weigh to the nearest 0.002 mg. With the disk in place in the electrolysis cell, introduce the active buffered solution. Electrolyze for 40 minutes at 1.5 to 1.8 volts at room temperature. Remove the still active solution through the glass side arm of the cell. Other deposits may be obtained from this solution, if desired. Rinse out the cell with doubledistilled water, and remove the disk, which nom contains oxide deposit. Wash the deposit with double-distilled water and remove adhering water droplets with a piece of filter paper. Allow the deposits to dry in air until no water is visible. Place in a 40” oven for 2.5 hours and weigh the cobaltic oxide trihydrate deposits. Deposits usually weigh about 1 to 2 mg. Determine the activity of the deposit by placing the disk in a planchet in position in the sample changer. Use a TO mg. per sq. cm. aluminum absorber. Correct for dead time, background, and efficiency on the observed activity and calculate the specific activity. All samples were counted for about 20 minutes. Table I s h o w that the method is satisfactory for a series of representative samples which vary considerably in composition.
standing is 30 minutes instead of the 24 hours that is usually recommended to ensure complete precipitation (4). This reduction is possible because a large percentage of the cobalt is precipitated in the first half hour and the isotope dilution technique does not require isolation of all the cobalt. Thus, even double precipitations may be made within a reasonably short time. Two or three deposits of cobaltic oxide trihydrate may be made from the same cobalt solution. The precipitation of potassium cobaltinitrite is satisfactory for the separation of cobalt from large amounts of iron, although several precipitations may be necessary (1, 7 ) . If the iron present in an alloy weighs a t least ten times as much as the cobalt, i t is convenient to perform a t least one ether extraction of iron before the precipitation. The volume of the solution was kept a t a minimum in order to increase the rate of cobaltinitrite precipitation. The combination of the two processes for reduction of volume (evaporation and sodium hydroxide treatment) was more rapid and convenient than either alone.
DISCUSSION
ACKNOWLEDGMENT
I n the separation of cobalt by the cobaltinitrite precipitation, the time of
The authors wish to acknowledge aid to Darnel1 Salyer in the form of a
predoctoral fellowship from the Cincinnati Chemical Works. LITERATURE CITED
(1) Faleev, P. V., Zaoodskaya Lab. 8 , 381 (1939). (2) Hague, J. L., Maczkowske, E. E., Bright, H. A , , J . Research iVatl. Bur. Standards 53, 353 (1954). (3) Harris, \Ir. F., Sweet, T. R . , ANAL. CHEM.26, 1649 (1954). (4) Hillebrand, W. F., Lundell, G. E. F., Hoffman, J. I., Bright, H. A., “Ap-
plied Inorganic Analysis,” 2nd ed., pp. 419-20, Wiley, Xew York,
1953. ( 5 ) Kallmann, s.,ANAL.CHERI.22, 1519 (1950). (6) Lundell, G. E. F , Hoffman, J. I . ,
Bright, H. -4., “Chemical Analysis of Iron and Steel.” a . 334. Wilev. Sew York, 1931. (7) Nikolow. C.. Przemusl Chem. 17, 46 I
I
I ,
’ (i933j. ’ (8) Salver, D.. Sweet, T. R., 4 x . 4 ~ . CHEM.28, 61 (1956). (9) Theurer, K . , Sweet, T. R., Ibzd., 25, 120 (1953). [lo) Young, R. S.,“Industrial Inorganic .4nalysis,” pp. 77, 82, Wiley, New Tork, 1953. i l l ) Young, R . S.,Pinkney, E. T., Dick, R., IND. ENG. CHEJI., ANAL.ED. 18, 474 (1946).
RECEIVED for review February 2 i , 1956. Accepted October 1 1 , 1956.
