sible volatilization of copper a t an elevated temperature or a n irreversible absorption of copper on the porcelain surface. It would appear, in light of the work reported herein, that a more satisfactory explanation should be based, primarily, on the presence of small and varying amounts of copper n ithin the porcelain cruciblc. As dry-ashing procedurt for the determination of copper in the latest edition of the AOAC methods ( 2 ) employs porcelain crucibles and thereby recommends their use, extreme caution should be exercised by proponents of this procedure in the use of these crucibles.
ACKNOWLEDGMENT
The authors wish to express their appreciation to A . L. Tester for allocation of funds supporting a portion of this study, to Donald McKernan and members of the staff of the Pacific Oceanic Fisheries Investigations for their assistance, and John J. Naughton for helpful comments and criticisms. They are also indebted to Kazuji Terada for technical assistance provided in the early stages of the study and to W. Y. Young of the Shipyard Laboratory, Pearl Harbor Naval Shipyard, Oahu, Hawaii, for spectrographic analvses.
LITERATURE CITED
(1) Assoc. Offic. Agr. Chemists, “Official and >,Tentative Methods of Analvsis, 6th ed., p . 122, 1945. (2) Zbid., 8th ed., p. 402, 1955. 1 3 ) Borchardt. L. G.. Butler. J. P.. ANAL.CHEX 29,’414 (19.57). ( 4 ) Cheng, K. L., Bray, It. H., Zbid., 2 5 , 655 (1953). (5) Sandell, E. B., ”Colorimetric Determination of Traces of Metals.” 2nd ed., p. 320, Interscience, i V e ~ York, 1950. (6) Tressler, D. K., Lemon, J. l l : , “Marine Products of Commerce. ’ 2nd ed., pp. 294, 299, Reinhold, S e i v York, 1951. >
,
RECEIVED for review August 22, 195i. Accepted Frbruary 21, 1958.
Automatic Titrator Based on Constant Current Potentiometric Titrations IRVING SHAIN and CALVIN
0.HUBER’
Chemistry Deportment, University of Wisconsin, Madison, Wis.
b An automatic titrator is described which is intended for use with constant current potentiometric titrations. These titrations ordinarily produce some sort of peaked titration curve. By restricting the applicability of the titrator to systems which produce this type of titration curve, it has been possible to build an instrument which is simple and inexpensive, yet sensitive and positive acting.
A
TITRATORS based on various kinds of electrotitrations can be classified into three main types: first, those involving automatic plotting of the titration curve; second. those involving termination of titrant flow a t a preset potential; and third, those involving termination of titrant flow on the basis of the shape of the titration curve (2, 5 ) . Automatic titrators of the third type are the most convenient and the easiest to operate. No detailed information (exact end point potential, exact rate of flow of titrant, and the like) need be known for the successful automatic operation of the titrator. This type of device was described b y Malmstadt and Fett (6), whose derivative circuit has been applied to several types of titrations. The automatic titrator described here is also of the third type, and was designed specifically for use with constant current potentiometric titrations. This means of end point detection
