Function Description
********************

	Name: PH_TITR 
	Function: f(x) = Sa + ([H]**n / ([H]**n + Ka**n)) * (Sah-Sa) 
	Parameters: pKa Sa Sah HC 

Notes
*****

	pH titration of NMR chemical shifts

	HC is the Hill coefficient

C-code for the function
***********************

 	b = pow(10.0, (-x)*HC);
	c = pow(10.0, (-pKa)*HC);
	answer = Sa + (b / (b + c)) * (Sah - Sa);