In Praise of Thiosulfate

R. J. Tykodi Southeastern Massachusetts Unlversity, North Dartmouth, MA 02747

The thiosulfate ion, S ~ 0 3 ~ - , is a very versatile chemical; its reactions in aqueous solution ( 1 4 ) are both interesting and educative-as a matter of fact, the aqueous chemistry of the thiosulfate ion can be used to illustrate many of the concepts we present to our students in general chemistry (7,8): gas formation (GF), precipitate formation (PF), complex forma- tion (CF), acid-base interaction (ABI), redox interaction (RDXj, time evolution of chemical processes, catalysis, stoi- chiometry. The reactions of thiosulfate make impressive lec- ture demonstrations and worthwhile laboratory experi- ments.

First, let us look a t a few reactions (1.3).

This is one of the reactions used to produce thiosulfates on a commercial scale.

RDX: 4%) + GOH-(aq) - 2s'-(aq) + S20,2-(aq) + 3HZ0

This reaction is perhaps better written as

RDX: 4S(s) + 40H-(aq) - 2HS-(aq) + S20,2-(aq) + H,O

to allow for the most recent view (9) on the HS-(ad, S2-(aq) equilibrium.

Partial oxidation of polysulfides, S,'-, n = 2 . 3 , 4 , . . . ,gives thiosulfates, whereas full oxidation gives SO,".

This last reaction should be written more adcurately as

RDX: 2Sz0,2-(aq) + 1,-(aq) - S40,2-(aq) + 31-(aq)

since iodine is not very soluble in water (0.029 g/100 g Hz0 a t 20 OC), and we complex i t with excess iodide to keep it in solution: I2 + I- - 13-.

Although accurate, i t is a bit of a nuisance always to refer to dissolved iodine as Is-; SO, for the purposes of this paper, I shall use the notation Iz(aq/I-) for dissolved iodine:

RDX: 2SZ0,2-(aq) + IJaqK) - S,O,2-(aq) + 2I-(aq)

Aqueous solutions of sodium thiosulfate exposed to the air undergo slow decomposition due to oxidation hy dissolved oxygen and, occasionally, to the growth of sulfur-consuming microorganisms (thiobacteria) (10). Many solid thiosulfates crystallize with water of crystallization, Na2SzO3 for exam- ple. Although the solid alkali-metal thiosulfates are reason- ably stable when dry, other solid thiosulfates are unstable when dry-CaSz03, for example, sometimes decomposes vi- olently and becomes a yellow, pasty mass (2).

Now let us look at some of the fundamental aqueous- solution reaction categories.

Gas Formatlon If we acidify an aqueous thiosulfate solution, the thiosul-

furic acid first formed is unstable, decomposing mainly into sulfur and sulfur dioxide (with lesser amounts of other prod- ucts as well):

RDX, GF, PF: S20,2-(aq) + 2H+(aq) - S(s) + SO,(g) + H,O

Ifthe reaction takesplaceina test tubeand if the test tube is then warmed, the odor of SO2 is unmistakable-more ahout this reaction later.

A solution containing a soluble azide, Ns-(aq), and dis- solved iodine, 12(aq/I-), very slowly liberates nitrogen-the rate of the reaction is so slow a t room temperature as to be scarcely perceptihle. If the solution contains a small amount of S20S2-(aq) [or SCN-(aq) or S2-(aq){HS-(aq)?)], however, the evolution of nitrogen proceeds briskly (5.6):

s,o,2- RDX, GF: 2N,Jaq) + I,(aqfl-) d 3N,(g) + 21-(aq)

Of this reaction too, more later.

Precipitate Formatlon Most of the thiosulfates that have been prepared are solu-

ble in water a t room temperature; the thiosulfates of Pb2+, Agf, Ba2+, and TI+, however, are very sparingly soluhle (5, 6). Lead and silver thiosulfates dissolve in excess reagent, forming thiosulfate complexes. A solution containing a thio- sulfate precipitate or complex of either lead or silver pro- duces the corresponding sulfide upon boiling:

A

RDX, PF: Ag,S,O,(s) + H,O -Ag2S(s) + 2Ht(aq) + SO?-(aq)

for exam~le The of BaS103(s) from moderately concen-

trated solution is a slow process that can he accelerated by ruhhing the inner wall oi the (glass) container with a glass stirring rod-the rubbing procedure makes a useful demon- stration.

