The chlorine solution produced by the castner process has 0.9M HCl and HOCl, both of which are gases at room temperature. That is a lot of gas. 200mb of that solution in a 3x3x3 vat cause the pressure to rise by 17kPa. I dont actually see any ways to fix that issue other than adding solution of gases in liquids.

A related issue (also caused by the fact that gases do not dissolve in liquids, and weirdness in how vats calculate reaction rates), is that when the chlorine solution is placed in a vat, the HCl stays mostly undissociated; this is probably because 200mb of water in 27000mb of vat is not a very high concentration. (i didnt actually run the math or test enough to check if this is the actual cause)