Thiosulfate An Interesting Sulfur Oxoanion That Is Useful in Both Medicine and lndustryBut Is Implicated in Corrosion S. W. Dhawale Division of Natural Science and Math, Indiana University East, Richmond, IN 47374 The family of sulfur oxoanions consists of several chemically active species, such as sulfite, thiosulfate, and tetrathionate (see Figs. 1-3). Tykodi described the thiosulfate ion as a versatile ion whose aqueous chemistry can illustrate several wnceots in eeneral chemistm. such as gas formation, a l d redox reaction; ti). Some additional interesting of thiosulfate ions are me- aspects sented in this paper. Problems and Applications in Industry Studies from various laboratories have shown that thiosulfate and other sulfur species are highly "aggressive", a term that corrosion engineers use to describe a chemically active species that provides an environment favorable for various forms of corrosion. The thiosulfate ion has been shown to be a factor in the ambient-temperature stress corrosion of pressurized borated-water reactors (2). This poses problems in the nuclear industry, where stainless steel tanks are used to store borated thiosulfate solutions. During emergencies or after a nuclear accident, these stored solution sprays may he used to react with iodine (one of the fission products). Thiosulfate is also found in some industrial effluents as an impurity or as a product of some reactions. Avrahami and Golding (3)found that thiosulfate is one of the prominent products of the oxidation of sulfide and polysulfide aqueous solutions. Like many other sulfur compounds, thiosulfate plays an important role in technology, pharmaceutical chemistry, and biochemistry. Sodium thiosulfate finds broad application in our society as shown below. an image-iirdng agent in photography an antichlor in the paper industry an antidote to cyanide and arsenic poisoning a treatment for some forms of parasitic skin disorders
The Stability of Thiosulfate Ions Metastable in Aqueous Solutions Aqueous solutions of thiosulfate may impair the localized corrosion resistance of some stainless steels. To understand the mechanisms of such localized corrosion, one must know the basic facts of thiosulfate behavior in aqueous solutions. Acwrding to a simplified Pourbaix diagram for the sulfur-water system at 25 'C and at 1atm, all sulfur species whose oxidation numbers lie between -2 and +6 are thermodynamically unstable, that is, in ametastable state. An exception is solid sulfur. Factors That Affect Stability Thiosulfate is a metastable anion that tends to undergo chemical decompositionin aqueous solutions. However, investigators have reported that thiosulfate solutions prepared in freshly boiled double-distilled water or distilleddeionized watekare very stable ifstored in air-tight bottles (41.Dilute solutions of thio~ulfaterO.01 M or lower, decompose more rapidly than the concentrated solutio& (0.1 M or higher) (5).Weak solutions of sodium thiosulfate are more stable when prepared and stored properly. Oxidation-state diamams show a clear thermodvnamic tendency for chemicai'disproportionation rewtions"at low pH. The following is one possible reaction.
These uses are briefly described later in a section that covers the miscellaneous uses of thiosulfate. Figure2. Sulfiteanion (SO;.). This anion and bisulfite (HSOi)are two ofthe natural products of thiosulfate degradation that are of industrial and medical importance.
