Environ. Sci. Technol. 1998, 32, 3178-3184
Redox Potential Monitoring as a Method To Control Unwanted Noble Metal-Catalyzed Hydrogen Generation from Formic Acid Treatment of Simulated Nuclear Waste Media R. B. KING* AND N. K. BHATTACHARYYA Department of Chemistry, University of Georgia, Athens, Georgia 30602 H. D. SMITH AND K. D. WIEMERS Pacific Northwest National Laboratory, Richland, Washington 99352
Simulants for the Hanford Waste Vitrification Plant feed containing the major nonradioactive components Al, Cd, Fe, Mn, Nd, Ni, Si, Zr, Na, CO32-, NO3-, and NO2- were used to study redox potential changes in reactions of formic acid at 90 °C catalyzed by the noble metals Ru, Rh, and/or Pd found in significant quantities in uranium fission products. Such reactions were monitored using gas chromatography to analyze the CO2, H2, NO, and N2O in the gas phase and a redox electrode to follow redox potential changes as a function of time. In the initial phase of formic acid addition to nitrite-containing feed simulants, the redox potential of the reaction mixture rises typically to +400 mV relative to the Ag/AgCl electrode because of the generation of the moderately strongly oxidizing nitrous acid. No H2 production occurs at this stage of the reaction as long as free nitrous acid is present. After all of the nitrous acid has been destroyed by reduction to N2O and NO and disproportionation to NO/NO3-, the redox potential decreases upon further formic acid addition. Hydrogen production typically begins to occur when the redox potential of the reaction mixture becomes more negative than the Ag/AgCl electrode. The experiments outlined in this paper suggest the feasibility of controlling the production of H2 by limiting the amount of formic acid used and monitoring the redox potential during formic acid treatment.
Introduction The most promising method for the disposal of highly radioactive nuclear wastes is a vitrification process in which the wastes are incorporated into borosilicate glass, the glass is poured into large stainless steel canisters, and the sealed canisters are buried in suitably protected burial sites for disposal. A method for feed preparation for the vitrification process uses formic acid for glass redox and melter feed rheology control. The operation of the glass melter and durability of the glass are affected by the glass oxidation state. Formation of a conductive metallic sludge in an overreduced melt can result in a shortened melter lifetime. An over-oxidized melt may lead to foaming and loss of ruthenium * Corresponding author phone: (706)542-2626; fax: (706)542-9454; e-mail:
[email protected]. 3178
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as volatile RuO4. Historically, foaming in the joule-heated ceramic melter is caused by reboil and is controlled by introduction of a reductant such as formic acid into the melter feed. Formic acid is also found to decrease the melter feed viscosity thereby facilitating its pumping. The reducing properties of formic acid can lead to a number of difficulties in the processing of nuclear wastes. Thus the catalytic decomposition of formic acid can lead to the unwanted generation of hydrogen by the reaction
HCO2H f H2v + CO2v
(1)
The catalytic role of the noble metals Ru, Rh, and Pd present in the wastes as uranium fission products (1) for hydrogen generation from formic acid is discussed in previous papers (2-4). In addition, formic acid can reduce nitrate to ammonia in the presence of the noble metals Rh and/or Pd under certain conditions as discussed in another previous paper (5). This paper discusses the correlation of the redox potential of the reaction mixture with such hydrogen and ammonia generation processes. Such information provides a basis for using redox potential control during feed preparation with formic acid to minimize these unwanted processes.
