QVANTITATIVE L E C T U R E E X P E R I M E N T S ON ELECTRO-CHEMISTRY BY UT.LASH MILLER APiD F R A N K B. K E N R I C K
T h e great advances made in electro-chemistry during the last ten years -both in theory and in technical applications have natixrally caused this subject to receive more attention in universities and colleges than has hitherto been the case. A satisfactory exposition of the principles of this essentially quantitative science, necessitates, however, the employnient of quantitative lecture experiments ; for the common practice of substituting tables of figures for actual measixrements in the lecture room, not only produces abstruse slid uninteresting lectures, but tends to obscure the experimental origin of the laws of the science. Unfortunately, the usual methods of measuring current, resistance, voltage, etc., cannot readily be adapted to the exigencies of the lecture table. T h e use of ordinary ammeters, voltmeters, etc., is possible only in rooins of very moderate dimensions ; while the standard laboratory methods of niensurenient are for the most part totally unsuitable. How, for instance, coiild Kohlrausch’s apparatus for measuring resistance be made use of, unless each student were provided with a telephone? A number of special arrangements have consequently been deLrised, by Liipke a i d others; these too fail in several particulars to conform to the requirements of the lecture room. I n the first place, they all involve the iise of a delicate galvanometer, which for demonstrations before a large audience should be aperiodic, of wide range, and big enough to be read at a distance. A11 instrument combining these characteristics still remains to be invented. Secondly, they are complicated by a maze of wires, cells, boxes, keys, etc., which is not only confusing to the
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W ; Lash Miller and Frank B. Kenrick
audience, but seriously hampers the lecturer, especially if several experiments are to be performed in succession. Finally, the results of the measurements have as a rule to be extracted from the experimental data by a more or less complicated mathematical operation. T h e effect of quantitative experiments carried out under such conditions is most undesirable. T h e attention of the students is directed so effectually to the details of the methods of measurement, that the real subject-matter of electro-chemistry the relations between chemical composition znd electrical properties- appears of but secondary importance. T h e writers have consequently endeavored to construct a measuring apparatus free from the defects enumerated above. T h e instrument, which has been continually in use during the past two years, is described in the following pages. It is provided with a dial two feet in dianeter, is “dead beat,” and reads directly ohms, mhos, volts, and amperes, their multiples and sub-multiples ; further it can be changed from any one of these uses to any other without delay. THE MEASURIKG I N S T R U M E K T
In carrying out measurements with the Wheatstone bridge, the resistance in one of the arms is varied until 110 current flows through a galvanometer. If the needle of the galvanometer incline to one side the variable resistance is too large : if to the other it is too small. In our instrument the galvanometer needle, unless exactly central, makrs a n electric com!acI‘, and thus by means of a relay, electro-magnets, etc., alters the variable resistance until the needle swings back to the central, or zero, position. A pointer moving on a dial records the change in the variable resistance, and the apparatus is so arranged that the figures on the dial give directly, in ohms, the resistance of the substance (wire, electrolyte, etc.), which is being measured. T h e instrument thus performs automatically the operations which an experimenter would carry out in measuring an electrical resistance. I t is also capable of making any other measurement that can be reduced to a zero method involving
Quantitative Lecfure Experiments 092 Electvo-chemistry 601 the alteration of m e resistance only -for example, the determination of electromotive force, conductance, os- current. T h e descriptiori of the instrument falls naturally under three heads: (A) the galvanometer, with contacts; (B) the mechanical device for altering the resistance and operating the pointer; (C) the subsidiary resistance coils, and the method of altering the connections rapidly when the function of the instrument is to be changed (e. g., from voltmeter to ohm-meter.)
