Standardization of the Boehm titration. Part I. CO2 expulsion and endpoint determination

The various steps required in the Boehm titration (CO₂ removal, agitation method, endpoint determination, etc.) are carried out in different ways by different research groups, making a comparison of the results between these groups difficult. Herein, the methods of CO₂ expulsion and endpoint determination for the Boehm titration were standardized. Blank samples of the three Boehm reaction bases, NaHCO₃, Na₂CO₃ and NaOH, were examined for complete CO₂ expulsion through sparging with an inert gas (N₂ or Ar), heating, or utilizing a N₂-filled glove box. Boehm titrations using NaOH as the reaction base were studied through direct titration and back-titration. It was found that to minimize errors both the NaOH titrant and HCl should be standardized prior to titration and that a back-titration is preferable for all three reaction bases. Additionally, the titration should be performed immediately after degassing for 2 h with N₂ or Ar, and degassing should continue during the titration. This is found to be particularly true of the NaOH reaction base, where the effects of dissolved CO₂ are the most noticeable and persistent. With sufficient CO₂ removal, there is no significant difference between pH electrode or colour indication endpoint determination, and either is satisfactory.     

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.     

Adsorption equilibria in the systems: Aqueous solutions of organics—oxidized activated carbon samples obtained from different parts of granules

The adsorption of organics from aqueous solutions on the oxidized activated carbon samples obtained by gradual abrasion of granules is studied. The low temperature nitrogen adsorption-desorption isotherms are applied to determine the parameters of porous structure of investigated materials. The chemistry of adsorbent surface is characterized using XPS technique and Boehm titration method. The adsorption isotherms of three organic substances from dilute aqueous solutions are also measured. The influence of changes of surface properties inside of a carbon particle on organic solute adsorption from aqueous solutions is analyzed and discussed.     

Assay of quaternary ammonium antimicrobial compounds by aqueous potentiometric titration

An automatic potentiometric titration method for the determination of quaternary ammonium antimicrobial compounds at the macro level is described. The procedure involves the use of standard sodium lauryl sulfate as the titrant and a nitrate ion-selective or surfactant electrode to detect the end point. The method, which includes a new, simpler means of titrant standardization, avoids the use of hazardous solvents such as chloroform, which are employed in two-phase titration methods currently in common use. In this study, four frequently produced antimicrobial quats were titrated potentiometrically and by a two-phase titration using bromphenol blue as the indicator. Comparison of results shows the automatic potentiometric method to be accurate, more precise, easier, faster, and generally better suited for use in a production laboratory than the two-phase approach. The nitrate and the surfactant electrodes appear to be equal in performance.     

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.     

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.     

Nonaqueous titrimetric analysis of sulfuric and alkylbenzene sulfonic acids in detergent intermediates

A single differential nonaqueous titration method is presented for the analysis of sulfuric and alkylbenzene sulfonic acids in a detergent intermediate material. The organic and sulfuric acids in the sample were quantitatively titrated potentiometrically with standard cyclohexylamine solution. With certain specified sample compositions, approximate sulfuric acid determinations may be made without sample weighing or the use of standardized titrant. The titration is based upon the ratio of the first and second end points of the differential titration curve. Because of its simplicity, the latter method adaption is especially useful for routine control type analyses.     

Textural and chemical factors affecting adsorption capacity of activated carbon in highly efficient desulfurization of diesel fuel

Two synthetic, polymer-derived carbons, and two commercial carbons were investigated as adsorbents of dibenzothiophene and 4,6-dimethyldibenzothiophene from simulated diesel fuel in dynamic conditions. The total concentration of sulfur was 20 ppm. The surface features of the carbons were evaluated using adsorption of nitrogen, potentiometric titration, Boehm titration, thermal analysis and FTIR. The polymer-derived carbons outperformed the commercial micro- and micro/mesoporous carbons from the point of view of adsorption capacity and selectivity. The latter was evaluated based on the adsorption of naphthalene, which was also present in the fuel used. It was found that the presence of arenes did not affect significantly the capacity measured. The results suggest that the amount adsorbed is mainly governed by the volume of micropores, where dispersive interactions are predominant. Acidic groups located in larger pores are also important to attract additional molecules DBT and 4,6-DMDBT via specific interactions with the progress of adsorption. These groups may also contribute to the reactive adsorption leading to oxidation of DBT and 4,6-DMDBT.     

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.     

