Neutralization of Strongly Acidic or Basic Samples

Determination of small concentrations of anions in samples containing 1 M sodium hydroxide and determination of trace cations in strong mineral acids are two examples where sample neutralization is needed. However, chemical neutralization with HCl or NaOH would introduce a high concentration of unwanted ions. A better way is to introduce the or OH needed for neutralization through an ion-exchange membrane. This process is called electrodialysis. 

Haddad, Laksana and Simons [3] described a device for off-line neutralization of strongly alkaline samples. The method uses an electrodialysis cell comprising three compartments separated from each other by cation-exchange membranes (Fig. 9.2). 

Cations can pass through the membrane but anions cannot. The cathode compartment of the cell was typically filled with 20 mL of 0.1 M sodium hydroxide, the anode compartment with 10 mL of 0.05 M sulfuric acid, and the sample compartment contained 2 mL of highly alkaline sample. Application of power ( 3 W) resulted in currents of 100-200 mA. As shown in the figure, water is reduced to H2 -t- OH in the cathode and is oxidized to O2 + H in the anode compartment. To complete the electric circuit cations must be carried from the anode through the sample solution to the cathode. 

Hydrogen ions entering the sample compartment will be neutralized by OH to form water, and Na from the sample compartment will move across the membrane into the cathode compartment. The net result is that both Na and OH are removed from the sample without disturbing other sample anions. 

Devices are commercially available that maintain sample integrity and trace detection limits, while eliminating interfering sample matrices. Dionex offers an Auto Neutralization Module that neutralizes concentrated acid with electrically generated hydroxide ions [4], 

---