SEPTEMBER 15, 1936
34 1
ANALYTICAL EDITION
volume to 25 ml. with water, and compare in a colorimeter against a standard chromate solution similarly treated. For samples containing up to 0.02 per cent of CrzOa a convenient concentration of standard solution is 0.01 mg. of Cr203per 25 ml. of final volume after treatment with acidified diphenylcarbazide. The sample and standard solutions should be treated with diphenylcarbazide at the same time, and the comparison made without undue delay, since the red-violet color of the oxidized reagent fades slowly on standing. The color development is very rapid and the solutions may therefore be read in the colorimeter as soon as they have been well mixed. It is not necessary to add sodium carbonate to the standard solution. If the amount of chromium present is less than 0.002 or 0.003 per cent of Crz08, the color comparison cannot well be made with a colorimeter. I n such a case transfer the vanadium-free solution of the sample which has been treated with diphenylcarbazide to a color comparison tube (see under Vanadium), and add to a similar tube 1 ml. each of diphenylcarbazide solution and 6 N sulfuric acid diluted with water to give approximately the same volume as that of the unknown. Then make the color comparison by adding from a buret a standard chromate solution treated with diphenylcarbazide to the comparison solution until the liquids in the tubes show the same color intensity when viewed against a white background from above. The standard solution may conveniently contain 0.01 mg. of CrzOsper 25 ml. of final volume. MOLYBDENUM.Special solutions are prepared as follows : Potassium thiocyanate, 5 per cent. Stannous chloride. Ten grams of SnClZ.2Hz0in 100 ml. of 1 to 9 hydrochloric acid. This solution should be freshly prepared. Standard molybdate solution, 0.05 mg. of MOOSper ml. Prepare by diluting an ammonium molybdate solution 10 or 20 times as strong. Obtain the exact strength of the stronger solution by standardization-for example, by precipitating as lead molybdate. Ethyl ether, treated with thiocyanate and stannous chloride. Shake reagent quality ether with one-tenth its volume of equal amounts of potassium thiocyanate and stannous chloride solutions the same day it is to be used. Tranvfer 50 ml. of the diluted filtrate and washings from the sodium carbonate melt (corresponding to 0.5 gram of sample) to a separatory funnel of suitable size, add slowly with agitation 8 ml. of concentrated hydrochloric acid, swirl to liberate excess carbon dioxide, and cool the solution to 20’. Add 3 ml. of potassium thiocyanate solution, mix, and then add 3 ml. of stannous chloride solution; after mixing allow the solution to stand for 30 to 45 seconds. Add 6 or 7 ml. of reagent quality ethyl ether to the separatory funnel, shake vigorously for 30 seconds, allow the liquids to separate, draw off the aqueous layer into a beaker, and run the ether into a color-comparison tube having a diameter of approximately 10 mm. (a small vial, 14 X 55 mm., can be used for the purpose). Return the aqueous solution to the separatory funnel, extract with 2 or 3 ml. of ether, and add the latter to the first extract in the comparison tube. Usually, for the amounts of molybdenum likely to be encountered in rocks, two extractions with ether will suffice. However, it is best to extract a third time and examine the ether layer in the funnel for a trace of color by looking through the great thickness, and to add this extract to the others if any color is apparent. To make the color comparison add the standard ethereal solution of molybdenum thiocyanate (prepared as described below) from a microburet to a second identical tube containing an appropriate volume of ether, treated with thiocyanate and stannous chloride, until the colors match when the tubes are viewed from above against a white background. Run a blank on the sodium carbonate used through all the steps of the procedure. To prepare the standard molybdenum thiocyanate solution, transfer 5 ml. of ammonium molybdate solution, containing 0.05 mg. of Moos per ml., and 50 ml. of 5 per cent sodium carbonate solution to a separatory funnel, add carefully 8 ml. of concentrated hydrochloric acid, swirl to liberate excess carbon dioxide, and cool to 20’ C. Then add 3 ml. each of potassium thiocyanate and stannous chloride solution, in the order named, mixing well after each addition. After 30 to 45 seconds extract with 10 ml. of ether and run the latter into a dry 25-ml. volumetric flask. Extract four times more with 5-ml. portions of ether (the last extract should be nearly colorless) and add the extracts to the flask. Make up the combined extracts to 25 ml. with ether that has been shaken with thiocyanate and stannous chloride. There is thus obtained a solution containing the equivalent of 0.01 mg. of Mooa per ml. Take great care to prevent changes in concentration of the standard solution as a result of evaporation; transfer the solution rapidly to the microburet and keep the latter well covered. Because of its instability, the solution cannot be kept longer than a day.
Literature Cited (1) Cazeneuve, P.,Bull. soc. chim., 131 23, 701 (1900). (2) Feigl, F., “Quantitative Analyse mit Hilfe von Tupfelreaktionen,” p. 343, Leipzig, Akademische Verlagsgesellschaft, 1931; Leitmeier, H., and Feigl, F., Mineralog. petrog. Mitt., 41,95(1931). (3) Geilmann, W., Wrigge, F. W., and Weibke, F., 2.anorg. allgem. Chem., 208, 217 (1932). (4) Hauptmann, H., and Balconi, M., Ibid., 214, 380 (1933). (5) Hevesy, G. von, and Hobbie, R., Ibid., 212, 134 (1933). (6) Hillebrand, W. F., A m . J. Sci., [41, 6,209 (1898); U. S. Geol. Survey, Bull. 167, 49 (1900); Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” p. 764, New York, John Wiley & Sons, 1929. (7) Hurd, L. C., and Allen, H. O., IND.ENQ.CHEM.,Anal. Ed., 7, 396 (1935). (8) Komarowski, A. S.,and Poluektoff, N. S., Redkie Metally, 2, No. 4,43 (1933). (9) Malowan, S.,2. anorg. allgem. Chem., 108, 73 (1919). (10) Montequi, R.,and Gallego, M., Anales SOC. espafi. fis. qudm., 32, 134 (1934). (11) Montignie, E.,Bull. SOC. chim., [41, 47, 128 (1930). (12) Moulin, A., Ibid., [31 31, 295 (1904). (13) Stanfield, K.E., IND.ENQ.CKEY.,Anal. Ed., 7, 273 (1935). (14) Winogradow, A. P., Compt. rend. acad. sci. U.R.S.S., 1931A, 249. RECEIVED April 13, 1936.
A Spindle for Insulating Wires JULIAN M. STURTEVANT Yale University, New Haven, Conn.
I
N THE construction of thermocouples and other electrical apparatus i t is sometimes desirable t o wrap a small bundle of wires with cotton or silk insulating thread. This is a rather tedious procedure when done by hand, but the simple and easily constructed spindle illustrated enables one to wrap about a foot of wire with one layer of thread in a minute. On a pulley, A , are mounted a spool, B , and a guide, C, at about the same distance from the center. The spool carries the thread to be used, and is supported by a machine bolt carrying a spring so t h a t the tension on the thread may be adjusted. The bundle of wires t o be wrapped is inserted in a glass guard, E , within the stationary hollow axle, D. The pulley is driven by a small motor, whose speed is conveniently controlled by a foot-operated rheostat of the type used with s e w i n g m a c h i n e s . If a large a m o u n t of w i n d i n g is t o be done, a n automatic feed for the wire could be rigged up. Also, more than one spool a n d g u i d e c o u l d b e mounted on the pulley if more than one layer of insulation was desired. The finished b u n d l e m a y b e rendered moisture-proof by being impregnated with shellac or paraffin, or, if none of the conductors is enameled, with glyptal varnish. C m 1 2 3 RBCEIVED July 31, 1936.
