82
INDUSTRIAL AND ENGINEERING CHEMISTRY
time necessary for a determination is less, than b y the regular Pregl procedure. Another reason for automatic combustion is the reduction of man-hour time necessary for operation. Automatic combustion makes the burning rate uniform and therefore more reproducible conditions are obtained. The total elapsed time for a single analysis is about 40 minutes, of which about 20 minutes requires the operator’s attention, including the time for weighing the sample. The results obtained have the same accuracy as those r e ported for the Pregl microprocedure. The apparatus is checked once or twice a week on standard compounds. The average value for nitrogen obtained on 20 analyses of a standard sample of 2-amino-1-naphthalene sulfonic acid was 6.29 per cent with a limit of error (2u) of ~ 0 . 1 3per cent. The theory for this compound was 6.28 per cent.
Acknowledgment The authors wish to acknowledge the assistance of A. F. Mincz and W. L. Hatton in the construction and design of
Vol. 14, No. 1
the automatic apparatus. Acknowledgment is also made to other members of the microchemical laboratory, especially to R. Koegel, for contributions made during the development of the procedure.
Literature Cited (1) Am. Chem. SOC.,Committee on standardization, Division of Analytical and Micro Chemistry, IND.ENG.CHEM.,ANAL.ED., 13,574 (1941). (2) Hallett, L. T., Ibid., 10, 101-3 (1938). (3) Milner. R. T.,and Sherman, M. S., Ibid., 8, 331 (1936). (4) Niederl, J. B., and Niederl, V., “Organic Quantitative Microanalysis”, p. 84, New York, 1938. (5) Ibid., p. 63. (6) Norton, A. R.,Royer, G. L., and Koegel, R., IND.ENG.CH~M., ANAL.ED., 12,121 (1940). (7).Pregl, F., “Quantitative Organic Microanalysis”, 2nd English ed., tr. by Fylernan, Philadelphia, P. Blakiston’s Son & Co., 1930. (8) Royer, G. L., Norton, A. R., and Sundberg, 0. E., IND.ENG. CHI&, ANAL.ED., 12,688 (1940). PRESENTED before the Division of Analytical and Xicro Chemiatry a t the 102nd Meeting of the AMERICAN CHE.WCAL SOCIETY. Atlantic City, N. J.
Colorimetric Microdetermination of Arsenic after Evolution as Arsine E. B. SANDELL, University of Minnesota, Minneapolis, Minn.
S
EVERAL methods for the colorimetric determination of arsenic after evolution as arsine have been described. Morris and Calvery (3) decompose the arsine b y passing i t through a heated fused silica tube, dissolve the deposited arsenic in nitric acid, and after the evaporation of the latter determine arsenic b y the molybdenum blue method. Taubmann (4) absorbs the arsine in silver sulfate solution, precipitates the excess of silver with chloride, oxidizes the arsenic in the filtrate t o the quinquevalent condition by boiling with bromine water, and finally applies the molybdenum blue method. The time required for a determination is about 3 hours. The following method is based on the absorption of arsine in an acid solution of mercuric chloride containing permanganate. Arsine is thus oxidized in one step t o arsenate, and arsenic can then be determined in the solution b y adding a n excess of ammonium molybdate-hydrazine sulfate and heating t o obtain the molybdenum blue. Cassil and Wichmann (9)have used mercuric chloride solution for the absorption of arsine in a microvolumetric method for arsenic based on iodometric titration. The following procedure is designed for amounts of arsenic ranging from 1t o 15 micrograms. Approximately one hour is required for a determination. The directions for the evolution of arsine are similar t o those of the A. 0. A. C. (1).
of about 0.5 mm. The absorption vessel, C, is drawn from a test tube, and should have a capacity of 8 to 10 ml., with the tapered rtion of such dimensions that 1.35 ml. of absorbing solution will c v e a depth of 6 to 7 cm. A short piece of glass tubing, D,having an internal diameter approximately I 111111. rester than the external diameter of tube B, is placed in the a%aorption vessel to break up the bubbles of gtw and provide more absorption surface; it should be completely covered by solution. The apparatus used should be tested for its ability to absorb arsine completely by running a known amount of arsenic.
