- - .- - -, GEORGEL. GILBERT Denison University Granville. Ohio 43023
The Arsenic(lll) S u l l i i Clock Reaction SuBMlTTm BY
Kenneth W. Watklns Colorado State University Fort Colllns, CO 80523
CHECKED BY
Ronald DlStetano NorthamptonCounty Area CommunRy College Bethlehem, PA 18017
Two colorless solutions, one containing sodium arsenite and acetic acid and the other containing sodium thiosulfate, are mixed in a beaker. After about 20 s, the mixture turns a brilliant yellow color. This color is due to colloidial arsenic(II1) sulfide, a substance that has been used for thousands of vears as a oaint oiement and that is known as kines . yellow, oipiment, and auripigment (I,2). The time inter& is re~roducibleand d e ~ e n d e non t the concentrations of thiosulfate and acetic acid. This clock reaction is well suited for determining the order of reaction and the activation energy.
Acetlc Acid Dependence Prepare the following solutions in two 250-mL beakers:
Al. 20 mL A, 78 mL HzO,2.0 mL acetic aeid A2.20 mL A, 76 mL H20,4.0 mL acetic acid Place 100mL of solution B into each of two 150-mLheakers. Follow the above procedures, and compare the time intervals as before. Materials for Temperature Dependence Solution C: Dissolve 6.0 g of sodium arsenite, NaAs02,in 270 mL of distilled water. Then add 30 mL of glacial acetic aeid. Solution D: Dissolve 45.0 g of anhydrous sodium thiosulfate, NazSz03, in 300 mL of distilled water. tbree 250-mL beakers tbree 150-mL beakers thermometer timer ice hot plate Procedure
Dependence Place 100 mL of solution C in each of three 250-mL beakers, and 100 mL of solution D in each of three 150-mL heakers. Maintain one C-D pair at room temperature, one at about 0 OC (ice-water bath), and one near 40 OC (hot plate). When ready to begin, take the beakers from the ice-water bath and place the thermometer in solutionC.Pour aolutionD intosolutionC, andsimultaneouslystart the timer. When the yellow color of arsenic(II1)sulfideappears, stop the timer and record the time and temperature. Repeat this procedure for the reagents maintained at room temperature and 40 'C, respectively. Temperature
Materials for Concentration Dependence Solution A: Dissolve 12.0 g of sodium arsenite, NaAsO*, in 300 mL of distilled water. Solution B: Dissolve 105 g of anhydrous sodium thiosulfate, Na&Oa, in 700 mL of distilled water (or 165 g of pentahydrate, Na&Oq. - - " 5Hv0. can be used). 126 mL e l a d acetic aeid eight 25;-&L beakers eight 150-mLheakers timer Procedure
of Arsenite Concentration Place three 250-mL beakers in a row on the lecture table. Lahel the beakers Al, A2, and A3. Place the following solutions into the beakers:
Effect
Al. 80 mL A, and 20 mL acetic acid A2.40 mL A, 40 mL distilled water, and 20 mL acetic acid A3.20 mL A, 60 mL distilled water, and 20 mL acetic acid Place three 150-mL beakers in a row directly behind the larger beaken. Place 100 mL of solution B into each of these beakers. Pour the contents of the first small beaker into beaker A1 and start the timer. After about 20 x the solution will auddrnlg turn n yrllow-gdd color. Stup rhr timer and rrcord the time. Kegeat this procedure tor rhe vther two concentrations, and compnre the time intervals for the yellow color to appear. of miosulfate Concentration Place three 250-mL beakers in a raw on the lecture table. In each of the three heakers prepare a solution exactly as in A2 of the previous procedure (40 mL A, 40 mL HzO, 20 mL acetic acid). Place three 150-mL beakers in a row directly behind the larger heakers. Place the followingmixtures involving solution B into these beakers:
The procedure descrihed here was adapted from several sources of demonstrations (3. . . 4). . The oreoaration of arsenic(II1) sulfide by this reaction was fir& reported by Vortmann in 1889 (5). However, he made no mention of the sudden appearance of AS&. The "clock feature" of this reaction was reported by Forbes e t al. ( 6 )in 1922 in a paper that describes the effect of reactant concentrations on the time interval. These workers suggested the use of this reaction as an alternative lecture demonstration to the iodine clock reaction. The details of the mechanism are still unknown; however, a plasible mechanism has been suggested and will be reviewed below. The formation of As& is related to the decomposition of thiosulfate in acidic solution which has also been used as a demonstration (7).
Effect
B1.25 mL and 75 mL distilled water B2.50 mL B and 50 mL distilled water B3.100 mL B Pour the contents of beaker B1 into the larger beaker containing solution A, and start the timer. As before when the solution turns yellow, stop the timer and record the time. Continue this procedure for solutions B2 and B3, and compare the time intervals for the yellow color to appear.
