"Does anybody know where i can find the official spezification of the RS232"

The Official Standard is TIA/EIA-232-F, Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange.

It's here - in the Top Selling Standards of the Telecommunications Industry Association (TIA):
http://www.tiaonline.org/standards/search_n_order.cfm
(I have aired my opinion on having to pay for Standards before)

"The RS232 is spezified for 20000bits/sec..."

Correct (section 1.3 of the spec); there are other Standards for higher speeds:

TIA/EIA-530-A High Speed 25-Position Interface for Data Terminal Equipment and Data Circuit-Terminating Equipment, Including Alternative 26-Position Connector (ANSI/TIA/EIA-530-A-92) (R98)
TIA/EIA-561 Simple 8-Position Non-Synchronous Interface between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange (ANSI/TIA/EIA-561-90) (R98)
TIA/EIA-574 9-Position Non-Synchronous Interface between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange (ANSI/TIA/EIA-574-90) (R98)

"...with a cablelength of 50ft"

Incorrect: the Standard specifically excludes any definition of cable length (section 3.2); rather, it defines the electrical requirements - so the actual permissible length will depend on the properties of the cable used.
Capacitance is the Big Issue here.