SEPTEMBER, 1953
AN INEXPENSIVE, PORTABLE ELECTROANALYZER PAUL S. FARRINGTON a n d ROBERT L,*
PECSOK University of California, Los Angeles, California
R E C E N T developments in the field of controlled cathode electrolytic devices, or "potentiostats," have revived interest in the determination of metals by electrolytic deposition. While such automatic devices are usually beyond the scope of the elementary analytical course, the principles involved can be covered adequately by simpler equipment controlling only the total applied e. m. f. Electrolytic copper determinations are an old standy-by for first year quantitative analysis student,s, and procedures are described in most standard texthooks. At the University of California a t Los Angeles we use a mixed copper-nickel unknown to demonstrate the possibility of separation as well as determination. The problem of a suitable current supply is a common one. Some laboratories are fortunate to have a house d-c. supply of adequate power and stability; however, many of these are subject to periodic shnt-downs and voltage fluctuations when other equipment is turned on or off. Commercially available electrolytic analyzers are quite satisfactory for simple determinations but are relatively expensive, particularly when a number of stations are required for large classes. Noncommercial devices can he very effective if well designed and properly constructed. This paper describes an instrument designed, constructed, and used in these laboratories for undergraduate work. Our objectives were as follows:
( 1 ) Cost: Minimum compatible with satisfactory performance. (2) Performance: -4 variable output of sufficient stability to assure an accurate determination with a maximum of 5 amp. a t 5 volts through each pair of electrodes. The current supply of each unit must be independently variahle and unaffected when other units are turned on or off. (3) Portability: We prefer that each student take the apparatus to his own work station where he can profitably use waiting time for other work. (1) Compactness: We store our instruments in unassigned student desks between classes. ( 5 ) General Appearance: A handsome, well-designed instrument usually commands more respect from the student. (6) Ease of Operation: Simplicity of controls is desirable for student use. DESCRIPTION OF APPARATUS
The complete apparatus, shown in the figure, consists of a power supply, an interchangeable electrode holder, two magnetic stirrers, platinum electrodes, and beakers. One unit with an electrode system a t each side is supplied to each student. Power Supply. Heath-Kit Battery Eliminators,
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JOURNAL OF CHEMICAL EDUCATION
Model BE-3, were puroliascd from tlw Heath Co., Benton Harbor, Rlichigan. Althongh the basic cirmiit xms not, altered, certain additions and subst.it~ntions~ v e r made. The meters provided were replared as indicated on the List of Pnrt,s, in order to increase the accuracy of reading. The out,put circuit wasdivided, right and left, earh side including a shunt for t,hr amm&r and an on-off switch for the elect,n~drsystem. The ammet,er &elector s\vit.cli permils the measurement of current,t,hml~glieit,herside independently. The voltsgp oontrnl and voltmet,er are common lo 110th sides. The prohlem of two elect,rode syst,ems supplied
-
TABLE 2 Copper-Nickel Determinations .
~
Copper---
Snnrpir (hppcr aiw
Unk. #I1 (36.32% C:u, 1 . 6I ) *
-0.10 -0.11
TABLE 1 Ripple Characteristics
- .- -
-
0 0 0 3 0 .73 I I1
Four banana pins project int,o sockets jn the insulat,ors on top of the power supply. Eby binding posts are mount,ed on the ends to hold the electrodes. Brass rods, running through the lucit,e, connect the binding post,s t,o the banana pins. Magnetic Stirrers. The prohlems arising from a rotat,ing elect,rode are eliminated through tthe use of rnagnet,ic stirring. Ahhough any commercial nnit, wonld be satisfactory, we constructed satisfactory models a t considerably less expense. This saving may not be appreciable unless one is const.rncting a dozen or more stirrers. The mot,or, with it,s speed-cont,rolling rheostat, and magnet are mounted inside an inverted stainless steel heaker. An aluminnm disc fast,ened t,o the heaker above the magnet int,roduces eddy current.^ which effectively reduce t,he t,op speed of the motor without affecting its starting torqne. POWER CHARACTERISTICS
-
0 1 2 5
--
1Jnk. $19 0.2102 021M) 0 . I R N O.IR.54 ( 3 6 8 8 % qu, 1 5 9 3 % NI)" a Eleotralytie determination with Snrgent-Slomin Anxlyzpr.
2 SPKP witches 1 SPI)T switch 4 Insolsfora, Johnson 135-4li.l Elrclrod~Assembly I,ur.ite, 21 X Z 5 / 4 X V4 in. Binding posts, Ehy Sliwers 2 Stainless Steel Beokrrs. BI.:BW13975 2 Midget motors, Rwrol DY,\l$ Xi9-3 2 Msgnets, Alnico V, V 4X '/. X 2 in. 2 Rh~v~stats, I500 ohma, Ohtnitr 015O 2 Stirring h a s
1,.
0.1606 O.l60T,
%
El'~07,
-0.18 +O.Oli
List of Parts The following list is compiled For n single unit providing two e l e c trode stat,ions. Lahnv m i l t s lmvr no1 I,P+-n estimated.
1 r.011.
-Tn111~.g. PomOl. .\'?:ckel-g. 0.1603 0.1607
-
I,oad, amp.
Tuken. g. Fo~md,g. E-w w . % 0.2771 0.2773 +010 0.2771 0.2771 0 0.3632 0.3632 O 0.3632 0.3631 -0.03 0.3632 0 3 6 4 1 +I125 0.3632 0.3637 +014 0 3 f i 3 2 0.3630 -0.06 031i32 0 3 6 3 1 -0.03 04X(i6 0.4860 +0.0T, (1.4297 0.4298 M.02
5 ~ 0 1 1 %1,.
0 0 0 1
1 45 i.5 5
The voltage regulation was dedermined by sett,ing the vokmeter at 5 volts with no load and then drawing successively larger currents, noting the voltage decrease. This test was made without varying the voltage ront,rol setting. With a 1-ampere load, the voltage decreased to 4.6 volts; 2 amp., 4.2 volts; 3 amp., 3.7 volts; 4 amp. 3.3 volts; and 5 amp., 3.0 volt,s. I t is possible t,o obtain 5 volts and 5 amp. a t each pair of electrodes. The maximum rat,ed output of this power snpplp is 10 amp. a t 6 volts for continnons use. The peak to peak ripple measured a t 1 and 5 volts under several loads is shown in Table 1. U7hile t,he filtering appears to he rather poor, the results obtained in analytical determinations indicate that this amount o f ripple can he bol~rat,edfor t,he procedures used.
SEPTEMBER, 1953 ANALYTICAL RESULTS
~h~ instrument has been tested by the assay of pure copper ,,,ire and the analysis of unknowns used in course work. The latter are mixtures of copper and nickel carbonates. The experimental procedure is described in Kolthoff and Sandefl.1 Results are compared with
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values obtained electrolyt~ically k i n g a SargcntSlomin Electroanalyzer, and confirmed by the analysis of a solution prepared cleterminately from pure copper lvirebee 2). ACKNOWLEDGMENT
It is a vleasure to arkno\dedae .. the assistance of Karl denaen and Ross Farmer, electronics technicians, and 'KOLTHOFP, I. M., A N D N. H. SANDELL., "Texthook of Quanti~ , M~ ~~C ~ ~ CO., I I ~ xew , , Fmnk Schuster, machinist, ~ h ocontributed many tative inorganic A , , ~ I I C~ V ~~ S O~ ~od., valuahle ideas and ronst,lwted the instrument. Yark, 1943, p. 4%.
