through the resistance r3; hence, for such a condition R = r2. If the value of R is such that E' is greater than E", current will flow through the electrolyte; for such a condition the quantity in parenthesis in the denominator of (3) will always have a positive fractional value, and as rs is always positive, it follows that the denominator of (3) will always be greater than r2. Hence R will always be less than r3 for such a condition. Thus in any event the equivalent resistance R can never be greater than r; and accordingly the voltage drop through the variable resistance or across the electrodes can never be greater than the value of b found by solving the proportion Thus if the service voltage V is 110 volts, R1 50 ohms, and rз is set at some definite value, say 2 ohms, it means that no matter what the resistance of the electrolyte may be the voltage drop across the electrodes cannot be greater than 4.25 volts in accordance with the proportion (50 + 2): 2 :: 110: b. The actual drop would be somewhat less than 4.25 volts and could easily be computed from equations (3) and (1) if the value of r1⁄2 and the counter e.m.f. E" were known. 338. Practical Set-up. Because of the different values of r2 that are encountered by virtue of different types of vessels, electrodes, etc., that are in use, it is advisable in the set-up to provide a certain flexibility in R1 and rз. With respect to R1 the simplest thing is to construct a lamp-board24 having four or five sockets in parallel, then by screwing in a successive number of lamps we cut down the resistance according to the rule for a parallel combination, i.e., where R represents the equivalent resistance of the parallel combination, and r', r", r'"', etc., represent the individual resistances which make up the parallel combination. Thus, using lamps of 24 Hard asbestos board, having a thickness of about inch, serves as an excellent material from which to make the lamp-board. 60 watts-110 volts, the corresponding equivalent resistances of the combination would be as follows: 25 1 2 3 4 5 Number of Lamps.. Equivalent Resistance (ohms).... 200 100 67 50 38 With respect to rs the simplest thing is to use a rheostat with a maximum resistance of 10 ohms and maximum carrying capacity of 5 amperes. In addition to the foregoing equipment a voltmeter with a range of 0-10 volts and an ammeter with a range of 0-5 amperes are indispensable. The plan of the total circuit is portrayed herewith. DIAGRAM OF ELECTROLYTIC CIRCUIT FOR USING HOUSE SERVICE Stands for supporting the electrodes are not represented in the diagram. The stands should preferably have a glass column because then they can be touched while the current is flowing. The size of wiring to be used for the lamp-board and other parts of the circuit should be in accordance with the following table: 25 To figure the resistance of a lamp from its specification in terms of watts and volts, divide the square of the number of volts 110 volts, we have for its resistance by the number of watts; thus for a lamp of 60 watts110 X 110 60 = 200 ohms (approx.). 339. Stirring. It is all important in any electrolytic determination to prevent local depletion of the electrolyte at the surface of the cathode and consequently a great many different schemes have been used to effect this end; among such schemes may be mentioned the rotating anode 26 the rotating cathode,27 the use of a magnetic field,28 etc. The various schemes, however, differ mostly in mechanism and not in principle as they are all based on the main idea of thorough and efficient stirring. There is, therefore, not much advantage of one scheme of stirring over another if both cause a thorough circulation of the electrolyte. One of the easiest ways to accomplish this desired result is by means of a small glass propeller which is driven by means of a motor. The glass propeller is easily made by flattening out the end of a stout stirring rod (6 mm. diameter) into a small oval blade, say 10 mm. X 8 mm., and then turning the blade so that one of its axes lies in a plane which is perpendicular to the stirring rod and the other axis cuts the plane at a slight angle. For use the propeller should be placed well below the level of the electrolyte and the beaker centered with respect to the stirring rod which forms the shaft of the propeller. The free end of the stirring rod should be rigidly connected to a driving shaft by means of an adjustable jaw. The speed at which the propeller should run is 500-700 revolutions per minute. See Fig. 47, § 341. 