Reaction of metal-selenide colloids with DTNB.

Mixtures containing 50 µM sodium selenide and 100 µM of the metal ion under study were prepared in an anaerobic glove box and left to incubate in this box for 10 min at room temperature (180 min in the case of Fe²⁺). At time zero, 100 µM DTNB was added to the samples in the glove box. (A) Data with Co²⁺, Cd²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺ ions. TNB absorbance was followed at 412 nm. Data with Ca²⁺ or Mg²⁺, which are similar to those with Mn²⁺ or without metal, are not displayed in the figure. (B) Data with Ag⁺, Cu²⁺ and Hg²⁺ ions. TNB absorbance was followed at 390 nm instead of 412 nm (see Materials and Methods). In all cases, turbidities of the metal-selenide mixtures were measured in the glove box before DTNB addition. Corresponding values are indicated on the left side of the figure. Next, turbidities were measured just after DTNB addition. Because DTNB slightly absorbs light at 390 and 412 nm, values measured immediately after DTNB addition (0.25 min on the figure) systematically exceeded the values before DTNB addition (−1 min on the figure). With Ag⁺, Cd²⁺, Cu²⁺, Hg²⁺, Pb²⁺ and Zn²⁺, the OD increments upon DTNB addition (0.065±0.005 and 0.24±0.01 at 412 and 390 nm, respectively) were close to those obtained in a control experiment without metal and selenide (0.060 and 0.24 at 412 and 390 nm, respectively). With Co²⁺ and Ni²⁺, the increments at 412 nm were slightly higher (0.14 and 0.16, respectively). With Fe²⁺, the increment was far higher (0.45 at 412 nm, see Results).