Boehm titration

Oxidation and incorporation of nitrogen usually affect the surface chemistry of carbons [12, 20, 24]. The types and numbers of basic and acidic groups evaluated using Boehm titration [3, 4]. are collected in Table 2. The data reported indicate that the initial carbons differ in their acidity as a result of the activation method and the type of an organic precursor [1]. BAX is more acidic than BPL. This is expected for a phosphoric acid activated carbon [25]. After modification with urea and heat treatment at 723 and 1223 K, the overall surface chemistry... 

Boehm titration results are presented in Table 1. As seen in Table 1, oxygen functional groups of surface-treat ACFs are increased. These results can be explained that the anodic oxidation treatment of the ACFs surfaces produces various oxygen functional groups, i.e., carboxylic, lactonic, and phenolic groups. 

Boehm titration pK of functional groups, their number... 

Contescu, A., Contescu, C., Putyera, K., and Schwarz, J.A. (1997). Surface acidity of carbons characterized by their continuous pK distribution and Boehm titration. Carbon, 35, 83—94. 

Boehm titrations" [9-10] aim at quantifying the acidic functions present on the solid surface and classifying them in different types by titrations with bases of different strengths. High care is needed for this characterization because of the small amount of solid, the high dilution of the solution used for the titrations and possible soda carbonation. Yet in this work, total acidity only is often measured by titration of the solid with soda. 

The total acidity of the solids is calculated from soda titration. Results are presented in Table 2 for Boehm titrations, where the number of acidic functions is expressed in meq/100 g of solid, and in Figs, la and lb for the Johnson Matthey titration. 

FTIR spectroscopy. Several absorption bands are mentioned in the literature [9-11] and attributed to vibrational properties of different surface groups. It is important to stress that this method does not make the difference between identical groups of different strength. In this section, we will only focus on the common features of the support and the catalyst. The phenolic groups seem to be more numerous than the others (quinonic, carboxylic, lactonic). The nitric treatment seems to increase the amount of quinonic groups on the support, and the quinonic and phenolic ones on the catalyst. Those results are in accord with these obtained in the Boehm titration. At this moment, we have no direct information on the basic surface groups. 

Table 2,1 Results of Boehm Titration (mmol/g) for Initial and Urea-Treated Carbons at 623 and 1223 K...

Figure 2.7 Number of acidic groups determined from Boehm titration versus potentio-metric titration. (From ref. 82, with permission from Elsevier.)...

This approach was used to characterize carbons [203,215,290]. The results showed good correlation with the data obtained nsing the Boehm titration method (Figure 2.15).The advantage of the IGC approach is that it provides information about acidity in a nonaqneons environment, and the resnlts are obtained in a rapid and reproducible way. 

As bases sodium bicarbonate, NaHCOs (pKa=6.37), sodium carbonate, Na2C03 (pKa.=10.25), sodium hydroxide, NaOH (pKa= 15.74) and sodium ethoxide, NaOCzHs (pKa=20.58) are used. It is assumed that sodium bicarbonate neutralizes carboxylic acids, sodium carbonate - carboxylic acids and lactones, sodium hydroxide - carboxylic acids, lactones and phenols, whereas sodium ethoxide will react with all oxygen species, even extremely weak acids (pKa< 20.58). fti practice Boehm titration is limited to the determination of carboxylic groups, lactones and phenols [192 197]. Sodium ethoxide is not used very often owing to the necessity of performing the experiment in nonaqueous media and oxygen-free conditions. One of the reasons why the sodium salts were chosen as bases is that they do not form precipitates after reaction with gaseous CO2 and their specific interactions with carbon surfaces are minimal [198]. 

The effects of heat treatments and chemical oxidation on the distribution of surlace groups on carbon materials have been widely studied and discussed based on the results from Boehm titration [14, 16, 44, 204—207]. Table 1 shows as an example, the Boehm titration of a series of activated carbons oxidized with nitric acid at different concentrations and a saturated solution of ammonium peroxodisulfate [207]. 

Oxygen content, point of zero charge and Boehm titration of a series of activated carbons oxidized with nitric acid and ammonium peroxodisulphate [207]. Reprinted with permission from C.O. Ania, J.B. Parra, J.J. Pis, Ads. Sci. Technol., 22 (2004) 337... 

One of the reasons why advanced approaches to the titrations of functional groups have been developed was the impossibility of obtaining a reliable estimation of surface chemistry based on potentiometric titration in aqueous media. Until the mid-90 s many attempts were made, but the curves obtained did not reveal any discrete end points useful for meaningful interpretation [74, 192, 198, 211, 212], It was believed that the smooth titration curves represent the continuum of closely interacting functional groups. The evaluations based on the amount of NaOH used to reach a certain pH are similar in their nature to Boehm titration [192, 196-199], The derivatives obtained in some studies were difficult for characterization due to the many maxima revealed when no well-defined end points were present [196]. 

The importance of basicity was also shown when the density of basic groups on the surface determined using Boehm titration method was linked to the MM removal ctpachy. The majority of carbons used for that research followed the linear trend. The discrepancy in some case was caused by the differences in the content of catalytically active phase, iron oxide, which will be addressed later in this section. 

Concentration (meq/g) of Surface Functional Groups Calculated Using Boehm Titration Method... 

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