Reagents
A
B
Apparatus Cassil and Wichmann ( 2 ) have described an apparatus for the evolution and absorption of arsine in mercuric chloride solution, which doubtless can be adapted to the procedure below. In the present work the apparatus consists of a 50-ml. Erlenmeyer flask closed by a one-hole rubber stopper from which leads a tube, A (Figure l), containing at its lower end one or two plugs of glass wool impregnated with lead acetate. This tube is connected by means of a short section of rubber tubing to the delivery tube, B, which is drawn down to a capillary tip having an opening
FIGURE^. APPARATUS
FOR
ABSORPTION OF ARSINE
Stannous chloride, 40 grams of stannous chloride dihydrate in 100 rul. of concentrated hydrochloric acid. Potassium iodide,. 15 grams in 100 ml. of water. Zinc, 20- to 30-mesh, “arsenic-free”. Mercuric chloride., 1.5 n-a m s in 100 ml. of water. Potassium permanganate, 0.03 N , 0.10 gram in 100 ml. of water. Discard the solution when a precipitate of manganese dioxide forms. Ammonium molybdate-hydrazine sulfate. Prepare fresh daily by mixing 10.0 ml. each of solutions A and B and diluting to 100 ml. with water. Solution A: Dissolve 1.0 gram of ammonium molybdate in 10 rnl. of water and add 90 ml. of 6 N sulfuric acid. Solution B: Dissulve 0.16 gram of hydrazine sulfate in 100 ml. of water. Standard arsenic solution. To prepare a 0.100 per cent arsenic solution, dissolve 0.1320 gram of arsenic trioxide in 2 or 3 ml. of 1 N sodium hydroxide, dilute with water make acidic with hydrochloric acid, and dilute to 100 ml. From this stock solution, prepare by dilution a standard solution containing 0.010 mg. of arsenic per milliliter.
ANALYTICAL EDITION
lanuary 15, 1942
83
Discussion The results obtained in the application of the method, mostly t o pure arsenic solutions, are given in Table I. Judging from the values obtained, 5- or 10-microgram quantities
I
MICROGRAMS AS PER ML.
FIGURE 2. CALIBRATlON CURVE (I-&. CELL)
Procedure The prepared sample must be free from substances that prevent the complete evolution of arsine. Transfer the solution of the sample, conveniently having a volume of 25 ml. and containing not more than 15 micrograms of arsenic (20 micrograms of arsenious oxide), to the 50-ml. Erlenmeyer flask and add sufficient concentrated hydrochloric acid to make its total volume 5 ml., 2 ml. of potassium iodide solution, and 0.5 ml. of stannous chloride solution. Allow the mixture to stand a t room temperature for 15 to 30 minutes to assure the complete reduction of arsenic from the quinquevalent to the trivalent condition. Alternatively heat to 80-90" C., keep a t this temperature for 5 minutes, and then cool to room temperature. Measure 1.0 ml. of mercuric chloride solution, 0.2 ml. of 6 N sulfuric acid, and 0.15 ml. of potasRium permanganate solution into the absorption vessel. hlix with a thin glass rod. Connect the dratvn-out delivery tube to the tube passing through the rubber stopper and lower the former into the absorption vesscl, so that its tip nearly touches the surface of the solution. The absorbing solution should not be allowed to enter the delivery tube, for thcn a coatin of mercury arsenide is likely to be formed later on the interior ofthe tube. When all is in readiness quickly add 2.0 grams of zinc to the flask. Immediately invert the stopper and lower the delivery tube into the abeorbin solution so that its tip barely touches the bottom of the vessel. %lorn the gases to bubble through the solution for 25 or 30 minutes without heating the flmk. A t the end of this time, the solution should still contain some permanpsnnte. With larger amounts of nrsenic (10 micrograms) the solution will be more or less turbid because of the Reparation of hydrated mnnanese dioxide; this does no harm. Disconnect the delivery tube from the rest of the npparatus, leave it in the abmr tion vessel, add 5.0 nil. of ammonium molybdate-hydrazine sulgte reagent, and mix well. Heat for 15 minutes in a water bath a t 95-100°, cool, transfer the solution to a 10- or 25-ml. volumetric flnsk, and make up to volume with