Experimental Approach Redox potentials of reaction mixtures were monitored using a P14805-DXK 58/120 Xerolyt combination redox electrode connected to a 2300 pH transmitter modified for oxidationreduction potentials (ORP). Both the electrode and the transmitter were purchased from Ingold Electrodes, Inc., Wilmington, MA. This setup provides a continuous readout of the potential of the reaction mixture relative to a Ag/AgCl reference electrode, which is +0.2222 V relative to the standard hydrogen electrode on which E° values of electrochemical half-reactions are based. Before use, the redox electrode was calibrated using standard redox solutions (Table 1) as redox buffers. In all cases, the observed potentials were consistent with the reported potentials for the redox buffers. All reactions were conducted in glass reaction vessels of ∼550 mL total capacity equipped with a threaded plug and O-ring adapter used in the previous work (2-5). A pressure gauge was attached to the system. The three sidearms of the vessel near the top were capped with rubber septa through which needles could be inserted to flush the system or to sample the gas phase. A fourth opening also capped by a rubber septum was used for the periodic withdrawal of liquid samples from the reaction mixture for ammonia analyses (5). The reaction vessel had two thermocouple wells. A thermocouple attached to a cycle heater was placed in one well to maintain a constant temperature. The other well was used to check the temperature of the reaction mixture independently with a second thermocouple. The reaction mixture was stirred magnetically during the course of the experiment. Feed simulants were prepared on a 1-L scale from reagentgrade metal chlorides or metal nitrates using the protocol summarized in Figure 1. The nominal compositions of the feed simulants used in this work are listed in Table 2. These compositions are based on a typical Hanford Waste Vitrification Plant (HWVP) neutralized current acid waste (NCAW) feed. Samples of these feed simulants (typically 40-50 mL) were treated with 88 wt % formic acid in a 550-mL closed glass reactor at 80-100 °C. The formic acid was added at the reaction temperature at a constant rate using 10-mL plastic disposable syringes driven by a Sage Instruments syringe S0013-936X(97)00951-6 CCC: $15.00
1998 American Chemical Society Published on Web 08/25/1998
TABLE 1. Redox Buffers Used in This Work composition
mV reporteda,b
mV measureda
quinhydrone in pH 4 buffer quinhydrone in pH 7 buffer 19.61 g of (NH4)2Fe(SO4)2‚6H2O and 24.11 g of NH4Fe(SO4)2‚12H2O treated with 28.1 mL of concn H2SO4 and diluted to 500 mL with water 2.64 g of K4Fe(CN)6‚3H2O, 2.06 g of K3Fe(CN)6, 1.44 g of Na2HPO4, and 0.87 g of NaH2PO4 diluted to 500 mL with water
264 ( 20 87 ( 20 468 ( 20
268 93 457
220 ( 20
235
a
At 20 °C.
b
Preliminary Instructions for the M/2350 ORP Transmitter; Report dated April 1992, from Ingold Electrodes, Wilmington, MA.
TABLE 2. Compositions of the Feed Simulants Used in This Work simulant
metal sources
Al
Cd
Cu
Fe
13M1+Cu 12M1 12M2 9M1XC 9M2XC 8M2XFeC
nitrates nitrates chlorides nitrates chlorides chlorides
0.226 0.226 0.226 0.226 0.226 0.226
0.03 0.03 0.03 0.03 0.03 0.03
0.004 0 0 0 0 0
0.45 0.45 0.45 0.45 0.45 0
nominal concentrations of components,a (M) Mn Nd Ni Si Zr 0.0314 0.0314 0.0314 0.0314 0.0314 0.0314
0.0264 0.0264 0.0264 0.0264 0.0264 0.0264
0.0392 0.0392 0.0392 0.0392 0.0392 0.0392
0.0854 0.0854 0.0854 0.0854 0.0854 0.0854
0.156 0.156 0.156 0.156 0.156 0.156
Na+
CO32-
NO3-
NO2-
0.801 0.801 0.801 0 0 0
0.125 0.125 0.125 0 0 0
0.116 0.116 0.116 b 0 0
0.435 0.435 0.435 0 0 0
a Hydroxide ion is also present from the NaOH used to precipitate the hydrous metal oxides. b No extra NaNO was used in the preparation 3 of this feed simulant. However, an undetermined amount of nitrate ion was present since the nitrate ion arising from the metal nitrates used to precipitate their hydrous oxides could not be completely washed out of the precipitates.
