V O L U M E 19, NO. 5, M A Y 1 9 4 7 lated values for oxygen are subject to still greater accumulation of errors, and show a still greater deviation from theory than does the determined value. The apparatus used for the direct determination of oxygen is similar in many respects to that used for the determination of carbon and hydrogen. However, the latter requires more manipulations, maintenance, and specialized technique. The titrimetric determination feature of the oxygen determination permits the simultaneous operation of multiple units, enabling one person to complete a greater number of analyses per day. Although the average time required t o complete duplicate determinations with the present apparatus is about 90 t o 100 minutes, it is believed that this time can be reduced t o about 60 t o 70 minutes. It appears that this method should be widely acceptable as a rapid and satisfactory method for the direct determination of oxygen in organic compounds.
351 ACKNOWLEDGMENT
The authors are indebted t o Ernest Turk, of this laboratory, for helpful suggestions during the progress of the work. LITERATURE CITED
(1) Brown, E. H., and Felger, M. M., IND.ENG.CHEM.,ANAL.ED., 17, 280 (1945). (2) Elving, P. J.,and Ligett, W. B., Chem. Revs., 34, 129-56 (1944). (3) Korshun, 31. O., Zavodskaya Lab. (U.S.S.R.), 10, 241-5 (1941). (4) Mellor, J. W., "Comprehensive Treatise on Inorganic and Theoretiral Chemistry", T'ol. 9, p. 754, London, Longmans, Green and Co., 1929. ( 5 ) Schutse, M., Z . anal. Chem., 118, 241-5 (1939). ( 6 ) Ibid., 118, 245-58 (1939). (7) Untersaucher, J., Ber., 73B,391-404 (1940). (8) Zimmermann, W., 2. a n d . Chem., 118, 258-63 (1939). before t h e Division of Analytical a n d Micro Chemistry a t t h e CHEMICAL SOCIETY,Chicago, Ill. 110th meeting 0 ) t h e AMERICAN
PREaExTEn
Test for Selenium Based on a Catalytic Effect FRITZ FEIGL AND PHILIP W. WEST' .Ministerio da .Igricultura, Laboratorio da ProduFzo .Mineral, Rio de Janeiro, Brazil The reducing power of alkali sulfides can be considerably enhanced by the addition of elemental selenium. This effect, a catalytic in nature, has been adopted as a sensitive and selective test for the detection of selenium, as well as for the detection of selenites after reduction to elemental selenium.
T
HE fact that certain redox reactions involving alkali sulfides are greatly accelerated by the presence of small amounts of elemental selenium seems hitherto to have been unknown. However, with certain reactions under selected conditions it is pobsible t o recognize this effect and to make use of it in chemical analysis. EFFECT O F SELENIUM ON REDOX REACTIONS O F ALKALI SULFIDES
The eKect of selenium on certain redqx reactions of alkali sulfides can be readily demonstrated, provided the reaction with sulfide alone proceeds slon-ly with the formation of distinctive reactiori products. Under such conditions it is necessary only to compare the reaction velocities of selenium-free and seleniumcontaining sulfide solutions under like conditions. Reduction of Chromates. Place 2 ml. of 4% potassium chromate solution in each of two test tubes. In one tube place 1 ml. of 0.1 &I sodium sulfide and in the other tube 1 ml. of 0.1 M sodium sulfide containing 0.01 mg. of selenium. Place both tubes in boiling water. I n the mixture containing selenium reduction will take place within 1 or 2 minutes, as evidenced by the formation of a precipitate of green hydrated chromium oxide. The mixture not containing selenium will remain clear, and only after additional heating or prolonged standing will any visible evidence of reduction be apparent. This demonstration can also be accomplished a t room temperature where the difference in reaction velocities becomes even more striking. Reduction of Picric Acid. Place 2 ml. of a 1%solution of picric acid in each of two test tubes, and sodium sulfide and sodium sulfide containing selenium as described above. Upon heating, the mixture containing selenium becomes red (formation of picraminic acid) much more quickly than does the selenium-free mixture. Reduction of Dichlorophenol Indophenol. Place 1 ml. of 0.1 AI sodium sulfide in a test tube, and in a second test tube place 1 ml. of 0.1 M sodium sulfide which contains 0.01 mg. of selenium. To each add 5 drops of 0.5% dichlorophenol indophenol solution. I n the tube containing selenium the blue dichlorophenol indophenol will be immediately decolorized (formation of the leuco compound), while in the other tube the color will persist for some minutes. 1 On sabbatical leave from t h e Coates Chemical Laboratories of Louisiana State Cniversity, Baton Rouge, La.
