FLC IS GIVEN A TWR FREQ TO BE USED FOR DEP THAT IS NOT PUBLISHED.

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ON TAXI OUT WE WERE #3 OR #4 IN LINE FOR RWY 32L FROM INTXN T10 WHEN WE SWITCHED TO N TWR FREQ OF 126.9. WE NOTICED THAT WE DID NOT HEAR THE TKOF CLRNC FOR DEPARTING ACFT. SWITCHED TO S TWR FREQ AND STILL HEARD NOTHING. RETURNED TO N TWR FREQ AND THEY TOLD US; AS WE WERE APCHING THE #1 POS; TO CONTACT TWR 132.7. THIS FREQ IS NOT PUBLISHED ON THE ARPT DIAGRAM CHART; 10-9. WE PROBABLY COULD HAVE WAITED TO CHANGE FROM GND CTL TO TWR; BUT THE XMISSION VOLUME IS SO LARGE AT ORD; WE ASSUMED THE N TWR FREQ WAS THE CORRECT ONE AND THAT THE TIMING WAS RIGHT FOR THE FREQ CHANGE. LAST CLRNC RECEIVED WAS 'TAXI TO RWY 32L; INTXN T10.' THE 132.7 FREQ IS NEW TO ME AS I'VE NEVER SEEN IT ON A CHART OR BEEN GIVEN IT BEFORE. CALLBACK CONVERSATION WITH ORD TWR AND ORD TRACON REVEALED THE FOLLOWING INFO: CALLBACK WAS MADE TO ORD TWR AND ORD TRACON. TRACON STATED THAT THE FREQ THE FLC WAS TOLD TO USE IS A #3 SPARE FREQ. THERE ARE 3 FREQS THAT ARE TO BE USED WHENEVER THE MAIN FREQ IS BLOCKED BY AN EMER; MAINT WORK; OR ACFT BOMB THREAT. THE FREQ ASSIGNED (132.7) WAS USED WITH NO PRIOR NOTICE; THEREFORE; ORD CTLR FELT THAT SOMETHING OCCURRED SUDDENLY THAT CAUSED THE ABNORMAL USE OF THE SPARE FREQ. CTLR FURTHER STATED THAT THE SPARE FREQS ARE NOT PUBLISHED ANYWHERE AVAILABLE TO FLCS.