This reaction is much more complicated than i t looks with other products being polythionic acids of the formula HzS,06 (x = 2-6) and hydrogen sulfide. Much of the sulfur occurs as S6 and Ss rings and as polymeric sulfur. T h e best summary of the results is by Davis (8). When thiosulfate is acidified in the presence of arsenite, the oroduct that forms is arsenic(II1) . . sulfide rather than sulfur. Several experiments were carried out in which the amount of sodium arsenite was gradually decreased from that given in the procedure. As the concentration of sodium arsenite was lowered, the color of the product gradually shifted from yellow to white. This shows that competing reactions are involved. Volume 64
Number 3
March 1987
255
exchange, and they have different oxidation numbers (12).
In the presence of arsenite the products are mainly arsenic(II1) sulfide, bisulfite (or SOz), and pentathionic acid (8-10). An equation that accounts for these products is 18Ht + 12SsOF + 2As02-
-
As2S3(s) + 6HS03- + 3H2SS06+ 4H20
Pentathionic acid H&Os is also formed to some extent in the acidification of S ~ 0 3 ~but - , its yield is enhanced by the addition of arsenite ion. Curiously, arsenite in an AsOz- to S?0? ratio of as little as 1to 20 will re vent the formation of &nificant amounts of free sulfur (6). The reason for the time interval heti~rethe sudden anmarance of AS& does not seem to have been explained. I t is possible that a time interval is necessary before the concentration of AS& builds up of a saturated solution. This would be similar to the "Old Nassau" and related reactions in which HgIz(s) is formed. The solubility of As2S3 is only M. 0.00005 g/100 mL H 2 0 at 18 'C ( I I ) , or about 2 X
A mechanism for the decomposition of thiosulfate in acidic solution to yield molecular sulfur, which also accounts for the formation of AS& when arsenite is added, has been proposed by Davis (8). In the first step of this mechanism, the sulfur atom of -2 oxidation state is protonated. Then a chain of sulfur atoms is built u p one atom at a time by a series of nucleophilic displacements of sulfite by thiosulfate.
..
and so on until Concentration Dependence
The low solubility of As& means that the concentrations of the reactants remain essentially constant during the time interval, and so the concentration and temperature dependences can be demonstrated using the method of initial rates. The rate of reaction is the reciprocal of the observed time interval. By varying the thiosulfate concentration as in the procedure, the rate was found to double when the concentration of thiosulfate doubled. This showed that the reaction was first-order in thiosulfate. Additional experiments in which the acetic acid and sodium arsenite concentrations were varied showed the complex nature of the reaction. The order with respect to acetic acid was not constant, varying between approximately first-order under the conditions described in the procedure to zero-order a t higher acetic acid concentrations. The results show that the reaction rate is zero-order with respect to arsenite. This is true as long as the arsenite concentration is above the minimum required to prevent the competing reaction which produces significant amounts of free sulfur. Temperature Dependence
The dependence of the reaction rate on temperature is easily demonstrated with this system. A plot of the log of the rate versus the reciprocal of the absolute temperature yielded a straight line from which the activation energy was calculated to be 46 kJ/mol. Similar experiments on the decomposition of thiosulfate in acetic acid solution (no arsenite present) gave the same activation energy as that for the formation of As2S3. The Mechanism
In the mechanism given below, thereactive form of arsenic is As3+ ion. In acetic acid solution arsenite is converted to arsenious acid. Arsenious acid is amphoteric and in acid solution will react stepwise to produce a small concentration of As3+. A ~ ( o H+) ~3HC2H3O2+ As3++ 3CzH302-
+ 3HzO
I t is important to distinguish between the two sulfur atoms in thiosulfate. Radioactive labeling experiments indicate that the two atoms are not equivalent. They do not 256
Journal of Chemical Education
This series is followed by the formation of the Ss ring in a unimolecular reaction.
Calculations of charge distributions in sulfane sulfonic acids support the suggestion that the nucleophile will always attack the sulfur atom adjoining the SO3 group, as in eqs. 2 and 3, because this atom is the most positive of the accessible atoms of the chain (13). . . The mechanism is also reviewed bv Kice (14). T o exnlain the formation of As&. a mechanism is suegested t i a t produces sulfide ion. T i e product of eq 2 also can nndereo " a rearrangement to form a sulfane monosulfonic acid. HSSS03- + -SSS03H
(7)
This is followed by a displacement reaction that yields pentathionate and bisulfide.
I t is a t this point that arsenic ion can react withsulfide ion to produce AS&.