Figure 1. The thiosulfate anion (s,o$-). This metastable anion is a natural sulfur donor, disproportionatingto form suHite and sulfuror an active sulfur species. Although this tendency is medically useful, it can contribute to the corrosion of metals and other materials. This compound is also subject to action by microorganisms. It can be p r o duced by certain bacteria under certain conditions, but it can also be degraded by other bacteria to form hydrogen sulfide. 12
Journal of Chemical Education
Figure 3. Telrathionate (s,o;-). This ion can be produced by the oxidation of thiosulfate, as in the paper industry. It is also a known metabolic product of sulfurbacteria. .,
Since wlloidal sulfur and bisulfite ions (HSOi) are metastable in an acid environment, they undergo further reactions to !zive various products. when; thiosulfate-solutionis titrated against standard sulfuric acid or hydrochloric acid, the solution becomes cloudy. Approximately &lo% of the total sulfur precipitates between pH 3 and 2. The solution also shows a strong buffering action, thus suggesting the formation of a weak acid and its salt (unpublished results from my laboratow). Also, the concentration of thiosulfate ions decreasesin the presence of copper (5) and copper-base alloys (unpublished results from my lab). In short, many factors influence the stability of thiosulfate solutions, as summarized below (5,6). the purity of the water used as the solvent the concentration and pH of the solution the presence of certain metals or alloys the presence of certain metals and sulfur bacteria exposure to ultraviolet light Corrosion and Thiosulfate Ions Corrosion chemistry is of vital importance to industries. Sulfur and sulfur comoounds are usuallv blamed for damaging a variety of str&tures and materiks. 'steam power plants cement old art structures almost all industrially useful common metals some alloys Stress-Corrosion Cracking of Stainless Steels
In the early 1950's intergranular stress-corrosion cracking of sensitized stainless steel became recognized as a technological problem in the petroleum industry. Even the highly corrosion-resistant 304 stainless steel was found to undergo stress-corrosion cracking in petroleum and other industries due to sulfur wmpounds. As discussed below thiosulfate ions have been implicated in various forms of corrosion. Isaacs et al. (9)and Newman et al. Dhawale et al. (7,s) (10) investieated stress-corrosion crackine of sensitized stainless steels, type 304, in the presence orthiosulfate solutions. The data from all these investieations indicate that intergranular stress-corrosioncracking occurs within a specific wtential ranee in the presence of thiosulfate solutions. ~;)rat?dsodium thiosulfate solutions, as dilute as 10" M to 10.' M. caused stress-corrosion cracking under the experimeutal'conditions. Pitting and Crevice Corrosion
Pitting of stainless steels in the paper and pulp industry is believed to be caused bv thiosulfate ions (11).Sodium hydrosulfite is used as a whitening or brightening reagent in the paper pulp industry, and it can produce thiosulfate ions. Newman (12) demonstrated that thiosulfate ions lowered the pitting potential of stainless steel alloys, making the material more susceptible to pitting corrosion. In addition, studies on crevice corrosion (13) indicate that very small amounts of thiosulfate ion can promote crevice corrosion of stainless steels. The generally accepted mechanism of these electrochemical forms of corrosion suggests that a disproportionation of thiosulfate ions oroducG an active sulfur snecies at a low pH. Active su1f;r species aid the dissolution of metd and prevent the repassivation of the metal surface, thus promoting corrosion. Thiosulfate also causes general corrosion of copper and copper-base alloys. A demonstration can be easily produced to convince students. Stainless steel, copper, and
brass specimenn that have been corroded in thiosulfate solutions obviouslv show the deteriorating- effects of sulfur compounds. Microbial Corrosion and Sulfur Oxoanions Thiosulfate and other sulfur oxoanions are common intermediates in the ahiological as well as biological oxidation of sulfide.Also, many microomanismscan reduce thiosulfate to hydrogen sulfide, whichcan then be excreted or retained. Thiosulfate stimulates growth of a marine pseudomonad growing on low acetate (14). Sulfur-reducing bacteria wuple the oxidation-reduction of organic carbon to the redox system of inorganic sulfur. Thiosulfate and tetrathionate have been detected in these culture solutions. Sulfate-reducing bacteria are responsible for the corrosion of type 304 and type 316 stainless steels in the paper industry (14, 15).Buried carbon steel and cast-iron pipelines can also undergo localized corrosion in the presence of anaerobic sulfur bacteria, accumulated thiosulfate, and other sulfur oxoanions. Miscellaneous Uses of Thiosulfate Photographic Development Photography involves various processes to produce permanent images. The development of exposed film can be summarizedas below AgBr + developer -t Ag + B F + oxidized developer Generally, the development process leaves unreduced grains on the parts of the film that were exposed to low light intensities. To prevent darkening of the film by the residual silver halide grains, these grains must be removed during the fixing process. Usually sodium thiosulfate solution (hypo)is the fixing agent. It converts residual silver halide to water-soluble complex ions that are removed during the washing process. Paper Making
Sodium thiosulfate is also used as a neutralizer of the chlorine wm~oundsthat are used as bleaching aeents in the paper iniustry One of the reactions used
