V O L U M E 2 4 , N O . 1, J A N U A R Y 1 9 5 2
209
Follo~ ing development the strips are removed, and the position of the cholesterol on the tracer strip is checked by dipping the strip into Liebermann-Burchard rea ent. Portions of the test strips bearing the cholesterol are t i e n cut out, carefullv folded, dropped into the micro-Soxhlets, and extracted with rhloroform for 0.75 hour on a steam bath. The cooled extracts are transferred to Evelyn colorimeter tubes immersed in a 50" C. water bath and evaporated to dryness. using the vacuum manifold mentioned above. C. Colorimetry. Ten milliliters of chloroform are added to c,ach of t h r colorimeter tubes. T h r tubes are equilibrated in :I 25" C. n-atcr bath for -capable of li~iiig :tpplied t o such qu:intities I i e r a u ~ c:t niillimicrogram of silver, for t ~ ~ m i p lRhould e, rcquiw 8.94 X IO-' roulomh in undergoing oxicht ion or rrduction. Thiii ~~orre.y)ontl.~ to :in average cwrrrrit of :iliiiiit 0.lpa. for 8.9 seconds. (-11 t ( i t h r present timr. three gcrr( w l t,vpes of coulometric tic.teriiiiir:~tioric;have berw npplictl. 111 t h i \ first tylw, n-hirh is cmployctl in thr present study, the iiumlwr of roulombs required to dissolve :L substanre is measured. GroIvrr (8) in 1917 arid later Fr:iiiris (7') and Ogarev ( 1 3 )utilized this method to tletermiric~the thirkiiws of metal coatings. Camptwll arid Thomas ( 2 ) similarly deterniitied the t,hirkriess of oside coating3 on metals. Zbinden (2f ), in t prorediire later employed by I-. S . .I.
riique has berii c,\panded and applied to a varietv of deterniinslions (1, 3. n, 6 , 1 1 , 12, 15, IC, 19). The method tlrsc~ibedin the present study is similar to that of Zakharrvskii and Zhinden, except that the deposit is dissolved by changing thP potcmtial uriiformly in a positive direction and mcvmuring the rurrent on an automatic rerording potentiometer. \Vhtw the chart spred is kno\\.n, integration of the resulting curi,rrit-tinir e u r v ~gives the number of coulombs, from which the amount of deposit may be calculated. It was felt that t,he constant current, method would be less satisfactory because very small currents would be required a n d relatively high residual currents might be encountered. \vliicsh \voultl rrsult in cwrrent efficiencieu less th:iri 100To. \PP4RATUS
,. 1i v o
rupidly intercharigeahlt~c*irctuits.oiic' for plating and the other for dissolution. havc Lieen incorporated in the instrument stion-ri i n Figure 1. Six rlcrtrolytic. cells can be operated simult:incvusly : ( r i d inticpentirntly and :iny one of the six can bc. witched instaritarir~oriil~ i n t o the dismliitioti vircuit n-ithout disturbing the other fivv Thc plating cirruit is el;writially :I voltusc. divider across a 6volt storage battery. .4 reversing switch is provided, so that either positive or negative potentials versus a reference half-cell mag be obtained. Similarly, the dissolution circuit consists of a B-volt battery connected across another voltage divider, whose output, in turn, is applied across a motor-driven slide-wire. This arrangement allows one to select the size of the span (range) of potential to be traversed by the slide-wire. The starting potential can be displaced in either direction by means of a separate 1.5volt dry cell and its associated voltage divider.
ANALYTICAL CHEMISTRY
210 The current is determined by measuring the potential droll across a precision resistor (Leeds & Sorthrup, S o . 4773 decade resistance box) connected in series with the electrolytic cell u m g a Leeds & Northrup Speedomax automatic recording potentiometer having a full scale sensitivity of 2 5 mv., a full scale travel time of 1.2 seconds, and a chart speed of 10 inches per minute. Khen the slide-wire and the chart are both moving, the resulting curve is effectively a current-time curve.
Table I. Effect of Electrode Area on Residual Current and Amount of Silver Found at Constant Plating Time Electrode Area. SQ. Cin.