340. Cathodes. Generally speaking, these should be of platinum. It is found that gauze cathodes give much better deposits than plate electrodes. The reason for this seems to be that the interstices in the gauze bring about a much better circulation of the electrolyte so far as conditions at the surface of the electrode are concerned, the better circulation serving to keep down any local depletion of the electrolyte that might tend to occur. 26 F. F. Exner, J. A. C. S. 25, 896 (1903). 27 F. A. Gooch and H. E. Medway, Amer. J. Science (4) 15, 320 (1903). CHAPTER XXI ELECTROLYTIC DETERMINATION OF COPPER 341. Practical Points. It is assumed that the student is familiar with the general theory of electrolytic determinations (Chapter XX), and is provided with the special apparatus which is required for this kind of work, namely, platinum electrodes, electrode stands (preferably of glass), a stirring device, a suitable electrical set-up for safely employing and regulating the current, an electrolytic beaker and a pair of split watch-glasses. With respect to the electrodes, the anode usually consists of a platinum wire having a thickness of about 1 mm. and bent into the form of a spiral (§ 334); the cathode should preferably be in the form of a rectangular piece of wire mesh about 8 cm. wide x 7 cm. deep, the mesh being composed of wires which have a diameter about 0.1 mm. and which are spaced about 0.5 mm. between centers. If wire mesh is not available, a thin rectangular plate about 8 cm. wide x 7 cm. deep X 0.1 mm. thick will answer.1 Whichever electrode is used, the gauze or the plate, it should be bent to correspond to the curvature of the electrolytic beaker. Before being used both the anode and cathode should always be immersed in hot 6 molar nitric acid for a few moments, then thoroughly rinsed with distilled water, and finally ignited to a bright red over the colorless flame of a Bunsen or Meker burner. By this treatment any grease or other impurity that might contaminate the electrode is removed and a fresh clean surface produced; this cleanliness of electrode surface, particularly with respect to the cathode, is very necessary for securing a satisfactory deposit. After being cleaned, the anode and cathode are used at once, the latter without being weighed; the reason for not weighing the cathode beforehand is found in the fact that the 1 Some analysts use a large platinum evaporating dish of about 100-150 c.c. capacity, making the dish serve the double purpose of containing the electrolyte and acting as the cathode at the same time. anode dissolves slightly during the electrolysis (§ 328) to the extent of several tenths of a milligram and this dissolved platinum oooo FROM AMMETER BALLARD ΤΟ FROM FIG. 47 eeee FROM PARALLEL TO BRANCH LEAD RESISTANCE ее DETAILED ASSEMBLY FOR ELECTRO LYTIC DETERMINATIONS is plated out on the cathode. Consequently the cathode is not weighed until the electrolysis is completed; it is then weighed with its deposited copper, after which the copper is dissolved off by means of 6 molar nitric acid, and the cathode itself weighed, all according to § 346. With respect to electrode stands nothing special needs to be said except that they preferably be made of glass and that they be provided with a firm base and an adjustable binding post (see Fig. 47). 342. In regard to a stirring device, we have already mentioned the several types in § 339 and have described there in detail the device which we regard as the simplest, namely, the motor-driven glass propeller. Some one of these devices is absolutely necessary where cathodic current densities of 0.01 to 0.02 amps./cm.2 (so-called "rapid" methods of deposition) are to be used, such current densities usually effecting a quantitative deposition inside of two hours; if cathodic current densities of 0.001 to 0.002 amps./cm.2 (so-called "slow" methods of deposition) which require twelve to fifteen hours are to be used, stirring may be omitted. In regard to the electrical set-up for adapting the house service to electrolytic determinations, we have already gone into this matter in §§ 335–338, but the matter of using only the proper kind of set-up is so important that we advise a perusal of these paragraphs again, especially where a procedure directs, as some procedures do, that before detaching the cathode, the electrolyte should be replaced by water with simultaneous washing of the cathode while the current is still kept flowing or, as other procedures direct, that the cathode be gradually lifted out of the electrolyte as it is, while the current is still flowing. |