Table 5.

Metal-chelate buffers for sub-μM Ca²⁺ and Zn²⁺ solutions

Chelator	Ca2+ buffering range at pH 7.4	Zn2+ buffering range at pH 7.4	Zn2+ buffering range at pH 7.4 with competing metal
EDTA	0.002 to 0.18 μM	0.003 to 0.3 pM
EGTA	0.006 to 0.5 μM	0.15 to 15 nM	9.7 to 1340 nM (with 2 mM Sr2+) 2 to 134 μM (with 2 mM Ca2+)
HEDTA	0.3 to 25 μM	0.15 to 15 pM	0.05 to 7.5 nM (with 2 mM Ca2+)
NTA	20 to 1670 μM