FIGURE 1. General procedure for the preparation of the feed simulants used in this work. pump (Model 355) purchased from Fisher Scientific. The standard rate of addition of formic acid was 0.0194 ( 0.0002 mL/min (1.164 mL/h) corresponding to 0.448 mmol of HCO2H/min or 26.9 mmol of HCO2H/h. The composition of the gas phase was analyzed periodically for H2, CO2, NO, and N2O using gas chromatography. The H2 analyses were performed using a Varian 90P gas chromatograph with a 20 cm × 6 mm column packed with a 40/60 mesh 13× molecular sieve material and argon as the carrier gas. The column temperature was maintained at 80 °C. The CO2, N2O, and NO analyses were performed using a gas partitioner (Fisher model 1200), which separates the gases on the basis of their size and polarity by means of two columns, a 2 m 80/100 mesh Columnpak PQ and a 3.3 m 13× molecular sieve column mounted in series using helium as the carrier gas. The temperatures of both columns were maintained at 50 °C. Sensitivity factors were determined from known amounts of pure certified samples of the gases of interest and were rechecked every week. Nitrogen dioxide could not be determined using gas chromatography because of the NO2 a N2O4 equilibrium. After the feed simulants had been treated with the formic acid, the resulting reaction mixture was analyzed for NH4+ produced by NO3- reduction (5). A 0.5-mL sample of the reaction mixture was diluted with 0.5 mL of distilled water and then was made strongly basic with 1-2 drops of 10 N NaOH in order to convert NH4+ to NH3. The NH3 concentration was then determined using a MI-740 microammonia electrode obtained from Microelectrodes, Inc., Londonderry, NH. This electrode is designed to measure NH3 gas and to
record the NH3 concentration as millivolts on a standard pH meter; an Orion 290A portable pH meter was used for these studies. The ammonia electrode was calibrated before each analysis using 0.1, 0.01, and 0.001 N NH4Cl solutions made basic with 10 N NaOH. A few experiments were done in which pH and redox potential were monitored simultaneously during the formic acid titration of the feed simulants. For this purpose, a ∼550mL glass reactor was constructed with three large necks so that the Ingold HA-405-DXK-58/120 combination Xerolyt pH electrode, the Ingold P14805-DXK 58/120 combination Xerolyt redox electrode, and the PT 100-S8-120 automatic temperature compensator could be accommodated in the same experiment. The pH electrode and the automatic temperature compensator were normally connected to the Ingold model 2300 pH transmitter; the redox electrode, which is less temperature sensitive, was connected to the Ingold redox transmitter. The connection of the temperature compensator could be changed from the pH transmitter to the redox transmitter for occasional temperature compensation of the redox potentials, which was relatively small.
Results Limited Component Experiments. The limited component experiments with the redox electrode system summarized below were performed in order to study some of the individual processes observed in the more complicated feed simulant systems. The Rh/NO2- and Pd/NO3- systems were chosen for this study since they are known (3) to generate H2 upon titration with formic acid at 90 °C. In addition, the Pd/NO3system is of interest since Pd can catalyze the reduction of nitrate to N2O by formic acid. (a) Titration of Sodium Nitrite with Acetic Acid (Figure 2). This experiment was performed to study the effect of nitrous acid production on the redox potential of the solution in the absence of any other components. Thus, titration of a solution of 0.8 g (11.6 mmol) of NaNO2 in 60 mL of water with 87.9 mmol of acetic acid at 90 ( 1 °C led to the production of NO. At the same time, the redox potential rose to +450 mV and remained in this range throughout the duration of the experiment. No gases other than NO were observed in this experiment, consistent with the lack of reducing power VOL. 32, NO. 20, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Titration of 0.8 g of sodium nitrite in 60 mL of water with 87.9 mmol of acetic acid at 90 ( 1 °C.