UTOXSTIC
1 Present address, Rockford College, Rockford, Ill.
1286
0
ANALYTICAL CHEMISTRY
has been described by Reilley, Cooke, and Furman ( 7 ) . Two indicator electrodes are polarized b y a small constant current, and the potential developed between the electrodes is measured during the course of the titration. The resulting titration curve frequently has a peaked shape (Figure 2,a). By restricting the applicability of the titrator to systems which produce some sort of a peaked titration curve, it has been possible to design an instrument which is simple and inexpensive, yet n-vhich is positive acting and sensitive. DESCRIPTION
OF TITRATOR
This automatic titrator (Figure 1) consists of an amplifier circuit (VI) and a trigger circuit (V2 and relays). The amplifier circuit is arranged so that only the potential change immediately after the end point reaches the trigger circuit to stop the flow of titrant. Amplifier Circuit. T h e potential developed between the electrodes in t h e titration vessel is amplified in triode V I A . Figure 2, a and b, illustrates t h e shape of a typical titration curve a t t h e grid and plate of V I A . This amplified signal is coupled t o t h e grid of V1B through C1. T h e grid of V l B , however, is clamped to ground by the silicon diode, X. The silicon diode has a very high resistance in one direction and a low resistance in the other. It is placed in the circuit so that any tendency of the grid of V1B to go negative is grounded. Conversely, if the signal goes positive (as after the end point), a very high resistance t o ground appears. Thus, the titration curve a t
the grid of B1B appears as in Figure 2,c. This signal is amplified in triode V l B , and the titration curve a t the plate of V1B appears as in Figure 2,d. Thus, during a titrat,ion the potential a t the plate of V1B remains constant until immediately after the end point. At that time a large decrease in potential appears a t the plate of T U ? . This sudden change in potential is used t o activate the trigger circuit which, in turn, stop2 the flow of titrant. Trigger Circuit and Relays. The duo-triode, P 2 , is arranged in a Schniitt-type, bi-stable trigger circuit ( 8 ) . In this circuit only one section of the duo-triode can conduct at any one time. Potentiometer R11 is set so that V 2 9 coilducts current before the end point. The large negative pulse a t t8he grid of 1-26 imniediately after the end point cause3 1’2.4 to cut off. T’2B then conduct>,and relay 1 closes. Relay 2, a lucking-type relay, also closes and operate5 tht. buret solenoid, stopping the flov of titrant. Because relay 2 is now locked in this position, the flow of titrant remains stopped until the next titration i.; Jtarted 117pressing the push hutton w i t c h , 82. Pilot lights are provided: *YE1 indicates the power on, and S E 2 indicates a completed titration. Cell Current. The cell current n.as supplied by a 4.j-wlt battery with a 5- or 20-megohin resistor in series with the cell. This allon-ed choice of either 9- or 2.2-,ua. cell current. The cell current also may be taken from the po\Ver supply. Power Supply. The titrator requires 110 volts alternating current for operation of the relay and solenoid circuit. T h e tulles require a total of about 6 ma. a t 300 voltsdirect current. An electronically regulated supply was
lOOVDC
-
inl
i
a.
-1 -Y b.
e
I
RELAY 2
M L.
I
I
ML.
Figure 2. End point portion of typical titration curve showing shape a t various points in titrator circuit a.
T O BOLLNOID'QPCRbTED BURETVbLVE
lit. R?. R3.
R4.
Figure 1. Circuit diagram of titrator 1 megohm C1. 1 pfd. 15 kohms C2. 0.1 pfd. 5.6 kohms S1. S.P.D.T. po\ver
switch
R5.
560 ltohnis
R6.
10 kohms
R7. R8. R9
10 kohms 27 kohms, 1 vatt 22 kohms. 1 watt
R10.
100 kohms
Rll. R12.
500 kohms 390 kohnis
I
S2. S.P.S.T. push-button sivitch X . Silicon diode 15300
(Raytheon) Vl. 12AX7 duo-triode V2. 12AU7 duo-triode NE1, .VE2. Keon pilot lamp, KE-51 Relay 1. 10,000-ohm, S.P.D.T.plate circuit relay Relay 2 . 110-volt, alternating current operated. D.P.S.T.+ S.P.D.T. locking relay
R13, R14. 56 kohnis All resistors 0.5 matt unless otherwise noted.
used in this work. The filament supply was 6.3 volts alternating current. Buret Assembly. Ordinary 25-ni1. burets were used. T h e titrant flowed from t h e tip of the buret, t,hrough a short piece of capillary tubing t o control t h e rate of flon-, through t h e solenoid-opera t,ed b w e t clamp, and t'hen through t h e delivery tip. Connections were made with rubber or plastic tubing. The solenoid clamp (Guardian, 110 volts, alt'ernating current) merely pressed closed the short piece of rubber t,ubing between the capillary tubing and the delivery tip. The action of this clamp was found to be rapid and reproducible. Titration Cell and Electrodes. T h e titration cells were 150- or 300-ml. beakers. T h e solutions were stirred during titration with a magnetic stirrer. Gold microelectrodes were used. T h e y were constructed by force fitting gold \\-ire (0.81-mm. diameter) into R hole in a small piece of Teflon ivhich \vas threaded to fit t h e end of a Teflon tube. Contact was made by :I copper wire soldered t o the gold heforc ass~mbling the two i
pieces of Teflon. The gold mire was cut off to a length of 11 mm. Placement of the electrodes and delivery tip was not critical for titrations a t the 1% error level. I n several of the titrations studied, the O.lY0 error level was realized by placing the drlivery tip and cathode opposite each other near the walls of the titration vessel. The anode position was not critical in these titr a t'ions. Titrations of ferrous ion n ith cerium(1V) were tried with both gold and platinum electrodes. Potentials reached in the course of this titration are quite oxidizing, and thus the effects of electrode oxidation were made more obvious. As expected (1, d ) , gold generally gave better results. The end point peak potential 1% as higher and less drifting of potentials occurred. RESULTS OF TITRATIONS
In order t o test the automatic titrator, titrations mere performed both manually and automatically using several different reactions (Table I).
