyanate was formerly used in pyrotechnics causing an effect known as the Pharaoh's serpent or Pharaoh's snake. When the compound is in the presence of a strong enough heat source, a rapid, exothermic reaction that produces a large mass of coiling, serpent-like solid is started. An inconspicuous flame, which is often blue but can also be yellow/orange, accompanies the combustion. The resulting solid can range from dark graphite gray to light tan in color with the inside generally much darker than the outside. The reaction has several stages as follows: Igniting mercury thiocyanate causes it to form an insoluble brown mass that is primarily carbon nitride, C3N4. Mercury sulfide and carbon disulfide are also produced.
