At B5U' C., a single additional peak was generated and was identified as n-C3Fe (2). Verification was obtained from the analysis of the pyrolysis products of Teflon, heated a t 650' C. (3, 6), which produced the identical peaks. 4 t 800' C., cis- and trans-CaF~-2 and C3Fs were identified, as was .SO-CJ?~,by the method described by Campbell and Gudsinowicz ( 3 ) . Between 700" to 800' C., the yield of C2F4 decreased sharply while the adjscent peak, which was assigned to C2F&( 2 ) , appeared and increased in size. DISCUSSION
A gas liquid chrom:itogram of a comples, gaseous, fluorocarbon mixture is shown in Figure 1. The individual components of cis- and trans-C4F8-2 niisture were identified by infrared
spectrometry. This c i s form gave characteristic absorption bands a t 5.78, 7.42, 9.0, 10.5, and 13.83 microns, while the trans form gave absorption bands at 7.74, 11.27, and 14.6 microns. Cyclo-CaFe and n-CaFs were not resolved. The small peak which emerges immediately before n-CaF8 was assigned to C2F2; it is difficult to conceive of another assignment. The small peak preceding iso-CdFs is probably octaBuoro-l-butene (C4F8-1). A gas solid chromatogram of a similar gas mixture is shown in Figure 2. The temperature was programmed from room temperature to 180' C. and the separation was completed in 60 minutes. The cis and trans isomers of C4F8-2 were not resolved, but n- and cyclicC,F6 could be resolved.
LITERATURE CITED
(1) Atkinson, B.,
J. Chem.
SOC. 1952,
2684.
(2) Atkinson, B., Atkinson, V. A., Ibid., 1957,2086.
(3) Campbell, R. H., Gudzinowics, B. J., ANAL.CHEM.33, 842 (1961). ( 4 ) Greene, S.A,, Moberg, M. L., Wilson, E. M., Ibid., 28, 1369 (1956). (5) Larcher, J. R., Tompkin, G. W., Park, J. D., J. Am. Chem. SOC.74, 1693 (1952). (6) Lewis, E. E., NayIor, >I. -4., Ibid.,69, 1968 (1947). (7) Reed T. M., 111, . ~ N A L . CHEX 30, 221 (1958).
STANLEY A GREENE FRANCIS M. WACHI
Materials Sciences Laboratory Aerospace Corp. P. 0. Box 95085 LOBAngeles 45, Calif.
The Pyrolytic Graphite Electrode as an Indicating Electrode for Pote nt io imet ric Titrations SIR: Preliminary investigation has been made of the pclssibility of using pyrolytic graphite as an indicator electrode. Although therz have been many instances of the use 0' various types of graphite and carbon electrodes ( I ) , the only reported use cf pyrolytic graphite as an electrode has been the work of Laitinen and Rhodes (4) in fused salts. Since pyrolytic graph ltcl is a relatively nen- material that has unique properties, the further investigation of its analytical applications is necessary. Information on the history, ,properties, and applications of pyrolytic graphite can be found in commercial publications and elsewhere ( 2 , 3 , 5 ) . Pyrolytic graphite k a polycrystalline form of carbon that l-as a high degree of orientation. The material is produced by vapor phase deposition. The result of the deposition is the growth of a material that has a metallic-type behavior in the plane of deposition and a ceramic-type behavior in the direction perpcndicular to the plane of deposition. The properties that rcnder pyrolytic graphite of interest as an indicator electrode are a high degree of impermeability, freedom from triipped gases and metallic contaminant:,, and chemical inertnew The particular pyrolytic graphite that \yas used in this experimental work was obtitined from High Temperature Materials, Inc., Boston, ;\insy. Potentiometric titmtions were performed as a means of investigation. The potentiometric apparatus consisted of a Beckman model 11-2 p H meter, a Beckman calomel e.ectrode, Model S o . 39170 with a fiber tip, and a Beck-
man glass electrode, Model S o . 40498. The calomel electrode was used as a reference electrode for pH and redox titrations; the glass electrode as a reference for redox titrations. A syringe-type buret that was obtained from the Micro-Metric Instrument Co., Cleveland, Ohio, was used for the addition of titrant. The buret was equipped with either a 1-m1.-per-inch or a 3-m1.per-inch syringe. Stirring of the solutions was accomplished by use of a magnetic stirrer and stirring bar. The indicating electrode was fabricated by sealing a '/4-inch cube of pyrolytic graphite into the flared end of a borosilicate glass tube, 7 mm. in diameter, with epoxy cement. Only the face of the "a" plane of the pyrolytic graphite
-6 bV.1
-600
,
, -400
I
1
-200
t a
203
433
600
I
I -10
..../'.2
,
1'4
Figure 1. Effect of H f ion concentration on potential developed b y pyrolytic graphite electrode vs. calomel electrode
was exposed to the solution. Two milliliters of Hg were placed inside the tube to make contact only with the other slightly-conducting face of the graphite. Care must be taken to seal the graphih so that the Hg comes into contact only with the top surface of the graphite. If the Hg can penetrate the epoxy on the side of the cube, the layers of the graphite may be split with a resultant increase in resistance. A Cu wire was immersed in the H g and connected to the pH meter. The electrode should be carefully examined to make sure that there is no pathway to physical contact of either the mercury or the copper wire with either the potential indicating surface or the solution in the titration cell. An H-type glass cell that was fitted with a fritted disk and a n agar plug in the cross-arm was used as a titration vessel for nonaqueous titrations. When the pyrolytic-graphite electrode and a calomel reference electrode were placed in solutions of differing pH, a variation in potential was noted. Figure 1 shows the linear variation of potential with the change in H + concentration. The potential varied 205 mv. in going from 1.ON to 1 X lo-" HC1 solutions. As the OH- concentration increased, the potential became more negative, -15 mv. with l X 10-3,v KaOH to -150 mv. with 1 X lo-" KaOH. Acid-base titrations were performed with the same electrode system as an end point indicator. I n Figure 2 the titration curve of 0.089N NaOH with 0.093N HCl is shown. The curves are reproducible with an abrupt potential shift of 200 mv. at theendpoint. The reverse of this titration, in which 1 ml. VOL 35,
NO. 7, JUNE 1963
929
- 200
0 s'
E
200
400
0.120 0.240 0.360 0.480 0.600 0.720 0.840 0.960 f.080 4.200 VOLUME OF TITRANT, ml.
