Boehm titration is a quantitative analysis of functional groups on the surface of carbon materials based on their reactions with basic solutions of NaHCO3 (pKa = 6.4), Na2CO3 (pKa = 10.3), and NaOH (pKa = 15.7). This is a cost-efficient method that gives absolute values with high precision of the accessible, mainly oxygen-containing functional groups on the surface. Originally, Boehm titration was developed for carbon materials like conductive carbon black (CCB), activated carbon, porous carbon, and graphite. Modern carbon-based materials like graphene, graphene oxide (GO), or carbon nanotubes can also be analyzed this way. GO is a carbon-based two-dimensional nanomaterial containing high amounts of functional groups. It is mainly used to form reduced graphene oxide (RGO) (e.g. exfoliated graphene) as a modern high-end material with remarkable mechanical and electrical properties, applied in nanocells, detectors, nanoscaled conductive devices, batteries, and more. 

The sample materials are weighed into different glass beakers to react with added bicarbonate, carbonate, or sodium hydroxide solution for two days. Blank samples must also be prepared for each base solution. 

OMNIS titrator with the digital pH electrode and a dosing module.
Figure 1. OMNIS titrator with the digital pH electrode and a dosing module.

Aliquots of the blank and sample were titrated against hydrochloric acid solution until after the last equivalence point (Figure 2). 

Figure 2. Exemplary titration curve of an aliquot of GO in NaOH solution with HCl as titrant.
Table 1. Summarized results for the functional group determination by Boehm titration of graphite and GO.
Base solution (n = 3)  Functional groups in graphite (mmol/g) Functional groups in GO (mmol/g)
Sodium hydroxide 0.1982 5.7354
Sodium carbonate 0.0628 4.1399
Sodium bicarbonate 0.0452 3.6967

Boehm titration is the easiest and most cost-efficient way to quantify the number of functional groups on carbon materials. As expected, the amount of functional groups found on GO is significantly higher (30 times) compared to graphite. Therefore, this method allows for quality control of carbon-based materials, and by using the high-end OMNIS platform, the results are directly calculated and displayed. Furthermore, analysis can also be automated reducing sources of human error and allowing a higher sample throughput. 

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