Table
V.
Effects of Various Anions on Recovery of Tin
Anion toTin, Tin Mole Recovered, % Anion Rstio Mmole Recovery’ Chloride 4 0.0974 100.3 15 0.0960 98.9 25 0.0943 97.1 25’ 0.0943 96.8 Fluoride 1 0.0963 99.2 4b 0.0969 99.8 10’ Interferes 1oC 0.0972 l%:1 Nitrate 10 0.0966 99.5 20 0.0970 99.9 30 0.0971 100.0 Phosphate 5 Interferes Citrate 5 Interferes ... Tartrate 5 Interferes .. . a 0.09707 m o l e of tin taken. Single determinations. * Longer time allowed for equilibrium. c Aluminum nitrate added to complex fluoride.
tion were 50 ml. of 2M sulfuric acid3M ammonium sulfate. Under these conditions excellent recovery is obtained (Table 111) and addition of a maximum suppressor such as gelatin is unnecessary. Reliability.
Results of nine 1-ml. portions of t h e standard tin solution carried through the cupferron titration compared t o seven iodometric results showed no significant difference between t h e two sets of d a t a a t t h e 95% confidence level. Based on these nine results, t h e coefficient of vari-
ation for a single determination is 0.5%. Effects of Diverse Metal Ions. T h c effects of various cations on the recovery of tin were studied by titrating standard portions of tin containing varying amounts of metal ions (Table IV). At 1 to 1 and 10 to 1 molar ratios of metal to tin, aluminum, cerium(III), cobalt, chromium(II1) , lanthanum, magnesium, nickel, and zinc were without effect. Mercury, lead, and uranium(VI) were without effect in equimolar amounts, but they interfered a t the 10 to 1 ratio. Tungsten (added as tungstate) showed a slight positive bias a t the 1 to 1 level. Elements such as antimony(III), bismuth, copper, iron(III), molybdenum, niobium, titanium, and zirconium which react with cupferron in 2M sulfuric acid interfere. Effects of Common Anions. The effects of nitrate, chloride, and fluoride ions were investigated by adding these anions as the corresponding acid (Table V). The nitrate ion was without effect a t the levels studied. 4 t molar ratios greater than 15 to 1, the chloride ion caused a premature end point. Fluoride ion in equimolar amounts did not interfere. A 10 to 1 ratio was permissible if aluminum was added as a masking agent. Strongly complexing anions such as phosphate, citrate, and tartrate interfered seriously. Preliminary Separation of Tin by Distillation. The possibility of a preliminary separation of tin from interfering cations by selective distillation of tin tetrabromide was investigated. Because of the detrimental effect of
chloride and presumably bromide ions on the titration, this approach was not feasible. A secondary separation by precipitation with ammonia (with aluminum hydroxide carrier) was tried, but it gave randomly low results due to carry-over of the halides. Bromide distillation and subsequent destruction of bromide by oxidation to bromine Rere not studied, because of time considerations and the hazardous nature of bromine. ACKNOWLEDGMENT
The authors express appreciation to Phillip J. Elving, University of Michigan, for valuable consultation at the initiation of this work. LITERATURE CITED
(1)Fritz, J. S.,Richard, M. J., Bystroff, A. S., ANAL.CHEM.29, 577 (1957). (2) Kallman, S., Liu, R., Oberthm, H., Zbid.,30, 485 (1958). (3)Kolthoff, I. M., Johnson, R. A., J . Electrochem. SOC.98, 138 (1951). (4)Ibid., p. 231. (5)Olson, E. C., Elving, P. J., ANAL. CHEM.26, 1747 (1954). (6)Zbid.,27, 1817 (1955). (7)Porter. J. T.,ANAL. CKEM.30, 484 (1958). ’ (8)Smith, G . F. “Cupferron and Xeocu ferron G. krederick Smith ChemicafCo., dolumbus, Ohio, 1938. ~
RECEIVEDfor review J u n e 20, 1958. Accepted August 3, 1959. Division of -4nalytical Chemistry, 133rd Meeting, .4CS, San Francisco, Calif., April 1958. Work performed under Contract No. AT(10-1)-205,. fpr the U d. Atomic Energy Commlssion.
iodometric Method for Determination of Persulfates NAG1 WAHBA, M. F. EL ASMAR, and M. M. EL SADR Biochemistry Department, Faculty of Medicine, Abbassia, Ain-Shams University, Cairo, Egypf
b An accurate, rapid, and simple iodometric method for the determination of persulfates is based on quantitative reduction of persulfates with a large excess of iodide ion in an atmosphere of carbon dioxide. An equivalent amount of iodine is liberated and titrated with a standard solution of sodium thiosulfate. Traces of ferric salts in the acetic acid (0.003 to 0.006%) accelerate iodine liberation. The method shortens reaction time to 15 minutes; the lndelli and Prue method needs one day.
