A simple method for determining the neutralization point in Boehm titration regardless of the CO2 effect

We report a simple method by which the neutralization point in the Boehm titration can be easily determined without going through a pre-screening process to remove the effect of atmospheric carbon dioxide (CO₂). The proposed method is based on the principle that the equivalence and the corresponding neutralization point of the reaction bases remains unchanged regardless of the dissolution of CO₂ in the reaction bases. This method was used to measure the surface functionality of acid-treated multi-walled carbon nanotubes with high precision.     

Standardization of the Boehm titration: Part II. Method of agitation, effect of filtering and dilute titrant

Multiple steps in the Boehm titration are carried out in a variety of manners by different research groups, thereby making results difficult to compare. The methods standardized in this paper include method of agitation, use of dilute titrant, carbon removal from reaction bases and the effect of air on NaOH standardization; uncertainty estimations are also shown. By examining the multiple agitation methods, it was found shaking was the optimal method for use in the Boehm titration as other methods (stirring and sonicating) affect the carbon surface. It was also found that filtering the carbon and reaction base mixture did not affect the titration, nor did the use of dilute titrant. Solutions must be freshly standardized prior to use since storage (even over a one week time period) results in a change in concentration.     

Effects of carbon dioxide and acidic carbon compounds on the analysis of Boehm titration curves

In the characterization of the surface functionality of acid-oxidized carbon nanotubes (CNTs) using the Boehm titration method, dissolution of acidic carbon compounds (ACCs) fragmented from the acid-oxidized CNTs and atmospheric carbon dioxide (CO₂) in the reaction base results in a somewhat over-estimation of the surface functionality of the CNTs. We developed a modified Henderson–Hasselbalch equation to show that the influence of the carboxylic and phenolic groups of ACCs and atmospheric CO₂ on the resultant titration curve can be identified and quantitatively measured, which makes it possible to determine the true surface functionality of acid-oxidized CNTs and ACCs.     

Differentiation and quantification of surface acidities on MWCNTs by indirect potentiometric titration

Wet-chemically oxidized carbon nanotubes (CNTs) generally exhibit both covalently-bound acidic functional groups on the surface and surface-adsorbed acidic substances, i.e. carbonaceous CNT fragments from the oxidation procedure. Direct potentiometric titration of oxidizable high surface area materials with dynamically desorbing acidic fragments is slow and inaccurate. Adsorbed acidic fragments are deprotonated by sodium hydroxide and form anions in solution which is not the case for covalently bound acidic groups on the CNTs, so the following filtration after NaOH treatment separates desorbable acidic substances from non-desorbable or covalently bound groups. For a known concentration of NaOH, titration of the eluate with hydrochloric acid (HCl) allows determination of the concentrations of both types of acidities. However, contrary to reports in the literature, the NaOH consumed by non-desorbed acidic groups has to be accounted for and impedes distinction of desorbed acidic groups via their pK_a values. Results are presented of a study on the information content and the reliability of indirect potentiometric Boehm titration for different oxidized multi-walled CNTs.     

Liquid Phase CO2-Analytics for Standardized Mass Transfer Characterization in Packed Columns

Mass transfer parameters are necessary for the design of absorption and desorption processes in packed columns. To determine the effective interfacial area and liquid side mass transfer parameters, CO₂ absorption and desorption are frequently used. Reliable analytics for concentration determination are essential to obtain correct results. In this work two methods of CO₂ liquid phase analysis are compared: first, the back titration of unreacted NaOH after prior precipitation of the bound CO₂; secondly, the inorganic carbon analysis with a commercial inorganic carbon analyzer.     

Methodological limitations to determining acidic groups at biochar surfaces via the Boehm titration

The Boehm titration is frequently employed to characterize acidic groups at biochar surfaces. However, biochars contain inorganic basic components (carbonates, oxides, hydroxides), inorganic acidic species (silica, alumina) and organic acids (carboxylic acids, phenols, humic-like substances) that can be differentially solubilized in the Boehm bases, rendering invalid results. This is demonstrated for two biochars. Prior to titrations (Boehm or continuous), biochar should be pretreated with NaOH to remove solubilizable acidic species, and then with HCl to remove solubilizable basic components and protonate carbon acid sites. Pretreatment is successful when direct and indirect titrations yield identical results and no precipitate is observed.     