Complex Formation Thiosulfate forms strong complexes with Cu(I), Cd(II),

Bi(III), Hg(II), Ag(I), Au(I), and Fe(II1) (6). In the fixation step in the production of photographic images, thiosulfate is used to dissolve away unreacted silver halide through the formation of soluble complexes such as Ag(Sz03)z3- and Ag(S~03)3~-:

The reactions of thiosulfate with Fe(II1) and Cu(I1) are quite interesting. The reaction (aqueous solution, room tem- perature) between Fe3+(aq) and S20a2-(aq) eventually leads to the formation of Fe2+(aq) and S40s2-(aq):

RDX: 2Fe3+(aq) + 2S,O;-(aq) - 2Fezt(aq) + S,0,2-(aq)

The reaction takes place in two stages. First, a deep violet complex anion, Fe(S203)2-, forms almost immediately; it is quite stable and decomposes very slowly, with a fading away of the violet color:

RDX: Fe(S,O,),-(aq) + Fe3+(aq) - 2Fezt(aq) + S,0,2-(aq)

146 Journal of Chemical Education

The presence of CuZ+ ions catalyzes the decomposition reac- tion, and the violet color fades much more rapidly than when copper is absent (5,6).

Thiosulfate reduces Cu(I1) to Cu(1) and complexes the C d I ) (1):

RDX: 2SZ0,2Xaq) + 2Cuz+(aq) -2Cu+(aq) + S,0,2-(aq)

-the characteristic blue color of CuVaq) thus fades away and a colorless solution containing t h e complex ion Cuz(SZO3)?2-(aq) results. If the colorless solution is boiled, CuzS precipitates.

Redox Interaction

Mild oxidants oxidize thiosulfate to tetrathionate,

whereas stronger oxidants oxidize it to sulfate,

[Mn04-, C r ~ 0 7 ~ - , and Hz02 (basic solution) act in the same way].

Acid-Base interaction

The decomposition reactions

RDX, PF, GF: S,O;-(aq) + 2Ht(aq) - S(s) + SO,(g) + H,O

RDX, PF, GF: BaS,O,(s) + 2Hi(aq) - BaZt(aq) + S(s) + S02(g) + H,O

are essentially acid-base interactions [formation of the un- stable HzS203(aq)] followed by the RDX, PF, GF processes.

Tlme Evolution and Calalysls In most of the demonstrations and experiments that our

general chemistry students see the chemical reactions ap- pear to be almost instantaneous. It comes as something of a shock, therefore, to those students who go on to study organ- ic chemistry to find that many organic reactions have to be "cooked" for long periods of time to produce a reasonable yield of the sought-after product.

By making use of the chemistry of thiosulfate, we can teach our general chemistry students two valuable lessons: (1) chemical processes take time-sometimes a fraction of a second, sometimes minutes, sometimes a truly long time; (2) catalysts can increase the rates of chemical processes.

Precipitation of BaS24 Mix moderately concentrated (ahout 0.5 M) solutions of

BaCIdaq) and NapSzOdaq). Put some of the mixture in each of two heakers. Let one beaker sit undisturhed. Rub the inner wall of the other beaker with a glass stirring rod. The "rubbed" solution should precipitate BaSlOs quite readily, whereas the undisturbed solution should take 5-10 min to develop sny significant amount of precipi- tate.

Decomposition of YS24 Take a solution of Na2SzO3-a few tenths molar-and add to it a

fewmilliliters of dilute HCI ( 1 4 M). The mixture will remain clear for a few minutes and then will gradually grow turbid, ultimately turning milky white and opaque.

The thiosulfuric acid first formed decomposes into (mainly) sul- fur and sulfur dioxide. It takes some time for the granules of sulfur to grow to such a size that they scatter visible light (turbidity)-the granules ultimately become a suspension of a finely divided sulfur precipitate (opaque solution).

Reaction of iron(1ll) with Thiasulfate Prepare some Fe(NOs)a(aq)-a few tenths molar-and put some

into each of three beakers. Prepare some NazS203(aq)--a few tenths molar; also prepare some Cu(N0a)daq)-a few tenths molar-in a smalI bottle with s medicine-dropper top. Add a small amount of NazS203(aq) to one of the beakers containing Fe3+(aq). The solution will immediately turn a deep violet color. During the next few minutes the violet color will gradually fade away and the color will return to that characteristic of Pes+(aq). Next adda small amount of NazSIOs(aq) to each of the remaining two beakers [containing Fe3+(aq)]. To one of the beakers add a few drops of Cu(N03)daq). The violet color in the solution containing the Cu2+(aq) catalyst will fade very rapidly, whereas the violet color in the uncatalyzed solu- tion will again take several minutes to fade away.

The Azide and iodlne Reaction Prepare solutions-a few tenths molar in each case-of

NaNa(aq), Idaq) in excess KI, and NazS20daq). Put the thiosulfate in a small bottle with a medicine-dropper top. Mix the NaNdaq) and 12(aqA-) solutions, and put some of the mixture in each of two heakers. To one of the heakers add a few drops of NazSzOdaq).

The mixturecontaining the S~03~-(aq) catalyst should vigorously evolve nitrogen. Theuneatalyzed mixture should show no activity at all.