Figure 2.
Potentiometric titration of
NaOH with HCI
Buret: 3-ml. per inch syringe type
of the HCl solution was titrated with S a O H , was carried out. The total volume of the titrated solutions was 26 ml. A sharp break in potential of over 100 mv. was obtained a t the end point. iilthough the end point was reproducible in a series of four titrations, the initial potential shon ed a slow rise from 320 to 421 mv. The same equivalence point is achieved with the pyrolytic-graphite electrode as is shown with the glass electrode. Sulfuric acid solutions of similar concentrations n ere also titrated. Duplicate titrations resulted in identical titration curves. S o change in the starting potential of the two titrations was observed. The possible use of the pyrolyticgraphite electrode as a substitute for the glass electrode in acid-base, nonaqueous titrations was considered. The glass electrode is unsatisfactory after protracted use in nonaqueous titrations bezause of the drying-out of the glass. Frequent soaking in water is necessary to restore the glass electrode to satisfactory working condition. Since the pyrolytic-graphite electrode contains no rrater and is relatively impermeable to liquids and gases, no deterioration of the electrode should occur because of dehydration.
-1s a test, nonaqucous titr at ions ' wrc carried out. A 1-ml, aliquot of a solution of o-phenyl phenol in isopropyl alcohol was diluted to 25 ml. 11-ith pyridine. The diluted solution was placed in one compartment of an H-type titration vessel. The pyrolytic-graphite indicat,or electrode rras immersed in the pyridine. A calomel, fiber-t'ip electrode was placed in the KCl solution in a separate compartment of the H-type cell. The two compartments were separated by a fritted-glass disk and an agar plug. An excellent titrat,ion curve was obtained as shown in Figure 3. -1i)Otential shift of more than 100 nil-. n-as obtained as shown a t the end point. The stoichiometry is in agreement iritli t,hat found by titration with a g1 electrode. The sensitirit,\- of the pyrol>-ticgraphite electrode toirard changes in H T concentration can best be explained by assuming that it functions as an oxygen electrode. -111 of the titrations were performed open to the air. Qualitative tests had shonn that the pyrolytic graphite electrode responds to changw in redox potentials. To establish thc value of the i,\-rolytic-grapliite
electrode as a redox indicator, a nuinbcr of potentiometric titrations of Fe(I1) in 1.ON HzSOa with KsCrOa solution were carried out. Glass or calomel electrodes were used as reference electrodes. I n Figure 4,the results of such a titration with a calomel reference electrode are plotted. Duplicate titrations reproduced so well that the titration curves superimposed. Titrations performed with a glass reference electrode gave equally reproducible curve3. -\ bresk in potential of slightly more than 100 mv. was obtained. Equilibrium was attained. The time interval between the adding of an increment of titrant and the taking of a reading x i s lees than a minute. The pyrolytic-graphite electrode nil1 probably find its chief uses in nonaqueous titration, in titration of highlyradioactive solutions where glass electrodes are damaged by radiation, and in titration of highly-concentrated salt solutions. LITERATURE CITED
(1) Elving, P. J., Smith, D. L., - 4 s ~ ~ . CHEX 32, 1819 (1960). ~ \ 2 General ) Electric Co., Pyrolytic Gvaphite, Preliminary Engineering Data, 1962, Metallurgical Products Dept., Specialty ,411oys Section. Detroit 32, 1Iich. ( 3 ) High 'l'emperrtture llaterials. Inc., P!/rd!/fic Graphifr, T'riipwtj- I ):ita, Oct. 6, 1061, High Teniper:itur(, 11:iterials, Inc., 130 Lincoln Street,Brighton,
-4nalytical Chemistry Division
Oak Ridge Sational Laboratory Oak Ridge, Tenn.
Oak Ridge National Laboratory is qlerated by Union Carbide Coy). fur the I-.S..4tomic Energy Commission.
800
coo
I
i >'
E 400
i400 300 I
I
2oo'
0IAO
0;40
Od60
0480 0 6 0 0
VOLUME O F TITFIANT
072C
0840
0960
Buret: 3-ml. per inch syringe type
938
tetrabutyl ammonium hydroxide in benzene solution ANALYTICAL CHEMISTRY
IOj20
0 2 4 0 3 6 0 0.480 O $ c 0 1 0 ~ 2 0 1 0 ~ ~ 0 1 0 $ 6 0 1 ( 0 1 8 0 1 ~ 2 0 0 VOLUME OF TITRANT, ml.
P .
Figure 3. Potentiometric titration of o-phenyl phenol with tetrabutyl ammonium hydroxide
Titrant:
O'
Figure 4.
Potentiometric titration of Fe+2 with yo,Cr04 Buret: 3-ml. per inch syringe type
Composition of tesi solution: 1 mi. 0.1 N ferrous ammonium sulfate, 25 m I 1 .ON HnSOc aoln. Titrant: O.099N K ~ C r 0 4s o h