(A)The Galvatzometer This is constructed on the d’Arsonval model, but as it is designed to indicate merely the presence and direction (not the intensity) of a current, one of the main conditions for t’ne efficiency of an ordinary galvanometer -constancy of the magnetic field-may be dispensed with. We have consequently employed a powerful electro-magnet in place of the usual steel magnets, thus increasing the delicacy of the instrument and ensuring good damping. The Contacts. I n our first experiments a horizontal platinum wire was attached to the frame of the coil, so that when the latter swung to the right or to the left the horizontal wire was brought into contact with one of two stationary perpendicular platinum wires ; thus completing a circuit and operating the right or left relay, as the casemight be. We found, however, that when contact had been made, the current apparently fused the wires together, and the galvanometer was unable to pull them apart. Three different means of overcoming this difficulty were tested. In the first place, the relay was re-wound with finer wire, in order to reduce, as far as possible, the current passing through the contacts ; next, attempts were made to replace the platinum wires by infusible substances, such as carbon, or by mercury ; and lastly the points of contact were moved as close as possible to the axis of the coil, in the hope that the increased leverage might be sufficient to pull the wires apart. T h e first change, though an obvious inprovenient, was in itself insufficient to obviate all ‘( sticking,” the second resulted in uncertain electrical
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Pi? Lash Miller aud F~aizkB. Kenrick
contact ; I and when the perpendicular wires were moved close to the coil, the slightest vibration of the latter was sufficient to make and break the contacts in the most irregular manner. Finally the arrangement represented in Figs. I and 2 ' was devised and found to give complete satisfaction.
Fig.
I.
T h e coil G carries the glass fork f; which moves the glass lever h pivoted at 72. Contact is made between the prolongation e of k , and a or i respectively. BY this means the advantage of leverage referred to above is secured ; while as the pivot 72 is held at both ends, accidental vibration of G cannot affect the contacts. DelaiZs. T h e pivot is made of two needle-points PZ, fastened with sealing-wax in a glass tube t, to which k is fused. IZ communicates electrically with J, a brass frame supporting Q, by means of a spiral of fine platinum wire. T h e contacts a and b may be adjusted in the direction to and from the coil, by means of the wood screw and slot w ; and in the direction to and from k. by the fine adjusting screws U and V. T h e bearings, y, are small pieces of window glass, in each of which, while red hot, a depression has been made with the point of a pin. They _ _ _ _ _ _ ~
The l'cohering '' effect of a wire from one pole of a small induction coil, although of some service, could not be relied upon.
QZL a iztiia live Lectzi ye Experimeit ts i i z Elect Yo-chemist ~y 603
are attached to J by Faraday’s cement. T h e screw Q serves to adjust 12. T h e field magnet M originally formed part of a small electric motor. The coil G is 60 mm high, 40 mni broad, formed of No. 40 silk covered copper wire wound on a
Fig.
2.
soldered copper frdme, and suspended by KO. 44 platinum wire. T h e stops s s, prevent too wide an excursion. T h e relay consists of two electro-magnets with cores of iron wire, each wound with 3400 ohms of No. 36 wire. T h e electrical circuits are shown in Fig. 3. ( B ) The Mecham’cal Rheostat
T h e duty of this part of the apparatus (See Figs. 4 and 5) is to iizcrease or decvease an electrical resistance, according as the galvanometer relay makes contact on the right or left side respectively. T h e variable resistance consists of a long wire, zigzagged on the surface of a wooden wheel, as shown in Fig. 5. S is a rubbing contact of spring k a s s , the Scraper ”, bent so ((
W Lash Miller and
I?
B. K e w i c k
I
I
I
ziod Fig. 3.
N D p
...........
C
Q fif 40
I Fig. 4.
Quantitative Lectzwe Ex#eriments
i ~ Electvo-chemistry 2
605
as always to touch two contact wires at once. T h e rollers U and Z are kept revolving in opposite directions by the motor E
and cog-wheels K. If the vzght contact of the relay be closed, a current flows through the solenoid-magnet R, which forces the wheel d against Z and W (as shown in Fig. 5), thus communicating the motion of Z to W, and increasing the resistance between [8] and S. T h e Zeff solenoid, actuated by a current through the left relay, causes motion in the opposite direction and decreases the resistance. A friction block B stops the wheel promptly when the solenoid ceases to act. A pointer P, attached to the axis of W, moves i n front of the dial D, which is divided into IOO parts. A corresponding set of numbers on the back of the wooden wheel and a small stationary pointer fixed to B indicate the readings to those behind the instrument. F is an iron fly-wheel, used to gear down the motor and to carry the commutator H (Fig. 4). T h e speed of the motor can be
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Pi7 Lash AkZler and I? B. K e n k k
regulated by a rheostat ; a convenient rate for most purposes is about IOO revolutions of F, or 3 of W per minute. The Resisfance Wheel (W Figs. 4 and 5) is built up of three thicknesses of wood, 40 nim in all, with the grain crossed to prevent warping. 201 pieces of No. I O copper wire bent and nicked as shown in Fig. 4a (C, contact wires) are fastened by small staples to the wheel ; and between these and a row of pins p , there is stretched backwards and forwards, 135 feet of naked resistance wire. Good contact between the latter and the contact wires is ensured by soldering. T h e total resistance of the wire is 203.7 ohms ; the resistance between t w o neighboring contact wires is one two-hundredth of that amount.' T h e construction of the solenoid magnets is shown in Fig. 6. T h e wire (250 ohms, No. 24) is wound on a brass tube, and the whole is enclosed in a piece of two-inch iron pipe. T h e wheel d is of brass, covered with a rubber band. Each solenoid is mounted on a block of Fig. 6 . wood, whose position on the bed of the apparatus can be adjusted by a slot and wood screw. (See Fig. 5). Fig. 3 shows the electrical connections. T h e 32-candle power lamp j serves as resistance and indicator.