Reduction of Cacotheline. Place 2 ml. of a saturated aqueous solution of cacotheline in each of two test tubes. Add sodium sulfide to one tube and sodium sulfide containing selenium t o the other tube and heat as described under the discussion of the chromate reaction. A color change from yellow t o violet (formation of a reduction product of nitrobrucine) occurs more rapidly in the selenium-containing solution. Reduction of Methylene Blue. Place 1 ml. of 0.1 M sodium sulfide in a test tube and in a second test tube 1 ml. of 0.1 M sodium sulfide containing 0.01 mg. of selenium. To each then add 2 drops of 0 . 0 5 5 methylene blue solution. The sulfide solution containing selenium immediately decolorizes methylene blue (formation of leuco methylene blue), whereas the selenium-free sulfide solution needs a t least 15 to 20 minutes to accomplish the same effect. These esperiments show that selenium can activate redox reactions of sodium sulfide in aqueous solution with compounds of lvidel>-different character. I n like manner other alkali sulfides can be activated. It is not impossible that this effect can be used in organic chemical preparations where alkali sulfides are employed as reduct,ors in alkaline media. THEORETICAL CON SIDERATION S
There is no doubt that in all the reactions mentioned above, sulfide ions act as reductors and selenium accelerates the reaction velocities. The general nature of the "selenium effect" can be elucidated by consideration of the mechanism of the methylene blue reduction. The decoloration of methylene blue by excess alkali sulfides can be formulated by the following equation: 2 11B
+ S-- + 2 H?O
----)
2 HMB
+ 2 OH- + So
(1)
where 1IB represents methylene blue and HhIB represents the leuco form of methylene blue. As an excess of sulfide ion is present, the free sulfur formed (Equation 1) is dissolved by formation of polysulfide:
so + s-- + is. . SO]-*
(2)
I n the case where selenium-containing sulfide solutions react \vit,h methylene blue, it is necessary t o consider the st'ate of the
ANALYTICAL CHEMISTRY
352 selenium. Selenium is dissolved by alkali sulfides to form yellow to brown solutions of selenosulfides: XazS. . . Seaor [S. . Sea]--. Therefore, the reaction of such solutions with methylene blue can be formulated according to the equation:
.
2 AIB
+ [S . . . Sea]-- + 2 H20 +2 HMB + 2 OH- 4So
+ Sea
(3)
The elemental sulfur and selenium formed are redissolved by the excess of sodium sulfide with the formation of polysulfides or selenosulfides :
s-S--
and
+ so + 1s. . . SO] -+ Sea [S . . . SeO] --
(4)
(5)
--f
occurs. This was confirmed by the fact that pure selenosulfate, which did not reduce methylene blue, after addition of sodium sulfide possessed the typical reactivity of selenium-activated sulfide solution. This difference in behavior of sulfite-decolorized solutions of polysulfides and selenosulfides is of fundamental importance in the detection of small amounts of selenium in sulfur. To identify selenites by means of the catalytic reduction of methylene blue, it is first necessary to reduce the selenium to the elemental state. In general, this reduction is accomplished in acid solution by use of various reducing agents-for example, with hydrogen sulfide SeOa--
+ 2 S-- + 6 H + +3 HzO + Sea + 2 So
The analogous reaction occurs when selenite solutions are Comparison of Equations 1 and 2 with Equations 3, 4, and 5 referheated in the presence of an excess of sodium sulfide. (30 shows that the difference consists in the fact that in seleniumences were found in standard treatises on analytical chemistry containing solutions the ion [ S . . . SeO]-- reacts ‘(Equation 3) indicating that this reduction of selenites in alkaline media ocand after reaction is regenerated (Equation 5 ) . It is evident that curs.) In this case a yellow or brown solution is obtained which the ion [S . . Sea]-- reacts more rapidly than the simple S-indicates that the reaction ion. Therefore, in spite of the preponderance of S-- ions in the system it is the [S Sea] ion which reacts preferentially. AcSe03-2 S-3 HzO +Seo 2 So 6 OHcording to this, the activating effect of selenium is catalytic in nature. The selenium is a catalyst because it acts in and is occurs and is followed by dissolution of Seo and So in the excess always regenerated to the form of the ion [S. . . Sea]--. of sodium sulfide with the formation of the corresponding selenoKot to be excluded is the possibility that the complex ion sulfides and polysulfides. Therefore by this direct procedure the [S, . . Sea]-- is in equilibrium with its isomeric form [Se . , . SO]-selenium is transformed to that form which is active catalytically. according to [S.. . Sea]--= [ S e . . . Sol--. The polysulfide formed in this reaction can be destroyed by Even when the equilibrium is shifted strongly towards the treatment with sodium sulfite as described above. left side, a more rapid reduction is possible if the Se-- ion reacts quicker than the S-- ion. The designation “increase of the reDETERMINATION OF SENSITIVlTY activity by complex formation” also is justified, for the primary The sensitivity-Le., the concentration and identification limformation of the complex ion [S . . . SeO]-- is undoubtedly necits-was determined by comparing the decolorization times of essary. methylene blue when added to solutions of 0.2 M sodium sulfide I n many cases of catalyzed reactions in homogeneous systems containing known quantities of selenium and 0.2 M sodium sulthe catalyst acts by formation of unstable, reactive, intermediate fide solutions which were selenium-free. compounds with one of the reactants. Often unstable complex compounds play the role of intermediates. I n the present case, Spot Plate Procedure. I n each of tivo adjacent depressions of Se-act relatively stable selenosulfide complexes-Le., Ka2S a white spot plate was placed 1 drop of the respective solutions as the intermediat.e compounds. The authors believe that this to be compared, and a drop of 0.01% methylene blue solution was added. The decolorization times were distinctly different when catalytic effect of selenium is an unusually clear demonstration the selenium-containing drop corresponded to that complex compounds can play an important role in the mechanism of catalysis. Limit of identification = 0.08 microgram of selenium Drop volume = o.08 ml. Limit of concentration = 1 to 1,000,000
.