Methoden zur bestimmung des maleinsäureanhydridgehaltes in acrylnitril-copolymeren in gegenwart initialer carboxygruppen

Three different titration methods were proved with respect to accuracy, reproducibility and handling for the determination of maleic anhydride (MSA) content in acrylonitrile (AN) copolymers in form of poly(AN-co-MSA) and poly(AN-co-styrene-co-MSA). The comparison comprises (i) the combined method A/H characterized by the reaction of anhydride groups (AG) with aniline and titration of the formed monoacid with methanolic KOH (A) and the complete hydrolysis of AG and titration of acid groups (SG) with aqueous NaOH (H), (ii) method B based on the conversion of AG with n-butylamine (BA) and back-titration of unreacted amine with HClO₄ in glacial acetic acid and (iii) method C consisting of the reaction of AG with p-chloroaniline (pCA) followed by Cl-determination after the Schöniger-decomposition. Whereas all mentioned methods are suitable for the determination of AG in the presence of initial SG, the combination of A/H additionally allows the simultaneous determination of AG and SG. In comparison with the other methods the combination of A/H is to be favoured due to the possibility to obtain additional information about SG and because of the better results in accuracy, reproducibility and handling. By means of FTIR spectroscopy the content of AG and SG was estimated qualitatively and a correlation between the spectroscopic and potentiometric data of AG was discovered.     

Effects of diluents on NO x formation in coflow CH4/air diffusion flames

The effect of diluent addition on NO _x formation in a laminar CH₄/air coflow diffusion flame was investigated by numerical simulation with experimental verification. The hydrocarbon fuel stream was diluted with N₂, CO₂, and Ar. The volume fraction of diluents systematically changed from 0.0 to 0.5. The simulation data agree well with the experimental one. The computational results indicate that overall the three diluents reduce the formation of NO and the effects vary from weak to strong in the order: N₂, Ar and CO₂. Differences between the influences of the various diluents are discussed in terms of the thermal, the dilution and the direct chemical effects, respectively. Further, the addition of CO₂ reduces the formation of NO₂, while the addition of N₂ or Ar has little effect on it. However, the formation rate of N₂O increases by each of the added diluents.     

Comparison of DFT characterization methods based on N2, Ar, CO2, and H2 adsorption applied to carbons with various pore size distributions

Adsorption isotherms of N₂, Ar, CO₂, and H₂ were measured on selected activated carbons representing various pore size distributions. Isotherm data were analyzed using the DFT method. A good agreement was obtained between N₂ and Ar results in a wide range of pore sizes. Both N₂ and Ar results agree with CO₂ analysis at 273 K in the range of small micropores where CO₂ analysis is most applicable. Also, in this range of pores the analysis of H₂ adsorption data at 77 K gives results which are consistent with the results obtained from the more conventional vapor-phase adsorptives. However, the range of PSDs calculated from H₂ data extends to pore sizes smaller than those that can be characterized by other adsorbates.     

Effects of uniaxial drawing and heat-treatment on gas sorption and transport in PVC

Sorption and transport measurements for various gases in rigid poly(vinyl chloride) were made following uniaxial drawing and heat treatment. The permeabilities of He, Ar, N₂, and CH₄ were found to be essentially independent of pressure in PVC while CO₂ showed a complex pressure dependence which varied with prior exposure and degassing history. Sorption isotherms were analyzed by the dual mode sorption model, and the parameters obtained were correlated with the Lennard–Jones potential-well depth of the gas. The Henry's law coefficient for CO₂ was found to be significantly larger than expected which is believed to be the result of a specific interaction with PVC. Uniaxial drawing of PVC above its glass transition caused significant reductions in gas permeabilities, of which roughly one-third is attributable to the accompanying heat treatment rather than molecular orientation per se. The physical state of the polymer was characterized by density, birefringence, and calorimetry. Changes in gas sorption and permeation behavior are discussed in terms of these results.     