water. Filter the solution through a small plu of fine glass wool in a small funnel and reject the first portion ofthe filtrate. Obtain the transmittancy of the clear solution with a photoelectric colorimeter, using a red filter (preferably one showin maximum transmission a t about TOO mp). I n the comparison c e j place a reagent solution obtained by mixing 1 ml. of mercuric chloride solution, 0.2 ml. of 6 N sulfuric acid, and 0.10 ml. of potassium r r m n n g a n a t e and heating in a water bath for 6 minutes. dd 5.0 ml. o f ammonium molybdate-hydrazine sulfate and heat for 15 minutes at 95". Cool to room temperature, make up to the same volume as the unknown, and filter. Since Beer's law holds for the color s stem (Figure 2), the reference curve can be constructed by finzng the transmittancy of a single solution of known arsenic concentration. Mix 1.00 ml. of 0.001 per cent arsenic (as arsenious oxide) solution, 1.0 ml. of mercuric chloride, 0.2 ml. of 6 N sulfuric acid, and 0.10 .ml. of ptassium perniangnnate solution, and heat at 95" for 5 minutes. hen add 5.0 ml. of the molybdate-hydrazine reagent and proceed as described for the unknown. Correct the result for a blank carried through the procedure.
of arsenic can be determined with a n accuracy of 5 per cent, and 1- or 2-microgram quantities with a n accuracy of 10 per cent. The results tend t o be low. Antimony in small amounts does not interfere appreciably, although it appears to lower the results somewhat (14 and 15, Table I). Phosphine, from phosphides, would be expected t o give high results, but such error can be prevented by subjecting the sample t o a preliminary oxidizing treatment. The composition of the molybdate-hydrazine reagent has been altered from t h a t recommended b y others (9) b y increasing the hydrazine sulfate concentration. The mixed reagent should be prepared fresh daily, because its reducing power decreases reIatively rapidly on standing. The component solutions are stable for a long time, so t h a t a reagent of constant reducing power is easily obtained by mixing these in the ratio specified, and t h e reference curve need not be reconstructed. However, the best practice is t o construct the calibration curve the same day the determination is made. The blank solution, for the comparison cell, has a slight brownish color, b u t this is of no moment when a photoelectric colorimeter is used. A very slight difference in hue between unknown and standard solutions may be expected if a Duboscqtype colorimeter is used. There is a slight precipitation of mercurous chloride when the absorbing solution is heated with the reagent. Although the precipitate is very small in amount, it must be filtered off when a photoelectric colorimeter is used. Filtration through a plug of fine glass wool is satidfactory. Repeated filtration of a reduced solution of arsenomolybdate through glass wool caused no change in the concentration of the colored compound. An acid solution of potassium permanganate alone will not absorb arsine completely. Ceric sulfate and potassium bromate cannot be used in place of potassium permanganate in the absorbing solution. Potassium bromate (in hydrochloric acid solution) allowed a precipitate of mercury arsenide to form, and the same was true for ceric sulfate, but t o a smaller extent. Even if ceric sulfate were effective as an oxidizer, its use would be inadvisable because of the common presence of phosphate in ceric salts.
TABLEI. DETERMINATION OF ARSENIC (Volume of final solution 10 ml.. in Nos. 1 and 2, 25 ml. in all others; cell thickness 1 om.) As Found y Error y No. As Taken y 1.0 1.0 2.0 2.0 5.0
5.0 5.0 7.5
9 10 11 12 13 14 16 16
4.9 4.8 7.3 . ._
9.8
9.7 9.8 10.3 15.1 9.6 9.5 9.8
-0.1 -0.2
-0.2
-0.2 -0.3 -0.2 +0.3 +o. 1 -0.5 -0.5 -0.2
Literature Cited (1) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis. (2) Cassil. C. C.. and Wichmann, H. J., J. Assoo. Oficial Agr. Chcm., 22,436 (1939). (3) Morris. H.J., and Calvery, H. O., IND.ENQ.CHBM.,ANAL.ED., 9,447 (1937). (4) Taubmann, G.,Arch. exptl. Path. Pharmkol., 176, 761 (1934).
