Metal chelation LLE

The metal chelate LLE was much more common 25 years ago when it was the principal means to isolate and recover metal ions from aqueous samples of environmental interest. The complexes were quantitated using a visible spectrophotometer because most complexes were colored. A large literature exists on this subject (14). The technological advances in both atomic absorption and inductively coupled plasma-atomic emission spectroscopy have significntly reduced the importance of metal chelate LLE to TEQA. However, metal chelate LLE becomes important in processes whereby selected metal ions can be easily removed from the aqueous phase. 

The APDC forms chelates with some two dozen metal ions (100). The extent of formation of the metal chelate is determined by the magnitude of the formation constant, The efficiency of LLE as defined by a metal chelate s percent recovery depends on the distribution ratio, D, of the metal chelate in the two-phase LLE. We use the fundamental definition of D to develop a useful relationship for metal chelate LLE. 

Our examples so far have focused on neutral organic molecules such as acetic acid. The majority of priority pollutant organics of importance to TEQA are neutral molecules in water whose pH is within the 5-8 range. Before we leave the principles that underlie LLE, the answer to the question just posed is yes Consider the significant difference in versus HOAc partition constants discussed earlier. Ionic compounds have little to no tendency to partition into a moderate to nonpolar organic solvent. If, however, an ion can be converted to a neutral molecule via chemical change, this ion can exhibit a favorable K. This is accomplished in two ways chelation of metal ions and formation of ionpairs. The mathematical development of a metal chelate is discussed in this section. 

We have assumed that the protonation of HOx as discussed earlier is negligible. Equation (3.18) states that the distribution ratio for the metal ion chelate LLE depends on the pH of the aqueous phase and on the ligand concentration. 2, and a are dependent on the particular metal ion. 

The mathematics for the general case of LLE involving metal ions and the neutral metal chelates that can be formed... 

Chromium(VI) can be quantitated without coprecipitation by forming a metal chelate. Method 7196A provides a procedure to prepare the diphe-nylcarbazone complex with Cr(VI) in an aqueous matrix. The method is not sensitive in that it is useful for a range of concentrations between 0.5 and 50 mg/L Cr. A more sensitive colorimetric method converts Cr(VI) to Cr(VI) chelate with ammonium pyrrolidine dithiocarbamate (APDC), followed by LLE into methyl isobutyl ketone (MIBK). The molecular structure for APDC is as follows ... 

TO WHAT EXTENT CAN A GIVEN METAL CHELATE BE RECOVERED BY LLE ... 

Upon examination of this relationship among D, Kd, and q hoac> it becomes evident that the distribution ratio depends on the extent to which a solute (in our example, acetic acid), distributes itself between two immiscible phases (e.g., ether and water). At the same time, this solute is capable of exhibiting a secondary equilibrium (i.e., that of acid dissociation in the aqueous phase), as determined by the fraction of all acetic acid that remains neutral or undissociated. We will introduce this concept of fractional dissociation as just defined when we discuss LLE involving the chelation of transition metal ions from an aqueous phase to a water-immiscible organic phase. 

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