Rei. Arca of Peak
Residual Current/ Electrode Area
.4a Plated/ Electrode Area
Residual Current / ig Plated
PROC EDUH 1;
Exploratory studies were carried out using 50-nil. portions of 1.00 X 10-6 A3f d v e r in 0.1 X acid (either nitric or sulfuric). The 10-5 Jf silver was madc up by diluting a standard solution of 1.00 X 10-3 JB silver sulfate or Filver nitrate (depending upon which acid was used). The st:indard solutions Tvere prepared from reagent grade chemicals in distilled water and were stored in the dark in black bottles. Dcposition took place on a platinum electrode connectcd through appropriate salt bridges t o a large saturated calomel refcrencc electrode (SCE),used as an anode. .4fter plating for :i fi\-ntl length of time (measured with a stop watch) from an unst,irrc:d solution, the cell was switched instantaneously to the dissolution circuit and the current-time curve was recorded. Intcgration of the area under the curve was performed with a planime-
5.00
2.97 ~
1. 5 -.
4 . r, .
3 0 -.
__
-
?~howconsiderable variation. :i smaller electrode tends to have, Imth n greater plating rate and a greater residual current in proportion to its apparent area. Also, the ratio of the number of coulombs duc to residual current to those duc to silver is larger for a small elect,rodc. However, the increase in rcsidunl current i q prnportionat,ely greater than that due to silvcr. Thus, a Ion-er. limit on electrode size is imposed by the relatively large size of t h c residual current. .\n electrode size, optimum conipro~nisehctween plating rate and size of blank, appear3 to be about 0.2 sq. cni. ter. Blank Determination. It was necessary in all caws to subtract VARIABLES the residual current from the curve ohtained with silver, as might be expected (Figure 2). Experience has shown that a witable The principal variables are the electrode area, the blank due to blank can he determined most accurately by plating for the snmc residual current, the rate of changing the voltage during dissolulength of time from a similar solution having.no silver. It is untion, the motion of the solution, the t.emperature, the time coniortunate that such a correction is necessary, becaupc it c o n ~ t i stant. of tht! recorder, and t,he reproducibility of measuring the tutcs an element of uncertainty, particularly n t high emeitiviaren. ties where the background is not Iincxr. Rate of Change of Potential during RECORDER LEADS Dissolution. Thc effect, of the ratc of CELL *I change of potential during dissolution 4 LEEDS AND NORTHRUP was evaluated by plating from a loi J I DECADE RESISTANCE BOX silver solution onto a 0.5 sq. cm. electrode for 1 minute and then adjusting the speed of the motor-driven slide-n irr to different rates. The range covcrctl during a sweep was from 0.00 volt (thc potential a t which plating was done) to PLATING 4 +1.00 volt. us. S.C.E As the area due PLATING VOLTAGE / REVERSAL SWITCH DISPLACEMENT to current f l o ~during dissolution theoretically remains constant regardless of polarization rate, it is obvious that the upper limit on the rate is determined by PARALLEL OR the epeed of response of the instrument, INDEPENDENT PLATING and the Ion-er limit by the accuracy with OPERATION VOLTMETER nhich one can measure the area of a curve having a low peak and a long OTHER CELLS base, the latter being corrected by a subCONNECTED 1 . 5 ~DRY CELL dantial blank. The optimum range a8 IN WRALLEL shown in Figure 3 appears to be between 0.04 and 0.06 volt per second. Movement of Solution. Any motion 6 v PLATING BAT of the solution will result in an increased plating rate over that obtained in a stationary solution. Attempts to control Figure 1. Plating-Dissolutiori Circuit agitation arising from vibration met with only limited success. Current-time curves made with n stationary platinum electrode showed a Electrode Area. The larger the ares of the plating surface, the. larger the background due to the generation or removal of gas, variation of =t4% in area for a plating time of 3 minutes in risuallg oxvgen or hydrogen, during dissolution, and the larger 10-3 ,li solution. The error due to vibration should be d e the amount of silver that will plate out of a particular solution in a creased by using an electrode rotated a t B rapid, constant rate (9), or eliminated entirely by carrying the deposition to given time. Silver was deposited from a 1.00 X 10” -11silver completion before dissolving the deposit. solution in 0.1 N nitric acid on platinum electrodes nrhose apparPlanimeter Errors. The planimeter used in this study was deent surface areas varied from 0.002 to 2.0 sq.cm. for exactly 1 signed to measure areas up t o 10 to iz0.01sq. inch. The alternaminute a t a potential of 0.00 volt versus the saturated calomel electrode. Under these conditions, one would expect the ratios of tive method of cutting out the curves and weighing the paper was discarded because it seemed undesirable, particularly in the prethe amount plated to the electrode area as well as the ratio of the liminary work, to take the chance of destroying a result by makresidual current t o the electrode area to be constant. The data in Table I show this to be only approximately so. While the data ing an erroneous correction for the residual current.
9
?
