edited by DARRELL H. BEACH
chemical of the month Hypo-Hydrated Sodium Thiosulfate Hypo, hydrated sodium thiosulfate, Na2S2O35Hz0 is also referred to as sodium thiosulfate pentahydrate. Method of Preparation
Sodium thiosulfate is a white solid formed by the reaction of sodium sulfite and sulfur upon boiling and evaporating:
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SO?-(aq) + S(s) SzOF(aq) The resulting substance was incorrectly called "hypo," from an old name, sodium hyposulfite. Uses of Hypo
Sodium thiosulfate has three very important uses: (a) As an antichlor to remove excess chlorine from bleached fahrics:
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SzO?(aq) + C1&) + HzO(l) Sod2-(aq)+ S(s) + 2 Cl-(aq) + 2 Ht(aq) (b) As a volumetric standard for the estimation of iodine 2 SzOz2-(aq)+ Iz(4 2 IWaq) + SdOs2-(aq) The reaction gives a quantitative yield of sodium tetrathionate, NazS40daq). (c) As a fixing agent in photography when it complexes with unchanged silver halide:
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Ag+(aq)+ 2 Ss0s2-(aq) [Ag(S20&l3-(aq) Of the three listed uses, the latter is the most common. Aqueous photographic fixing solutions are made by adding about 240 g Na2Sz03.5 HzOfi of solution. Acidification of aqueous hypo gives a sulfur precipitate
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Sz0s2-(aq)+ 2 Hf(aq) S(s) + HzSOdaq) As sulfur is produced, the solution becomes opaque to a mark located below the reaction vessel. This reaction.has made possible an alternative to the iodine clock reaction. I t is the so called sulfur clock. History
Daguerre, a French theatrical scene painter, treated polished silver plates with iodine fumes. He obtained beautiful pictures by exposing these plates to light. He then dissolved away the unaffected silver iodide by means of hypo. These
The Culver Academies Culver, Indiana 46511
pictures were called Daguerreotypes. Sodium thiosulfate pentahydrate was first used this way around 1826. Physical Properties
Na?S70q.5H?O - - .. - has a molar mass of 248.18 "e mol-'. I t has a denait? of 1.729 g ~ m - Its ~ .tnelung retnperature if herween 10 to43"C. Its boil~nrtemoeruture less t h e watersot hydration is 100°C. I t dissolves 79.4 g1100 cm3 of water when ;old and 291.1 g1100 cm3 of water when the water is hot. Structure of Thlosulfate Ion
The structure of the S ~ 0 3 ion ~ - is most interesting. The two sulfurs are not eauivalent. The ion has the form of a sulfate ion, Sod2-, in which one of the oxygen atoms has been reolaced bv a sulfur atom:
The shaoe of the SgOn2- ion like the SOa2- ion is tetrahedral. It has one sulfur atomat the center of a &ahedron. The other sulfur atom and the three oxvgen atoms are located at the -corners of a tetrahedron. cost
Three chemical supply houses listed prices ranging from $8.50 to $9.50 per pound (1980 prices). The same chemical companies showed current prices for five pounds ranging from $19.75 to $25.14. All of the foregoing prices are for Reagent, ACS crystals. A large photographic supply company lists photographic grade hypo at $0.95 for 170 g, $1.25 for 354 g, and $2.10 for 709 g. It is interesting to note that a January 1937, invoice from Central Scientific Company showed a price of $2.20 for a five-pound container of CP crystals. General Comments
Silver halide decomposition by light and their solubility in "hypo" are fundamental to the chemistry of photography. Photographic films are coated with an emulsion of the silver halides sinelv and in combination with eelatin. Unon exoosure to light, a h i t i o n (or all depending on I'ntensity and exposure time) will beein to decom~ose.After develooine with a re~ )un, ducing agenisuch as hydroquinone ( c ~ H ~ ( o H ~the changed silver halides are removed by hypo before further
Introduction to "Chemical ol the M o n t h with &is issue of ME JWRNAL we begin a new feature entitled "Chemical of the Month" The intent of this monthly feature is to put more descriptive chemistry back onthe scene in our modern chemistry courses. This editor sees this featwe as w i n g several purposes t a w busy demisWteachet: ( I ) increasing me knowledge of andlar reviewing descriptive chemistry far his or her awn background; (2) assigning wrinen and/or wal student reports on desuiptive chemistry; (3) hlmishing monmly bulletin be=& teatue topics; and (4) providing materials on descriptive chemistry which ultimately may become a part of the course syllabus. Each monthly issue will provide a short history of me chemical belng featured. Also included will be the memcds of preparation, physical p r o p srties and structure, chemical properties, uses, amount prcduced in the United States, and me current price. Not all of these categories will be u d each time; nor will each division experience the same depth and breadth. Not all chemicals will be well-known ones. In fact, sameobscurechemicals which serve useful purposes will be featured as well as the "big ones." The chemical of the month may be an elemant or a campound. it may be From the organic or the inorganic division of chemistry. Hopefully,all at us should learn from Um materials pwwtnted in mis column. Our sMents should consequently benefit directly or indirectly from the same.