M
reported in the literature for determination of persulfates are based on oxidation-reduction ti-
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ETHODS
-
ANALYTICAL CHEMISTRY
trations. Alkali persulfates can be evaluated by adding to their solutions a known excess of an acidified ferrous salt solution (5) and determining the excess of ferrous iron by titration with standard potassium permanganate solution. Another procedure utilizes standard oxalic acid solution ( 5 ) . A sulfuric acid solution of a persulfate is treated with an excess of standard oxalic acid solution in the presence of a little silver sulfate as catalyst. The excess oxalic acid is titrated with standard potassium permmganate solution. Both methods need many freshly prcpared standard solutions and require heating. Consequently, a simple method for persulfate determination is required. The most satisfactory and
more direct method appears to be iodometric. The persulfates slowly hberate iodine from solutions of potassium iodide ( 2 ) . The liberation of iodine may be made suitablc for quantitative R orh onl) under certain conditions. In the present method, conditions are established so that the iodine liberated 1s quantitatively equivalent to the amount of persulfate added. The liberated iodine is titrated with a standard solution of sodium thiosulfate. This is the basis of a simple and accurate iodometric method for the determination of prrsulfates. Traces of fernc salts in the acetic acid and potassium iodide accelerate iodine liberation from the reaction of persul
fates with iodide ion, thus permitting an improvement of the Indelli and Prue method ( I ) . The new procedure emIlloys 0.003 i 0.006% ferric chloride hcsahydrate in glacial acetic acid or 1 . O N sulfuric acid as solvent. Reaction time is 15 minutes. The method is more direct than methods currently in use and gives accurate results. REAGENTS
Acetic acid, Merck analytical reagent grade, redistilled in all-glass fractionating column. The fraction boiling at 117’ C. is used. Ferric chloride hexahydrate (0.1% solution, 0.25 gram), dissolved in 250 ml. of acetic acid. Specified concentrations of ferric chloride in acetic acid are prepared by pipetting appropriate volumes of the 0.1% stock solution into volumetric flasks containing acetic acid. Sodium thiosulfate, standard solution, 0.l.V. Sulfuric acid. standard solution, 1.Oh’. Potassium iodide solution (iodatefrer) 167i. Starch solution 1%, freshly prepared or properly preserved as follows: Make a paste of 1 gram of soluble starch with a little water, pour, with constant stirring, into 100 ml. of boiling water, and boil for 1 minute. Allow the solution to cool, and add 3 grams of potassium iodide. This solution may be preserved for a long period under a h y e r of toluene in a stoppered bottle. The addition of 0.1 giam of thymol to 100 ml. of boiling water before mixing 1 grani of starch produces a starch solution that keeps for several months. PROCEDURE
Weigh nccuratel;. 0.2 to 0.3 ?;ram of potassium persulfate into a 250-nil. conical flask with accurate ground-glass stopper, dissolve in 20 ml. of distilled water, and add glacial acetic acid (15 ml.) containing the desired percentage of ferric chloride (0.003 to 0.00670), followed by 10 ml. of 16% potassium iodide solution. Ilisplace the air by carbon dioxide, close the flask with t’heglass stopper, m d mix the contents thoroughly. Leave in the dark for 15 minutes a.nd titrate the liberated iodine with a standardized solution of 0.1.1sodium thiosulfate; when the liquid has bccome a pale yellow, add 1 ml. of frrshly prcparecl starch solution and continue the titration until the blue color just disappears. From the amount of 0.1N sodium thiosulfate required to titrate the iodine librrated, calculate the percentage purity of the sample. Ten milliliters of A sulfuric acid can be used instead of glacial acetic acid. 1 ml. of 0.1N Na&Os = 0.01352 gram of Ka2Oa.