-
-
IT23
t
V O L U M E 2 4 , N O . 1, J A N U A R Y 1 9 5 2
21 1
H y u&ig triaiiyles of several different sizes with kiiown are:ia. it was fouiid that the reproducibility and accuracy depended t o ? large extent on the skill of the operator. The precision of sucressive readings was greater for equilateral triangles than with those whose height was several times greater than the base. The ratios of the area to the distance traveled by the planinieter (thn circumference) should be kept as large as possible. The area of figures whose height was approsiniately twice the base (roughly thc dimensions of the curves nrt,iinlly rnconritered) rould be rlrtcrmined to zk0.01 sq. inch. Recorder. With a chart speed oC 10 iiiclics pcr i i i i n u t c :iiicl :L pen travel iimc of :itjout 3 second to cross the clinrt, :I siza1)h' (war n-odd be introduced if the current, changed rapidly during dissolution. Duiing exploratory runs it was found that the rur(1 a n d derrcnsrd relatively sloir-ly, providcd that thr, rate of ih:irigt. of voltage ivns not, too rnpid. ITcrrcr, t i t i - i w o r i l l trodii~~cd Ijy 1 hc rivordcr :ipprard t o hr, nrgligililic 5.6 L''OI
The cathode consists uf :I 1 sq. mi. piece of platinum foil, to which 3 copper wire is fused. The foil is embedded in beesm-ax in a 1 X 1 X 0.75 inch polystyrene box, keeping a fixed area of the foil uncovered to act as the active plating surface by placing a lightl!. greased S o . 1 cork borer against the center of the foil Ivhile t h r \\-asis poured. The technique of waxing the entire surface and then scraping away some of the \Tax to expose a n active surface proved unsatisfactory. The copper wire is coiled to ab$orb vibrations which might loosen the coating of was when thr w l l is being cmrinected into the circuit. Protection against evaporatioii is afforded bj- covering the box and by placing a feiv drop.: i l f miter in dcpre.csions i n the wns on either side o f thr foil. PLATINUM FOIL
7
,POLYSTYRENE
BOX
~
BEESWAX
HEAVY COPPER WIRE ( 13)
g. Ag. COPPER WIRE ( 20 )
.
0 '/2 L 2 INCHES
I
PLATINUM FOIL
SIDE VIEW
TOP VIEW
Elertrol3-is Cell for \ l i r m l i t f A i
Figure 4.
'
Toliirnc-
of
Snlu t i c i n
T h r salt l)ri(lgc : i i r t l rcictreiic~1 ~ 1 1 lie\\ II i i i I i g u r ~ t ~ conPist ol :in ag:ir-fillotl i ~ ~ i l l a tip q - joined I,! :i saturated aqiirnus potaesium sulfate solution to another agar tube. which rii1)s into :I connecting solution of potassium mlfate. .\n agxr bridge sattirated Tvith potassium chloride lorms the fiunl link to thr
SECONDS
1.00
0.75
0.25
0.50
saturated cnlomel electrode. The sulfate solution Ijrtwcn the tip and the rest of the bridgc :dlon-s for day-to mntraction of the Ppstem n.ithout pushina The
0
E vs S.C.E. E'ipitrc 2.
(:nulometric I)issnlutinn Curve fnr Silver
F), FGy
Temperature. It is knon-ii from polarography that thc diffuriori wefficicnt :tlonc increases 2 to 5% per C. ( 9 ) , so that i t tvab highly desirable to carry out a11 depositions a t :ipprosimately thc same temperature. The fluctuations in rooni temperature were ncvrr more thnn 1 0 . 2 ' c'. during a given set of determinations. Tlir tcini)craturo f+Ttli*t.of rourse, would be of minor importanre i f plating n-ew viirriid to c~~niplction.
SAT
K p SO4 SOLUTlOh
VAT \SAT.
K2S04
0
1
T
KCL AGAR BRIDGE
Z
A
R TYGON TUBING
2
w
TYGON TUBIN K z so4
LUTION
z
1.20
CAPILLARY TIP
__
MICROELECTROLYSIS
CELL
a 4
0
c" 1.10 a W a a
t
0 9om
Figure .5.