( C ) The SfafionaryResistance Coils At the back of the dial are .fastened a number of coils of wire of various resistances, the ends of which communicate electrically with mercury in 3 2 mercury cups arranged in two rows in the wooden beam A. This takes the place of a switch_ _ _ _ ~ _ _
After the apparatus had been in use for some tinie, it was noticed that some of the sections of wire C-J3 (Fig. 5 ) were slack, owing to warping ; this was remedied by inserting a cord under the wires, and pushing i t toward J3. It would probably be better to make S movable and W stationary. Warping of W could then he prevented by heavy braces, and as the weight of the moving parts would be inconsiderable, weights or springs might be used as the source of power. The apparatus as described was constructed in odd hours with the material next to hand, and, although rather heavy, works very well inpractice.
board. T h e necessary connections are made by stout copper wires (KO. IO) amalgamated at the ends, and fastened in place on wooden blocks, Sigpz Blocks one of which seen from the back, is represented in Fig. 7. On both sides of an upright sheet of tin attached to the sign block is written the value of one division on the dial when the connections are made ; so that the very process of inserting the Volts,” for exaniple, sign makes the connections that convert the apparatus into a voltmeter. Stops and guides prevent false connections being Fig. 7. made, as for instance by shifting the block the width of one mercury cup to the right or left. T h e construction of the fifercuvy Cups is shown in Fig. 4. An iron screw conveys the electricity from the binding post to the cup, while a small rubber washer effectually prevents any leakage of mercury. )),
((
Arrangement o f the Resistance CoiZs Table I gives the arrangement of the stationary resistance coils. T h e mercury cups are denoted by figures and letters in square brackets ; binding posts on the bed of the machine are represented by symbols in rouizd brackets ; while uizbvacketed izuinbers give the resistance in ohms of the various coils. Dotted lines indicate electrical connections between the cups, posts, or resistance coils whose symbols they join.
TABLEI
[I].
. .o. I . ..[z] .. .0.9.. . [3]. . . 9 . . . [4]. . .90. . .[5]. . .goo.. .[6]. . .gooo. . .[7] [SI.. .2oj.7 ohms on wheel. . [ 9 ] . [IO]. . . r o ~ . S ...[ a ] . . .[ 111.. .203.7.. . [ 121.. . [b] . . .1833... [rg] (G) ...[c] ...[ 141 ...1000. . . [ ~ ] . . .roo0 ...[ e] ... 1000 ...[151 . . . [ ~ ] ( h )...[161... 1000...[ O V . ] ...( k ) [ 171 .. . ( I I O v. ) [ z o ] ...Rightsolenoid. [24] ...(- X ) . [IS] . . . ( - - 2 V . ) [I91..*(+2 v
. .
[ z I ] . .Inner Ring. [2 2 1 . .Outer Ring. [23]. . . ( A X ) .
..Scraper. [26]. .g,Fig. 9. [2j].
.
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W. Lash Miller and F. B. Kenrick
( 0 17.) and ( 1 1 0 V.) are binding posts attached to the negative and positive of the I LO volt circuit. ( - y 2 V. j and ( - 2 V.) are binding posts communicating with the positive and negative terminals of an isolated two volt circuit (storage battery). ( + X j and (- X ) are the terminals of the instrument, to which the electromotive force, resistance, etc., to be measured is attached. ( h ) and ( K ) are binding posts by means of which [16] and ( 0 V.) may be connected directly. ‘ ‘ Inner ring, ” etc., refers to the Commutator, Fig.
