Q. How can we manage and control stabilization of alkalinity reserves and acid acceptance level of perchloroethylene in metal cleaning applications?–K.B.
A. Your chlorinated solvent distributor should be able to get you a method directly from the solvent manufacturer to measure this based on the specific type, amount and control range of acid acceptors in your particular solvent. While control ranges may vary between the suppliers, the method for performing the acid acceptance test should be relatively similar, regardless of solvent type or acid acceptors.
Unless they are supplied from your distributor, you should first make reagents for titration, since the titrants used for this procedure cannot be readily purchased. To do this, mix sodium hydroxide into a methanol solution and hydrochloric acid into isopropyl alcohol (hydrochlorination reagent). Contact your supplier for specific directions to make these reagents. The hydrochlorination reagent is mixed with the chlorinated solvent sample from the degreaser in a 50:50 mixture and allowed to stand. In this stage of the test, the hydrochlorination reagent is added in excess to the chlorinated solvent sample such that it reacts with acid acceptors in the solvent. An indicator such as bromophenol blue is added, then that sample is titrated with the sodium hydroxide solution from above until the color changes from yellow to blue. Consider this value “A.”
This titration is repeated to establish a blank value. However, in this case, it is done without the chlorinated solvent. The exact same volume of hydrochlorination reagent (with bromophenol blue added) is titrated with the sodium hydroxide solution to the same endpoint. Consider this value “B.” Using the standard specific gravity for the chlorinated solvent you are working with (in this case 1.623 for perchloroethylene), you can then calculate the amount of acid acceptors in the solvent, generally expressed as a percent equivalent of sodium hydroxide. This can be done by subtracting the titration amount A from the blank value B, then dividing that difference by the product of the volume (in mL) of solvent times the specific gravity of the solvent. Summarized below:
Acid acceptance (as % NaOH) = (B – A) / (mL solvent × specific gravity of solvent)
As mentioned, I would contact your distributor to assist you in setting up this titration and possibly supplying you with the reagents so you do not have to make them yourself.