The first three or four columns will be presented by invited authors. It is hDpd that in this way we can establish a common format. Conbibutions are welcome and should be sent to me feature editor at the above address. Darrell H. Beach brings with h ~ mto the JOURNAL,numerous awards, citations, and 24 years of secondary school teaching. He recsivsd his B.A. in Chemistry from Central Michigan University in 1957, his M A , from the University of Michigan in 1963, and his Ed.D. from the University of Sarosota (formerly Laurence University) in 1973. Dr. Beach is presently a Master Instructor of Chemistry and the Eugene C. Eppley Chairholder of Chemistry at the Culver Academies. He was the 1977 recipient of the Outstanding Chemistry Teacher from me Purdue ACS Section and was me 1979 regioMl MCA Award winner. He has also been an NSF and GE Fellow. Dr. Beach has authored three books and has aver two dozen other publications. He has served on the ACS-NSTA Level I1Examination Committee since 1968 and has been chairman of the group since 1974.
Volume 58
Number 4
April 1981
325
of the negative to light' The sodium thiosulfate dissolves the remaining silver halide by forming the [ A g ( S 2 0 3 ) 2 ] 3 - complex. At one time thiosulfuric acid, H2S203,WBS to he a product of the reduction of sulfurous acid, H z S 0 3 , and was therefore termed "hyposulfurous acid." The name "hyposulfites" for the salts (i.e., N a z S 2 0 3 ) of thiosulfuric acid is wrong, and yet most individuals know the chemical of the month as just plain "hypo." Even if it is a misnomer, it contributes much to our daily lifes.
General References Asimav, Isaac, "A Short History of Chemistry." Amhor Books. Doubleday and Company. Inc, New York, 1965, p. 159. Beaeh,DarrdlH., CHEMIBNEWS, 102, 16, (1979).
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Journal of Chemical Education
Bunzell, H. H., "Everyday with Chemistry.). Groaset and Dunlsp, New Yark, 1937.p. 84.
Cmphell.J.Arthur."ChemicdSystemsEnergeti~DmmicsStmdure:W.H.Freeman
end co., san~ranei~eo. 197n.p. 437. "ChemscoiEn~inrrringNeus."58, Ill]. 12 (March 17,1980). Dvia. L., et "1.. "Inveatigatiw Chemistry," Heinemann Educational Books Lrd, Iandon, 1976, p. 296. Gelender. Maxwell, "Review Text in Chemistry," ~ m a c oschool ~ub~iration. hc.NW ~ o r k , 1 9 5 9 , ~241. . Greemtone, Arthur W.. and Harris. Sidney P., "Concepta in Chemistry: 3rd. Ed., Hacourt Blsee Jovsnouich. New Ymk. 1975, p. 496. pading, L ~ U"coliege , chemistry: 3rd. ~ d .W.H. , ~resmanandco., sari ~ r a n e k o1964, , P. 397. Faaeliffe, C. T., and Rawson, D. H.. "Principles of Inorganicand Theoretical Chemistry: 2nd. ~ d .~,e i n e m a n n~ d u c a t i o n d~ o o k LU., s ~ o n d a n 1975, , p. 314. Rernv. .. H... "Treatise on lnoresnie Chemistrv." . Volume 1. Elsevier Publishin." Comoanv. ~ ~ ~1956, P. t 720.~ ~ d ~ ~ , Sienko, Miehell J., and Plane, nobrt A,, "Chemistry," 5th. Ed., Mffima.-Hill Book CornPBDY,New York, 1976.p. 393. Weat. Robert C. (Editorl,"Handbookof Chemistry and Phyaies," SmhEd., TheC h a m i d Rubber Co., Cleuelsnd, 1969. p. B-161.
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