the 0.1% stock solution. Samples of potassium persulfate are estimated. I n the !%bsence of iron, reaction occurs quantitatively in 1 hour, but in 15 minutes with 0.003 and 0.006% ferric chloride hexahydrate. The optimum concentration of ferric chloride hexahydrate is 0.003 and 0.00670. Above and below these concentrations, secondary reactions occur and iodine reappears after a certain time. Uniformity of Results. The percentage purity of several samples \ as determined 100 times. The difference in results was negligible ( =t0.2yo) and due t o experimental error. Comparison of Results by Three Methods. As procedures involving ferrous sulfate and oxalic acid are commonly employed in analytical work, the percentage purity of potassium persulfate in various samples was determined by these methods and by the proposed method (Table I). All three methods agree within the experimental error, although the iodometric method agrees more closely with the oxalic acid method. Determination of Optimum Potassium Iodide Concentration. A large excess of iodide ion must be present to liberate iodine quantitatively. Solutions of 4, 8, and 16% potassium iodide are prepared and 10 ml. of each used. The optimum concentration was found to be 10 ml. of 16% potassium iodide. With 4% reaction occurred to the extent of only 7Oy0 and with 8% only SO%, and iodine reappears after a certain time. Stability of Potassium Persulfate Solution. Potassium persulfate is not stable for a long time in solution. I t easily decomposes to potassium sulfate, especially on heating. Storage in a dark brown bottle has no effect on stability. DISCUSSION OF RESULTS
If a strong oxidizing agent is treated in neutral or more usually acid solution
RESULTS
with a large excess of iodide ion, the iodide reacts as a reducing agent and the oxidant is quantitatively reduced ( 5 ) . I n such cases, an equivalent amount of iodine is liberated and is then titrated with a standard solution of a reducing agent, usually sodium thiosulfate. The complete reduction of persulfates with the iodide ion is difficult and the reaction may be made suitable for quantitative work only under certain conditions. For accurate results the following experimental conditions must be observed:
Determination of Optimum Iron Concentration. Solutions of 0.001. 0.0015, 0.003, 0.006, 0.009, 0.012, anti 0 . 0 1 8 ~ 0ferric chloride hexahydrate ir, glacial acetic acid are prepared from
Increase of the iodide ion concentration. Trace of iron salts in the acetic acid. Replacing the air in the titration vessel by carbon dioxide, Leaving for 15 minutes in the dark.
Table I.
Ferrous Sulfate Method
Percentage Purity of Persulfates Oxalic IodoAcid metric Method Method
98.82 95.48 97.22 99.92(A.R.) 99.96(A.R.) 96.58
99.42 95.82 97.46 99.94 99.98 96.62
99.65 95.62 97.44 99.94 99.97 96.60
If these conditions are not strictly followed, iodine liberated is less than the theoretical amount and lower results are obtained. Sulfate can be easily detected by ordinary tests after the titration. The reaction is believed to be as follows : K&Oa
+ 2KI = 2K2S04 + 1%
Potassium persulfate solution is unstable. It is easily converted to potassium sulfate, especially on heating. Indelli and Prue (I) added a trace of ferrous sulfate, leaving the solution for a day and titrating the liberated iodine against sodium thiosulfate. It is reported (3) that persulfates are powerful oxidizing agents, oxidizing ferrous to ferric salts 2Fef2
+ &08-z = 2Fefa + 2S04-2
Silbert and Swern (4) found t,hat trurrs of ferric salts in the acetic wid and sodium iodide accelerate iodine libcrat,ion from the reaction of pcwattxrs with iodide ion and shorten reaction times as compared to other nwthods. The present authors found that traces of ferric salts reduce rraction times to 15 minutes as comparrd to 1 hour in the absence of iron salts and 1 day if a trace of fcrrous sulfat,e is addcd. I n the Indelli and Prue method, pcrsulfatcs first oxidize ferrous to ferric salts, which then accclrrate iodine libvration from the reaction of persulfatcs with iodide ion. I t is simpler and more convenient to add traccs of ferric chloride directly. LITERATURE CITED
( 1 ) Indelli, A., Prue, J. E., J . 1959, 107.
C h . SOC.
( 2 ) Parkes, G. D., Mellor, J. W., "Melior's
Modern Inorganic Chemistry,” rev. ed.,
p. 473, Longmans, Green, London, 1947.
(3) Partington, J. R.,,,“TextBook of Inorganic Chemistry, 6th ed., p. 486, Macmillan. London, 1953. (4) Silbert, ’L. S., Swern, D., ANAL. CHEM.30, 385 (1958). (5) Vogel, A. I., “Text Book of Quantita-
tive Inorganic Analysis,” 2nd ed., pp. 286, 357, Longmans, Green, London, 1953.
RECEIVEDfor review March 31, 1959. Accepted August 3, 1959. VOL. 31, NO. 1 1 , NOVEMBER 1959
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