Gravimetric Determination of Thallium with Tetraphenylarsonium Chloride WM. T. SMITH, JR., University of Tennessee, Knoxville, Tenn.
A satisfactory gravimetric determination of thallium may be accomplished by precipitation with tetraphenylarsonium chloride. The thallium must be in the trivalent state and in the presence of an excess of hydrochloric acid when the precipitant is added. Hydrogen peroxide serves well to oxidize the thallous ion. The precipitate is washed with hydrochloric acid solution, as distilled water causes some hydrolysis, and is dried to constant weight in an oven at 110' C.
T
H E methods for determining thallium were reviewed recently by Chr6tien and Longi (I), who concluded that of fifteen gravimetric methods only the determination as thallous chromate (6) is worth retaining. The solubility of thallous chromate in water is so great that the precipitation and washing must be carried out under very strictly standardized conditions if reproducible results are to be obtained. A method for determining rhenium depends on the formation of an insoluble perrhenate according to the following equation: (C6H5)&+
+ ReOd-
--+
(C&15)rAsRe04
THALLIUM SAMPLES C.P. thallous sulfate was dissolved, filtered, and recrystallized twice from water. Thallous perrhenate was prepared by mixing equivalent amounts of thallous sulfate and potassium perrhenate in solution. The relatively insoluble thallous perrhenate was successively recrystallized from water until a reproducible melting point waa obtained for the salt. All samples analyzed were dry weight samples of the above two salts.
PROCEDURE
In an examination of the method ( 7 ) Willard and Smith found that several ions may interfere, especially if the concentrations are appreciable: permanganate, periodate, perchlorate, thiocyanate, nitrate, iodide, bromide, fluoride, and the complex halide anions which can be formed from some cations. Cations that form insoluble chlorides also interfere. When trivalent thallium is present in solution with an ewess of hydrochloric acid, a white precipitate is obtained on adding the tetraphenyhrsonium ion. The weight of precipitate is in excellent agreement urith the formation of an insoluble trtraphenylarsonium chlorothallate: ( C B H ~ ) ~ - 4~ S TlClr+
solution ( 7 ) ; 0.1 N potassium permanganate; and 30% hydrogen peroxide.
+ (CGH~)~AST~C~~
The filtrate obtained gives a negative test for thallium with ammonium sulfide ( 6 ) and with 10% potassium iodide (8). Although the chlorothallate ion is apparently present in solid chlorothallic acid (4), no other compounds containing this ion have thus far been isolated. The interfering ions are those mentioned above and in addition the perrhenate ion.
The thallous compounds were oxidized with 2 ml. of hydrogen peroxide in the presence of sodium hydroxide solution, the mixture was then acidified with hydrochloric acid, and an excess of the acid was added. At this point a white precipitate, apparently a thallous-thallic complex ( 4 ) , forms, but redissolves after the addition of 1 ml. of hydrogen peroxide to the acid solution. Some of the samples were oxidized with standard permanganate sohtion in the presence of hydrochloric acid (3),in order to obtain a check on the thallium content. Care was taken to leave no excess of permanganate in the solution, as the permanganate ion interferes. The solutions were made from 0.5 to 2 X with respect to hydrochloric acid and then the solution containing an excess of tetraphenylarsonium chloride was added. The precipitation mixtures a t this point varied in volume from 25 to 75 ml. The mixtures obtained were heated to boiling to coagulate the white precipitates and then allowed to stand overnight before filtering on sintered-glass funnels. These precipitates were dried in an oven a t 110a C.