...
a
+
+
+
+
. ..
APPLICATIONS
The catalytic effect of selenium on the system methylene bluealkali sulfide is so distinctive that it is readily applicable for tests to detect elemental selenium, even when present as traces. Because alkali selenites are readily reduced to elemental selenium, a delicate test for selenious acid and selenites is also possible. The following facts were necessarily taken into consideration in developing the analytical procedures. The catalytic effect of selenium is due to the coordination of elemental selenium xvith sulfide ion. Therefore, coordination of elemental sulfur with sulfide ion could be expected to produce the same effect. This was found to be the case, but the effect is not of magnitude comparable with that produced by equivalent amounts of selenium. Furthermore, any deleterious effect due to the presence of great amounts of polysulfides is readily obviated by addition of sodium sulfite and heating, whereby the yellow solution is decolorized and thiosulfates are formed which have no interfering actions. Tellox or brown selenosulfide solutions are also decolorized by sodium sulfite when heated, indicating the formation of selenosulfates. I n contrast to thiosulfate solutions, selenosulfate solutions obtained in this way act in the same manner as the original selenosulfide. As it was found that selenosulfates prepared directly by dissolving selenium in sodium sulfite do not reduce methylene blue, it was apparent that the reversible reaction
[S
.. . Sea]-- + SO,--
e S--
+ SSeO,--
Test Tube Procedure. In two test tubes were placed, respectively, 2 ml. of the solutions to be compared. To both tubes were added 2 drops of 0.05% methylene blue solution and the tubes were shaken briefly to ensure mixing. The following results were obtained : Limit of identification = 0.3 microgram of selenium Limit of concentration = 1to 6,500,000 In general, the test is performed by using a mixture of 0.2 iM sodium sulfide containing 5% sodium sulfite, so as to eliminate any deleterious effects due to large amounts of polysulfides (compare with preceding remarks), The sensitivity of the test performed with sulfite-containing solutions was found to be the same as that obtained x i t h sulfite-free solutions. DETECTION OF SELENIUM IN PRESENCE OF SULFUR
To detect free selenium in the presence of a large amount of free sulfur it is necessary to heat the sample in the presence of an excess of sodium sulfide, thus forming polysulfide and selenosulfide. After decolorization of the solution so obtained by treatment with sodium sulfite, the conditions are suitable for applying the new test. To obtain an orientation of the limiting proportions of selenium to sulfur a t which selenium still could be detected, the following experiments were made:
A solution of 0.2 M sodium sulfide was saturated with sulfur by heating it in the presence of excess sulfur and filtering. The amount of sulfur dissolved was approximately 12 grams per liter. To 2 ml. of this polysulfide solution 0.05 ml. of a standard selen-
V O L U M E 19, NO. 5, M A Y 1 9 4 7 ium solution in 0.2 M sodium sulfide was added (corresponding to 0.5 microgram of selenium). This mixture was then decolorized by adding solid sodium sulfite and heating. After the solution mas cooled, methylene blue was added and comparison was made with a blank of 2 ml. of 0.2 M sodium sulfide containing the same quantity of sodium sulfite. Whereas the blank test retained a distinct blue color for a t least 20 minutes, t h e solution containing selenium J m s decolorized within 1 minute. This experiment shows that selenium can be detected easily a t a ratio of 1 part of selenium to 48,000 parts of sulfur. Calculated to percentages on a dry weight basis, this corresponds to 0.002% selenium in sulfur. DETECTION OF SELENIUiM DIOXIDE
Solutions of alkali selenites can be reduced by heating them in the presence of sodium sulfide, whereby polysulfide and selenosulfide are formed. After addition of sodium sulfite and heating, the solution is in the proper condition for testing by means of the catalytic reaction. By such a procedure,using 1ml. of the solution to be tested and 1 ml. of 0.4 M sodium sulfide containing 5% sodium sulfite, the following values were obtained: Limit of identification = 0.4 microgram of SeOs Limit of concentration = 1 to 2,000,000
353 DISCUSSION
The catalytic action exerted by selenium on the reduction of methylene blue by sodium sulfide in the presence of sodium sulfite is specific. Other strong reducing agents-Le., sodium hypophosphite, sodium phosphite, and sodium n i t r i t e 4 0 not interfere. I n the presence of thiocyanates the normal color of the methylene blue is changed to a lavender color, owing to the formation of the thiocyanate of methylene blue, but this does not prevent observation of the fading effect caused by the presence of selenium. Cyanides interfere because under the conditions of the test selenocyanides are formed instead of selenosulfides. Tellurium does not interfere. This is to be expected, because tellurium, in contrast to sulfur and selenium, does not dissolve in alkali sulfides to form complex compounds. ACKNOWLEDGMENTS
The junior author wishes to thank Louisiana State University for granting him a sabbatical leave to make this research possible. He also wishes to express his appreciation to the Research Council of that university for providing additional funds in support of these studies. Both authors wish to thank Mario da Silva Pinto, director of the Laboratorio da ProducBo Mineral, for his interest and support.
