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Pyrolytic graphite cup as both vessel and electrode for micro redox and acid-base potentiometric titrations
MICROCHEMICAL
Pyrolytic
JOURNAL
8,234-240
Graphite
for Micro
(1964)
Cup
Redox
and
as Both Vessel Acid-Base
and
Electrode
Potentiometric
Titrations’ PAUL F. THOMASON Oak Ridge National Laboratory, Oak Ridge, Tennessee Received
July
2, 1964
INTRODUCTION
A microtitration cell that does not require an internal electrode is needed for potentiometric titrations of highly radioactive materials. Such a cell would avoid the effect of radiation on electrodes; also, it would permit microvolumes of solutions to be titrated, because the volume of liquid usually needed for adequate contact with internal electrodes would not be required. Miller and Zittel (4) showed that pyrolytic graphite gives good results when used as an indicator electrode in polarography and voltammetry. It therefore seemed worthwhile to evaluate this material for use simultaneously as both a titration vessel and an electrode. This paper describes the fabrication of two pyrolytic graphite cells and their use, together with an in-burette platinum wire as the reference electrode. The tip of a microburette is the only object other than the vessel that touches the solution being titrated potentiometrically. Redox and acid-base titrations were carried out both manually and automatically. For microtitrations with color indicators, Kirk (3) recommends a shallow white dish as a titration vessel. Berret (1) used glass cells with diameters of 8,11,16, and 24 mm that contained 0.15-10~~ of liquid for microtitrations. For measurement of pK, and molecular weight of microsamples of organic acids, Wilson and Munk (6) used a Metrohm (Brinkman Instruments) S-15ml microtitration cell, but never for more 1 Research sponsored by the US. Atomic the Union Carbide Corporation.
234
Energy
Commission
under contract
with
PYROLYTIC
GRAPHITE
TITRATION
235
VESSEL
than 10 ml of liquid. Linderstrom-Lang and Holter (2) titrated 4 X lOedX 10P4 mg of chloride with an in-burette silver electrode versus another silver electrode in the solution being titrated. MATERI.%LS
AND
METHODS
Apparatus Pyrolytic graphite cups. Two titration cells (Fig. 1) were fabricated from carbon rod and were coated (both inside and outside) with a layer of pyrolytic graphite -l/16-inch thick. The coating was applied accord-
FIG. 1. tion cells.
(ORNL
photograph
No. 65441). Photograph
of pyrolytic
graphite
titra-
ing to the methane-deposition method. One cell is g-inch OD by s-inch long and has a hole $-inch ID by s-inch deep; it contains -800 ~1. The other cell is s-inch OD by s-inch long with a hole s-inch ID by vs-inch deep; it holds ~500 ~1. To make electrical contact, each cell is fitted tightly into a brass hose clamp of appropriate size. A wire for connecting the cell to either the potentiometer or the automatic titrator is soldered to the clamp, Stirring bars are short pieces of 20-gage nickel wire sealed into a glass capillary. The cell and clamp are electrically insulated from the magnetic stirrer by a thin sheet of mica.
236
PAUL
F.
THOMASON
Microburette. Manual titrations were performed with a Micro-Metric microburette (Micro-Metric Co., Cleveland, Ohio) fitted with a tip that has a platinum wire sealed into the bend to provide good electrical contact with the titrant. The orifice of the burette was sealed with a flame and was then gradually filed away until a very small aperture (~40 p diameter) was obtained. The small size of the opening prevents diffusion of the titrant into the titration vessel. Burette syringe. A 0.200-ml per inch syringe was used in the burette assembly; thus it was possible to dispense as little as 0.2 ~1 of titrant per division of burette displacement. Potentials for manual titrations were measured by means of a Beckman model 1019 research pH meter. Figure 2 is a photograph of the manual-titration apparatus.
FIG. 2. ratus.
(ORNL
photograph
No.
65442).