( G ) is a binding post connected to the point marked “ T o [14] ” in Fig. 3. The cups [a] to cf] are situated at one end of the switchboard. If [a] be joined to [b] by a sign-block, ‘‘ the Doubler ”, . the value of one division on the dial when measuring voltage or current is doubled. When measnring olims or mhos, the resistance in series with the galvanometer may be lessened by coniiecting [f]to [PI ’ [d 1 or I.[ ’ The coils 0 . I , 0.9, and I O ohms are made of heavy wire mounted on wood ; the remainder, of fine silk covered special resistance wire ” wound non-inductively on glass tubes. 8. ( S e e page G I I.)
‘ (
Connections on the Szkn Blocks ’
In the following table the first column gives the value of one division on the dial, the second the connections effected by the corresponding sign block.
Qua ~zt it a tiue L ectzire Experin2enis it2 Elect ~po-chemistry 609 ______ I ~ I O O OVolt 1,500 Volt ' I / I O O Volt 11 I O Volt OneVolt One+ I , roovolt I/IOCO Ampere 11500 Ampere I I / I 00 Ampere I / I O O Ampere I I , I O Ampere ' 'One Ohm IO Ohms 1 '1000hms 'I'IOO,OOOMho 1'10,000 Mho "IiioooMho ' I / I O O Minute 1
An example will make the meaning of the table clear. When the One Volt " block is set in place the following connections are made by it : Mercury cup No. I with cups S o s . 8, 18, and 24 ; No. 5 with No. 14 ; S o . 7 with No. 23 ; No. 9 with No. I I ; and No. 1 2 with No. 19. Reference to Table I shows that these connections complete the circuit shown in the following diagram, Table 111, in which the connections made by the block ((
TABLEI11 (--XI
=-IOO
il
1;
ohms
(-.2V) = -
9900 ohtns = (-CX)
II
Galvanometer
I I
Scraper Resistance Wheel = 203.7 ohms = (+2Y)
K i t h these blocks the '' Doubler " may be used. With these blocks ( h )and ( k ) should not be short-circuited. ( h )and ( k ) may he short-circuited only when high resistancesare being measured. The internal resistance of the ammeter. With this block in position, the instrument acts as a stop-match. The pointer is set in motion when contact is made hetween the terminals [ X ) and stops imniediately the contact is broken. The speed of the motor can be adjusted (rheostat and speed indicator) so that the pointer moves over the dial from o to 100 in one minute. The momentum of the fly-wheel is so great, and the friction of the resistance-wheel ( W ) 011 its bearings and on the scraper is so slight, that no appreciable change in rate is caused by bringing thesolenoid into operation. I
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W. Lash M i l k aizd E;. B. Keizrick
are denoted by double lines, the permanent connect?ons by single lines. When the mechanism comes to rest, the voltage between [8] and the scraper S (Fig. 5) will evidently be one-hundredth of that between the terminals ( + X and -X) of the instrument. But, as every two contact wires on the wheel (one division on the dial) correspond to one-hundredth of a volt, the dial reading gives the potential difference between the terminals in volts. Alfenzative Connections In the connections as made by the ohm and mho blocks of Table 11, the resistance of the rubbing contact between scraper and wires is added to the variabb resistame in the third arm of the Wheatstone bridge. Table IV gives a scheme of connections
TABLE IV One Ohm I O Ohms
* ~/roo,oooMho I/IO,OOO M h o I / I O O O Mho
I
I , I j , 2 2 ; 8, 16, 21 I , 15, 2 2 ; 8, 16, 21 I , 8, 1 6 ; 5 , r j . 2 2 ; I , 8, 1 6 ; 4, I j , 2 2 ; I , 8, 1 6 ; 3, I j , 2 2 ;
; 6, 13, 1 7 ; 9 , ; 7, 1 3 ~ 1 7 ; 9, 9 , 1 1 : 13, 17,
g, 9,
11; 11;
13, 17, 13, 1 7 ,
11. 11. 21. 21. 21.
in which this is avoided. Our experience shows, however, that no appreciable error is introduced into the measurements by this contact, even in cases where the variable resistance amounts to but one or two ohms. There is, moreover, a drawback to the use of the blocks of Table IV : the readings on the dial do not give ohms, mhos, etc. with absolute correctness. T h e greatest deviation is in the center of the scale ; the point 25 on the scale corresponding to 23.2 ohms, 50 to 47.6, 75 to 73.2, etc. This is partially compensated in some cases, by polarization of the electrodes when the conductance of electrolytes is measured with a direct current. With block i‘ 10-5 mho”, for instance (for resistances of 1,000to IOO,OOO ohms), the potential difference between the electrodes is in the neighborhood of IOO volts ; and as the polarization with fair-sized electrodes is from two to three With these blocks ( h ) and ( k ) should not be short-circuited.