FIGURE 3. Titration of 0.8 g of sodium nitrite in 60 mL of water with 84.9 mmol of formic acid at 90 ( 1 °C.
of acetic acid in aqueous solution. The observed amount of NO at the end of the experiment was 7.3 mmol as compared with the 7.7 mmol of NO expected from 11.6 mmol of sodium nitrite by the disproportionation reaction:
3NO2- + H2O f 2NO + NO3- + 2OH-
(2)
(b) Titration of Sodium Nitrite with Formic Acid (Figure 3). Titration of a solution of 0.8 g of NaNO2 (11.6 mmol) in 60 mL of water with 84.9 mmol of formic acid at 90 ( 1 °C led to the production of NO as the redox potential increased to +460 mV. Reduction of nitrous acid by formic acid to N2O then took place with the concurrent production of CO2. During this phase of the reaction, the redox potential fell rapidly to +50 mV. This suggests that the reduction of nitrous acid by formic acid to N2O is associated with a significant decrease in the redox potential of the solution. A very small amount of H2 production was observed starting 100 min after termination of the formic acid addition (Figure 3). (c) Titration of Sodium Nitrite with Formic Acid in the Presence of Rhodium Trichloride (Figure 4). Titration of a solution of 0.8 g of NaNO2 (11.6 mmol), 0.014 g of RhCl3‚ 3H2O, and 60 mL of water with 84.9 mmol of formic acid at 90 ( 1 °C led to the usual increase of redox potential to +450 mV during the initial stages of the reaction when nitrous acid was produced and decomposed to NO. The redox potential then dropped abruptly to -100 mV when ∼3.2 mmol of NO was produced. When the potential reached -100 mV, steady evolution of H2 began to occur. This potential for evolution of H2 was about 150 mV more negative than that observed in the feed simulant systems possibly due to the absence of iron and therefore the Fe3+/Fe2+ redox system in the limited component systems. The final pH was 2.6, and 0.1 mmol of ammonia was observed at the end of the experiment. (d) Titration of Sodium Nitrate with Formic Acid in the Presence of Palladium Dichloride (Figure 5). Titration of a solution of 1.9 g (22.4 mmol) of NaNO3, 0.013 g of PdCl2, 3180
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FIGURE 4. Titration of 0.8 g of sodium nitrite and 0.014 g of RhCl3‚ 3H2O in 60 mL of water with 84.9 mmol of formic acid at 90 ( 1 °C. and 60 mL of water with 84.9 mmol of formic acid at 90 ( 1 °C did not lead to an initial increase in redox potential since none of the oxidizing nitrous acid was generated in the absence of nitrite ion. Instead generation of 0.3 mmol of H2 was observed in the early stages of the reaction during which time the redox potential dropped to -280 mV, which is approximately the potential of the standard hydrogen electrode relative to the Ag/AgCl reference electrode. After
TABLE 3. Product Information in the Formic Acid Titrations of the 9M2XC and 8M2XFeC Feed Simulants mmol of gasa simulant 8M2XFeC/NaNO2/Na2CO3 9M2XC/NaNO2/Na2CO3 8M2XFeC/NaNO2/Na2CO3 9M2XC/NaNO2/Na2CO3 8M2XFeC/NaNO2/NaNO3/Na2CO3 9M2XC/NaNO2/NaNO3/Na2CO3 8M2XFeC/NaNO3/Na2CO3 9M2XC/NaNO3/Na2CO3 8M2XFeC/NaNO3/Na2CO3 9M2XC/NaNO3/Na2CO3 8M2XFeC/NaNO3/Na2CO3 9M2XC/NaNO3/Na2CO3 8M2XFeC/NaNO3/Na2CO3 9M2XC/NaNO3/Na2CO3
noble
metalb
none none RuCl3 (14) RuCl3 (14) RhCl3 (14) RhCl3 (14) none none PdCl2 (14) PdCl2 (14) 5% Pd/C (115) 5% Pd/C (115) 5% Rh/C (115) 5% Rh/C (115)
