Reagents and methods for determination of calcium within a sample. Specific embodiments include a reagent for determination of calcium comprising a mono-nitro substituted BAPTA-type chelator (BAPTA=1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid). Methods for accurate determination of calcium in a sample, such as a blood, whole blood, plasma or serum or any other aqueous liquid sample including cerebrospinal fluid, lymph, salivary juice or urine. Embodiments are also useful for clinical diagnoses.