What Is a Polysulfide?

Polysulfides are compounds containing polysulfide ions. Polysulfides can be obtained by reacting sulfides or ammonium sulfides of alkali metals and alkaline earth metals with sulfur. With the increase of sulfur atoms, the color gradually changes from yellow to orange to red. ^[1]

Contains polyphosphine ions. Soluble sulfide (containing S ^2- ) interacts with S to form polysulfides. The polysulfide ratio solution is generally yellow.

Although polysulfide is not an anion in itself, it is correct to include polysulfide in view of its important connection with other sulfur anions. The general formula of polysulfides of alkali metals and ammonium is M ₂ S _x , where x> 1. Sodium polysulfides are known to have M ₂ S ₂ , Na ₂ S ₄ and Na ₂ S ₅ , and potassium polysulfides , The number of x reaches 6.

Polysulfides are important in the paper industry. In alkaline pulping, the common method is to apply sulfate digestion solution. The active components of this "caustic soda" are sodium hydroxide and sodium sulfide containing small amounts of sodium carbonate, sodium sulfate, sodium thiosulfate and sodium sulfite.

For sulfate pulping, the digestion liquid is recovered. This process involves separating the "black liquor" and "black liquor" from the pulp by evaporation and burning to recover various inorganic components. The solution of combustion and smelting is called "green liquor", which contains ferrous sulfide, manganese dioxide and the like. This "green liquor" was then converted into a "caustic soda liquor" that was then used to digest the pulp.

Polysulfides are also used in plant protection and as chemical intermediates. Polysulfide (and other sulfur anions) is often required in process water applied to blast furnace slag. ^[3]

Polysulfides often coexist with other sulfur-containing materials such as sulfides (monosulfides), thiosulfates, and sulfites. The current discussion will focus on the determination of polysulfides in these mixtures. In the analysis of polysulfides, it is usually necessary to distinguish between total sulfur, monosulfide sulfur, and polysulfide sulfur.

The analytical methods have evolved from mainly classical gravimetric methods to classic titration methods to more recent potentiometric titration methods. Nevertheless, many of the older basic reactions used to separate and identify various sulfur anions are still in use today.

Polysulfide gravimetric method

Polysulfide-type sulfur is easily converted into elemental sulfur, which is then determined by gravimetric method. This method is to neutralize the aqueous solution of polysulfide to methyl orange, or to heat it in benzene or carbon disulfide, and then evaporate the organic solvent.

Another gravimetric method is the determination of total sulfur, which is oxidized with basic peroxide or bromine, and the sulfate produced is measured gravimetrically as barium sulfate. The above method does not take into account the content of thiosulfate in the polysulfide, which is often present. Gravimetry is sometimes used in combination with iodometric titration, which only measures the amount of monosulfur. On the other hand, monosulfides can be separated and measured gravimetrically in the form of cadmium sulfide.

Modern methods tend to avoid the use of more time-consuming gravimetric methods.

Polysulfide titration

1. Iodometric method

Schulek proposed a standard method for the determination of sulfides in 1925.

The sample solution was boiled with alkaline oxides in a osmic acid medium to generate hydrogen sulfide, each sulfide-type sulfur atom forming a thiocyanate. Excess cyanide is removed by boiling in an acidic solution, while thiocyanate and thiosulfate remain in the solution without being affected by this treatment. Bromine can oxidize thiocyanate to cyanogen bromide. Excess bromine is destroyed by phenol. Potassium iodide was added and the released iodine was titrated with standard thiosulfate.

2. The acid-base titration method can be used to determine the total sulfur content of polysulfides by using the basic peroxide oxopoly fluid. Feil and Berthold developed a titration method based on this principle. Titrate excess sodium hydroxide with standard sulfuric acid. ^[3]