Photograph
of manual-titration
appa-
Automatic titrator. For the automatic delivery of titrant, an ORNL model 1728 automatic titrator (5) was used with the same syringe as was used for the manual titrations. This instrument has an anticipation circuit that prevents overshooting the equivalence point. Also, the instrument can dispense as little as 0.2 ~1 of titrant per increment, because the titrator comes to a halt during potential changes of only a few millivolts. Reagents Standard solution of ceric ammonium sulfate, 0.0881 N in Ce4+. Ceric ammonium sulfate (G. F. Smith Chemical Co., Urbana, Illinois) was
PYROLYTIC
GRAPHITE
TITRATION
VESSEL
237
dissolved in 1 I%’H2S04. The solution was standardized versus a National Bureau of Standards (NBS) standard sample of As-OS. Ferrous ammonium sulfate solution, -0.06 N in Fe++. Ferrous ammonium sulfate (Baker and Adamson, reagent-grade) was dissolved in 1 N HaSO+ Standard solution of sodium hydroxide, 0.1137 N NaOH. Carbonatefree 50% NaOH solution was diluted to give -0.1 11: NaOH. The solution was standardized against an NBS standard sample of potassium acid phthalate. Hydrochloric acid solution, -0.1 LIT HCl. J. T. Baker reagent-grade HCl was diluted. Titration
Procedure
For both the manual and automatic titrations, the 0.200 ml per inch syringe of the burette was filled with 0.0881 N Ce4+ solution, care being taken to exclude air bubbles. By means of calibrated micropipettes, from 100 to 200 ~1 of the Fe++ solution was pipetted into the larger pyrolytic graphite titration cell, and 20- and SO-p1 aliquots were pipetted into the smaller cell. The burette was lowered until the tip just touched the surface of the solution in the cell, the stirring motor was started, and the potential of the system was measured after each addition of Ce4+ solution. RESULTS
AND
DISCUSSION
The curve obtained in the manual titration of 20 ~1 of Fe++ solution with Ce4+ solution is shown in Fig. 3. The total volume in the smaller cell at the end of the titration and after the pipette was washed with 1 N HGS04 was - 200 yl. The larger cell usually contained N 400 ~1 of solution at the end of the titration. The addition of 0.2 ~1 of titrant at the equivalence point (0.076 inch X 0.200 ml = 15.2 ~1) produced a change in potential of 513 mV, which can be measured readily. The normality of the ferrous ammonium sulfate solution was found to be as follows: Aliquot (~1)
N
20 50 100 200
0.0666 0.0666 0.0668 0.0666
238
PAUL
F. THOMASON
800 -
700
-
600 ‘; E ; a c z p g
500 500-
400
-
300 -
200 -
100 -
BURET
PLUNGER
DISPLACEMENT
(in.1
FIG. 3. (Unclassified ORNL Dwg. No. 64-5434). Curve for the manual titration 20 ~1 of Fe+ + solution with 0.0881 N CeZ+.
z I + 0.861
I POTENTIAL
FIG, 4. (Unclassified potentiometric titrat.ion
I IN VOLTS
k+o.too+l I I
I
( t?G.E. vs In- Buret
I
of
I
Electrode)
ORNL Dwg. No. 64-.%X54). Curve obtained in the automatic of 20 p1 of .- 0.06 N Fe+ ,t solution with 0.0881 N Ce4+.
PYROLYTIC
GRAPHITE
TITRATION
239
VESSEL
To the pyrolytic graphite cell was added 100 ~1 of Fe++ solution and 100 ~1 of 1 N H&Oh. A small Beckman S.C.E. was placed in the solution; the measured potential of the cell was +0.393 V vs. the S.C.E. Thus, the in-burette electrode is about +O.SOO V more positive than the reference S.C.E. Figure 4 is the curve obtained in the titration of 20 ~1 of 0.0666 N Fe++ with 0.0881 N Ce4+ by means of the ORNL automatic titrator. Solutions of HCl were titrated in the same manner with 0.1137 1v NaOH as titrant. The results of the manual and automatic titrations agreed within 3 ppt. Figure 5 is the curve from the automatic titration of 200 c;l of 0.1 N HCl. with 0.1137-v NaOH.
i --pa 4.5
pH IN ARBITRARY
UNITS
(l?G.E.
