Coulometric Titrations with Electrically Generated Ceric Ion requirecl to reach a poLeritiornetric end point was nieasured. l3) emplojing a current source which n as regulated electronically, a controlled current, constant to H ithin O.Ol%, was realized. The average error of the coulometric method in the titration of ferrous sulfate was O . O i ~ c . The results indicate that this coulonietric process is practical and that i t ma? be extended to other titrations utiliiing ceric ion.
Becaube ceric salts are usuall) prepared b? electro1 j tic oxidation of cerous salts, the application of this process to coulometric analysis seemed possible. This oxidation w a s found to proceed at essentiallj 100% efficiencj. The resulting ceric ion was used to oxidize samples of ferrous sulfate present in the electrolyzed solution. The electrol?tic oxidation w a s carried out at a cwnstant k n o w n current and the time
THE
The t,iti,iitionb\\ere c,ai,riedout in a 3 -\L sulfuric acid mrtlium ivhich gave a sonie\diat sliarpw and more rapidly att,ained ( A n d point t,han the 1 .\- c~~iicrntrtlt,ioii~ uruuily used. The end point could be easily detcrmiiiwl to within 0.05 sec'iinci in all instances nit,hout plot,ting the potential ( ~ v e s .This w i s possible because in coulometric proceduree much smaller increments of reagent call be grnerated in tlie vicinity of t'he end point t81iancan be added with the use of a buret. In the titration of 0.1 iv ~ ~ h t i o i i s using a current of 25 nm., amounts of titrating agent can be added which are equivalent t,o 5 X in]. of 0.1 reagent (generation Lit,erval, 0.02 second). The addition of such small quantities of reagent. would otherwise be extrmiely difficult in macroprncdures. The potential values m a r the erid point were rapidly at,tained after the addition of each incrrnient when a sulfuric arid concentration of 3 *\- was used. The precision with which t l i v end point ran be ohtained is showii in Figurr I ,
rlectrolytic oxidation of cerous salts lias long beeii used I t has been shown that, such oxidation niay proceed a t 100yo current efficiency ( 4 ) . It therefore set~iirdadvantageous to s h d y the coulometric generation of ceric sulfate and the use of this reagent in analytical oxidations by the process of coulometric titration or in other ways. The use of chlorine, generated coulometrically, as a strong oxidant has been investigated ( 1 ) . Ceric sulfate is a more powerful oxidant in acidic solutions and is inore generally applicable. In t h k investigat,ion, ceric ion was generated quantitatively a t a platinum anode and Rpplied to the coulometric titration of solutions of ferrous ion. The elect,rolytic oxidation w a s carried out a t constant current, using an electronic circuit (S). In this way, the value of the current could be controlled automatically with high precision. The end point was determined potentiometrically. As oxygen was found to interfere tro a slight extent, the titrations were carried out in an inert atmosphere. A negat,ive error equivalent to about 0.01 ml. of 0.1 .V Ferrous solution was found if air was not, excluded. Chloride ion, Iwcause it has a lower oxidation potential than ceric sulfate, was foulid to he anodically oxidized. Large amounts of sodium ion cniisrti a preripitatioii of a slightly soluble Sa,SO,.cc.,(So,),.2~€~0~ as a method for the preparation of ceric salts.
A\
41'P 4RA'I
Electrical Circuit. The electronic circuit used to niaiiitain the current through the cell has been described ( 3 ) . During a 15minute run, t,he value of the current varied less than 0.01%. Current Measurement. The value of the current was determined by measuring the voltage drop across a standard resistanrc by means of a potentiometer. A resistance box rated a t 0.01 yo ( 0 t h \Voulff, Berlin) wap used as the resistance standard. The 10-ohm resistance in this box was checked against a Woulff 10ohm standard which had a value 10.000. The other resistances in the box were then checked against t.hie 10-ohm value in a manner similar to calibration of a set of weights. .41,eeds & Northrup st,udent potentiometer was calibrated a t a Jingle point, against R new standard Eppley cell, and was subsequently uaed only in thc neighborhood of the calihratrd point. This was accon1plisht.d by choosing an appropriate value of the resistanre over whic-h the voltage drop was to be taken. Time Measurement. An electric clock (Standard Electric Time Co., Type S-6) employing il synchronous motor was uwd to time the generation intc,i.vals. This clock is equipped with a magnetic clutch tvhich has a start-stop error of about 0.01 second per operation. Titration Cell. The titration cell used in this work is shown i n Figure 2. The vai,ioux r.lectrodtts ustd were madt. iritt:rcliangeable to increase the versatilit). of thr, iippararus. The indicator anti reference rlrctroder could Ijr reniovecl for cleaning, :is NX~II:IS t,he cathode a d cathode compartnient. The large ~)Iatinuni anode could not be removed from the apparatus, but it \VNP mounted with an interchangeable ground glass joint, so that the electrode could be easily replaced in case of damage. il fift,h ground-glass joint was provided, so that the ~olutioncould be swept with carbon dioxide. Stirring was accomplished b y nieans of a magnetic stirrer. The mercury-mercuric suffate (ca. 107~ sodium sulfate) electrode was found to be 0.41 volt positive t.o the saturated calomel electrode (0.66 volt I ' S . -V hylrogen elrctrode). The platinum anode a t which the ceric ion was generated was a 2.5 X 2.5 cm. piece of No. 28 B. and S. foil. A 1 X 2 em. piece of foil was used ae the cathode, but the size of this electrode is unimportant.