One‐step production of CO‐ and CO2‐free hydrogen from biomass

The reaction between a biomass (cellulose, sucrose, glucose, starch, cotton, or Japanese paper) and NaOH in the presence of water vapor produced pure hydrogen without CO and CO₂ at temperatures in the range 473–623K. The addition of Ni/Al₂O₃ or Rh/Al₂O₃ catalyst to cellulose enhanced the production of hydrogen at <573 K. The reaction between cellulose and NaOH can be written as: C₆H₁₀O₅ + 12NaOH + H₂O = 6Na₂CO₃ + 12H₂. The reactivities of alkali metal hydroxides were: KOH > NaOH ? LiOH. Copyright © 2004 Society of Chemical Industry     

The use of the Nb-Nb2O5 as an oxide ion indicator electrode in fused salts

The use of the Nb-Nb₂O₅ electrode as an oxide ion indicator electrode in potentiometric titrations between acids and bases in fused salts was investigated. Two oxide ion acceptors (acids), NaPO₃ and Na₄P₂O₇ were titrated with oxide ion donors (bases) in fused KNO₃. The reaction was carried out at 625 K and the bases used in these titrations are K₂CO₃ and Na₂O₂. The electrode was found to serve as an excellent indicator for acid base potentiometric titrations in fused salts. It serves also as an oxide ion indicator for the titration of mixtures of acids in which chemical transformations are taking place during the course of titration. The experimental results were discussed in terms of the potential arrests and potential shifts upon the addition of the oxide ion donor to the acid.     

Study of the carbonization temperature for a chemically activated carbon: influence on the textural and structural characteristics and surface functionalities

This study aims at investigating the relationship between the porosity and the surface functional groups of an activated carbon during the pyrolysis process. Several activated carbons are prepared by the carbonization of a mixture of olive waste and ZnCl₂ at different temperatures, which are chosen by taking into account the different phenomena occurring during this treatment. The porosity of the prepared carbons is investigated using both N₂ and CO₂ isotherms at 77 and 273 K, respectively. Long-range and short-range orders are studied respectively by X-ray diffraction (XRD) and Raman spectroscopy. The investigation of the carbon functionalities is carried out by Boehm titration, Fourier transformed infrared spectroscopy (FTIR) and water adsorption measurements. The effect of carbonisation temperature on the adsorption of cyclohexane provides additional information on the oxygen functional groups and especially the carboxylic proups. To summarize, the results show that a carbonisation at 573 K is sufficient to obtain a well-developed porosity and to ensure already the formation of acidic surface groups and especially the carboxylic groups.     

对用于活性炭表面含氧官能团分析的Boehm滴定法的几点讨论

不同的研究者在采用Boehm滴定法测定活性炭表面含氧官能团的含量时在操作方法上存在差异.本文探讨了Boehm滴定法中CO2的去除,直接滴定与返滴定,振荡时间,溶液的浓度对滴定过程的影响,提出适宜的操作建议.     

Feasibility of Na-based thermochemical cycles for the capture of CO2 from air—Thermodynamic and thermogravimetric analyses

Three Na-based thermochemical cycles for capturing CO₂ from air are considered: (1) a NaOH/NaHCO₃/Na₂CO₃/Na₂O cycle with 4 reaction steps, (2) a NaOH/NaHCO₃/Na₂CO₃ cycle with 3 reactions steps, and (3) a Na₂CO₃/NaHCO₃ cycle with 2 reaction steps. Depending on the choice of CO₂ sorbent – NaOH or Na₂CO₃ – the cycles are closed by either NaHCO₃ or Na₂CO₃ decomposition, followed by hydrolysis of Na₂CO₃ or Na₂O, respectively. The temperature requirements, energy inputs, and expected products of the reaction steps were determined by thermodynamic equilibrium and energy balance computations. The total thermal energy requirement for Cycles 1, 2, and 3 are 481, 213, and 390 kJ/mol of CO₂ captured, respectively, when heat exchangers are employed to recover the sensible heat of hot streams. Isothermal and dynamic thermogravimetric runs were carried out on the pertinent carbonation, decomposition, and hydrolysis reactions. The extent of the NaOH carbonation with 500 ppm CO₂ in air at 25 °C – applied in Cycles 1 and 2 – reached 9% after 4 h, while that for the Na₂CO₃ carbonation with water-saturated air – applied in Cycle 3 – was 3.5% after 2 h. Thermal decomposition of NaHCO₃ – applied in all three cycles – reached completion after 3 min in the 90–200 °C range, while that of Na₂CO₃ – applied in Cycle 1 – reached completion after 15 min in the 1000–1400 °C range. The significantly slow reaction rates for the carbonation steps and, consequently, the relatively large mass flow rates required, introduce process complications in the scale-up of the reactor technology and impede the application of Na-based sorbents for capturing CO₂ from air.     

