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The type and concentration of the carrier for metal ion transporting is another important factor that should be selected properly. In facilitated BLM transport, the carrier forms a complex with the guest cation and helps it to transfer through the hydrophilic membrane (Ajwani et al. 2012). For this purpose, the behavior of two crown ethers (18-Crown-6 and DB-18-Crown-6) was studied and the results are shown in Table 2. It seems the greater flexibility of 18-Crown-6 works better than the hydrophobicity of DB-18-Crown-6 (possessing two benzenes) in the chloroform membrane for magnesium transport. It should be noted that DB-18-Crown-6, due to possessing two benzene groups, is more hydrophobic than 18-Crown-6. For the same reason, it is easier for 18-Crown-6 ether to adjust its structure for better reacting with Mg2+, and this leads to its greater flexibility.

Table 2

Effect of carrier type, tested concentration is 0.01 M in chloroform

%EStructureCrown type
11.86  DB18C6 
18.36  18C6 
%EStructureCrown type
11.86  DB18C6 
18.36  18C6 

Experiment conditions: [Mg+2] = 0.059 M; membrane solvent = chloroform; temperature = 25 °C; feed pH = 4; stripping reagent = 0.1 M of SCN; pH of the receiving solution = 3; extraction time = 2 hr.

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