Sodium sulfate is produced on a very large scale as a by-product of several important industrial processes. In many cases, disposal of this material is difficult. As a result, there have been many efforts to use electrolytic salt splitting to recover both sulfuric acid and sodium hydroxide. Because of major technical and economic problems, none of these efforts has been successful. The technical problems are caused by the acid strength of sulfuric acid:

(1) Even low-level conversion of sodium sulfate to sodium bisulfate lowers the pH of the anolyte below 2, which precludes the use of high-efficiency membranes containing a carboxylic barrier layer. The use of a sulfonic barrier layer (Nafion 324) limits the maximum caustic concentration generated as the catholyte to about 15%.

(2) Competition by hydrogen transport: As sodium ions in the anolyte are removed by current transport through the membrane, they are being replaced by hydrogen ions generated by the anodic oxydation of water with the release of oxygen gas. At first, most of these hydrogen ions are captured by sulfate ions to form bisulfate ions, which are not completely dissociated in the presence of excess sulfate ions. At these low conversions, loss of CE due to transport of hydrogen ions through the membrane is not a serious problem. As the anolyte is depleted of sulfate ions, the dissociation of bisulfate becomes more pronounced and at the point of complete conversion of the anolyte to sodium bisulfate in other words, when one half of the original sodium content of the anolyte has been removed, CE drops to an unacceptably low value. The use of a three-compartment cell with the sodium sulfate feed first entering the center compartment and then overflowing to the anode compartment can extend the maximum practical conversion to about 60%. Alternatively, with a sufficiently high flow through the center compartment, it may be possible to maintain its pH above 2.5, thereby allowing the use of a high-efficiency membrane between the center and the cathode compartments.

The economic problems are related to the cost comparison with sodium hydroxide generated by the electrolysis of salt (NaCl). While the operating costs of the two processes are almost the same, the NaCl electrolysis generates chlorine as a valuable by-product. The value of oxygen generated by salt splitting is comparatively minor.

In view of these considerations, the following conditions must be met in order for the electrolytic splitting of sodium sulfate to be successful:

(1) Both acid, in the form of a sodium bisulfate solution, and caustic, in the form of a 15% NaOH solution, must be reusable on site with full or almost full credit for their value.

(2) Sodium sulfate must be available at a negative value; that is, there must be some credit for disposing of an undesirable by-product.