“s In-Buret
Electrode)
FIG. 5. (Unclassified ORNL Dwg. No. 64-5253). Curve obtained in the automatic potentiometric titration of 200 uI of N 0.1 N HCl with 0.1137 N NaOH. SUMMARY A potentiometric titration vessel of pyrolytic graphite is described for use in microtitrations, which are usuahy performed with coIored indicators. The vessel also serves simultaneously as the indicator electrode, the reference electrode being
240
PAUL
F. THOMASON
an in-burette platinum wire. Thus, the need for internal electrodes is eliminated. The vessel has been used satisfactorily in both manual and automatic titrations. ACKNOWLEDGMENTS The author is grateful to F. J. Miller for his helpful suggestions and to Richard L. Heestand of ORNL Metals and Ceramics Division, who deposited the pyrolytic graphite on the carbon cups. The author thanks Mrs. Helen Raaen for her careful editing of the manuscript. REFERENCES I.
BERFCET,R., Sur un Appareillage de Microtitrimetrie et ses Applications. Bull. Sot. Chim. France, Ser. 5 1960, 271-275 (1960). 2. GLICK, D., Recent advances in histological chemistry by the Linderstrom-Lang, Holter technic and development of ultramicro technics. J. Chem. Educ. 16, 68-76 (1939). 3. KIRK, P. L., “Quantitative Ultramicroanalysis,” pp. 37-38. Wiley, New York, 1950. 4. MILLER, F. J., AND ZITTEL, H .E., Fabrication and use of pyrolytic graphite electrode for voltammetry in aqueous solution. Anal. Chem. 35, 1866-1869 (1963). 5. WAGNER, E. B., Automatic potentiometric titrator, velocity-servo, ORNL Model Q-1728, Method Nos. 1 003028 and 9 003028, ORNL Master Analytical Manual, TID-7015, Sects. 1 (April, 19.58) and 9 (Sept., 1958) and Suppls. 2 (May, 1960) and 4 (June, 1962). 6. WILSON, A. M., AND MUNK, M. E., Automatic apparatus for pK, and molecular weight determinations of micro samples of organic acids. Anal. Chem. 34, 443-444 (1962).
Pyrolytic
JOURNAL
8,234-240
Graphite
for Micro
(1964)
Cup
Redox
and
as Both Vessel Acid-Base
and
Electrode
Potentiometric
Titrations’ PAUL F. THOMASON Oak Ridge National Laboratory, Oak Ridge, Tennessee Received
July
2, 1964
INTRODUCTION
A microtitration cell that does not require an internal electrode is needed for potentiometric titrations of highly radioactive materials. Such a cell would avoid the effect of radiation on electrodes; also, it would permit microvolumes of solutions to be titrated, because the volume of liquid usually needed for adequate contact with internal electrodes would not be required. Miller and Zittel (4) showed that pyrolytic graphite gives good results when used as an indicator electrode in polarography and voltammetry. It therefore seemed worthwhile to evaluate this material for use simultaneously as both a titration vessel and an electrode. This paper describes the fabrication of two pyrolytic graphite cells and their use, together with an in-burette platinum wire as the reference electrode. The tip of a microburette is the only object other than the vessel that touches the solution being titrated potentiometrically. Redox and acid-base titrations were carried out both manually and automatically. For microtitrations with color indicators, Kirk (3) recommends a shallow white dish as a titration vessel. Berret (1) used glass cells with diameters of 8,11,16, and 24 mm that contained 0.15-10~~ of liquid for microtitrations. For measurement of pK, and molecular weight of microsamples of organic acids, Wilson and Munk (6) used a Metrohm (Brinkman Instruments) S-15ml microtitration cell, but never for more 1 Research sponsored by the US. Atomic the Union Carbide Corporation.
234
Energy
Commission
under contract
with
PYROLYTIC
GRAPHITE
TITRATION
235
VESSEL
than 10 ml of liquid. Linderstrom-Lang and Holter (2) titrated 4 X lOedX 10P4 mg of chloride with an in-burette silver electrode versus another silver electrode in the solution being titrated. MATERI.%LS
AND
METHODS
Apparatus Pyrolytic graphite cups. Two titration cells (Fig. 1) were fabricated from carbon rod and were coated (both inside and outside) with a layer of pyrolytic graphite -l/16-inch thick. The coating was applied accord-
FIG. 1. tion cells.