Oxidation of H2S on activated carbon KAU and influence of the surface state

Properties of the oxidized activated carbon KAU treated at different temperatures in inert atmosphere were studied by means of DTA, Boehm titration, XPS and AFM methods and their catalytic activity in H₂S oxidation by air was determined. XPS analysis has shown the existence of three types of oxygen species on carbon catalysts surface. The content of oxygen containing groups determined by Boehm titration is correlated with their amount obtained by XPS. Catalytic activity of the KAU catalysts in selective oxidation of hydrogen sulfide is connected with chemisorbed charged oxygen species (O_3.1 oxygen type with BE 536.8–537.7 eV) present on the carbons surface.

Formation of dense sulfur layer (islands of sulfur) on the carbons surface and removal of active oxygen species are the reason of the catalysts deactivation in H₂S selective oxidation. The treatment of deactivated catalyst in inert atmosphere at 300 °C gives full regeneration of the catalyst activity at low temperature reaction but only its partial reducing at high reaction temperature. The last case is connected with transformation of chemisorbed charged oxygen species into CO groups.

The KAU samples treated in flow of inert gas at 900–1000 °C were very active in H₂S oxidation to elemental sulfur transforming up to 51–57 mmol H₂S/g catalyst at 180 °C with formation of 1.7–1.9 g S_x/g catalyst.     

Characterization of oxygen functional groups on carbon surfaces with water and methanol adsorption

We propose a new method, using the ambient temperature adsorption of methanol in the Henry law region as a molecular probe, to determine the concentration of surface oxygen functional groups (C_α) on carbon adsorbents. Two adsorbents: porous A5 and non-porous Carbopack F have been chosen here as experimental examples. The theoretical Henry constant (K_α) is the product of the intrinsic interaction between a methanol molecule and one functional group, α, having a concentration (C_α) and is estimated by carrying out a volume integration of the Boltzmann factor for methanol interacting with the functional group. Our results show that C_α determined with methanol for A5 compares well with that determined previously with water as the molecular probe (Nguyen, 2014 [1]); and both are in good agreement with results from Boehm titration. For Carbopack F, we find that C_α, determined by methanol adsorption, gives more realistic concentrations than those obtained by Boehm titration, which are subject to large errors because of the very low concentration of the functional groups. The method proposed here is fast and easy to implement, and serves as an alternative to the Boehm titration technique.     

活性炭表面热氧化对其吸附二苯并噻吩性能影响

本文主要研究活性炭表面氧化对其吸附二苯并噻吩性能的影响。将活性炭在不同低温下氧化制得表面氧化活性炭，用静态吸附法进行了二苯并噻吩在初始及氧化活性炭上的吸附等温线，应用Langmuir方程对吸附等温线进行拟合，用漫反射红外谱（DRIFTS）表征活性炭表面含氧基团，用Boehm滴定测定活性炭表面官能团含量，讨论了活性炭表面化学性质对其吸附二苯并噻吩的影响。结果表明：活性炭表面酸性含氧基团对二苯并噻吩的吸附有重要影响，酸性含氧基团越多，其吸附量越大。低温气相氧化活性炭提高了活性炭表面酸性含氧基团，提高了其对二苯并噻吩的吸附。氧化温度越高，其表面含氧基团含量越多，其对二苯并噻吩的吸附量也越大。Langmuir吸附等温线可适用于描述二苯并噻吩在活性炭表面上的吸附。     

活性炭表面官能团表征进展

对活性炭表面官能团的种类和各种定性定量分析方法如Boehm滴定法、零电荷点法(pHPZC)、傅里叶变换红外色谱法(FTIR)和X射线光电子能谱(XPS)以及程序升温脱附法(TPD)等进行了综述,并比较了各种方法的优缺点。     

Some new developments in surfactant analysis

Three new analytical methods for surfactants have been developed and evaluated, and the stoichiometry in cationic titration of a dianionic surfactant has been determined. The three methods include: (a) a rapid method for pyrolytic cleavage of hydrophobic groups from surfactants to hydrocarbons, which can be used for quantitative identification of the hydrophobic groups; (b) a molecular weight method for alkylphenoxy and alkoxy polyethoxylates based on sulfation followed by cationic titration; and (c) a microcationic titration method designed for low levels (1–10 microequivalents) of anionic surfactants, which possesses certain advantages over conventional methods. Previously stoichiometry in cationic titration has been reported only for monoanionic surfactants. In a study of a dianionic surfactant,n-octadecylbutane 1,4-disulfate, each sulfate group was found to react in a 1:1 equivalent ratio with a cationic titrant.     