Chronopotentiometric Analysis in Fused Lithium Chloride-Potassium Chloride H. A. LAITINEN and W. S. FERGUSON Department of Chemistry and Chemical Engineering, University of Illinois, Urbana, 111.
b
This chronopotentiometric investigation was prompted by the need for analytical procedures that are directly applicable in molten systems a t high temperature. With platinum microelectrodes, measurements upon the chlorides of bismuth(lll), cadmium(ll), silver(l), and copper(1) in a eutectic mixture of lithium chloride-potassium chloride a t 450” C. showed the theoretical relationship among applied current density, concentration, and transition time. The mixtures bismuth(lll)-silver( I) and bismuth(ll1)-copper(1) also gave the expected results. For times less than 5 seconds the mass transport was not As the complicated b y convection. diffusion field of the electrodes used was not physically constrained, the response of very small electrodes departed from linear diffusion control The diffusion even within 5 seconds. coefficients of bismuth(lll), cadmium-
4
ANALYTICAL CHEMISTRY
(II),silver(l), and copper(1) were, respectively, 0.6, 1.7, 2.6, and 3.5 X 10-5 sq. cm. per second. The chronopotentiometric method of analysis in fused salt solutions using electrodes without physically constrained diffusion fields was performed with an accuracy within f.2.6‘70. procedures capable of direct application to molten systerns a t high temperature are of increasing importance in present-day technology. S e e d for such procedures is evident, for example, in metallurgical slagging, the manufacture of glasses. metallurgy and electrorefining, and the use of fused salt solvent systems in some types of atomic reactors. Potentiometry (9, 12, 17, 20) and polarography (1, 2, 6, 15, 14) have been applied to analysis in molten systems. SALPTICAL
The chronopotentiometric method has inherent advantages which warrant its application to analysis in melts. Analysis by potentiometry is limited to a single component of a mixture, whereas chronopotentiometry is applicable to the simultaneous determination of several components. At high temperature the polarographic method must in general be carried out a t a solid microelectrode under conditions such that mass transport is a t best a mixture of diffusion and convection. Furthermore, the product of the polarographic process is frequently a solid metal of dendritic form which grows out into the solution, causing the limiting current to be variable n-ith time and nonreproducible. The chronopotentiometric method can, in contrast, be carried out under such conditions that (virtually) all mass transport is by linear diffusion, and the total amount of electrolysis product
formed is too minute to produce troublesome dendritic deposits. The specific objective of this work was to apply the chronopotentiometric method t o analysis in a lithium chloride-potassium chloride eutectic solvent at 450’ C. Chronopotentiometry is a method in which the potential of a polarizable electrode is measured during the constant-current electrolysis of a depolarizer in an unstirred solution containing a supporting electrolyte. During the electrolysis the availability of the depolarizer a t the electrode surface is assumed t o be limited only by seniiinfinite linear diffusion from a uniform bulk solution to the electrode surface. As the concentration of the depolarizer a t the electrode surface approaches zero, the potential a t the electrode changes very rapidly. The duration of the electrolysis required t o produce this effect is termed the “transition time.” Recent interest in transition time measurements as a means of chemical analysis has developed since Gierst and Juliard pointed out the analytical utility of this measurement (10). Delahay and Namantov (4) suggested the name “chronopotentiometry” for the method and reviewed the analytically significant aspects of the theory. The chronopotentiometry of systems containing one, tn-o, and three depolarizing species in various aqueous solution combinations \vas investigated a t platinum and mercury pool electrodes by Reilley, Everett, and Johns (18). Gierst and Xechelynck have reported upon a general chronopotentiometric study of aqueous systems ( 2 1 ) . Xicholson and Karchmer applied the method to a determination of lead in nitric acid (16). Finally, certain titration end points have been detected by chronopotentiometric means (19). As presented by Delahay and llaniantov (4),the fundamental relationship of analytical significance in chronopotentiometric work under the above conditions is
APPARATUS AND PROCEDURE