H2
CO2
N2O
NO
0.0d
19.3 17.1 20.5 17.7 22.5 22.2 8.4 8.5 23.6 28.0 22.0 29.3 10.9 19.9
5.2 5.2 6.3 5.5 6.1 6.1 0.0d 0.0d 2.2 2.3 1.7 1.3 0.0d 0.0d
8.3 7.2 5.6 6.3 6.9 5.9 0.0d 0.0d 0.0d 0.0d 0.0d 0.0d 0.0d 0.0d
0.0d 0.0d 0.0d 1.6 1.6 0.0§ 0.0§ 0.6 0.5 0.8 1.1 0.3 0.1
NH3
0.1 0.1 0.1 0.1
0.4 0.8 0.6 1.0c 0.3 0.5
final pH 4.1 3.9 4.1 4.0 4.3 4.2 3.2 3.4 3.8 5.3 3.7 5.1 3.4 4.8
a The detection limits for these products are estimated to be 0.1 mmol. b RhCl and RuCl were added as their commercial hydrated trichlorides 3 3 whereas PdCl2 was anhydrous; the weight of the noble metal compound in mg is given in parentheses. All of the experiments were performed at 90 ( 1 °C using 50 mL of the feed simulant and 10 mL of water with 84 mmol of formic acid being added over a period of ∼190 min. c 1.6 mmol of N2 was also observed in this experiment. d Below detection limit.
experiments when formic acid reduction of nitrous acid begins as indicated by the production of N2O. (b) During the time H2 is evolved, the potentials are rather negative (-100 to -200 mV in these experiments). The presence of a sufficiently strong oxidant such as nitrous acid (eq 3) appears to be sufficient to prevent H2 evolution. This observation may account for the occurrence of H2 evolution only after destruction of nitrite. Feed Simulant Experiments with Formic Acid. This work used the 9M2XC feed simulant containing iron or the ironfree 8M2XFeC feed simulant (Table 2). Each of these feed simulants was prepared from metal chlorides and free from nitrite, nitrate, and carbonate, which were added as 1.5 g (21.7 mmol) of NaNO2, 0.5 g (5.9 mmol) of NaNO3, and/or 0.7 g (6.6 mmol) of Na2CO3 when these ions were desired as additives in experiments using 50 mL of the feed simulant. The product information from these experiments is summarized in Table 3. Details of changes in redox potential in the experiments summarized in Table 3 are given in Table 4. The results from a typical experiment are depicted in Figure 6.
FIGURE 5. Titration of 1.9 g of sodium nitrate and 0.013 g of PdCl2 in 60 mL of water with 84.0 mmol of formic acid at 90 ( 1 °C. completion of hydrogen evolution, the redox potential rose to -15 mV. The final pH was 1.8, and no ammonia was observed at the end of the experiment. The following general observations can be made from these experiments: (a) Treatment of nitrite-containing systems with formic or acetic acid to give nitrous acid generally leads to an increase in the potential, typically to ∼400 mV, apparently a consequence of the oxidizing potential of nitrous acid, i.e. (ref 6)
HNO2 + H+ + e- ) NO + H2O
E° ) +1.00
(3)
This positive potential drops precipitously in the formic acid
The most abundant redox active component in the feed simulant is iron, which is likely present as hydrous Fe2O3. The experiments summarized in Tables 3 and 4 with the feed simulants 8M2XFeC and 9M2XC are similar except for the absence of iron in the experiments with 8M2XFeC and the presence of iron in 9M2XC. These experiments thus provide information on the effect of iron on the product distribution obtained from the formic acid titration of feed simulants in the presence of various noble metal species. In this connection, the presence or absence of iron is seen to have relatively little effect on the product