Titration of Nitric Acid Neutralization

Q. We use soda-ash tanks for neutralization in our nitric passivation process. We need to be able to determine the sodium hydroxide percentage (2–5 percent) in solution. Is a titration using HCl (hydrochloric acid) for the titrant and phenolphthalein as the indicator the proper method? If so, can you give me an example of the standard titration method and an example of the percentage calculation once I determine the amount of HCl required to achieve an endpoint?—D.Q.

A. From your description, I am assuming you are simply neutralizing your nitric acid passivation solution that is spent and then trying to dispose of it, either through your wastewater treatment system or by off-site shipment. In this case, I am not sure that you need the precision of a titration and that you may be better off with a simpler pH measurement. Typically, the pH measurement is what is used to classify something as hazardous due to corrosivity. 

If you really need to perform a titration, it is not clear to me what endpoint you intend to titrate to. You are using the correct chemical, sodium carbonate (a weak base), to neutralize the nitric acid solution (a strong acid). However, I am unclear what the significance is of a 2-5 percent sodium hydroxide equivalent, since you are using sodium carbonate for the neutralization, a much weaker base than sodium hydroxide. 

If you need to follow through with a titration on this bath, it will require you to make a known standard each time you need to perform this neutralization, since I would assume you will not be starting with an identical nitric acid concentration. A standard acid-base titration uses a titrant of known concentration to titrate a tank of unknown concentration. 

However, if you have two unknowns (the starting nitric acid concentration and the amount of sodium bicarbonate addition) you would need to first determine the concentration of the nitric acid with a standard acid-base titration.