Cell. 911 parts of t h e cell were borosilicate glass, except as otherwise noted. The cell components were contained within a side-arm test tube 330 X 60 mm. in outside diameter. This tube, filled with the solvent to a depth of 45 mm., was mounted rerticallv, with 100 mm. of the lower end inside an electric furnace. Portions of the solvent within the container were compartmented one from another, yet maintained in electrolytic contact by several smaller test tubes having fritted-glass bottoms. A KO. 13 rubber stopper containing three 16-mm. holes and four IO-mni. holes closed the top of the container and supported within the tube a thermocouple sheath, a n inert-gas delivery tube, and various electrodes. The solvent was maintained under a continuous slow flow of dry oxygen-free argon, which escaped via the side arm of the large tube: the holes in the stopper lvere closed by glass plugs when not in immediate use. Standard additions of solute were made to the solvent in the compartments of interest with the aid of a platinum spoon mounted in a long glass handle, manipulating through the 16-mm. holes of the stopper. At the conclusion of a n experiment the conipartments containing the fused salt qolutions n-ere removed from the system and their contents analyzed for chloride by a llolir titration. Assuming for the case of dilute solutions that the total number of moles present was equal to the moles of solvent present and that the density of the solution n-as equal to the density of the solvent, the compositions of the solutions nere expressed as Weight percentage 1Iole fraction
(4)
100 IP mJ1, ~
(3)
17
Msm ~
02’
R
This equation, which was first derived by Sand (RI), expresses the relationship betu-een the bulk concentration, C. of a depolarizer having a diffusion coefficient D and the transition time of an electrolysis proceeding a t constant current density io. The other symbols have their conventional electrochemical meanings. If a second depolarizer, undergoing reaction a t a potential different from the first, is present in the solution, two transition times will be shown. The relationship betiveen the transition time and the bulk concentration, C2, of the second depolarizer is
=
=
here TI’
solute weight in grams moles of chloride in the solution M e = average formula weight of the eutectic solvent d = density of the eutectic solvent in grams per cc. (22) 11- = formula weight of the solute. TU
= =
The conversion factors are: (4)to ( 5 ) . 29.i; ( 5 ) t o ( 3 ) ,0.0337. Reference Electrode. A platinum foil in equilibrium with a dilute solution of platinuni(I1) i n t h e fused salt solrent was used as t h e reference electrode in this 11-ork. This system has been shown t o determine a stable, nonpolarizable reference potential and has been described in various modifications
( 7 , 16). The reference system was confined t’o one of the comparbment’s of the cell. Indicator Electrodes. Five platinum electrodes were prepared and their projected areas determined b y optical micromet’ry. -411 of t h e glassto-plat’inum seals were made using Corning 0120 glass. Electrodes 1 a n d 4 consisted of, respectively, 18- (1.024 nim. in diameter) and 26- (0.405 mm. in diameter) B B: S gage wires projecting symmetrically from the sealed ends of 6-nim. glass tubes for 2.691 and 0.518 mni. Their total projected areas were 9.48 and 0.788 sq. mm. Electrodes 3 and 5 consisted of, respectively, 18- and 26-gage Tykes sealed symmetrically into the ends of 6-mni. glass t’ubes, which tbsequenbly ground and polished hour” metallurgical polishing alumina to yield only cross-sectional exposure of the wires having projected areas of 0.823 and 0.129 sq. mm. Electrode 2 consisted of a rectangular piece of 2-n~iIfoil 3.133 X 2.785 mm.. haring a total projected area of 17.5 sq. mm. I n use this electrode was suspended just below the surface of the solution in a veitical plane by a 5-mil wire. Working Electrode. A platinum foil 1 cni. square, which was in one of the compartments of t h e cell, was used as the working electrode in t h e electrolyses. Furnace. Cenco-Cooley S o . 13627, I l j - v o l t , iOO-watt’ (Central Scientific c‘o., Chicago, Ill.). A separate heating circuit equivalent t o 2 5 7 , of t’he original furnace heating capacity \vas installed in t h e bottom of t h e furnace cavity. Only this auxiliary circuit mas operated by t h e furnace controller. Controller. Wheelco indicating controller l l o d e l 241-P (Barber-Coleman c‘o.. Rockford, Ill.). Amplifier. Doelcani direct current amplifier. Xodel 2 HLA-4 with AH106 plug- in unit (Doelcani Division of llinncapolis Honeywell, Boston, Mass.). Recorder. Varian graphic recorder, Ifode1 G-10. 