Telegraph GK 1000

[Escutcheon]

Number of hydrogen atoms replaceable by a base in an acid.

[boxtops]

It seems to depend on the acid http://dictionary.ref...e.com/browse/basicity

It would be helpful to know the clue number, how many letters in the answer, and whether you have any letters already, please

[factor-fiction]

The answer may well be basicity (reminds me of Bay City Rollers) but without details of the number of letters and any known ones we can't be sure 

[Mosaic]

I love your avatar

[argorstran]

Could it be pH ?

[Bellerophon]

Basicity