Titration of Quadrivalent Tellurium with Thiosulfate RALPH A. JOHNSOU
ARD
D O N i L D R . FREDRICKSONi
D e p a r t m e n t of C h e m i s t r y , C'nicersity of Illinois, t rbana, I l l .
For the reductometric deterniination of tellurous acid, on11 reagents unstable i n air-namely, chromous and titanous salts-ha\ e been reported. 1 successful titration with thiosulfate, analogous to the Norris-Fa> titration, has not been reported. The reaction of telluriuni(1V) with thiosulfate is induced b? the addition of iodide to form the iodotellurite complex. On this basis, a feasible titration has been worked out. ilthough direct titration is possible, better results are obtained when an excess of thiosulfate is added and back-titrated with iodine. The method jields accurate and reproducible results, is relati\el> simple, and emploj s stable reagents.
'
T
ELLUROUS acid reacts with thiosulfate t o form monot.elluropentathionate ion, Te(S20a)2--,which is usually called telluropentathionate. According t o Foss ( 2 ) , it is a dithiosulfate complex ion of divalent, electropositive tellurium. The reaction of tellurous acid with thiosulfate is analogous t o that which is the basis of the Norris-Fay (6) method for selenium determination. Unlike the latter reaction, the reaction of thiosulfate with tellurous acid is too slow to permit its use in a titration. This difficulty is overcome by conversion of tellurous acid t o the iodotellurite complex, which reacts rapidly and quantitatively. The reaction presumably proceeds according t o the folloiving equation to forni telluropentathionat,e: Tern--
+ i S 2 0 3 - - = 61- + Te(S?Oa)n-- + Sa06--
Thus, through the agency of iodide ions in inducing the reaction, thiosulfate is made available for the quantitative volumetric determination of quadrivalent tellurium. In comparison to other reducing agents used for the titration of quadrivalent tellurium-namely, chromous (6) and titanous (8) ion-thiosulfate possesses the considerable advantage of stability t o air oxidation. I n the direct titration with thiosulfate, the deep color of the iodotellurite complex fades rather rapidly as the end point is approached. Although it serves t o signal the approach of the end point, the disappearance of the color is not sharp enough for visual end-point detection. T o some extent, the color change is obscured by the formation of elemental tellurium, which takes place more and more rapidly as the end point is approached. The formation of tellurium is probably due t o a hydrolysis reaction followed by dismutation ( 2 ) as folloxs:
+
factory, however, probably because iodine reacts with telluropentathionate ( 2 ) . PREPARATIOR- OF MATERIALS
The tellurium solutions were prepared from potassium hesabromotellurite ( I ) , which was twice recrystallized and dried in a vacuum desiccator. The salt was dissolved in moderately concentrated hydrochloric acid and diluted to volume: concentration of the acid after dilution varied from 0.3 to 1 S,according t o the tellurium concentrations. The concentration of tellurium in the stock solutions was checked by the gravimetric method of Lenher and Homberger ( 4 ) . Standard thiosulfate solution wa9 prepared by dissolving sodium thiosulfate crystals in freshly boiled distilled water. The solution was standardized against a primary standard iodate solution. Iodide was added as a solution of the reagelit grade potassium salt. Standard iodine solutions were prepared from resublimed iodine and reagent grade potassium iodide. TITRATIOS TECHSIQUE ATD A P P R A T Z T S
Titrations tvere carried out in 150-ml. beakcrs. Titrants !?ere added froni 10-in1. or 28-ml. burets. 1Iagnetic stirring was employed. A bright platinum indicator electrode was used in conjunction with a saturated calomel electrode. The salt bridge contained saturated potassium chloride solution. Potentials were read on a Leeds and Yorthrup potentiometer in the usual manner. CONDITIONS OF TITR.ATI0S