50-mv., 1-second full scale deflection (Varian Associat’es, Palo d l t o , Calif.). The chart speed of this instrument was increased t o 4 inches per second by direct drive with a Botline capacitor motor T y p e KYC23RB. 115-volt, 60 r.p.ni. (Bodine Electric Co., Chicago, Ill.). -4 timedelay relay circuit, energized by t h e pon-er t o the chart-drive motor, was installed t o control initiation of t h e constant current electrolysis. Current Source. A battery-poweyed source of t h e type described by Reilley. Everett, and Johns (18) u-as used. During a n experiment the source was a r ~ i n g e dto deliver current a t all times, either through the experimental vel1 or bypassed around it, so that the 1,esistors of the current, source came to temperature equilibrium. The electrolysis current was determined a t leisure before the electrolysis was initiated from the I R drop across the standard resistor, the “bypassing” current being in series n i t h a potentiometer which was adjusted to simulate the back electroVOL. 29, NO. 1 , JANUARY 1957
5
motive force exhibited by the cell during the actual electrolysis. Potentiometer. Student potentiometer (Leeds & S o r t h r u p , Philadelphia, Pa.). Chemicals. T h e anhydrous chlorides used as solutes were kindly supplied b y C. H. Liu of this laboratory, and had been prepared as follons. Cadmium Chloride. Vacuum desiccation of analytical grade dihydrate over anhydrous magnesium perchlorate. Bismuth(II1) Chloride. Commercially available anhydrous sample stored over anhydrous magnesium perchlorat?. Cuprous Chloride. I’repared according to the procedure of “Inorganic Syntheses’’ (8).
Silver Chloride. able sample.
of the depolarizer a t the electrode suiface approaches zero, the potential of microelectrode A again changes very rapidly. The transition time is determined by meisuring the displacement between the index of zero time and the second break in the potential curve. The microelectrode is stripped free of any metal plating before beginning the next transition time messurement bj. switching DPDT to position 2 . This short-circuits the plated microelectrode to the reference electrode through a potential which is adjusted so as to he positive n-ith respect to dissolution of the plate but negative u-ith respect to platinum metal diqsolution.
Commercially availEXPERIMENTAL RESULTS A N D DISCUSSION
Solvent Preparation. A practical method has been norked out for the preparation of the fused salt solvent, 11hich prevents hydrolytic decomposition during t h e fusion, yielding a melt suitably pure for microelectrode n o r k ( 7 ) . This topic is to be treated in detail in a separate paper. Procedure for Measurement. Figure 1 is a block diagram of the equipment used for measuring the transition times. K i t h DPDT in position 1, microelectrode -4is shorted to reference electrode B through a 2-megohm resistor. A microvolt signal from part of the IR drop between A and B is taken as the input to the amplifier where the signal is amplified 1000-fold. The output of the amplifier drives the pen and ink recording potentiometer. The voltage divider is properly adjusted to make tlie recorder correspond to 2 volts full scale. -4 transition-time measurement is started by switching poner to the recorder chart drive motor. Kithin 1 second the rate of the chart drive comes up to its full value of 4 inches per second, after which a 1-second time-delay relay switches the constant-current source from its “short-circuit” position (bypassing the cell) to its “output” position. The potential of microelectrode A shifts rapidly to the deposition potential for cadmium chloride and serves as an index of zero time. R h e n the Concentration
The x-alidity of Equation 1 n a s investigated for reductions of solutions of bismuth chloride, cadmium chloride, silver chloride, and cuprous chloride. As the polarographic reduction of cadmium chloride under the conditions of these experiments was knonn to be essentially reversible and the reduction product is a liquid metal, this solute was chosen as the model upon n hich the most extensive chronopotentionietric data were taken. The standard potentials of these metal ion-metal couple. in the eutectic solvent us. the platinum reference a t 1 M platinum(I1) concentration have been estimated in this laboratory to be: Bi(II1)-Bi, -0.46 volt; Ag(I)-Ag, -0.70 volt; CU(I)-CU, -0.91 volt; and Cd(I1)-Cd, - 1.15volts. Mixtures consisting of bismuth chloride-silver chloride and bismuth chloride-cuprous chloride \vere used as systems upon which to test the applicability of Equation 2 to consecutive processes. Bismuth(II1) reduction was the initial process in each case. Figure 2 is an example of a n experimental potential-time curve for cadmium chloride reduction. A construction line has been drawn through the potential break marking zero time for the electrolysis. The transition time is measured, as indicated by the construction lines, to the point of (virtual)
I
0 DC
0
AMPLIFIER
Figure 1.
