Boehm titration

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The Boehm titration is a wet chemical method for the quantitative analysis of oxygen-containing functional groups on the surface of porous carbon materials and goes back to Hanns-Peter Boehm .

history

In the work Surface Oxides of Carbon published in 1964, Boehm, Diehl, Heck and Sappok dealt with oxygen-containing surface groups on amorphous carbon materials. The surface functionalization was investigated by neutralization with different strong bases. For this, the carbon samples with 0.05 N to 0.1 N solutions were , , or neutralized. The remaining amount of base was then determined by titration with dilute hydrochloric acid. They observed that for the same carbon material, the amount of bases required for neutralization increased with the strength of the base. They attributed this to the presence of four differently acidic functional groups. Through various organic detection reactions (for example reaction with diazomethane and subsequent saponification with hot hydrochloric acid), the groups were finally identified as carboxyl, lactone and phenol and carbonyl groups.

principle

With Boehm titration, oxygen-containing functional groups on carbon materials can be quantified by volumetric titration . A total of four reaction bases can be used for this. Solutions of sodium hydrogen carbonate , sodium carbonate and sodium hydroxide are used as aqueous reaction bases . An anhydrous sodium ethoxide solution can also be used as the reaction base. In principle, acidic functional groups can be neutralized with the aid of the aqueous reaction bases. In this way, carboxyl , lactone or lactol and phenol groups can be quantified. The amount of carbonyl groups can also be determined by using a sodium ethanolate solution . The reason for this is the p K S values ​​of the reaction bases . It is assumed that the base neutralizes all groups that are more acidic than the base itself. With p K S = 6.4, sodium hydrogen carbonate is the weakest base and can therefore only deprotonate carboxyl groups. The next stronger base sodium carbonate (p K S = 10.3) can both deprotonate carboxyl groups and react with lactone groups with hydrolysis. Sodium hydroxide is the strongest base with p K S = 15.7 and can react with the aforementioned functional groups and phenol groups. It is assumed that anhydrides hydrolyze to two carboxyl groups without the influence of a base; these can then be neutralized by the reaction base. The carboxyl and phenolic groups are Brønsted acids . The reaction is therefore an acid-base reaction. Lactone groups, however, have no acidic protons; instead, the reaction is a base-induced hydrolysis. The ethanolate is the strongest base used with p K S = 20.6. However, the reaction with the carbonyl groups does not follow the Brønsted acid-base concept. The carbonyl group is a Lewis acid. The ethanolate ion reacts as a Lewis base , producing a deprotonated hemiacetal.

Individual evidence

  1. ↑ a b c H.-P. Boehm, E. Diehl, W. Heck, R. Sappok: Surface Oxides of Carbon . In: Angewandte Chemie International Edition in English . tape 3 , no. October 10 , 1964, ISSN  0570-0833 , p. 669-677 , doi : 10.1002 / anie.196406691 ( wiley.com [accessed January 31, 2020]).
  2. ^ HP Boehm: Surface oxides on carbon and their analysis: a critical assessment . In: Carbon . tape 40 , no. 2 , February 2002, p. 145–149 , doi : 10.1016 / S0008-6223 (01) 00165-8 ( elsevier.com [accessed January 31, 2020]).
  3. ↑ Jan Schönherr, Johannes Buchheim, Peter Scholz, Philipp Adelhelm: Boehm Titration Revisited (Part I): Practical Aspects for Achieving a High Precision in Quantifying Oxygen-Containing Surface Groups on Carbon Materials . In: C . tape 4 , no. 2 , April 6, 2018, ISSN  2311-5629 , p. 21 , doi : 10.3390 / c4020021 ( mdpi.com [accessed January 31, 2020]).
  4. ↑ Jan Schönherr, Johannes Buchheim, Peter Scholz, Philipp Adelhelm: Boehm Titration Revisited (Part II): A Comparison of Boehm Titration with Other Analytical Techniques on the Quantification of Oxygen-Containing Surface Groups for a Variety of Carbon Materials . In: C . tape 4 , no. 2 , April 11, 2018, ISSN  2311-5629 , p. 22 , doi : 10.3390 / c4020022 ( mdpi.com [accessed January 31, 2020]).