Deliberately Losing Control of C−H Activation Processes in the Design of Small-Molecule-Fragment Arrays Targeting Peroxisomal Metabolism

Combined photochemical arylation, “nuisance effect” (S_NAr) reaction sequences have been employed in the design of small arrays for immediate deployment in medium-throughput X-ray protein–ligand structure determination. Reactions were deliberately allowed to run “out of control” in terms of selectivity; for example the ortho-arylation of 2-phenylpyridine gave five products resulting from mono- and bisarylations combined with S_NAr processes. As a result, a number of crystallographic hits against NUDT7, a key peroxisomal CoA ester hydrolase, have been identified.     

The adsorption of dibenzothiophene using activated carbon loaded with cerium

Cerium-loaded activated carbon was prepared by classical soaking impregnation method and tested for dibenzothiophene adsorption from model fuels. The new adsorbents showed much better adsorption capacity and selectivity towards DBT than the virgin carbon. The adsorbents were characterized by N₂ adsorption, Boehm titration and FTIR. The improved performance is mainly due to changes in surface chemistry. The results show that the performance of activated carbon as desulfurization adsorbents can be considerably enhanced by a simple cerium loading method.     

Buffer gas optimization in CO2 laser ablation for structure control of single-wall carbon nanohorn aggregates

We succeeded in morphology-selective preparation of single-wall carbon nanohorn (SWCNH) aggregates by changing the buffer gas (760 Torr) in CO₂ laser ablation of graphite: He for seed type, N₂, Ar and Ne for dahlia type, and Kr for petal dahlia type. In Xe, aggregates of thin graphene sheets were the major product. The degree of graphitization increased with the mass number of the buffer gas. We specifically compared the structure and properties of SWCNHs prepared in N₂ with those prepared in Ar for possible costs reduction for production by substituting N₂ for Ar buffer gas. As-grown SWCNH aggregates prepared with N₂ had smaller specific surface area than those prepared in Ar, however, this discrepancy almost completely disappeared with heat treatment at 1200 °C. By additional oxidation treatments, the specific surface area of the former became larger than the latter, and this is the largest surface area ever reported. We inferred that as-grown SWCNH aggregates (N₂) had less number of petal-like graphenes, therefore after the removal of non-SWCNH carbons by the heat treatment and/or oxidation, leading to the large surface area.     

Gas Phase Reactions in Silent Electric Discharges Part III: Investigation of Chemical Reactions Involving Electron-Molecule Collisions in Various Mixtures of O2 with Ar,N2 or CO

Electron-molecule collisions play a significant role in chemical transformations in a silent electric discharge. The reaction rate coefficients of electron-molecule collisions in various gaseous mixtures of Ar, N₂ or CO with O₂ have been investigated. These coefficients can be determined experimentally and calculated theoretically. The measured values are in good agreement with those calculated by theoretical methods. The reaction rate coefficients of electron-O₂ molecule collisions increase with decrease of the amount of O₂ in Ar/O₂ mixtures, but decrease with decrease of the amount of O₂ in N₂/O₂ or CO/O₂ mixtures.     

Effect of nonreacting gases on the desorption of reaction-created CO from graphite

Temperature-programmed desorption (TPD) was used to determine whether inert gases influence the desorption step in the oxidation of carbon by CO₂. Surface oxides were formed on a graphite by reacting it at 1200 K with CO₂, and then quenching the reaction. The oxides then were removed by TPD to 1373 K, using He, Ar and Kr as carrier gases in separate desorptions. The oxides appeared as CO₂ and CO in the desorption carrier gases; some CO₂ was observed between 800 and 1050 K, but above 1050 K the product was almost entirely CO. The CO₂ desorption showed no effect from changing the carrier gas, but temperature of CO desorption peak and amount of CO desorbed by 1373 K both depended on which gas was used as a carrier. This shows that nonreactive gases affect the desorption step of the C-CO₂ reaction. Involved in the desorption step is movement of some surface oxide species to desorption sites; the nonreactive-gas influence occurs because the nonreactive gases affect this surface transport. In the presence of Ar and Kr, transport rates of surface oxides to desorption sites were higher than they were in the presence of He. Under reaction conditions this can result in greater CO desorption rates and faster overall reaction rates in the presence of Ar and Kr than occur in the presence of He.