The samples that were washed with distilled water all weighed from 0.8 to 2.6 mg. less than the theoretical. The average deviation was approximately 1% low for these samples. Examination of these determinations indicated that the low values were probably not due to impurities in the standard samples, inasmuch as REAGEYTS the deviations were independent of sample size. The solubility of the precipitate or the concentration of the acid did not account 0.1 iV sodium hydroxide; concentrated hydrochloric acid; solution of 6.7 grams of tetraphenylarsonium chloride in 100 ml. of for the difference, a$ the volumes and the hydrochloric acid concentrations were each varied by about three times without resulting- in a consistent deviation of the weight Table I. Gravimetric Determination of Thallium of precipitate. One of these precipitates, Volume of Ppt. when rewashed with 100 ml. of distilled Std. Calcd. Ppt. Found water, lost several milligrams in weight, Sample Oxidizing HC1 (CaHa)&L!l, Wt. of Found, Calcd.,Sample Wt., Mg. Agent Concn., N M1. Ppt., Mg. Mg. Mg. Wash and a faint brownish color, very much T1tS0. 25.8 KMnO4 1.0 5 74.5 71.9 -2.6 HzO like a trace of thallic hydroxide, stained 99.6 HiOi 2.0 10 287.8 285.7 -2.1 Hz0 45.4 KhlnOd 1.3 5 131.2 129.4 -1.8 HzO the white precipitate. This indicate? 47.9 HtOr 1.o 5 138.4 137.2 -1.2 H10 that the low values were due to hydroly115.4 XMnO, 1.0 10 333.5 332.1 -1.4 HzO 59.6 KMnO4 2.0 5 172.2 173.4 -0.8 HpO sis of the thallium compound on vash33.1 HiOi 1.5 10 95.7 96.1 +0.4 1 N HC1 99.7 Hior 1 .o 10 288.2 289.4 +1.2 1NHC1 ing. Subsequently all precipitates were 57 0 .. 5 9 Hi01 0.5 10 &$:: ;$ washed with approximately 1 N hydro8 HzO1 0.7 10 TlReO4 409.7 KMnO4 0.7 25 1228.3 1220.8 -7.5 Hi0 chloric acid; no loss of weight on 58.2 HaOr 2.0 10 174.5 174.4 -0.1 1 N HC1 194.7 Hs01 0.5 10 583.7 583.8 +0.1 1 NHC1 successive washing mas observed. The volume of wash liquid employed varied
;$:::
+E;;
937
ANALYTICAL CHEMISTRY
938 from 20 to 40 ml. The average deviation from the theoretical weight was 0.1% for precipitates that were washed with hydrochloric acid solution. The factor 2.890 converts weight of thallous sulfate to tetraphenylarsonium chlorothallate. The weight of thallous perrhenate is converted to the equimolecular mixture of tetraphenylarsonium perrhenate and tetraphenylarsonium chlorothallate by the factor 2.998. Thallium was not separated from rhenium but vias simultaneously precipitated with tetraphenylarsonium chloride. SUMMARY
Thallium may be quantitatively determined by precipitation from hydrochloric acid solutions containing the element in the trivalent state. The oxidation of the thallous ion may be accomplished with hydrogen peroxide or any other effective oxidizing agent which introduces no interfering substance. The
precipitating reagent is a water solution of tetraphenylarsonium chloride. The amount of excess of this reagent is not critical. The precipitate obtained should be washed with hydrochloric acid solution to prevent hydrolysis, and dried in an oven a t 110” c. LITERATURE CITED
(1) ChrBtien and Longi, Bull. SOC. chim., 11, 241 (1944). (2) Feigl, “Qualitative Analysis by Spot Tests,” 2nd ed., p. 93, New York, Kordemann Publishing Co., 1939. (3) Hawley, J . Am. Chem. SOC.,29, 300 (1907). (4) Meyer, Z . anorg. Chem.. 24, 321 (1900). (5) hloser and Brukl, Monatsh, 47, 709 (1926). (6) Scott, “Standard Met,hods of Chemical Analysis,” 5th ed., p. 942, N e w York, D. Van Nostrand Co., 1939. (7) Willard and Smith, IND. ENG.CHEM.,ANAL.ED., 11, 305 (1939). RECEIVED February 5 , 1948. Contribution 52 from the Chemistry Department, University of Tennessee, Knoxville.
