August 15, 1941
563
ANALYTICAL EDITION
Summary
Literature Cited
Gold in a n aqua regia solution is determined very conveniently by adding sodium hypochlorite or potassium &lorate, diluting, and boiling out the free chlorine formed with these substances. The acid is then neutralized with a n excess of sodium bicarbonate, potassium iodide is added, and the liberated iodine is titrated with standard sodium arsenite, using starch if desired.
(1) French, H., M i n i w Enc. Wmld, 37, 853-5 (1912). (2) Gooch, F. A., and Morley. Ii. H., Am. J . Sci., (4)8, 261-6 (1899). (3) jkpher, H. p. oesterr. 2. B ~ ., ~ .yiittenW., ~ . 1880, 182-3. (4) Lenher, Y., J . Am. chem. SOC., 35, 733-6 (191, ( 5 ) Peterson. H., 2. ano~c.Chem.. 19.59-66 (1899 w. B., BuLz.Insf. Met. (La 331, pp. 23-5 (1932). (7) Rupp, E., Der.,35, 2011-15 (1902). (8) Scott, W. W., "Standard Methods of Chemioal Analysis", 5th ed., Vol. 1, pp. 453, 1203, New York, D. Van Nostrand Co.. 1939. (9) Su&&dy, L., and vioaiin,B., Oesferr. Ghem..zfo., 41, 431-5 (1938).
Acknowledgment Appreciation is expressed for C. B. F. Young's aid in providing materials necessary for the work.
An Improved Tangentimeter HOWARD P. SIMONS D c y a r t m e n t of Chemical Engineering, West Virginia University, Morgantown, W.
I
N PERFORMING calculations with data from drying
and filtration experiments, computing partial molal heat contents, partial heats of solution, and the like, it is frequently necessary to differentiate a function represented by a curve for which a n equation is not easily obhinable, in order to obtain rates or instantaneous values a t any desired point. Such differentiations are most conveniently performed by one of the well-known graphical methods, especially that involving the construction of a tangent to the curve and subsequently determining the slope of the tangent. It has heen recognized that the accurate location of tangent lines is attended with some little difficulty, particularly where the tangent is t o be constructed to a line of small curvature, and that the errors resulting from such inaccurate construction may often be of considerable magnitude.
Va.
possibility of confusion even when a large number of tangents are to be determined on a small portion of curve, and eliminating the necessity of defacing the curve that is being differentiated. Moreover, the instrument may be constructed very simply and inexpensively, is rugged, and may be made to any desired size, so that tangent intercepts may be read accurately and quickly from curves of large dimensions. ,. .,
..,
Various meehaniral aids to the accurate construction of tangent lines have been used from time t o time. Latishaw ( 1 ) described n simple tsnaentimeter in which a piece of speculum metal at-
ing in of a h g e n&b& of tangent lines, as is freduently required, may often result in sohe little confusion, as well as defacing the curve. In 1930, Richards and Roope (8) described a tangent meter Jrhich made use of il prism mounted an a. vernier protrxtor scale, and this instrument was introduced commercially. Such an instrument, while accurate, made necessary the use of a factor to correct the observed tangent t o the curve scale. In addition, it m m somewhat expensive. The tangentimeter here described is a modification of that described by Latishaw, being capable of a similar degree of accuracy. It possesses the additional advantages of not,requiring the construction of any lines, thereby eliminating.tlic
FIGURE2. TANGENTIMETER IN USE
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Vol. 13, No. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 1shows the details of construction of the tangentimeter. A , A are two pieces of transparent Lucite, 2 inches (5.0 em.) wide and 0.375 inch (0.95 cm.) thick, and of any desired length. The ends of the plates should be highly polished to make them completely transparent. B is a metal mirror 2 inches (5.0 cm.) wide and 1inch (2.54 cm.) high, which is set upright between the ends of the two Lucite plates and is soldered into shallow slots cut in the brass binding strips, C , C. These strips are 0.375 inch (0.95 cm.) wide and 0.06 inch (0.16 cm.) thick. The binding strips carrying the metal mirror are fastened to the plates by means of small machine screws, D, which fit into holes drilled and ta ped in the Lucite plates. E, E is an index line scribed on the rower side of the lates, equidistant from the edges and exactly perpendicular to t i e metal mirror. The line may be easily scribed on the soft surface of the plastic by means of a machinist’s scriber, a narrow engraving tool, or a sharp needle. Figure 2 shows a photograph of the instrument in use. The tangentimeter is placed on the curve in such a position that the
intersection of the index line and the mirror coincides with the point on the curve at which the tangent is to be located. The instrument is rotated about this point until the portion of the curve in front of the mirror and its reflection in the mirror appear as a continuous, smooth, unbroken line. When this position is obtained, it will be seen that the index line on the instrument is exactly tangent to the curve. The intercepts of the tan ent line will be easily visible, and the slope may be very simply cafculated. This instrument has been used in this institution for the past year, both for research work and for student computations, with pronounced success.