6
Block diagram of transition-time apparatus
ANALYTICAL CHEMISTRY
linearity of tlie r2corcied potential-time function. Theoretically the steepest angle calculated from the chart speed and the pen speed which the potentionieter is capable of recording is 51’20’. As the linear portion of the curve in Figure 2 is rising a t an angle of about j l ” , the apparatus is shown to be operating a t its mavimum efficiency. The precision with which the transition times can be measured is of interest. Table I is a set of 11 successive determinations of the transition time of a cndniium chloride solution.
Precision of Transition-Time Measurements (CdC1:. 2.08mM. Current density. 15.8 pa. per q q . mm.) 71’2 Run 7, S O . See. Sec.i ’ 2 1 922 1.386 1 1 430 2 047 2
Table I.
3 4 5
ci-
i9
10 11
Average Std. dev.
2 2 2 2 2 2 2 2 2 2 +O
188 219 156 078 063 031 000 047 094
Oii 084
(+4 0 % )
1 4i8 1.488 1.467 1 440 1 436 1 423 1 414 1 430 1 4-16 1 440 f 0 029 (+2 0%)
The data of Table I shorn that for transition times on the order of 2 seconds the square root of the transition time \vas determined with a standard deriation of +2.0%. The transition times in Table I increase to a maximum a t run 4 and then decrease to a minimum a t run 9. This drift may be the result of the long period of the furnace controller “off-on” cycle. The practical limitations upon the range of the transition times which could be measured in these experiments were. for short times, the response of the recorder and, for long times, the interference of convection with diffusion transport of the depolarizer. From an examination of Figure 2 , it is apparent that transition times shorter than about 0.5 second are determined with relatively poor accuracy. Experience showed that whenever the transition time was shorter than about 5 seconds the io+’* product v a s constant for a particular bulk concentration of the depolarizer, as required b y Equation 1. The values of ior’ began to show positive deviations whenever the transition time exceeded about 5 seconds, irrespective of the depolarizer concentration over the concentration ranges investigated. According to Equation 1, the product iod‘*should be proportioiial to the bulk concentration of the depolarizer.
\
Ln J I-
0
>W U
z W
1.6 y. W
d
fz -
1.2
’
k
\
\
J
n
0.8
>
w I 0.4 I
I
0.5
1.0
0
TIME, Figure 2.
SECONDS
Experimental curve for cadmium chloride reduction
The espcriniental results presented in Figures 3 and 4 shoiv this to be the case for reductions of cadmium chloride, bismuth chloride, silver chloride, and cuprous chloride. Each of the points plotted is the average of from 8 to 30 taken from 4 to 13 observations of different current densities for each concentration of depolarizer. The data for cadmium chloride reduction make up two sets (one for electrode 1 and another for electrode 2 ) of 10 points each over the concentration range 2.05 to 79.0 niM. Least square lines were calculated and are drairn in for the t n o sets of points. The liiiearity of the plots is j udged satisfactoryall but 2 of the 20 points fall vithin +2.670 of their least square line. Mass transport to electrodes 1 and 2 is evidently under the same conditions, because the electrodes show essentially identical response per unit area, although their gross geometries are, respectively, cylindric and planar. The transport process is concluded to be a good approximation to linear diffusion. An assumption of linear diffusion means that diffusion is constrained to a direction perpendicular to the plane surface of the electrode. Kithout physical constraint linear diffusion applies only if the dimensions of the electrode are so large that the electrode surface approsimates a n infinite plane as compared with the diffusion layer thickness. The points determined for the electrolysis of bismuth chloride, silver chloride, and cuprous chloride also lie 011 linear plots with good consistency. These data were all taken with the use of electrode 1. All four of the solutes investigated in the fused salt system are thus shown to be amenable to chemical analysis by the chronopotentiometric method.