007
006
005 0
004
a03
9
POLARIZATION R A T E (VOLTS / S E C O N D )
Figure 3. Effect of Rate of Change of Potential during Dissolution on Area of Current-Time Curves
' S A T CALGMEC ELECTRODE
Standard C.cll and Salt. Bridge 3licroelrctrolysis Cell
for t - s e w i t h
I~xccsssolution is held in 3 reecrvoir n-hich IS open to atmospheric pr s u r e . In operation, the sample is.placed in the exposed foil and the cnpillarv tip of the salt bridge IS introduced into the drop through :i hole in the crll cover. IYlieri the salt bridge is not i n use, the tip is kept immcrscd in a saturated eolutioii of pot:iasiurv sulfate to prevent the agar from drying out. The solutions were inensured out hy rnr:iw o f : t lO-,ul micro-
212
ANALYTICAL CHEMISTRY
pipet in conjunction with a Clay-Adams 1-nil. pipetter. Pipetting Reproducibility. To determine the reproducibility of the reWas improved by grinding the tip down t o a point and thinly coatsults, five runs n.ere made each of several different electrodes ing it on the outside with silicone stopcock grease, thereby reover a period of a week. In each case, a 10-11. sample of 1.00 X ducing the size of the drop that adheres to the tip. After the sohtion in the pipet has been brought to the mark, excess liquid on 10-6 .?I silver was electrolyzed for 46 minutes and redissolved as the tip can be removed easilv without withdrawing any liquid described above. The results in Table I1 show that the average from inside the pipet by q u i c ~ l ybrushing the tip at an angle of areas for the dissolution curves obtained with different electrodes about 45" against a piece of filter paper. were different. The theoretical area obtainable from 1.00 X lo-'" K h e n a newly constructed cell is first placed in operation, the background is excessively high. Reduction of the background Inole of silver is 1.93 sq. inches at :L recorder sensitivity of 0.833 can best be accomplished by making three or four depositions to in condition^^ the surface, The background for p : ~ per . inch deflection, while the average actually obtained \vas than for freshly boiled nitric acid of the same concentration ; t,hewfore, sulfuric nrld was used in most of there studies. Per Cent Deposited L I S . Time. l i i 2 2 50 a IO-'' to 1.00 x IO-" v) I 1.00 IO-" to 2 50 a IO-1o ordcr to df.trrininc the length of tinic MOLES OF SILVER I MOLES OF SILVER 0 required t 1 1 dcpopit silver completely, several runs were made on samples consisting of 10 p l , of M silver followed by 10 pl. of 0.1 .V d f u r i c a acid wash solution. The electrode iil 050mas held a t a potential of 0.00 volt i's. cl SCE a t a temperatureof 28'C., and thp $ I ooz 0 dissolut,ion operation covered the range 5i 0 2 5 from 0.00 to +1.00 volt v8. SCE at a (? 050rate of 0.05 volt per second. The reil I I 1 I d t s (Figure 6) show that essentially 8 0 025 0.50 075 103 I 5 all the silver was removed from the 2 OUANTITY OF SILVER (MOLES x 101') 0 05 IO 15 20 25 solution a f t w 1 hour, but t,he areas QUANTITY OF SILVER (MOLES x do) indicate t,hat only about 80% of the Figure $. Calibration Curves Using Microliter Volumes Of s o l u t i o n t,heoretical amount of silver \vas ar1. 1.00 x 10-11 to 2.50 x 10-10 mole of.ailver counted for even after 1.5 hours of 2. 2.50 X 10-12t o 1.00 X 10-11 mole of silver plating. This value corresponds to a
-
'
-
5
z
100.
(
0
x)
M)
90
ficiency of a particular electrode increased gradually with usagc, 011 the order of 3% for 30 cycles of plating and dissolution. Thc normalized average deviation of areas of the disRolution curves for all the electrodes was 2.3Y0, corresponding to +0.033 sq. inch. Calibration Curves. Calibration curves were prepared for quantities of silver ranging from 2.50 X 10-l2 to 2.50 X lo-'" mole of silver in 20 p I . of 0.1 .Y sulfuric acid (1.23 X 10-1 to 1.25 x 10-1 ,I[). Because of the necessity of keeping the recorder pen on the chart a t all times, the curves were broken into tn-o range?, one from 2.50 x 10-12 to 1.00 X lo-" mole, and the other from 1.00 x 10-11 to 2.,50 x 10-10 mole. The sensitivity of the rerorder in the former case was set to 0.0833 pa. per inch deflection, and in the latter case to 0.833 pa. per inch. Figure 7 shows the calibration curves for the two ranges. I n the upper range, the relationship is linear and passes through the origin. I n the lower range, the relationship is linear down to 5 X mole of silver, hut the carve does not pass through the origin. This phenonlenon
V O L U M E 2 4 , NO. 1, J A N U A R Y 1 9 5 2
213