distribution or final pH in the experiments with feed simulants containing nitrite. This suggests that iron does not participate in redox processes under these conditions. However, the experiments using the nitrite-free iron-containing feed simulant 9M2XC are seen to lead to significantly more CO2 and NH3 production and to a significantly higher final pH than corresponding experiments with the nitrite-free iron-free feed simulant 8M2XFeC. The additional CO2 produced in the presence of iron is much more than that expected from the production of the extra NH3, suggesting that some of this additional CO2 arises from the formic acid reduction of Fe(III) to Fe(II). It thus appears that reductions of nitrate to ammonia and Fe(III) to Fe(II) are “synergistic” processes in that noble metal systems VOL. 32, NO. 20, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. Redox Potentials (mV Relative to Ag/AgCl) at Various Stages in the Experiments Summarized in Table 3 simulant
noble metal
room temp
90 °C
HCO2H addna
NO evol
8M2XFeC/NaNO2/Na2CO3 9M2XC/NaNO2/Na2CO3 8M2XFeC/NaNO2/Na2CO3 9M2XC/NaNO2/Na2CO3 8M2XFeC/NaNO2/NaNO3/Na2CO3 9M2XC/NaNO2/NaNO3/Na2CO3 8M2XFeC/NaNO3/Na2CO3 9M2XC/NaNO3/Na2CO3 8M2XFeC/NaNO3/Na2CO3 9M2XC/NaNO3/Na2CO3 8M2XFeC/NaNO3/Na2CO3 9M2XC/NaNO3/Na2CO3 8M2XFeC/NaNO3/Na2CO3 9M2XC/NaNO3/Na2CO3
none none RuCl3 (14) RuCl3 (14) RhCl3 (14) RhCl3 (14) none none PdCl2 (14) PdCl2 (14) 5% Pd/C (115) 5% Pd/C (115) 5% Rh/C (112) 5% Rh/C (115)
20 262 43 265 75 25 20 115 42 94 43 67 60 52
-140 7 -98 -4 -125 -73 -150 -23 -140 -26 -193 -250 -165
-145 (4) -27 (4) -121 (3) -34 (3) -150 (3) -106 (8) -568 (4) -463 (7) -175 (8) -488 (8) -369 (9) -350 (7) -525 (9) -600 (4)
250 395 240 380 220 250
a
N2O evol 250 395 240 380 220 250
-248 -320 300 -250
H2 evol
NH3 evol
237 167
-250
-320 -320
-325 -488 -271 -350 -415
-315
The figures in parentheses indicate in minutes the time after the start of the formic acid addition that the indicated potential was recorded.
pH range of 2.0-4.7 encountered during most of the formic acid treatment is
Fe2+ + 3H2O f Fe(OH)3 + 3H+ + eE°25°C ) 1.057 - 0.1773 pH - 0.059 log[Fe2+]
FIGURE 6. Titration of 50 mL of 9M2XC, 1.5 g of NaNO2, 0.7 g of Na2CO3, and 14 mg of RhCl3‚3H2O with 83 mmol of 88% formic acid at 90 ( 1 °C. capable of catalyzing formic acid reductions can catalyze both the formic acid reduction of nitrate to ammonia and the formic acid reduction of Fe(III) to Fe(II). This suggests a formato-metal derivative as a common intermediate in the mechanisms of both reduction processes. The higher final pH, typically by ∼1.5 pH units, in the experiments containing nitrate and iron relative to corresponding experiments in iron-free systems can relate to the basic properties of Fe(OH)2 generated by reduction of the much weaker base Fe(OH)3 under these conditions. Table 4 indicates that the trends in redox potentials are similar in the presence and absence of iron. However, many of the observed redox potentials in the absence of iron are 50-150 mV less positive than corresponding redox potentials in the presence of iron in accord with the absence of the Fe(III) oxidant in the iron-free systems. In particular, the sharp rise in potential in nitrite-containing systems associated with nitrous acid production typically increases only as far as 220-250 mV in the absence of iron in contrast to ∼400 mV in many of the systems containing iron. In this connection, the iron redox reaction (7) most relevant in the 3182