The Reaction of lmnilll) with Iodide Prepare solutions-a few tenths molar in each case-of

Fe(NOd,(aq), Kl(aq), and Na?S!O,(aq). Also, put some 2% starrh solution in a small bottle with a medicine-dropper top: do the same with some dilute Cu(NOd.(aql. In one beaker put some of the FcrNO,J.,(aq);in another beaker put a mixtureof Fe(N01Maq) and Kl(aq1, with the KI somewhat in excess. Watch the two beakers for several minutes: the mixture gradually develop a seddiah hrownish color that intensifies as time goes on

The reaction of Fe3+(aq) with 1-(aq) is surprisingly slow (11):

RDX: 2Fe3+(aq) + 21-(aq) - 2Fe2+(aq) + I,(aqn-)

To demonstrate the presence of Iz(aqn-) in the mixture, add one or two drops of Cu(N03)daq) and a few drops of the 2% starch solution to the reaction mixture. The starch will turn hlue, showing the presence of Iz(aqA-). Put some of the Na2Sz03(aq) in s squeeze bottle with a fine delivery-tip. Run a stream of thiosulfate into the reaction mixture until the starch color vanishes:

RDX: 2Sz0:-(aq) + Iz(aqA-) - S40,2-(aq) + 21-(sq)

If there is still some residual Fe3+(aq) in the solution and if you have not too seriously overshot the endpoint in the thiosulfate "titration," the hlue starch color may return after a while as more I d a d - ) is formed in the reaction mixture. If the color does not return, add a little mare Fe(N03)3(aq) to the mixture and watch the hlue starch color gradually form and intensify as more Iz(aqA-) is liberated.

Stolchlometry

The reaction

plays a key role in many analytical chemical procedures. In discussing the stoichiometric aspects of the following analyt- ical procedures, I shall make use of de Donder ratios and the de Donder relation (8).

For a general chemical reaction

let nf(. . .) stand for the number of moles of a reactant con- sumed, or the number of moles of a product formed, in the given reaction. The de Donder ratios, then, are the ratios nf(A;)lai, n'(B;)lP,, and the de Donder relation is the state- ment that all the de Donder ratios are equal to one another (8):.

n'(A,) n'(A,) -- n'(BJ n'(B,) - _ = , . , = - = _ _ . . . 9 81 82

Volume 67 Number 2 February 1990 147

Finallv. let FM(. . .) stand for the formula mass (molecular weighi) of the substance shown in parentheses.

Now let us look at some analytical procedures.

Copper Ore Low-grade copper ore can be concentrated by a flotation

process to the point where the concentrated ore can be smelted economically. The ore is ground to a fine powder; the metal-bearing particles are separated from the gangue (dross) by agitation with water and suitable flotation re- agents, so that the froth that forms carries with it the valu- able part of the original ore. The froth is made to overflow the flotation vessel and is collected.

In one method of analyzing the concentrated ore for its copper content, the reactions

RDX, PF: 2CuZt(aq) + 4I-(aq) - 2CuW + I,(aq/I-)

are made use of: excess iodide added to a dissolved ore sample liberates a stoichiometric amount of iodine, which is then titrated with a standardized thiosulfate solution.

The stoicbiometric calculations are pretty straightfor- ward, so I shall not say anything more about this use of thiosulfate.

Standardization of an Acid An interesting method of standardizing an acid is based on

the reaction

which only proceeds in the presence of an appreciable amount of hydrogen ion.

In using this method we dissolve a weighed amount of KI03 in a solution that contains excess KI and sufficient Na2S203. 5Hz0 to react with the Iz liberated duringtitration with the acid:

The liberated iodine, therefore, does not color the solution; and, after the addition of enough acid to decompose all the iodate, further addition of acid will cause the color change of an indicator such as methvl vellow, methvl orange, or bro- . . . mocresol green.

Suppose you wish to determine the molarity M(HC1) of a hydrochloric acid solution. Proceed as follows. Weigh out z g of KIO*. and dissolve i t in 50 mL of water: add excess KI and .---- sufficient Na2SzOs. 5H20, and titrate the thusly prepared solution with the hydrochloric acid, using methyl orange as indicator. Suppose it takes y mL of the HCI to reach the endpoint.

How should you express the molarity of the hydrochloric acid, M(HCl), in terms of z , y, and FM(KIOd?

