Electrophoretic mobility metals

At slightly acidic pH values weak dibasic acids H2L give on dissociation anions HL , forming ion pairs MHL with metal ions. These ion pairs are neutral for M(l), which is the case of Na(I), K(I) and ammonium ions, and electrophoretically mobile for M(ll), such as Ca(II) and Mg(II). A chromophore BH/B consisting of a weak base B, which at slightly acidic pH values is in equilibrium with its conjugate acid BH, also has electrophoretic mobility due to the latter ion and may serve for indirect UVD of the M(ll) ions. These principles have been applied as a CE method for determination of trace concentrations of Ca(II) and Mg(II) in aqueous solutions containing more than 5000-fold concentrations... 

To evaluate + for each metal ion, values of p8 are required at each concentration. While this can often be evaluated from electrophoretic mobility data, the high ionic strengths—Le., pH < 2—preclude meaningful measurement of mobilities. However, it can be seen that when ij/s and cf)+ are equal and opposite then adsorption is reduced to zero. The adsorption of Na+ is reduced to zero at the z.p.c. since, in this case, + is negligibly small. With Ni2+ and Cu2+ the pH must be reduced—i.e., made more positive—by 1.3 pH units to effect zero adsorption. Since near the z.p.c. ips and i//0, the total double layer potential, are approximately equal and given by the Nernst Equation, then... 

Metal-Binding/Electrophoretic Mobility Shift Affinity CE (ACE)... 

Indirect detection is commonly used for metal cations because most of these cations lack the high UV or visible absorptivity needed for direct photometric detection. Waters Associates introduced UV Cat 1 for indirect detection. This is an amine cation that absorbs strongly in the UV spectral region and has an electrophoretic mobility similar to 1+ and 2+ metal cations. Protonated phenylethylamine or 4-methylbenzyla-mine are suitable visualization reagents for indirect detection of metal cations at moderately acidic pH values. 

Our ability to separate free metal cations by CE is limited because many of the metal ions have similar electrophoretic mobilities. An excellent way to enhance the separation of metal ions is to add a relatively weak complexing ligand (L ) such as tartrate, lactate or a-hydroxyisobutyric acid (HIBA) to the BGE. Now part of each metal ion will remain as the free ion (M, for example) and part will be converted to a complexed form (ML , ML2, ML3, for example). The total mobility (p) will be the sum of the mole fraction of each species (a) multiplied by its mobility. 

The kinetic potential is usually denoted as the zeta (0 potential and it is determined from the electrophoretic mobility of the extremely dilute particles in an electric field. More recently, the nse of electrokinetic sonic amplitude (ESA), acoustosizer (AZR), or colloid (or ultrasonic) vibration potential (CVP) has become available for the determination of the potential in rather concentrated particle suspensions. Again the potential may be measured as a function of either the metal concentration or the pH. In the latter case the point where the mobility ceases is denoted the isoelectric point (pH,Ep Fignre 8.27). It correlates particnlarly well with the stability of the sol. 

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