Studies have also been made of the deposition and dissolution of cadmium from micro volumes of solution using mercury electrodes. T h e results t,o date look very promising. DETERMINATIONS IN LARGE VOLUMES
In Forking with large volumes (25 ml.) of very dilute solutions, deposition onto a stationary microelectrode 1% ould not only be very time-consuming but also inaccurate, owing to variations arising from any irregular motion of the electrode or the solution. .\ccordingly, these studies were performed using a microelectrode rotated at a constant speed of 600 r.p.m. by means of a synchronous motor which, from polarography, is known to result in faster and more reproducible deposition. In these experiments, the silver was contained in 2 5 - d . portions of 0.1 potassium sulfate. Sulfuric acid was found to beunsuitable as a supporting electrolyte because hydrogen evolved during deposition of the silver resulted in a second dissolution peak which overlapped that for silver. ils extreme dilutions of silver \rere used, it \vas necessary to remove all possible contaminants by doubly distilling the ivater and pre-electrolyzing solutions of potassium sulfate using a mercury cathode. The deposition and dissolution were carried out in the manner described pre-
I
I
I
0
LITERATURE CITED
I
12
8
4
I n order to deposit silver from very dilute solutions, a 1 of -0.1 volt us. S.C.E. was required, so oxygen was removed before deposition by bubbling prepurified nitrogen through the samples. Variation of Dissolution Area with Time of Deposition and Area of Electrode. For a given electrode, the variation in the quantity of silver deposited, as measured by t.he dissolution area, is linear with the plating time, as shown in Figure 8. I n going from one electrode to another, the slope of the line increases with increasing electrode area. Contrary to Khat one might ordinarily expect, the lines intersect below and to the right of the origin. The fact that the intersection point lies to the right of the origin is probably due to the failure to dissolve the first layer of silver atoms deposited onto the platinum in the same regian of potential as the rest. It has been 'shown ( 1 4 ) that a considerably more positive potential is required to dissolve this layer than to dissolve those deposited onto silver. The fact that the lines intersect a t a point below zero indicates that although the best blank possible ha3 been used, an overcorrection has been niade. Variation of Dissolution Area with Concentration of Silver. Figure 9 shows a linear relationship between concentratioii and dissolution area for a particular rlectrode (3 mm. long) in the utnge from 2 X lo-' .If to 1 0 F .I/ silver 3t u constant plating time of 5 minutes a t a potential oi -0.10 volt us. SCE. This figure confirms the expectation that changes in plating time have exactly the same effect as corresponding changes of concentration. The curve has a reproducible translation corresponding to 0.41 sq. inch. By extending the plating time to 30 minutes, a similar calibration curve can be obtained with concentrations aa Ion as 5 X J I . By extending the plating time even further, correspondingly lower concentrations should be measurable. In general. the technique appears to be useful because it can be both qualitative and quantitative. By control of the cathode potential, one can select an element to be deposited; by measuring the coulombs during dissolution using an integration procedure, one run determine millimicrogram amounts or submicromolar concentrations of an element.
20
16
PLATING TIME (MINUTES)
(1)
Brown, It. A . , and Swift, E. H., J . Am. Cheni.
Soc., 71, 2717
(1949).
Figure 8. Dissolution Area us. Plating Time for Rotating Platinum lllicroelectrode
(2) ('ampbell, I\'. E., and Thoma*. V. R., Trans. Electrochem. Soc.. 76, 303 (1939). ) Cooke, JV. D , , and Furnian. S . H., . i s . 4 ~ CHEM., . 22, S96 (1950). 1 Elema, R., Antonie z'on Leeuwenhoek, J . Microhiol. S e d . . Jubilee T 7 d . Albert J . Kluyver, 12, 243 (1947). 13) Epstrin, J., Sohrr. H. d.,and Silver, S. D., ANN.. CIHEM., 19, 675 (1947).
(6) Farrington, P. S..and Swift, E. H., Ihitl., 22, 889 (19.30). (7) Francis, H. T., J . Electrochem. Soc., 93, 79 (1948). ( 8 ) Grower. 0.G . , Pruc. .4m. Soc. Testing Matcriols, 17, 129 (1917).
19) Kolthoff, I. AI., and Lingane, J. J.. " P o l a r o g r a p h y , " Ken, York
Interscienee Publishers, 1939.
(.lo) Linganc. J. J.. J . Am. Chern. Soc., 67, 1916 (1945). :11) Meier, D. J.. IIyers, K. ,J., and Swift. E. H., Ihid.. 71, 2340 (1949). (12) Myers, Ilass. 'The author. are indebted t o t h e Office of Naval Research a n d t o the l t o n i i c Energy Commission for supporting this work.
