Theoretically, value of pH could range from - infinity to + infinity but practically the limits in aqueous solutions are from -1.2 to +15
pH is basically not equals to -log[H+] bt it actually depends upon the thermodynamics activities rather than [H+] ie on log aH+ where aH+ =[H+]F+ . in dilute solution F+ is near enough to unity and thus aH+ =[H+]. thus ph of 10m solution can't be calculated n its near to zero