Role of microporosity and surface functionality of activated carbon in methylene blue dye removal from water

Activated carbons have been prepared from jute stick by both chemical and physical activation methods using zinc chloride and steam, respectively. They were characterized by evaluating surface area, iodine number, pore size distribution, and concentration of surface functional groups. The chemically activated carbon largely featured micropore structure, while the physically activated carbon mainly featured macropore structure. The specific surface area of chemically and physically activated carbons was 2,325 and 723 m ² /g, while the iodine number was 2,105 and 815mg/g, respectively. The concentration of surface functional groups was determined by Boehm titration method, which suggested that different types of surface functional groups are randomly distributed on chemical activated carbons, while it is limited for physical activated carbon. The microporosity along with surface functional groups provided a unique property to chemically activated carbon to adsorb Methylene Blue dye to a large extent. The adsorption of dye was also affected by the adsorption parameters such as adsorption time, temperature and pH. Comparatively, higher temperature and pH significantly facilitated dye adsorption on chemically activated carbon.     

Catalytic properties of activated carbon surface in the process of adsorption/oxidation of methyl mercaptan

Activated carbons of different origins were used as adsorbents of methyl mercaptan (MM). Before the MM breakthrough capacity tests were carried out, the surface of carbons was characterized from the point of view of its chemistry (Boehm titration) and porosity (adsorption of nitrogen at its boiling point). The results showed that the ability of carbon to adsorb methyl mercaptan depends strongly on its surface chemistry, particularly on the presence of basic oxygen-containing groups and ash content. Catalytic effect of one metal, iron, was studied in more details. It was found that introduction of iron enhances the removal capacity significantly as a result of electron transfer reaction in which thiolate ion is oxidized to dimethyl disulfide (DMDS). This reaction likely involves the reduction of iron sites, which are regenerated after further re-exposure to oxygen. DMDS as a main reaction product is strongly adsorbed in small pores. Water is required for the formation of DMDS since it facilitates the dissociation of MM. That dissociation occurs in water film when the pH of the local system is greater than the apparent pK_a of MM in the confined pore space.     

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.     

表面改性对活性炭吸附性能的影响

采用浓硝酸对活性炭进行表面氧化处理,将得到的样品分别浸渍在银氨、硫酸铜及咪唑溶液中进一步改性,然后采用Boehm滴定法对改性活性炭表面酸性基团的含量进行测定,研究改性对活性炭吸附性能的影响。结果表明：通过上述改性,活性炭表面酸性基团发生了显著变化,特别是羧基含量增加较多,从而活性诙的吸附性能发生了相应的变化。     

Potentiometric titration of long chain quaternary ammonium compounds using sodium tetraphenyl borate

Current methods for determining the activity of long chain quaternary ammonium compounds (QACS) are based either on dye partition, titration, or colorimetric analysis. The two major disadvantages of these methods are the disparity of partition coefficients among differently constituted QACS and the difficulty in detecting visual end points. Some potentiometric titration methods for QACS have been reported in the literature. However, back titration techniques, as well as complicated electrode systems, are generally involved. A new potentiometric titration system is presented which uses aqueous sodium tetraphenylborate (TPB) solution as a titrant and a platinum-platinum electrode system to detect the end point. Standard potentiometric titration instruments may be used for this method. This new potentiometric method is superior in precision and accuracy to visual (colorimetric) methods. Original version presented at the 1986 Annual AOCS Meeting in Honolulu, HI.     

Automatic potentiometric titration of surface acidity of carbon black

Acid strength distribution on commercial carbon blacks and chemically oxidized carbon blacks was measured by automatic titration using sodium hydroxide solutions as titrants. The titration curves show that the weaker acids with pKa between 7.0 and 11.0 occur in the amount of 0.025–0.490 m eq/g, the stronger ones with pKa between 2.0 and 7.0 in the amount of 0–0.475 m eq/g and trace amount of pKa range below 2.0. The acid strength distribution obtained by this method coincides with that by the Boehm method. The observed amounts of total acid exceed active hydrogen contents obtained by the Zerewitinoff method in the carbon blacks. Three different oxidizing agents were used to determine their effect on carbon black acidity. Nitric acid produced acidic groups at the highest surface density; H₂O₂ produced a predominance of weaker acids; hot air produced stronger acids with an increase of specific surface area.