Colorimetric Determination of Antimony in Copper-Base Alloys ALBERT C. HOLLER', United States Metal Products Company, Erie, Pa. A rapid and accurate colorimetric method for the determination of antimony in copper-base alloys is described. The sample is dissolved in nitric acid and the antimony and tin hydrous oxides are filtered off and dissolved in sulfuric and hydrochloric acids. Antimony is determined by colorimetric iodide method.
I
T IS desirable to have a rapid and accurate colorimetric method for the control analysis of antimony in copper-base alloys whose chemical compositions are within the limits as given in Table I (2). After a thorough search of the literature, it was decided that the method best suited for these alloys would be based upon the fact that in an acid solution antimony reacts with the iodide ion to produce a yellow complex, the iodoantimonite ion (SbIt-) ,which is capable of colorimetric determination (4,5,6). APPARATUS AND REAGENTS
Apparatus. The transmittancy measurements were made with a Coleman Model 11, Universal spectrophotometer. The spectral band width was 35 mp. hbsorption cells with a 2.05cm. solution thickness were used. The calibration of the instrument was checked by using a Coleman mounted disk of didymium glass. Reagents. Standard Antimony Solution. Dissolve 0.1250 gram of C . P . antimony metal in 25 ml. of hot sulfuric acid (specific gravity 1.84),cool, and dilute to 250 ml. with distilled mater. All work done was based on a 5-gram sample; therefore 1 ml. of the standard solution would equal 0.01 % antimony. Potassium Iodide-Sodium Hypophosphite Reagent. Dissolve 100 grams of C.P. potassium iodide, and 20 grams of S.F sodium hypophosphite in 100 ml. of distilled water and filter before use. Sodium Thiosulfate Solution. Dissolve 5 grams of c P. sodium thiosulfate in 100 ml. of distilled water. Starch Solution. Dissolve 0.5 gram of "soluble" starch in 100 inl. of boiling distilled water. 1
Present address, 4733 Pleasant .*\e. S., LIinneapolis 9, X i n n .
METHOD
Preparation of Calibration Curve. Measure into 250-ml. volumetric flasks quantities of the standard antimony solution ranging from 5 to 30 ml. Add sulfuric acid (specific gravity 1.84), so that the total amount of the acid is 10 ml., dilute to 100 ml. with distilled water, and add 10 ml. of hydrochloric acid (specific gravity 1.19). Cool to 20' C., dilute to 250 ml. with water, mix, and transfer a 25-ml. aliquot to a 50-ml. volumetric flask. Add 10 ml. of the potassium iodide-sodium hypophosphite reagent, mix, and add 1 to 2 drops of starch solution. If a starch-iodine color forms, add the sodium thiosulfate solution dropwise until the color is discharged, and add 1 drop in excess. Dilute to the mark with distilled water, and mix thoroughly. The final temperature should be 20" =t1' C. Transfer a portion to a n absorption cell and measure the percentage transmittancy of the yellow
Table I. Chemical Composition of Copper-Base Alloys (2)
.
Element
Per Cent Present
Copper Lead Tin A41uminum Manganese Iron Nickel Antimony Arsenic Phosphorus Silicon Zinc
50 a n d over
0.0 to27.0 0.0 t o 2 0 . 0 0 . 0 to 12.0
0.0 t o 6 . 0
0.0 t o 3 . 0 0.0 t o 5 . 0 0 . 0 t o 1.0 0.0 to 1.0 0.0t01.0 0.0 t o O . l
Remainder