Iodide Concentration, Because the complesation of quadrivalent t,ellurium makes possible its rapid reduct,ion with thiosulfate, iodide is an important component in the titration. The minimum concentration of iodide is 0.1 11 for good results. When the concentration of iodide is less than 0.1 M, equilibrium is more slowly attained, and there is a corresponding loss in precision of the titration. Titrations in 0.1 t o 0.3 M concentrations of iodide are satisfactory and shon- no effect due t o variation of the iodide concentration. Iodide concentrations greater than 0.3 J1 should be avoided because of the danger of the side reaction of iodide with oxygen in the air t o form iodine. Somewhat better results are obtained in the back-titration method if the iodide is added after the thiosulfate. Effect of Acid. Hydrogen ions catalyze two side reactions, both of which lead to positive errors in the tellurium values: the decomposition of thiosulfate and the air osidation of iodide. Error due t o too great a,cidity may be minimized by carrying out the titration as rapidly as possible. Solutions with hydrogen ion concentrations up t o 1 S have been so titrated without serious error. I n addition t o inducing side reactions, larger hydrogen ion concentrat,ions decrease the sharpness of the end point. A moderate hydrogen ion concentration is necessary t o induce the reaction between iodotellurite and thiosulfate and t 6 repress the hydrolysis of iodotellurite and telluropentathionate. In 0.3 X hydrogen ion concentrations, equilibrium is attained almost immediately upon addition of iodide. At 0.15 .VIabout 5 minutes
++ +
Te(S20&-2H20 = Te(OH), 2 H + 2 S 2 0 3 ~ 2Te(OH)2 = TeO HBTeOa Ha0
+
As thiosulfate and tellurium(1V) are released in amounts stoichioinetrically equivalent, t o those of the titration reaction, no error is introduced by the side reaction. Kevertheless, the side reaction has an undesirable effect on the rate of attainment of equilibrium and on t,he sharpness of the end point. The titration is feasible if the end point is determined potentiometrically, however ( 3 ) . Better results are obtained if an excess of thiosulfate is added and back-titrated with iodine. KO formation of elemental tellurium was observed even after long standing when an escess of thiosulfate was present. Apparently, the excess thiosulfate stabilizes the dithiosulfate complex against hydrolysis. Equilibrium is rapidly attained in the back-titration, and the end point is sharp and reproducible when determined potentiometrically. Some moderately good end points may be determined with starch indicator in titrations of more concentrated tellurous acid-i.e., 0.1 X. I n general, starch end points are not Fatis1 Present address, American Meat Institute Foundation, University oi Chicago, Chicago 37, Ill.
866
V O L U M E 2 4 , NO. 5, M A Y 1 9 5 2
867
*
should be allowed for the reaction to go to completion before beginning the back-titration; satisfactory results are obtained, however, in the latter case. As the pH of the solution is raised above 1, the reaction becomes so slow as to render the determination impractical. The titration is best carried out in solutions of 0.3 to 0.4 N hydrogen ion concentration. However, with proper attention t o certain details, good results are obtainable in the range 0.1 to 1 S hydrogen ion. Best results are obtained in sulfuric acid. Hydrochloric acid is nearly as good. Perchloric acid is satisfactory, although in some cases the concentration of potassium ion may be sufficient to cause precipitation. Nitric acid yields good results if oxides of nitrogen are removed by addition of urea before addition of iodide.
handle under the stated conditions because of the marked tendency of such solutions t o form insoluble tellurous acid. EFFECTS O F OTHER IONS
Interfering ions are principally in two categories: (1) those which oxidize or reduce the reactants, especially oxidizing agents which react with iodide or thiosulfate, and (2) those which form slightly dissociated compounds with the reactants-e.g., cations which form iodide or thiosulfate complexes. In Table I11 are shown the results of titrations carried out in the presence of various ions. The salts of common strong acids and bases are without effect. The divalent ions which form slightly dissociated compounds with iodide yield low results. Citric acid, a typical hydroxy acid, does not affect the results.
RECOMMENDED PROCEDURE
Adjust a sample containing 1 t o 100 mg. of tellurium in the quadrivalent state t o a volume of 40 to 50 ml. and a hydrogen ion concentration 0.15 N t o 0.5 N . To this add quantitatively an excess of standard thiosulfate solution, then 10 ml. of freshly prepared 1 iV potassium iodide solution. Allow t o stand for 3 to 5 minutes. Back-titrate with standard iodine, determining the end point potentiometrically. Calculation: RIg. of Te = Ar thiosulfate X ml. of thiosulfate X 31.90 (The normality of thiosulfate is equal t o the molarity in this reaction.)
Table I.