Extrapolation of the plots of Figures 2 and 3 to zero depolarizer concentration shons that there is a “residual” transition time r h i c h must be associated n i t h the solvent alone. The source of the depolarizers responsible for the residual transition time may well be minute amounts of impurities in the colvent, nhich in fact constitutes a supporting electrolyte a t least 30 times more concentrated than ia coni entionally employed for aqueous chronopo1s. C a t tentiometry. Plots of particular current densities show that * to be expected a t the masimum zero depolarizer concentration is be-
tneen 0.3 and 0.1 see.’ 2. corresponding to 0.09 and 0.01 second for 7, the measured quantity. Oscillographic registration would be necessary to measure these small times so that direct corrections could be applied. Some points determined from measurements on the bismuth chloridesilver chloride and bismuth chloridecuprous chloride niixtures are includetl in Figure 4. These points lie reasonably close to the lines drawn through the points determined from measurements on the solutions of the individual respective solutes. Equation 2 is thus shon-n to be applicable to these mixtures. L-nder the conditions of the consecutive process experiments, the second process involves the codeposition of silver metal (or copper metal) along n i t h bismuth a t a liquid bismuth surface. The thermodynamic relationship for such a deposition n-ill be different from that for the deposition of pure silver (or pure copper) on a platinum surface and this difference should be reflected in the potential a t which the consecutive process deposition occurs as compared to the potential required for the pure metal deposition. The data presented above are all taken from examples of processes of the type nhere metal ions are being reduced to metal-that is, of examples TT here a nen phase appears as a result of the process. Some experiments of R preliminary nature were performed using the reduction of chroniiuni(II1) chloride to chromiuni(I1) chloride as the depolarizing reaction. I n this case the reduction product i; soluble in the solution and diffuses a n a y from the
I
0
. -
500
A
ELECTRODE I
N
W U
400 N I
I I 3 0 0
:
0 LY
9 200 r+=
:
tb
.d
IO0
CdC12 0
20
1 0
Figure 3.
30
0 IO CONC ENTRAT 1 0 N,
BiC13
50
60
70
20 3 0 MILLIMOLAR
40
50
40
Relation of i o d / 2 to C for cadmium and bismuth chlorides A.
CdClr, least squares line for electrodes 1 and 2
6. BiCls VOL. 29, NO. 1, JANUARY 1957
7
the points representative of the higher concentrations. Data are available for the response of electrodes 3, 4, and 5 in some of the same cadmium chloride solutions which were used to obtain the data for curve A of Figure 3. Thus the response of electrodes 3, 4, and 5 can be compared directly to the response of electrodes 1 and 2, which were previously noted to be essentially identical. The observations of for each of electrodes 3. 4, and 5 were averaged a t each concentration investigated. Then the deviation of each average value from the least square value (Figure 3) a t the same concentration \vas noted. The deviations were calculated to give percentage deviations. Values of the percentage deviation for electrodes 1 and 2 were also determined for inclusion in the comparison which is presented as Table 11.
electrode surface. The iorl product for a particular chromium(II1) chloride concentration decreases markedly with increasing current density, indicating that the reaction is diffusion-controlled. It was impossible to investigate the validity of Equation 1 over a nide range of concentration of chromium(II1) chloride at a single current density because of the limited range of measurable transition times mentioned above. Equation 1 n a s found valid for acceqsible transition times within the concentration range 7.33 to 74.2 niM a t each of five current densities. The diffusion coefficients of cadmium chloride, bismuth chloride. silver chloride, and cuprous chloride have been evaluated from the slope of the plots of Figures 3 and 4. This was done by setting the slopes equal to the constant appearing in Equation 1, nFa1 ?D1 ?, 2 . and solving for D. The values n.ere in the order bismuth(III), cadniiuni(I1). silver(I), and copper(1)-respectively. 0.6. 1.7, 2.6, and 3.5 X 10-5 sq. cni. per second. These values shon- a marked upiwrd trend n ith decreasing oxidation state of the depolarizer. which may be indicative of the relat i r e amounts of chloride being carried along by the various diffusing species. Delimarski, Markov, and Berenhluiii (6) have reported a diffusion coefficient for silver chloride in the lithium chloride -potassium chloride eutectic a t 450’ C. Their value was about 4 X sq. cm. per second for 3 to 18 m M solution. and was reported to increase manyfold as the concentration decreased from 3 to 1 mM. This concentration dependence was not verified b y the present a ork. The points resulting from mea to C for silver and cuprous chlorides A. 6.