Determination of Alkyl Sulfides and Disulfides SIDNEY SIGGIA AND R . L. EDSBERG, General Aniline & F i l m Corporation, Easton, Pa. A procedure for determining alkyl disulfides, which was found to work equally w-ell for alkyl sulfides, involves oxidation with bromine. Alkyl sulfides and disulfides can be determined in the presence of each other by the application of a second method. Samples containing less than 10 mole % thiol can also be determined. The procedure is precise to about *0.3% in the best cases and goes as low as *l.Oqc in samples where the end point is rather slow.
K
OLTHOFF et al. (4) devised a method for determining alkyl disulfides which involves reduction to the corresponding thiols (mercaptans) and amperometric titration of the thiols u ith silver nitrate. This method has the disadvantage of being reproducible to only about 2% in the optimum range of sample size and of yielding results that are about 5% low. A procedure yielding a higher precision and accuracy was sought, and the following oxidation method was devised. The method described involves oxidation of the disulfide with bromine to the formation of the corresponding sulfonyl bromide. RSSR
+ 5Brz + 4H20 + 2RS02Br + 8HBr
(1)
The oxidation is brought about by adding standard bromatebromide solution to an acid solution of the disulfide. As soon as the bromate-bromide strikes the acid solution, the bromine is liberated and is consumed by the disulfide. The end point is taken as the appearance of the first permanent bromine coloration. The usual method of adding excess bromate-bromide and determining the excess reagent iodometrically cannot be used in determining disulfides because of substitution reactions which consume bromine and cause high results. The substitution reactions do not interfere as long as there is no excess bromine present. This same procedure was found to be applicable to the determination of alkyl sulfides. The reaction is the oxidation of the sulfide to the sulfoxide.
RzS
+ Brz +R2S.Brz-+
Hz0
RzSO
+ 2 HBr
(2)
I n this case, as with the disulfides, using an excess bromatebromide and determining the excess cause high results. In the case of the sulfides, the trouble is not only substitution but further oxidation of the sulfoxide to the sulfone. Neither of these reactions interferes as long as excess bromate-bromide is avoided, A similar procedure was used by Sampey, Slagle, and Reid (5), who titrated the sample, in benzene solution, with a solution of
bromine in water. The main disadvantage of this method lies in the fact that bromine water is not a very stable standard solution. Bromine vapors are continually lost from the solution. Mixtures of alkyl disulfides and sulfides can be determined by first applying the bromine oxidation, which yields both the disulfide and the sulfide. Then, by using the method of Kolthoff el al. (4) the disulfide alone can be determined; the sulfide is then obtained by difference. The extent of the application of this method is limited by the relatively low precision and accuracy of the reduction method, and by the fact that the final result depends on a subtraction and that the disulfide oxidation involves 5 moles of bromine while the sulfide oxidation involves 1 mole. The best results are obtained on samples low in disulfide and high in sulfide; here, the error in the disulfide result does not figure too prominently in the final results. Thiols are also oxidized by bromine and act as interferences if they are not determined separately and the results corrected for their presence. 2RSH -+ Br2 RSSR 5BrZ +2RS02Br
HzO This procedure can be used to determine thiols. The precision is high (*0.5%), but the accuracy is low. When pure ethanethiol was run by this method, the results were very precise but were 3% low. 1-Pentanethiol results were 8’3 low. However, if the sulfide or disulfide contains thiol equivalent to 10% or less of the total titration, the absolute error in the first result is not significant. For instance, if the sample contains ethanethiol equivalent t o 10% of the total titration, then the error in the .total analysis is only 0.3Ye. In the case of 1-pentanethiol, the error would be 0.87e. The error introduced by the thiol increases as the amount of thiol in the sample increases. Samples of disulfide or sulfide containing thiols are determined by the bromination procedure described below, which yields both the thiol and the disulfide or sulfide; the thiol alone can be