(ORNL
photograph
No. 65441). Photograph
of pyrolytic
graphite
titra-
ing to the methane-deposition method. One cell is g-inch OD by s-inch long and has a hole $-inch ID by s-inch deep; it contains -800 ~1. The other cell is s-inch OD by s-inch long with a hole s-inch ID by vs-inch deep; it holds ~500 ~1. To make electrical contact, each cell is fitted tightly into a brass hose clamp of appropriate size. A wire for connecting the cell to either the potentiometer or the automatic titrator is soldered to the clamp, Stirring bars are short pieces of 20-gage nickel wire sealed into a glass capillary. The cell and clamp are electrically insulated from the magnetic stirrer by a thin sheet of mica.
236
PAUL
F.
THOMASON
Microburette. Manual titrations were performed with a Micro-Metric microburette (Micro-Metric Co., Cleveland, Ohio) fitted with a tip that has a platinum wire sealed into the bend to provide good electrical contact with the titrant. The orifice of the burette was sealed with a flame and was then gradually filed away until a very small aperture (~40 p diameter) was obtained. The small size of the opening prevents diffusion of the titrant into the titration vessel. Burette syringe. A 0.200-ml per inch syringe was used in the burette assembly; thus it was possible to dispense as little as 0.2 ~1 of titrant per division of burette displacement. Potentials for manual titrations were measured by means of a Beckman model 1019 research pH meter. Figure 2 is a photograph of the manual-titration apparatus.
FIG. 2. ratus.
(ORNL
photograph
No.
65442).
Photograph
of manual-titration
appa-
Automatic titrator. For the automatic delivery of titrant, an ORNL model 1728 automatic titrator (5) was used with the same syringe as was used for the manual titrations. This instrument has an anticipation circuit that prevents overshooting the equivalence point. Also, the instrument can dispense as little as 0.2 ~1 of titrant per increment, because the titrator comes to a halt during potential changes of only a few millivolts. Reagents Standard solution of ceric ammonium sulfate, 0.0881 N in Ce4+. Ceric ammonium sulfate (G. F. Smith Chemical Co., Urbana, Illinois) was
PYROLYTIC
GRAPHITE
TITRATION
VESSEL
237
dissolved in 1 I%’H2S04. The solution was standardized versus a National Bureau of Standards (NBS) standard sample of As-OS. Ferrous ammonium sulfate solution, -0.06 N in Fe++. Ferrous ammonium sulfate (Baker and Adamson, reagent-grade) was dissolved in 1 N HaSO+ Standard solution of sodium hydroxide, 0.1137 N NaOH. Carbonatefree 50% NaOH solution was diluted to give -0.1 11: NaOH. The solution was standardized against an NBS standard sample of potassium acid phthalate. Hydrochloric acid solution, -0.1 LIT HCl. J. T. Baker reagent-grade HCl was diluted. Titration
Procedure
For both the manual and automatic titrations, the 0.200 ml per inch syringe of the burette was filled with 0.0881 N Ce4+ solution, care being taken to exclude air bubbles. By means of calibrated micropipettes, from 100 to 200 ~1 of the Fe++ solution was pipetted into the larger pyrolytic graphite titration cell, and 20- and SO-p1 aliquots were pipetted into the smaller cell. The burette was lowered until the tip just touched the surface of the solution in the cell, the stirring motor was started, and the potential of the system was measured after each addition of Ce4+ solution. RESULTS
AND
DISCUSSION
The curve obtained in the manual titration of 20 ~1 of Fe++ solution with Ce4+ solution is shown in Fig. 3. The total volume in the smaller cell at the end of the titration and after the pipette was washed with 1 N HGS04 was - 200 yl. The larger cell usually contained N 400 ~1 of solution at the end of the titration. The addition of 0.2 ~1 of titrant at the equivalence point (0.076 inch X 0.200 ml = 15.2 ~1) produced a change in potential of 513 mV, which can be measured readily. The normality of the ferrous ammonium sulfate solution was found to be as follows: Aliquot (~1)
N
20 50 100 200
0.0666 0.0666 0.0668 0.0666
238
PAUL
F. THOMASON
800 -
700
-
600 ‘; E ; a c z p g
500 500-
400
-
300 -
200 -
100 -
BURET
PLUNGER
DISPLACEMENT
(in.1
FIG. 3. (Unclassified ORNL Dwg. No. 64-5434). Curve for the manual titration 20 ~1 of Fe+ + solution with 0.0881 N CeZ+.