&)
ANALYTICAL CHEMISTRY
AgCl CUCl
(1) Black, E. D., DeVries, T., ANAL. CHEM.27, 906 (1955).
(2) Chovnyk, N. G., Doklady B k a d . S a u k . S.S.S.R. 87, 1033 (1952). (3) Delahay, P., “New Instrumental Methods in Electrochemistry,” p. 180, Interscience, Ken, York, 1954.
Lengyel, B., Sammt, A., Z. physik. Chem. 181A, 55 (1937-8). Lyalikov, Y. S., Zhur. Anal. Khim. 5 ,
(4) Delahay, P., Rlamantov, G., ANAL. CHEM.27, 478 (1955). (5) Delimarski. Y. K.. Csvekhi Khim. 23, 766 (1954). I
.
323 (1950); 8, 38 (1953).
Delimarski, Y. K., Markov, B. F., Berenblum, L. S., Zhur. Fiz. Khim. 27, 1848 (1953).
Ferguson, W. S.,. Ph.D. thesis, University of Illinois 1956. Fernelius, W. C., “Inorganic Syntheses,” Vol. 2, p. 1, RlcGraw-Hill, New York, 1946. Flood, H., Forland, T., Acta Chem. Scand. 1, 592 (1947). Gierst, L., Juliard, A., J . Phys. Chem. 57, 701 (1953).
Gierst. L.. hlechelvnck. P. H., Anal. C h i h . Acta 12, f 9 (1955)
Nachtrieb. M.. Steinberg. R l . . J . Am. Chem. hoc. ‘70, 2613-(19-18); 72,
3558 (1950). (15) Nicholson, M. M., Karchmer, J. H., ANAL.CHEM.27, 1095 (1955). (16) Osteryoung, R. A,, Ph.D. thesis, University of Illinois, 1954. (17) Porter. B.. Feinleib. M..J . Electrocheh. Sdc. 103, 300 (1956). (18) Reilley, C. S . ,Everett, G. W., Johns, R.H.. ANAL. CHEM.27.483 (1955). (19) Reilley,‘C. N., Scribner, iV. G.; Ibid., 27, 1210 (1955). .
I
(20) Rochow, E., Didtschenko, R., J . Am. Chem. SOC.76, 3291 (1954). (21) Sand, H. J. S., Phil. Mag. 1, 45 (1901). (22) Van Artsdalen, E. R., Yaffe, I. S., J . Phys. Chem. 59, 118 (1955).
RECEIVED for review June 22, 1956. Accepted September 29, 1956. Division of Analytical Chemistry, 130th Meeting, ACS, Atlantic City, N. J., September 1956. Taken from a thesis submitted by W. S. Ferguson to the Graduate College of the University of Illinois in partial fulfillment of the requirements for the Ph.D. degree in chemistry.
Thermochemical Titrations Enthalpy Titrations JOSEPH JORDAN and T.
G. ALLEMAN’
Department of Chemistry, Pennsylvania State Universify, Universify Park, Pa. Based on careful fundamental considerations, a method is described for the titrimetric determination of heats of reaction and a corresponding theoretical equation is derived. The change in temperature during the titration vs. mole ratio of reactants was recorded automatically with the aid of a high sensitivity thermistor bridge circuit, a potentiometer, and a synchronously coupled constant-flow buret. Under judiciously controlled experimental conditions the shape of the titration curves was a function of enthalpy differences between reactants and products. Extrapolated zero ordinate intercepts can be used for rapid and convenient analytical determinations. Potentialities and limitations of enthalpy titrations are illustrated with their applications to the determination of divalent cations with ethylenediamine tetraacetate. From the titration curves the relevant heats of chelation were evaluated. The method is applicable to concentrations as low as 5 X M, yielding an accuracy within 3%. A precision and accuracy within 1% can readily b e attained in 1 0-2M solutions. An enthalpymetric sensitivity index is defined and discussed.
