[Anthony]

This is long but you can just skip to the end for the actual problem.

Decades ago I could have done this in my sleep but 18 years ago I suffered an injury that's slowed my thinking down a bit so I'm asking for help.  If someone could just outline the way to solve the problem I can do the math. 

Twenty years ago I began cleaning used mordant (ferric chloride) for etching copper and brass by using electricity. One to two volts followed by passively cleaning it with iron.  I'm still doing it by following my notes.  The problem is the density of the finished ferric chloride is a bit less than what I started with and I'd like to fix that.

What I do.  First I pour the used mordant, Fe[III]Cl (ferric chloride) (not 100% sure about the new nomenclature), into a large container with pieces of iron in it (nails).  After a month or so I filter the solution through paper towels and put it into the electric reactor.  After three or four days all the copper has plated out on the cathode and the reactor is filled with Fe[II]Cl (ferrous chloride) and blue green but with a small amount of ferric chloride (orange yellow) around the anode.  The solution gets filtered (again) into a second this time wide mouthed container also with pieces of iron in it and sits there for a few days or weeks.  At that point the solution is a very blue-green and has dehydrated somewhat.  From there I filter it into a third container and add equal amount of pool grade 31.45% HCl (muriatic acid) then using a fish tank air pump bubble air through it for a month or two.  At that point I have fresh ferric chloride which goes back into use.  There are collectively about three gallons in the sequence.  At the moment the I've got about 1.5 gallons ferric chloride, a gallon of filthy used mordant at step one and half a gallon of clean ferrous chloride (I haven't measured the density yet) I have in the past dehydrate the ferrous chloride to the point of crystallization. I was trying to experiment my way to the numbers I wanted and figured if the solution had crystals in it it was fully saturated.  The internet says that that has a density of 1.93 g/ml of ferrous chloride tetrahydrate.  My solution came in at 1.8 g/ml.  I have no idea why that is and besides I would rather not have to dehydrate the Fe[II]Cl to the point of crystallization.

So here's the problem (finally)  What density ferrous chloride do I need and how much pool grade hydrochloric acid must I add to end up with a 40 baume solution of ferric chloride? 

Thank you everyone.