INDUSTRIAL AND ENGINEERING CHEMISTRY
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Table I shows that when reducible metal ions such as Fe+++ and UOz++ are present, and an amount of titanous chloride is added which is sufficient t o reduce the metal to a lower valence state and leave an excess of titanous ion, the catalyzed reaction proceeds much more slowly than in the absence of the reducible metals. Accordingly the detection of tungsten by the method described fails when appreciable amounts of such metals as Fe”’, Uvl, and 5” are present. Cations which are reduced to the metal or form insoluble chlorides (Ag, Hg, Au, P t , etc.) must of course be absent. Although molybdenum catalyzes the reaction, as already mentioned, its effect is much less than that of tungsten, so that in small amounts i t hardly interferes (Nos. 28 to 30, Table I). Moreover, the presence of molybdenum is revealed by the appearance of a yellowbrown color, which fades rapidly, when titanous chloride is added to a 0.01 N hydrochloric acid solution containing molybdate. K h e n molybdenum and phosphate are simultaneously present, a permanent strong red-brown color is produced when titanous chloride is added to the acid solution, but the reduction of malachite green hardly proceeds more rapidly than in the absence of phosphate. The catalysis described may be applied in detecting tung-
VOL. 10, NO. 11
sten b y the spot technic. A 0.05-ml. drop of test solution (free from the interfering substances mentioned) which may be neutral or 0.1 N in hydrochloric acid is treated on a spot plate with 0.01 ml. of 1 per cent titanous chloride solution, and 0.01 ml. of 0.005 per cent aqueous malachite green solution is then added. It is important to add the titanous chloride first to the test drop, because if the malachite green is added before the titanous chloride the catalyzed reaction proceeds more slo~vly. The test drop becomes colorless (very pale violet) more or less rapidly, depending upon the tungsten concentration. With one part of tungsten in 100,000 of neutral solution, the blue-green color fades in about 3 seconds, whereas with one part in 500,000 decoloration occurs in 1 to 1.5 minutes. A blank remains green for 4 to 5 minutes. A tungsten concentration of 1 to 500,000 may, under these conditions, be considered the limit of the reaction, because a t a concentration of 1 to 1,000,000 decoloration requires 3 minutes. When the test drop is 0.1 N in hydrochloric acid, the limiting concentration may be set a t 1 to 250,000 (decoloration in 2 to 3 minutes, blank remains colored 3.5 to 4 minutes). RECEIVED July 29, 1938.
Buret Top for Precise Control of the Rate of Outflow E. P. WHITE, Victoria University College, Wellington, New Zealand
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T IS GENERALLY recognized that tapless burets have many advantages over those with small taps a t the lower end. Several methods of controlling the f l o of ~ liquid in tapless burets have been developed, notably that of Schilow (2-5) in which a manometer is used, and that of Benedetti-Pichler (1,s) which employs a capillary to break the flow and a small clip on rubber tubing to stop the flow. The accuracy of control of the latter buret was found to be limited by movement of the rubber tubing and by alteration in the position of the clip; the present paper deals with a buret top, which, while using the basic principle of Benedetti-Pichler’s method, allows a much greater exactness of control.
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FIGERE1.
DIAGRAM
The actual buret tube used was a 5-ml. graduated pipet drawn out at the bottom t o a jet delivering about 0.015-ml. drops, and ground flat at the top. Equally satisfactory control a a s obtained using 10-mi. or 1-ml, tubes. The buret top consists of a T-tube with the end of its vertical arm ground flat and fitted closely to the tube by rubber tubing. A full-scale section of the buret top is shown in Figure 1. On one side of the horizontal arm is a tap, a; filling is done by closing the regulator and opening this tap, which connects to low pressure from a filter pump. Into the other side of the T-tube is ground, and cemented with sealing-wax, a fine-bore heavy
capillary tube, h, ground smooth and beveled at its free end. On this is cemented a threaded collar, c, having 30 turns per 2.5 cm. (1 inch), screwing into a brass cap, d, turned from solid rod and having a raised milled portion, e . Into a small depression inside the end of the cap fits a cylindrical pad of firm rubber, j , having its project,ing end perfectly flat, while a fine hole, 9, through the brass wall allows free entry of air. -4 slight’turn of the regulating mechanism alters the pressure of the pad on the capillary tip, giving a very accurate control of the liquid flow. As the jet of the buret was always kept in the solution being titrated, this fine control allowed a very smooth approach to the end point impossible with a tap-type buret. This buret has been used for routine work on halogenation, involving iodine-thiosulfate titrations; the precision of the titer was determined by the accuracy to which the tube could be read, and by the sensitivity of the indicator. Discrepancies introduced by these two factors were far greater than any due to the control mechanism. Using a 5-ml. tube and lighting from a small bulb behind thin paper a t the back of the scale, the titers were repeatable to 0.01 ml., and with reasonable care a n approximation to the third place could be obtained. The device proved useful for the accurate adjustment of the liquid level in micropipets, especially if dangerous liquids were being handled. A furt’her very practical use was found in low-pressure distillat’ion; the buret top was used in place of the traditional screw clip on rubber tubing a t the top of the fine capillary tube dipping into the liquid. With this control the pressure could be adjusted rapidly to any desired level, and could be kept reasonably constant throughout the distillation.
Literature Cited (1) Benedetti-Pichler, A , , Z . anal. Chem., 73, 200 (1928). (2) Glaser, Ibid., 69, 461 (1926). (3) Mitchell, C. A,, “Recent Advances in Analytical Chemistry,” Vol. 11,pp. 355-6, Philadelphia, P. Blakiston’s Son & CO., 1931. (4) Schilow, E., Z . anal. Chem., 70, 23 (1927). (5) Schilow, E., Z . angew. Chem., 39, 582 (1926). RECEIVED August 13, 1938