~
Quantitalive Lecture Experiinenls iiz EZectpo-chewzislry 61I volts, the conductance as measured is 2 to 3 percent less than the actual conductance. T h e readings obtained with the block of Table IV would consequently be nearer the truth than those with the combination of Table 11.
Com mutator T h e most accurate results, however, may be obtained bj7 using the blocks of Table I1 and alternating the current in that branch of the Wheatstone bridge which contains the electrolytic resistance. T h e commutator designed for this purpose is shown in Fig. 8. I t is mounted on wood, and consists of three concentric brass rings, one of which is divided into eight isolated sections (+ and -) by small blocks of vulcanite, ZI. A pair of brushes set the inner and outer rings respectively in communication with mercury cups [21] and [22] ; and two brushes on the center ring, one behind the other at an interval of one section, communicate with the electrolytic resistance. A s the commutator revolves, the direct current to and from the mercury cups is interrupted ; while in the circuit containing the electro-
W
Fig. 9.
lyte it is transformed into a, series of pulsations in opposite directions. In order to prevent these interruptions from affecting the galvanometer, connection with the latter is broken at the right moments by small pieces of vulcanite, v,let into the margin of the outer ring. All five brushes are mounted on a hinged frame (Fig. 9) ;
when thrown back so as not to touch the commutator rings, thev make perinanent connections between [21] and [23], [22] and [24], [26] and the galvanometer coil (G). QCANTITATIVE LECTURE EXPERIMEXTS IN ELECTRO-CHEIMISTRY
ITith the aid of the instrument just described, it is an easy matter to illustrate a course of lectures on electro-chemistry by a series of quantitative experiments. ,411 the apparatus, etc., described in the following paragraphs ha5 stood the test of actual use in the lecture room, most of it having been employed in the lectures on elementary physical chemistry during the winters I 898-1899,and I899-1 900.
Electrobsis
(I). Electmdysis, Curveizt, Resistance. -Two of the common (Hofmann’s) electrolysis apparatus may be set up in series with a 32-c. p. incandescent lanip, the first being filled with inaxitnuin conducting sulphuric acid (one vol. acid to five vols. water) and the second with a mixture of one vol. acid and forty vols. water. If the terminals be connected to the positive and negative of the I I O volt circuit, the potential difference between the electrodes of the first cell will be found to be about ~ o v o l t s , and of the second about 2 2 volts. But although resistance, voltage, and concentration are thus different in the two cells, and current alone is the same in both, the volume of hydrogen liberated in the first cell is the same as that in the second. (11). Electi-o-chemical Epivaleizts. - T h e following cells are arranged in series, and the terminals connected to the 1 x 0 volt (better 2 2 0 volt) circuit. ( a ) Apparatus, Fig. I O , filled with sodium chloride sol’n. - 70 cc sat. sol’n. and 30 cc water. ( 6 ) Apparatus, Fig. I O , filled with nitre sol’n. -Nitre one * part, water five parts. ( c ) Hofmann’s electrolysis apparatus, filled with tnax. conducting sulphuric acid. When about I O cc hydrogen have been liberated, the current is stopped, the central pinch-cocks are closed, and the solu-
Qziantz'tative Lectzim Ex-eriinents z'7z Electvo-chemistry 61 3 tions from the cathodes are run out and neutralized with 1/50 normal acid. If two burettes be used, clamped close together, i t d l be apparent that the amount of acid needed is the same in each case. By adding a little litmus to the acid in the burettes, the column of liquid may be made more visible. Finally, the amount of alkali equivalent to the hydrogen liberated may be calculated. (111). Faraday's Constant. -The determination of this constant may be effected by electrolyzing sulfuric acid solution and measuring time, current, and amount of gas evolved. I n order to obtain a large volume of gas i n a short time the decomposition is carried out under reduced pressure. For this purpose the cell shown in Fig. 11 may be employed. T h e apparatus is filled
Fig.
TO.
wFig.