Comparison of Results by Two Methods
25 00-ml portions of solutions of potassium hexabromotellurite were ana-
l ~ z e d T h e gra\lmetric method was t h e SOz-hydrazine method of Lenher a n d Homberger ( 4 ) . T h e volumetric method was according t o t h e above recommended procedure Iodine and thiosulfate standard solutions were standardized against reagent grade potassium iodate. n‘orrnality of Te(1V) Method SO2-h) drazine Thiosulfate Individual 0 1140 0 1143 results 0 1140 0 1144 0 1139 0 1138 0 1140 0 1142 XIean normality 0 1140 0 1111 -
~~
-
_
_
~ ~
___
-
RESULTS
Results oi a series of titrations carried out according to the above procedure are compared in Table I with those obtained bjthe gravimetric method of Lenher and Homberger ( 4 ) . In the latter method, elementary tellurium is formed by reduction with sulfurous acid and hydrazine and is weighed in this form. The agreement is within the experimental error. The d a t a i n Table I1 indicate the precision of the method over a range of concentrations. Greater amounts of telluriuni than 100 mg. are difficult to
Table 11. Precision and Range (Titration& carried out according t o recommended procedure) Te(IT) Te(IY) Error, Added, Recovered, P a r t s per Mg. AI& Thousand 1.001 1,005 2.002 2.012 3 ,003 2.997 -2 4.004 4.010 +2 5,005 4.976 -6 10.01 10.08 +7 15 3 3 13 31 -1 15.30 +1 15.33 0 15.33 -2 15.35 0 90 9 91 2 +3 91.2 +3 90 8 -1 90 9 0 !IO 1
L 2
Table 111. Effects of Other Ions Solutions to be analyzed contained 15.35 mg. of tellurium(1V) in 40 ml. of 0.3 N hydrochloric acid. T o each were added 25.00 ml. of 0.02032 N sodium thiosulfate and 10 ml. of 1 Npotassium iodide. Excess thiosulfate was backtitrated with 0.02004 N iodine t o t h e potentiometric end point. T e RecovError, Other Substance ered, hlg. and Concentration Rfg. 0.00 15.35 1 .Ifcitric acid 15.35 0 00 0.5 A4 PiaZSOa +o. 02 15.37 -0 01 15.34 1 .If XaBr 0 02 1.5.33 15.36 +o 01 15 32 -0 03 1 .If SH6Cl 15.35 0 00 15.36 +o 01 15.32 - 0 03 15.32 -0 03 120 reduction -0 13 15 22 15.25 -0 10 - 0 01 15 34 4 0 03 15 38
Table I V ,
Titrations of Selenium and Tellurium Mixtures
Determinations were carried out by addition of 25.00 ml. of 0.02032 S s o d i u m thiosulfate a n d 10 ml. of 1 LVpotassium iodide to a solution of selenium(1V) and ‘or tellurium (IV) in 40 ml. of 0.3 N hydrochloric acid. Excess thiosulfate \vas back-titrated with 0.02006 5 iodine t o the potentiometric end point. Added, Found, Me. hIe. Run 1 Run 2 Av. Se alone 0.1810 0.1806 0.1812 0.1809 Te alone 0.1925 0 1934 0,1910 0,1922 0 3740 Se Te 0.3735 0.3722 0.3731 (sum of above) 8e Te 0.3744 0 3720 0,3732 (titration of
+ +
m i y t urel
XIIXTLRES O F TELLURIUM(1V) AND SELENIUM(1V)
t-nder the conditions of the tellurium titration, selenium is also titrated. When both selenium(1V) and tellurium(1V) are present. the equivalent of thiosulfate required is exactly equal t o the sum of the equivalents of both substances as shown in Table IV. Direct potentiometric titration of selenious acid with thiosulfate may be carried out in the presence of tellurous acid, according to Someya ( 7 ) . In 5 N hydrochloric acid, the end point is sharp enough to yield quantitative results for selenium. -4combination of the Sonieya titration of selenium and the titration of selenium and tellurium as herein described should provide an estimate of the tellurium present,. LITERATURE C I T E D
(1) -krchibald, E. H . , “ P r e p a r a t i o n of P u r e I n o r g a n i c C o m p o u n d s , ” S e w York. J o h n Wiley & Sons, 1932. ( 2 ) Foss. O., S c t a Chem. Seand., 3, 708 (1949). (3) F r e d r i c k s o n , D. R., senior thesis. U n i v e r s i t y of Illinois, 1951. (4) L e n h e r , V., a n d H o m b e r g e r , A. W., J . A m . Chem. Soc.. 30, 387 (1908).
(5) L i n g a n e . J. J., a n d S e i d r a c h , L., Ibid.,70, 1997 (1948). ( 6 ) S o r r i s , J. F., a n d F a y , H., Am. Chem. J . . 18, 705 ( 1 8 9 6 ) : 23, 119 11900). (7) Porneys. K.. Z. nnorg. u.allgem. C h e m . , 187,337 (1930). (8) T o n i i c e k . O., BUZZ. w e . chim., 41, 1389 ( 1 9 2 7 ) . R E T D I X -for F : ~rei-iem December 21, 1951. Accepted 1Iarch 3 , 1952