Answer. The de Donder relation tells us that

and

ClzlC102 Mixture Suppose you have on hand a gaseous mixture of Clz and

C102, and you want to determine the mole fraction of C102, X(CIO2). in the mixture. Proceed as follows. Introduce a sample of the mixture into a gas-handling apparatus, and cause the sample to bubble into an aqueous solution of KI- repeating the procedure if necessary-until all of the sample has been absorbed:

RDX: c~,(g) + 2KI(aq) - ~,(aqn-) + 2KCl(sq) (1)

RDX: 2C102(g) + 2KI(aq) - 2KClO,(aq) + I,(aqK-) (2)

Titrate the liberated iodine with a standardized solution of sodium thiosulfate, say 0.0891 M,

Suppose i t takes 17.66 mL to reach the endpoint (titration a). Then acidify the solution with HC1, releasing more io- dine:

RDX: KClO,(aq) + 4KI(aq) + 4HCKaq) - 2I,(aqK) + 5KCl(aq) + 2H,O (4)

Titrate the newly liherated iodine withmore of the 0.0891 M thiosulfate solution. Suppose the new titration requires an additional 21.30 mL to reach the endpoint (titration b).

What is the mole fraction of C102, X(C103, in the gaseous mixture?

Answer. To distinguish the numbers of moles consumed or produced in each of the four reactions, subscript the n'(. . .) quantities: let n2'(Iz), for example, represent the number of moles of 1 2 produced via reaction 2. The various de Donder relations that we shall need are:

where na,/(In), e.g., refers to the moles of I2 consumed via reaction 3 in titration a.

We see that

I - n3,t,'(Sz0:-) n,'(CIO,) = n,'(KCIOz) = n,'(KClO,) = - -

2 4

[ni(KC1O2) = nd(KC1O2) since the KC102 produced via reaction 2 is all consumed via reaction 4. And of course n4'(12) = n3,bT(12)-it is the 12 liherated in reaction 4 that is titrated in titration b].

We also note that

148 Journal of Chemical Education

Finally, we have

Unresolved Question Lest our general chemistry students conclude that chem-

istry is "finished business", we can point out to them one of the many unresolved questions in chemistry.

Although thiosulfuric acid is unstable in aqueous solution, i t can be made in the dry way at low temperature (-78 OC) by the direct combination of H2S and SO3 and by the reac- tion of chlorosulfuric acid with hydrogen sulfide(4):

H,S + SO3 - H,S20s

H03S-CI + H-SH - H03SSH + HCI

(The thiosulfuric acid so made decomposes into H2S and SOs if warmed to room temperature.)

Once having made thiosulfuric acid, the question arises, Where do the protons go? Both on oxygen atoms or one on oxygen and one on sulfur? Different authorities interpret the available evidence differently: some favor the symmetric placement HO(SS0)OH (12, 13), whereas others favor the unsymmetricplacement HS(OS0)OH (14). Clearly the mat- ter calls for further attention-perhaps an NMR study of the product could distinguish between -SH and -OH, there- by resolving the point at issue.

Also, there is some question as to whether the direct com- bination of HnS and SO3 a t low temperature truly yields thiosulfuric acid rather than an isomeric adduct H2S . SO3 (12).

Conclusion The chemistry of thiosulfate illustrates many of the topics

we develop in general chemistry. In showing our students aspects of thiosulfate chemistry, we introduce them via quite impressive. demonstrations and experiments to the fuuda- mental themes of aqueous solution chemistry.

Literature Cited I ' n ~ ~ d d l . F. P.: n d l . W. T. Analvticol Chamiafrv Vol. I. Quolitolive Analysis. 9th .. ~ ~ ~ - - ~ ,

ed.: Wiley: New YI 2. Eohrsim. F. Inoraon

. . 1973: Section 23.

6 . Suehla. G.. Ed. voaolg Terfboak of Macro and semimicm Qudifotiua i n a w n i c A&& 5th ed:;longman: London. 1979.

6. Burns. D. T.; Tamahend. A.; Carter. A. H. Inorganic Reaction Chemlstrv, Volume 2, neactionr o/the Elemenla and Their Compounds: Horwood: Chichester, E n g h d . 1981.

7 Tukr*li R I.,. Chem Edur 1987.64 243-246. .. ~~~~ ~ ~

8. Tyk0di.R. J .J . Chrm.~dur . 1 9 8 7 ; ~ ; 958-960. 9. Myers, R. J. J. Chpm Educ. 1986.63,687-690.

10. ~ d t h o f f , I. M.; Sandell. E. B ~ e r t b o n k o/ Quanfifntiue Inorganic Anolyslz. 3rd od.; Macmillsn: New Yark. 195tp562.

11. Hershey, A.V.: Bray, W. C. J.Am. Chem Soc. 1936,58,1760-1772. 12. Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements: Pergamon: New York,

19% PP 835,846. 13. Cotton.F.A.; Wilkinson,G.~duonepdlnorganicChrmi;iry,5thed.; Wiley New Yolk.

1988; p 520. 14. Sherpe. A. G. lnorponic Chemistry. 2nd ed; Lonpan: London, 1986: p 398.

Volume 67 Number 2 February 1990 149