Input signal (titration curve)
b. Signal a t plate of V1A c. Signal a t grid of VlB d. Signal a t grid of 1'2-4
Ferrous Ion with Cerium(1V). T h e ferrous-cerate titration was studied by Reilley, Cooke, and Furman (7'). It was performed b y adding a 15inl. aliquot of 0.1.11 ferrous sulfate solution to 50 nil. of 1Jf sulfuric-0.5V phosphoric acid solution. and then titrating with 0.1111 cerium(1V) sulfate. The manual titrations FTere performed by reading the potentials between the electrodes n i t h a vacuum tube voltmeter. Automatic titrations were performed a t a flow rate of 4 ml. per minute and s cell current of 2.2 pa. Making the colution 1-11 in hydrochloric acid affected neither the precision nor accuracy of the titration. Manganous Ion with Permanganate. T h e constant current potentiometric titration of manganous ion 11i t h permanganate m s studied by Huher and Shain ( 3 ) . The adaptation of the Lingane-Iiarplus method !vas used here. The titration was performed by adding 10-ml. aliquots of 0.03-If manganous sulfate to 100 ml. of saturated sodium pyrophoydiate, adjusting the p B to 5 to 7 , and titrating with 0.02-If potassiuni permanganate. The manual titrzitions n ere performed by reading the potential betneen the electrodes with :Lvacuum tube voltmeter. Automatic. titrations virre performed a t a flon rate of 2 ml. per minute and cell current of 2.2 fin. F l o ~rates greater than 2 nil. per minute increased the error rapidly, as the chemical reaction is rather slow. Thiosulfate with Cerium(1V). T h e constant current potentiometric titration of thiosulfate with cerate in t h e presence of excess iodide has been described b y Reilley, Cooke, and Furman ( 7 ) . This titration was performed by adding a IO-ml. aliquot of 0.231 sodium thiosulfate to 100 ml. of 0.1M potassium iodide, and titrating with 0.1N cerium(1V) sulfate. The VOL. 30, NO. 7, JULY 1958
1287
Table 1.
Precision and Accuracy of Automatic Titrator on Various Titrations
Results, Ml. Manual Std. dev. Automatic Titration“ 10.01 13.47 13,46 Fe++ with Ce(1V) 5.90 5.90 *o. 01 Mn++ with MnOa11.73 11.75 10.02 S2O3-- with Ce(1V) 11.72 11.74 10.01 C1- with Ag a Details given in text. * Averages and standard deviations of at least five replic,ate titrations. +
Table II. Effect of Flow Rate on Automatic Iron(l1)-Cerium(lV) Titrationa
Flow Rate. AIL p& Minute
End Std. Dev., Point, 1Il.b M1.b 1 5 90 1 0 01 2 5 92 i 0 01 1 0 01 4 5 92 6 5 93 i 0 02 12 5 90 1 0 07 a For manual titration: end point, 5.91 ml.; std. dev., 1 0 . 0 1 nil. Results are from at least five replicate titrations. averages and standard deviations of at least five replicate titrations.
