Surface Compounds of Carbon

This section is devoted to chemical compounds formed on the surface of the graphite crystal. The internal structure of the crystal is not significantly changed by the surface compounds, which is not the case with graphite compounds. 

OXYGEN COMPOUNDS REACTING AS ACIDS IN AQUEOUS SOLUTIONS 

Preparation involves heating the carbon for many hours in a stream of Og at a temperature as near as possible to the ignition point, i.e., 500°C. During oxidation, the carbon must be vigorously shaken or, better still, fluidized by the Og stream. Since oxidation produces COg, which is then absorbed. It is recommended that the oxidation product be subjected to long heating at 300°C in high vacuum. It should be remembered that the oxidation is accompanied by activation with Og. 

If a highly adsorptive carbon is used, as much as 15 g. of Og can be taken up by 100 g. of the carbon. Under the same conditions, steam produces acid groups whose concentration may reach 700 meq. of H+ ions per 100 g. of the preparation. The material is tested by shaking 0.1 g. of the carbon with 100 ml. of 0.05N alcoholic KOH. The H ions can be replaced by CHg groups through methylatlon with diazomethane. Because of the acidic surface oxides, the carbon is readily wetted by water and poorly by benzene, as contrasted with carbon having no acid surface oxides. Above 500°C, Og is released as CO and COg. 

OXYGEN COMPOUNDS REACTING AS BASES IN AQUEOUS SOLUTIONS 

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Most important and best known among the surface compounds of carbon are those with oxygen and with sulfur. Other elements, e.g., chlorine and hydrogen, can also serve as end groups. ... 

The term surface compound of carbon is restricted to chemisorption, i.e., linkage of the adsorbate to the surface by covalent bonds. Surface groups on graphite are situated on edge carbon atoms or on defect sites there is no attack of basal planes . Carbon-oxygen surface compounds are present on any black carbon in contact with air there are also surface compounds with S, H, N or halogens bonded to carbon... 

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