ANALYTICAL CHEMISTRY
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eter. Ra is wound from Manganin wire and immersed in oil. The degree of regulation made possible by the device is demonstrated by the fact that during the course of 30-minute runs, the variation in current has never been found to exceed 1 part in 25,000. In addition to applications in which precise energy measurements are to be made, the regulator is useful for maintaining constant currents during the calibration of resistances and direct current meters. I t seems possible that for the maintenance of constant current during moving boundary and electrophoresis experiments, a modified regulator based on the principle described might be superior to the one used by MacInnee and Longsworth [Chem. Reu., 11, 189 (1932)].
The eimplest way is to rotate the tip against a high-speed Carborundum wheel. If the tip is held lightly, and the grinding is not hurried, this method yields a satisfactory tip in a minimum of time. This rough-ground tip may then be fine-ground by lapping it in a rotating tapered cylinder into which are fed water and emery powder, as a t Q. Second Method. Lap out the desired portion of the tip from the excess glass on the head of the tip by means of a rotating slotted tube, mounted in a drill press, to which are fed water and emery. Cement the tip by plaster of Paris to a block of wood and clamp it to the table of the drill press. For a lap w e a piece of brass tubing of appropriate inside diameter to reduce the tip to the diameter desired. Run the rotating lap tube, slotted a t the cutting end to admit the grinding compound to the cutting edge, down over the stem of the bubbler tip and continue lapping, I , until the tip is Iert, as a t K . Finally polish the tip, if desired, and dissqlve the wires with narm nitric acid.
Preparation of Bubbler Tips. Charles L. Gordon, Sational Bureau of Standards, Washington 25, D. C.
Tips made in this manner can be h i s h e d to the same diameter as the capillary tubing to enter any apparatus into which the tubing alone can be inserted. For very small tips the smaller area of the seal in the glass between the wires of the spider requires a more positive method of fusing the glass around the wires, and platinum or palladium is more appropriate for the spider. For holding and spreading the spider wires, it is convenient to solder them together with gold a t the spreading point.
glass bubbler tips described by Branham and Sperling T [J. Research Natl. Bur. Standards, 701-5 (1939)l are convenient and efficient for dispersing a gas into an HE
22,
absorbing solution. They are made by sealing a number of fine copper wires between a cone formed on the end of a glass tube and a circular glass disk. The ends of the wires are then exposed by grinding and the wires are dissolved in acid. Their construction is rugged, yet their availability has been limited because of the difficulty of manufacture. A less exacting method of manufacture is given here. A
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Methodof Construction. Pre are a spider of copper or palladium wires of the size chosen for theioles by twisting the desired number of wires together so that about 0.5 inch (1.25 cm.) of untwisted wire remains as illustrated a t A . Clip off the twisted portion to reduce the total length to about 1.5 inches. Hold the wires over a flame momentarily to anneal them, then insert the twisted end into a jig and clamp it. Splay out the untwisted wires, as shown a t B , to radiate uniformly. Uniform arrangement is best done by slotting the jig, but arrangement by eye gives a satisfactory bubbler. Select a short length (about 2 inchep) of capillary tubing of such bore that it will easily pass over the twisted wires and yet not be over twice their combined diameter. Heat one end of this tube in an oxygas flame and flare the opening to 5 to 6 times the diameter of the twisted portion of wires. Place the inverted spider on a sheet of brass near the flame. Heat the flared end of the tube until it is very soft, then quicklv push it donrn over the fitem of the spider onto the fan of wires, as a t C. If thc glass was hot and soft enough (and not too large in internal diameter), the spider of wires will adhere to it, as a t D,on lifting the tube. Keep this assembly warm by holding in the warm blast far out beyond the end of the flame. Heat one end of a glass rod until a soft ball of glass forms and becomes very fluid a t the outer surface. Line up the tube held in one hand with the rod in the other and immediately press them together, as a t E. The soft glass unites with the warm exposed glass between the spider wires, as a t F. Soften the rod back of the flared head and draw it off, as a t G. Soften and paddle (or marver) the remaining knob of glass to form a flat end over the spider, as a t H , and anneal the whole end in a soft yellow flame. Finish the bubbler tip by removing the excess peripheral glass.
After proceeding as to H ( L to P ) fuse the whole end of the tip until it is nearly spherical, as a t P . Then rough-grind the tip to a slight cone as a t R by rotating against a high-speed Carborundum wheel. (This method, while quick, sometimes breaks the glass.) Then lap with emery in a slightly tapered tube as a t Q until the diameter ha8 been reduced to the desired pize. The spiders made of palladium have a tendency to be surrounded by bubbles in the glass, caused by gas escaping from the wire a t the higher temperature of fusion. The capillary tubes formed by platinum nire are neater, but a longer time is required to dissolve the wires nith aqua regia (4 parts of hydrcchloric acid, 1 part of nitric acid, and 1 part of water). The time for dissolving the wires is not unduly prolonged unless the wire has been hardened with iridium. In this case the tip can be sealed in a tube of borosilicate glass together with the acid mixture (without the 1 part of water) and heated a t 110" C. Tips with 20 and 30 openings formed by No. 44 B.&S. wire gage L = have been made using tubing of 3-mm. outside diameter. MP e r f o r m a n c e . Flow tests were made on a 9-mm. tip made from borosilicate glass capillary tubing of N 6mm. outside and 1-mm. inside diameter using 20 strands of No. 36 0B.&S. gage copper wire. A pressure of 24 cm. of water was required to P start the flow of air through the tip when immersed in water a t room temQ perature. With a pressure of 35 cm. of water, air flowed through all the openings a t a rate of 2.2 ml. per R second. Above this the rate of flow W&S a linear function of the logarithm of the pressure up to about 4.5 ml. per second a t a pressure of 52 cm. of water. The main difference in the delivery characteristics between the example used here and the Branham-Sperling tips is in the length of the capillaries produced by the wire. The Branham-Sperling tips are, in general, more uniform and the length of embedded wire is shorter. The pressure required to bubble gas through the tip made as described here is, therefore, greater than those made previously. This can be decreased by grinding the periphery away to produce a shorter restricted passage. With care a bubble can be blown around the center of the spider to decrease the restricting path and increase diameter of tip head.
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