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(4)
For example, at pH ) 4 and 0.2 M Fe2+, the E° is 1.057 - 0.709 + 0.041 ) 0.389 V versus the hydrogen electrode or 0.389 0.222 ) 0.167 V versus the Ag/AgCl electrode. Thus at potentials more positive than 0.167 V under these conditions, more than half of the iron is present as Fe(III); whereas at potentials more negative than 0.167 V, more than half of the iron is present as Fe(II) at thermodynamic equilibrium. This potential of 0.167 V ) 167 mV is in the middle of the potential range observed in many of these experiments. The effect of copper on redox potentials was also investigated in view of the previously observed significant effect of added Cu2+ on the N2O/NO ratio. In this connection previous work (8) indicated that the addition of only 1.0 g of Cu(NO3)2‚3H2O (0.004 M Cu) to 1 L of feed simulant (i.e., 13M1+Cu in Table 2) affected significantly the N2O/NO ratio when such a feed simulant was titrated with formic acid. Thus at the end of a titration with ∼85 mmol of formic acid, the copper-free feed simulant 12M1 gave an N2O/NO ratio of 0.95 whereas the otherwise similar copper-containing feed simulant 13M1+Cu gave an N2O/NO ratio of only 0.38. To investigate the effect of copper on the redox potential changes, 50 mL of the 0.004 M copper-containing feed simulant and 10 mL of water spiked with 1.5 g of NaNO2 and 0.5 g of NaNO3 was titrated with 85 mmol of formic acid at 90 ( 1 °C in the usual manner in the absence of noble metals. The redox potential and gas production were monitored. The initial potential of 39 mV decreased to -80 mV upon heating to 90 °C and then to -90 mV after addition of formic acid for 3 min. The redox potential then began to increase with evolution of NO and N2O at ∼340 mV, consistent with the usual formation of nitrous acid. The potential then started to decrease, leading to a potential of -172 mV and a pH of 4.1 at the end of the reaction. The final N2O/NO ratio was 3.6/12.7 ) 0.28, consistent with previous experiments with feed simulants containing Cu. A similar experiment was then performed but with the addition of 14 mg of RhCl3‚3H2O. The changes in redox potential during the formic acid titration were very similar to analogous experiments with an otherwise similar copper-free feed simulant (e.g., Figure 6). The N2O/ NO ratio at the end of this experiment was 0.40 as compared with an N2O/NO ratio of 1.02 for an otherwise similar experiment with the copper-free feed simulant. These
TABLE 5. Effect of the Amount of Formic Acid Added on the Reaction with 50 mL of 12M1 in the Presence of RhCl3‚3H2O at 90 ( 1 °C amount of gases (mmoles)b
evolution potentials and pH values NO
c
N2O
H2
formic acid mmoles
mV
pH
mV
pH
51 55 59 68 76 (6/21/94)a 76 (6/29/94)a 84 41 + 25 mL 12M1c
435 393 441 440 353 368 380 493
4.5 4.5 4.5 4.5 4.6 4.8 4.5 4.1
444 402 450 449 379 380 385 497
4.4 4.4 4.4 4.4 4.3 4.6 4.4 4.0
mV 70 15 -51 -60 40 35 129
potential (pH) pH
max
end
H2
CO2
N2 O
NO
5.0 4.8 4.6 4.1 4.2 3.9 3.8
459 (4.2) 412 (4.3) 463 (4.2) 465 (4.1) 379 (4.3) 407 (4.2) 392 (4.3) 499 (3.9)
114 (5.6) 48 (5.1) -9 (4.8) -18 (4.6) -82 (4.3) 18 (4.3) 10 (4.0) 88 (3.9)
0.0