I , , ,d,, ,
0 , 3 1
?
,
0
>
, ,
0.2 314.0
514.5
31%
TIME SECONDS
Figure 1. Titration Curve at 25 Milliamperes
945
ANALYTICAL CHEMISTRY
946 The cathode was isolated from the solution that was being titrated in a separate cathode compartment filled with 15% solution of ammonium sulfate. The level of this solution was kept above that of the solution being titrated to prevent diffusion losses. Electrical connection was made through a sintered-glass disk prepared according to the method of Kirk ( 2 ) . This disk should not have too high an electrical resistance, because the heat generated would cause the solution to boil. On the other hand, it should not be so coarse that the catholyte runs out too rapidly. A disk with a resistance of 100 to 250 ohms a t 100 ma. was found to be satisfactory.
Table I . Titrations w i t h Ferrous Ion Fe
Taken
Mu. 54.70 50.21
27.22
A
24.88 10.800
7hS
5.376
Current Ma. 127.60 127.59 127.68 127.71 127.71 127.54 127.56 127.51 127.56 127.84 127.88 127.84 39.79 39.79 39.78 39.78 39.78 39.78 39.78 39.76 39.76 39.76 39.76 39.76
Time Sec. 741.6 741.4 679.9 678.3 679.8 679.6 368.9 368.6 369.0 336.1 335.8 336.0 469.0 468.4 469.4 469.3 469.0 469.3 233.9 233.6 233.5 233.7 234.1 233.6
Fe Found Me. 54.76 54.74 50.22 50.13 50.24 50.16 27.23 27.20 27.24 24.86 24.85 24.86 10,798 10.785 10.808 10.805 10.798 10.805 5.385 5.374 5.372 5,377 6.386 5.374
Error
% 0.11 0.07 0.02 -0.16 0.06 -0.10 0.04 -0.07 0.07 -0.08 -0.12 -0.08 -0.02 -0.15 0.08 0.05 -0.02 0.05 0.17 -0.04 -0.07 0.02 0.19 -0.04
water. This solution was stored under carbon dioxide and discarded after a few hours. PROCEDURE
- - -- - - _ F i g u r e 2.
Pt
Titration Cells and Replaceable Electrodes
For the potentiometric indication of the end point, a platinum wire indicator electrode was used in conjunction with a calomel or a mercurous sulfate reference electrode. Measurement of the potential across the electrodes was followed with a Beckman Model G pH meter. Other and simpler means could presumably be used for indication. REAGENTS
Cerous Sulfate Solution. A saturated solution (ea. 12%) of reagent grade cerous sulfate octahydrate (G. F. Smith Chemical Go.). Standard Ferrous Ammonium Sulfate Solution. A 0.1 N solution of ferrous ammonium sulfate was standardized twice daily against a solution of Bureau of Standards potassium dichromate. To increase the accuracy of this titration, the ferrous ammonium sulfate solution was made slightly stronger than the potassium dichromate standard solution. Fifty milliliters of each solution were then pipetted, with the same pipet, into a flask. The excess ferrous ion was titrated potentiometrically (platinumcalomel) using a 5-ml. microburet. A precision of 0.01 ml. or 0.02% was obtained in this fashion. A 0.02 S solution of ferrous ammonium sulfate was prepared by dilution of standardized 0.1 N solution. This dilution was carried out in an atmosphere of carbon dioxide, using oxygen-free
Twenty milliliters of saturated cerous sulfate solution and 5 ml. of 18 N sulfuric acid were placed in the titration cell, and a rapid stream of carbon dioxide was passed through the solution for a few minutes. Tank gas was used without purification because the error introduced by air oxidation is small. The sample to be titrated was then pipetted into the cell, and the electrolysiv begun. The potential of the platinum indicator electrode was followed and the titration carried out as in an ordinary potentiometric titration, except t h a t the oxidant was added electrically. I n the vicinity of the end point it was necessary to wait about 5 to 10 seconds for the potential to reach equilibrium. -4sharp potential break was obtained and the end point could be easily noted without plotting the potential curves. If care was taken to stop the titration just a t the end point, another sample could be pipetted into the cell and titrated without removing the original. The results of twenty-four titrations with amounts of ferrous ion between 5 and 50 mg. are shown in Table I. The average error of these determinations was 0.07%. ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance, in the form of a postdoctoral fellowship to one of them (W.D.C.), of the National Research Council and the Atomic Energy Commission. LITERATURE CITED
.
Farrington, P. S., and Swift, E. H., ANAL.CHEM.,22,889 (1950). Kirk, P.L.,IND.EKG.CHEM.,ANAL.ED.,7, 135 (1935). Reilley, C. N., Cooke, W. D., and Furman, E.H., ANAL.CHEY., 23, 1030 (1951).
Smith, G. F., “Cerate Oxidimetry,” Columbus, Ohio, G. F. Smith Chemical Co.,1942. RECEIVED Sovember 7, 1950. (4)