z I + 0.861
I POTENTIAL
FIG, 4. (Unclassified potentiometric titrat.ion
I IN VOLTS
k+o.too+l I I
I
( t?G.E. vs In- Buret
I
of
I
Electrode)
ORNL Dwg. No. 64-.%X54). Curve obtained in the automatic of 20 p1 of .- 0.06 N Fe+ ,t solution with 0.0881 N Ce4+.
PYROLYTIC
GRAPHITE
TITRATION
239
VESSEL
To the pyrolytic graphite cell was added 100 ~1 of Fe++ solution and 100 ~1 of 1 N H&Oh. A small Beckman S.C.E. was placed in the solution; the measured potential of the cell was +0.393 V vs. the S.C.E. Thus, the in-burette electrode is about +O.SOO V more positive than the reference S.C.E. Figure 4 is the curve obtained in the titration of 20 ~1 of 0.0666 N Fe++ with 0.0881 N Ce4+ by means of the ORNL automatic titrator. Solutions of HCl were titrated in the same manner with 0.1137 1v NaOH as titrant. The results of the manual and automatic titrations agreed within 3 ppt. Figure 5 is the curve from the automatic titration of 200 c;l of 0.1 N HCl. with 0.1137-v NaOH.
i --pa 4.5
pH IN ARBITRARY
UNITS
(l?G.E.
“s In-Buret
Electrode)
FIG. 5. (Unclassified ORNL Dwg. No. 64-5253). Curve obtained in the automatic potentiometric titration of 200 uI of N 0.1 N HCl with 0.1137 N NaOH. SUMMARY A potentiometric titration vessel of pyrolytic graphite is described for use in microtitrations, which are usuahy performed with coIored indicators. The vessel also serves simultaneously as the indicator electrode, the reference electrode being
240
PAUL
F. THOMASON
an in-burette platinum wire. Thus, the need for internal electrodes is eliminated. The vessel has been used satisfactorily in both manual and automatic titrations. ACKNOWLEDGMENTS The author is grateful to F. J. Miller for his helpful suggestions and to Richard L. Heestand of ORNL Metals and Ceramics Division, who deposited the pyrolytic graphite on the carbon cups. The author thanks Mrs. Helen Raaen for her careful editing of the manuscript. REFERENCES I.
BERFCET,R., Sur un Appareillage de Microtitrimetrie et ses Applications. Bull. Sot. Chim. France, Ser. 5 1960, 271-275 (1960). 2. GLICK, D., Recent advances in histological chemistry by the Linderstrom-Lang, Holter technic and development of ultramicro technics. J. Chem. Educ. 16, 68-76 (1939). 3. KIRK, P. L., “Quantitative Ultramicroanalysis,” pp. 37-38. Wiley, New York, 1950. 4. MILLER, F. J., AND ZITTEL, H .E., Fabrication and use of pyrolytic graphite electrode for voltammetry in aqueous solution. Anal. Chem. 35, 1866-1869 (1963). 5. WAGNER, E. B., Automatic potentiometric titrator, velocity-servo, ORNL Model Q-1728, Method Nos. 1 003028 and 9 003028, ORNL Master Analytical Manual, TID-7015, Sects. 1 (April, 19.58) and 9 (Sept., 1958) and Suppls. 2 (May, 1960) and 4 (June, 1962). 6. WILSON, A. M., AND MUNK, M. E., Automatic apparatus for pK, and molecular weight determinations of micro samples of organic acids. Anal. Chem. 34, 443-444 (1962).