manual titrations were performed by titrating to the starch iodine end point. The automatic titrations were performed at a flow rate of 4 ml. per minute and a cell current of 2.2 pa. Chloride with Silver. I n order t o test t h e automatic titrator on a precipitation titration, t h e titration of chloride ion with silver was investigated. I n this titration t h e potentials before t h e end point are determined b y t h e chloride oxidation for the anode and t h e reduction of dissolved oxygen for the cathode. As the chloride concentration becomes
Std. dev. fO.O1
f 0 . 03 10.05 1 0.03
low just before the end point, the anode shifts to more positive potentials and the anode reaction becomes the evolution of oxygen. Just after the end point, the cathode also shifts t o r a r d more positive potentials and the cathode reaction becomes the reduction of silver ions. These potential changes result in the peaked titration curve required for operation of the titrator. The titrations were performed by adding 25-ml. aliquots of 0.04M potassium chloride to 25 nil. of 1X nitric acid in the titration vessel and titrating with 0.111f silver nitrate. The manual titration was performed to the dichlorofluoroscein end point. The automatic titration was performed a t a flow rate of 3 nil. per minute and a cell current of 9 pa. Gelatin was added (0.05%) to prevent reduction of colloidal silver chloride. The flow rate of titrant in this titration is severely limited by the efficiency of the stirring. Effect of Flow Rate and Dilution of Solutions. Because the ferrouscerate titration involves a fast reaction and is relatively free from complications, i t was used t o test the effects of varying t h e rate of titrant flou- and diluting the solutions. Table I1 indicates t h a t for this ideal
titration, flow rates as high as 12 ml. per minute can be used and t h e results remain within the 1% error level. For slower reactions, of course, the maximum flow rate will be lower. \Then the concentrations of both the ferrous and cerate solutions Tvere reduced to 0.01JI and the titrations performed as before, the error increased considerably. Standard deviations increased from 0.02 ml. for 0.1S solutions to 0.09 nil. for 0.01.Y solutions. ACKNOWLEDGMENT
The authors wish to thank E. H. Schraut of the University of IYisconsin Electrical Engineering Department for his suggestions during the development of this titrator. The work was supported in part by the Research Committee of the University of \Irisconsin with funds provided by the Kisconsin Alumni Research Foundation. LITERATURE CITED
(1) Baumann, F., Shain, I., ANAL.CHEM. 29, 303 (1957).(( ( 2 ) Delahay, Paul, Ken- Instrumental Methods in Electrochemistry,” p. 382. Interscience. Sew York. 1954. (3) Hubei., C. O., Shain, I., ASAL. ’CHEV. 29, 1178 (1957). (1)Iiolthoff, I. M.,Tanaka, S . j Ibid., 26, 632 (1954). ( 5 ) Lingane, J. J., “Electroanalytical Chemistry,” p. 128, Interscience, S e x Tork, 1953. ( 6 ) lIalmstadt,, H. V., Fett, E. R., .$SAL. CHEM. 26, 1318 (1954). ( i )Reilley, C. S . ,Coolie, K. D., Furman, S . H., Ibid.,23, 1232 (1951). 181 Schmitt,. 0. H.. J . Sci. Instr. 15. 24 (1938). \
I
RECEIVEDfor review July 29, 1057. ;\ccepted February 6, 1958.
Precipitation of Cadmium Sulfide from Acid Solutions by Thioacetamide DAVID F. BOWERSOX and ERNEST H. SWIFT California Institute of Technology, Pasadena, Calif. ,The precipitation of cadmium as sulfide by thioacetamide from acid solutions has been studied. In solutions having p H values of 2 or less the rate of precipitation is controlled by the hydrolysis of the thioacetamide to give hydrogen sulfide and is first order with respect to the hydrogen ion and to the thioacetamide concentrations. In solutions having pH values from 6.3 to 3.3, a direct reaction occurs in which the rate of sulfide precipitation is first order with respect to both the cadmium ion and the thioacetamide concentrations, and is in-
1288 * ANALYTICAL CHEMISTRY
Swift and Butler (4). They found that at p H values less than approximately 3 the rate of precipitation was hydrolysis controlled and was first order - d[Cd(II)]/dt = with respect to both the thioacetamide k[Cd(II)][CHBCSNH~]/[H+]”* and the hydrogen ion concentrations. is 8.1 X literliz min.-’ The precipitation of lead sulfide at a t 90” C. in 0.15M sodium formate higher p H values was governed b y a solution. The energy of activation for direct reaction which was dependent this reaction was calculated to b e 20.8 upon the lead ion and on the thioacet=k 1.2 kcal. per mole. amide concentrations to the first order, and on the hydrogen ion to the inverse half order. I n the precipitation of triHE precipitation of lead sulfide no evidence was positive arsenic (0, from solutions similar to those in found for other than the hydrolysisthis investigation were studied by
versely half order with respect to the hydrogen ion concentration. The velocity constant, k, in the expression
T