XI.
with maximum conducting sulfuric acid, which had been boiled to expel air, and a column of mercury is drawn u p into the graduated tube by connecting z to a filter-pump. T h e electrical connections are so arranged that inserting a plug closes the following circuits : ( a ) Positive of I I O volt circuit, - 16 c. p. lamp, - cell Fig, 1 1 , filled with max. cond. sulfuric acid - negative of I I O volt circuit.
W. Lash MiZZer and I;. 3. Kenvick
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( 6 ) ‘I’erininals of measuring apparatus, with block ‘‘ I / I W minutes.” When about 10-15 cc gas have been collected (half a minute or so), the plug is withdrawn, and the temperature, pressure, volume of gas, and duration of electrolysis, are noted. Next the block I ~ I O Oampere” is inserted in the measuring instrument, and the current determined under the same conditions as before ((
Colzductivity
(IV). Ohm’s LawforEZectrolykes.-If a 1.5 pct copper sulfate solution be electrolyzed between copper plates (3 in. by 6 in.) I $ inches apart, in series with a rheostat, measurements of the voltage at the electrodes, and of the current passing through the cell, for different amounts of resistance in the rheostat, show that these quantities are proportional to each other. (Use I / I O O volt an2 I / I O O ampere blocks). T h e necessary changes in the connections niay be made conveniently by means of a 3-pole double-throw switch ”, or a substitute constructed of wire and mercury cups. ((
(V). Conductance and Dimensions of Cell. - T h a t the conductance varies inversely as the distance between the electrodes, may be shown by filling a one-liter graduated cylinder (70 mm diameter) with copper sulphate solution (one grain of blue vitriol crystals in the liter) and measuring the conductance (10-4 mho block) between two circular copper plates, at various distances apart. As measuring cylinders ” are usually not cylindrical near the bottom, it is better to fix the lower plate at about the 2 0 0 cc mark. A piece of stout copper wire soldered to each plate and covered loosely with glass tubing, serves as a handle, and a t the same time establishes electrical communication. T h e relation between conductance and area of conductor, may be shown with the cell represented in Fig. 12. If the electrolyte be allowed to escape through the tube a, it will be fonnd that the conductance decreases, and that it is proportional to the height of the meniscus. ((
(VI). Conductance ai2d Temjevatuve.-By using the io ohms block, and connecting ( h ) and ( K ) through a rheostat, the ((
’)
Quantitative Lecture Experinzeizts i i ~ Electro-chemistry 61j resistance of a 16 c. p. lamp may be determined at temperatures from full white heat down to blackness. T h e extreme ratio is about 3 : 5. Electrolytes may be measured, using dip-electrodes”, in a beaker ; or if the conductance is too great, in a U-tube warmed in a beaker of water. T h e contrast between the temperature coefficient of dilute sulphuric acid and that of I O pct phosphoric acid is very marked. ((
(VII).Conductance and Conceztration - Ostwald’s Law.By using the cell represented in Fig. 12 the molecular conductivities of solutions of various concentrations (or numbers proportional to these) may be read directly on the dial of the measuring apparatus. T h e cell is made of glass and wood, the joints being kept tight by a piece of rubber tubing compressed by the wood screws as shown. T h e dimensions are: one centimeter through, IO cm wide, and j o cm high. Platinum foil fastened to strips of glass ( I cm x 50 cm) by Faraday’s cement serves as electrodes (6 6, Fig. 12). If 2 j cc 1/25o-normal hydrochloric acid be introduced through a funnel, and the electrodes be connected to the binding posts of the measuring apparatus (10-5 mho block) the pointer will indicate the molecular conductivity in mhos directly.’ Addition of water causes no alteration in the reading. (Stirring may be effected by blowing through a glass tube.) If, on the other hand, 25 cc of normal acetic acid be taken, and the volume be increased by successive additions of water from 25 to jo, 100, 2 0 0 , and 400 cc, it will be found that the conductance is proportional to the square root of the volume ; the conductance for volume 400 cc for instance, is double of that for IOO cc. and that again double the conductance of the 2 j cc originally taken. (L
(VIII).Isohydric Solutions. -T o prepare (approximately) isohydric ” solutions of acetic and hydrochloric acids, add water
When working with small volumes (25 C C ) , the current shoiild be shut off as Soon as the measurement is completed : otherwise the temperature of the electrolyte is apt to rise.