E
is tentatively proposed as a designation for volumetric methods depending on heats of reaction. This terminology was selected in accordance with Mellon’s suggestion (7) that analytical nomenclature should consistently identify procedures by the characteristic property involved. Specifically, automatic titrations are presented which yield in a n adiabatic system a plot of temNTHALPY TITRATIONS
Present address, Procter and Gamble Co., Cincinnati, Ohio.
perature change us. moles of added reagent. Similar techniques have previously been called “thermometric titrations” (5) and “thermal titrations” (3). However, the Committee on Nomenclature, Division of AnalyticalChemistry, AMERICANCHEMICALSOCIETY, has discouraged use of the adjective “thermometric” (4). A comprehensive review of relevant work published prior to 1953 is given by Linde, Rogers, and Hume (6)and additional references are summarized by Eming ( 3 ) . Enthalpy titrations have been applied to the accurate evaluation of heats of chelation of divalent cations with (ethylenedinitri1o)tetraacetate (ethylenediamine tetraacetate, EDTA). This is believed to be the first report of a successful titrimetric method for the determination of heats of reaction in aqueous solutions. Chelatimetric procedures for the quantitative determination b y enthalpy titrations with E D T A of lead, cadmium, cupric, nickelous, calcium, zinc, cobaltous, and magnesium ions are applicable t o concentrations as low as 0.0005M, and to some binary mixtures, EXPERIMENTAL
Materials. Reagent grade chemicals were used throughout. I n all experiments E D T A mas supplied to the titration mixtures as a solution of the tetrasodium salt of ethylenediaminetetraacetic acid, which was prepared by adding stoichiometric amounts of carbonatefree sodium hydroxide to the disodium salt of EDTA. obtained from the Bersworth Chemical Co., Framingham, Mass. Apparatus. Instrumentation was similar to that recommended by Linde, Rogers, and Hume (6). Certain refinements were incorporated, which in-
creased by a factor of about 30 the sensitivity of the temperature-monitoring signal, compared to what has been reported hitherto in thermal titration studies. As shown in Figure 1, the titrations were carried out with a horizontal, constant-flow, automatic syringe-buret patterned after the one described by Lingane (6). The unit was driven by a Model SG15, 110-volt, 6O-cycle1 600r.p.m. synchronous motor supplied by the Merkle-Korff Gear Go., Chicago, Ill. With the aid of interchangeable precision-machined gears, flow rates between 0.2 and 0.6 ml. per minute were readily obtained, the latter being used exclusively in the studies described here. A volume of titrant not exceeding 1 ml. was used in each titration, with a view to minimizing variations in the heat capacity of the system. The buret was calibrated in terms of both a revolution counter geared into its mechanism, and the volume of liquid delivered in a given time. With either method, volumes of titrant of the order of 1 ml. (corresponding under the experimental conditions to a delivery time of about 2 minutes or to 1500 counts) were measured with a precision and accuracy within 10.0015 ml. (*0.15%). The syringe-buret was connected by a three-way stopcock either to a titrant reservoir or to a tube with a capillary delivery tip which was immersed under the surface of the solution titrated. The titrations were performed in a 250-ml. wide-mouthed Dewar closed with a stopper with appropriate holes for inserting the buret tip, a glass stirrer (operated a t 600 r.p.m. by a Sargent synchronous rotator, Type KYC-22), and a temperature-monitoring device. The latter was a Western Electric 14B thermistor which had the following characteristics: sensitivity in the 25’ C. temperature range, -0.04 ohm per ohm per ’ C.; approximate resistance a t 25’ C., 2000 ohms; thermal time lag, less than 1 second. VOL. 29, NO. 1 , JANUARY 1957
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