The formation of AS& causes the equilibrium in eq 8 to shift to the right, thus favoring the formation of pentatbionate in the presence of As3+ ion. This shift siphons off intermediates that would lead to the formation of sulfur in the absence of arsenic ion. Finally, it is of interest to note that the antimony analog of this reaction has been described (15,16). Dlsposal
Solutions containing arsenious acid are highly poisonous and should not be ingested nor allowed to come in contact with mucous membranes or skin. Arsenic compounds are considered a hazardous waste by the EPA. Most municipalities have local laws governing disposal of hazardous waste. The arsenic sulfide precipitate should be disposed of ac-
cording t o the hazardous waste procedures established a t t h e reader's institution. If necessarv arsenic sulfide can be dissolved in dilute NaOH. In ~ o ; t Collins, hazardous waste from the Department of Chemistry and from chemistry classes a t the three local high schools can be stored for collection and proper disposal by the CSU Environmental Health Services. In the absence of established procedures the reader can consult t h e recommendations of the NRC Committee on Hazardous Substances in the Laboratory (I7) for disposal of As2S3 Llterature Cited I. Friedstein. H. G. il Chem. Educ. 1981.58.291. 2. Windhnlz, M., Ed. The Merck Index, 9th ed: Merck: Rahuay, NJ. 1976. 1. Chen. P. S. Entrifainina and Educational Chamieal Demonsrrorions; Chemical Elements: Carmarillo. CA, 1974. 4. Bailey, P. S.,e l al. J. Chem. Educ. 1975.52.524. 5. Vorfmann,G.Chem. Ber. 1889.22.2307. 6. Forbes,G.S.; Estil1.H. W.; Walker.0. J. J.Am. Chsm.Soe. 1922.44.97. 7. Alyea. H. N.: Dutton, F. B. Testad Demonsfroiionr in Chemisfgv, 6th ed,Jaurnsl of ChemicalEducation: Easton, PA, 1965; pp41.113. 8. Davis, R. E. J . Am. Cham. Sac. 1958.80. 3365,and references therein. 9. Rieaenfeld, E. H.; Sydor, G.; Z onorg. oIl@m. Chem. 1928,175.49: Chem Abatmcfs
1929.23.152. Sidwick,N. V. The Chemical Eiamsnfs ond Their Components;Oxford Uniu.: London, 1850:VOI. 2, pp siasw 11. Woast, R.C.,Ed. Hondbook ofch~mislry ond Phyrics.60th ed: CRC: BocaRaton,FL, 10.
1979:p B~S8. 12. Schmidt. M.; Siehert, W. In Comprekensiue Inorganic Chemistry:Bailar, J. C..et al.. Edn; P~rzamnn:New Yark, 1973;Vol.2, Chapter 23, p 885. 13. Meyer. B.:Peter, L.:Spitrer,K, l n o r i Chem. 1977,16.27. 14. Kice,d. L. In Sulfurin Orgonicandlnorgonic Chemirtgv:SennigA.,M; Dskker: New Ynrk, 197L: Vol 1 , p 153. 15. Meyer. M. J. Am. Cham.Sac. 1922.44.1438. 16. lyor, R. V.: Dwivedi, J. K. J. Indian Chem. Sor. 1958.36 295. 17. National Research Cnuncil. Prudent Practices /or Disposol 01 ChomVob from Labnmlorisr: Nations1 Academy: Washington. DC, 1963;pp 8042.
An Easily Demonstrated Photosensitive System S U B M I ~ Esv D
Davld 0. Cooke Fire Servlce College Moreton in Marsh, Gloucestsr, GL56 ORH England CHECKED BY
Luther K. Brlce, Jr. Virginia Polytechnic Blacksburg, VA 24061 This demonstration convincinelv - " shows the effect liaht can have on a chemical system. T h e aqueousreaction system described is easilv handled and observation does not resuire a n ultraviolet light source.
Materials and Equipment Manganese(I1)sulfate solution (4 gl2.50 mL) Hydrogen peroxide (30%by weight) Sulfuric acid solution (1M) Potassium iodate solution (5.35 gl250 mL) Phenylmalonic acid solution (2 g1100 mL) (Aldrich Chem. Ca.) Distilled water 2 100-mL beakers Glass stirring rod Card box to cover a 100-mL beaker
Procedure To 40 mL distilled water in a 100-mL beaker add 8 mL potassium iodate solution, 8 mL phenylmalonic acid solution, 4 mL 1M sulfuric acid and 2 mL 30% (by weight) hydrogen peroxide. Have ready the second beaker and a box to cover it. Add 10 mL manganese(I1) sulfate solution to the mixture, mix well, and pour about half of the resultant solution into the second heaker. Quickly place this heaker under the box to exclude light. In diffuse daylight' iodine steadily accumulates in the unshielded beaker. After about 60 s iodine is clearly visible (Absorbance 0.5 at 460 nm, l-em cell, 25 "C).After 2 or 3 min remove the boa covering the second beaker to reveal a colorless solution. Iodine now slowly accumulates in this heaker. Remarks I n this system iodine is produced via the manganese(I1)catalyzed iodate-peroxide reaction. I n the absence of light and earlv in the reaction ~henvlmalonic acid removes iodine . . as fait a s it is produced. In the prestmce of light it is suggwtell that ~hotodissoriafion utiodovlienslmalunic arid lt!ads lo a low concentraan addsional source of iodine. ~ b o v e - very tion of iodine, t h e iodination rate of ~henvlmalonicacid is independrnt the iodine conrentracim is required by an enolizatim mechanism. T h e two sources of iodine nuw allow the iodine production rate to exceed t h a t of consumption.
of
'
The reaction proceeds rapidly even at 350 Lux (for photographers 1/30 s at f4 using 400 ASA). If the room does not have sufficient da~iiqhtan ordinary anqlepoise lamp with 60-W bulb 0.5 m from the beak& is satisfac