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616
to 2 5 cc normal acetic acid in the cell (Fig. 121, until the conductance is equal to that of 2 j cc 1/2 50-normal hydrochloric 7 acid in the same apparatus. Then add water to the hydrochloric acid until its volume is the same as that of the acetic acid. If these two acids be mixed in the cell, the conductance of the mixture is the sum of those of the acids separately ; if more concentrated acids be mixed, the slim is less ; if more dilute, greater. T h e differences are well marked, and in accordance with the calculations.
(IX). The SoZudiZity o f Lead Sz@haie, etc.-may readily be calculated from measmements of the resistance of a saturated solution, using dip electrodes ". '(
Fig.
12.
'
EZectromotive Force As the least current that will operate the galvanometer contact is about 10-5 ampere, the internal resistance of the cells whose electromotive force it is desired to measure must be low. Siphons, capillaries, and cotton-wick connections are consequently inadmissible. T h e following forms have given satisfaction.
(X). ' ( CaZomeZ CeZZ ". --A glass tripod such a5 are used in exsiccators, but with legs only 2 cm long, stands in a crystallizing dish and supports a circular sheet of amalgamated zinc K O cm in diameter. T h e bottom of the dish is covered with mercury, and a solution of zinc chloride mixed with calonlel is poured in. Connection is made with the mercury by platinum wise sealed through a glass tube which is fastened by a split cork to the edge of the crystallizing dish. Two of these cells, with solutions of different strengths, may be connected zinc to zinc, and the resultant electromotive force measured directly in I/IOOOvolts. (XI). Amwl'gnwz CeZZs. -The
amalgams
(I
pct and
I/IOO
Qizaiztitative Lecture Exjerimeizis in Electro-chemistry 617 pct zinc amalgam) are contained in small crucibles standing in a crystallizing dish filled with the electrolyte ( I O pct zinc sulphate). (XII). Diajhragm Cells. - Small porous pots ” of unglazed porcelain standing in beakers are useful when strong acids are to be used; they are, however, difficult to clean, especially if precipitates have been formed in the walls. A most satisfactory cell may be made from the lower half of a thick glass bottle (one pound potash bottle) by splitting it .longitudinally with a hot iron. A piece of parchment paper is laid between the two halves, and the whole bound together with rubber bands. T h e edges of the glass need not to be ground smooth. This apparatus serves for concentration cells (normal and I/Ioo-normal silver nitrate, with silver electrodes), precipitation cells (silver nitrate on both sides of the diaphragm, sodium chloride in addition on one side) and cells with complex salts (silver nitrate, and silver nitrate with potassium cyanide). When the diaphragm gets dirty, it can quickly be replaced by a fresh piece of paper.I ((
Pola riza tio i z
(XIII). Coizstaizt andIzcomtaizt Cells. -If a Grove’s cell of the ordinary form be short-circuited through the nieasuring apparatus ( I/IOO ampere block,introducing one ohm into the circuit), the current falls off but I/IOO ampere in a minute. If, on the other hand, the nitric acid in the porous cell be replaced by zinc sulphate, the fall in current amounts to 1 / 1 0 ampere in the same time. (XIV). Polarizatioa by Curreizt. -If after measuring the conductance of hydrochloric acid solution in the cell of Fig. 12, with direct czwrent, the 10-5 mho block be quickly replaced by the I/IOO volt block, the polarization may be measured and its gradual disappearance noted. T h e effect of short-circuiting the cell, or of reversing the current through it, may also be shown. For cells whose E. M. F. lies between one and two volts, m e the rlroo volt block with doubler ; or the block I t I / I O O volt (range fron one to two volts in I ~ T O Ovolts). With cells of high resistance the latter cannot be ernployed.
618 Quantitative Lecture Exjerirnents in EZecfro-chemistry (XV). Maximum Electromotive Force o f PoZariza&m.Two platinum wires connected with the terminals of the measuring apparatus ( I / I O O volt block with doubler) are dipped just below the surface of a solution of silver nitrate (lead acetate, etc.). As the resistance wheel is turned in the direction of the arrow in Fig. 5 the potential difference between the two platinum wires increases until at last a current of 10-5 ampere flows through the electrolyte, and the left galvanometer contact is made. In order that the instrument may automatically indicate this maximum electromotive force of polarization ” the left solenoid is attached by the cord w (Fig. 5 ) to the contact T,which when closed, completes the circuit operating the right solenoid. By replacing one of the wires by foil, the polarization at the anode and at the cathode may be measured separately. ((
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The Chenzical La6orutoi.y of the University of Toronto, July, zpoo