Literature Cited (1) Latishaw, J . Am. Chem. Soc., 47, 793 (1925). (2) Richards and Roope, Science, 71, 290 (1930).
A Device for Starting the Swinging of a Balance RICHARD E. VOLLRATH, Allan Hancock F o u n d a t i o n , University o f S o u t h e r n California,
S
EVERAL methods are in use for starting the swinging of a balance in precise weighing. A current of air may be produced near one of the pans by a fanning motion of the hand, one of the pans may be pushed down by means of a fiber such as horsehair attached to a matchstick, or the beam may be given a push by means of a fiber attached to the rod carrying the rider. Guthrie (3) and Arzberger (1) have suggested that a puff of air be directed against the underside of one of the pans by squeezing a rubber bulb connected to a tube projecting through the base underneath the pan. None of these methods is altogether satisfactory, because the amplitude of the swing cannot be conveniently controlled. Furthermore, in the first two methods the balance case must be opened, and this is time-consuming and obviously unsatisfactory when hygroscopic objects are weighed. The writer has used for some time a simple and convenient device which makes i t possible t o start the beam swinging without opening the case and to control the amplitude evzn while the beam is swinging. There are no references to previous use of the device except a rather obscure statement (1, 2) in a work on balances. The present device makes use of the pull of a small electromagnet on a piece of soft iron or steel mounted on the beam or the pans of the balance. When
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F I ~ U R1.E DETAILS OF ELECTROMAGKET
Los Angeles, Calif.
the electromagnet is excited by closing a switch for a few seconds the pull exerted deflects the beam and starts it swinging. The balance used by the writer is a type T analytical balance made by Wm. Ainsworth & Sons and has a sensitivity of 0.05 mg. at a load of 200 grams. It was furnished by the maker with certain minor changes to permit the mounting of the device described below: a 0.79-cm. (6/la-inch) hole drilled in the black glass base just below the center of the left-hand pan for mounting the electromagnet, a 0.62-cm. (0.25-inch) hole in the front of the case for mounting a small push-button switch, and steel buttons on the underside of the pans in place of the aluminum buttons against which the pan arrests press in this make of balance. The steel buttons were plated to avoid possible corrosion. I n Figure 1 is shown the electromagnet mounted under the base of the balance and concentric with the center of the left pan, the core projecting through the hole in the base. A brass nut screwed on the threaded upper end of the magnet serves to hold it in place. The core of the electromagnet and the spool on which the exciting winding is wound are all in one piece turned out of cold-rolled steel bar. The spool is wound with about 350 turns of number 25 double cotton-covered magnet wire, impregnated with shellac and baked dry. The ends of the winding are soldered to lugs on a strip of Bakelite to facilitate connecting the coil into the exciting circuit. The presence of the stiffening rib indicated in Figure 1 necessitated making the magnet core longer than would otherwise have been necessary. The distance between the upper end of the core and the pan arrest was about 0.62 cm. (0.25-inch)-large enough to allow the pan arrest to be locked in the down position as indicated by the dotted lines of Figure 1. The drawer in the base had a notch cut out of the back end, to allow it to pass the coil which projects into the drawer compartment. The push-button switch was made to operate with slight pressure and is shown in Figure 2. It consists of two strips, A and B , of 28-gage spring phosphor bronze 0.62-em. (0.25-inch) wide with a contact point of copper on one of them. These sprin s are mounted on a strip of Bakelite which is clamped to the insife of the balance case by means of a nut that screws on the bushing in which the brass rod of the push button slides. A slight pressure on the push button pushes the two springs into electrical contact, thereby allowing the energizin current to flow through the electromagnet, as may be seen &om the wiring diagram (Figure 3). The small Bakelite knob on the end of the push rod near the spring prevents the metal parts of the balance from coming into electrical contact with the circuit when the push button is operated. The electrical wiring is concealed in the space below the base and to the left of the drawer. A lamp cord passes through a small hole in the back of the balance and is connected to a 50-watt lamp serving as a current-limiting resistor and a plug for connecting to the 110-volt line. The most convenient place for the push button is to the left of the drawer. The weighing is carried out in the usual way until balance is established closely enough to permit final determination of the
