My can of sparex says 2 1/2 pounds of powder plus 7 pints water
equal 1 gallon of pickle. This translates to put powder in a gallon
jug. Fill with water. (Distilled please)
Distilled? Hardly needed. Tap water will do just fine. You’re not
concocting a plating bath. Just a mild acid dip. Any minerals or
impurities in the water won’t hurt it. The worst it can do is create
a little yucky surface scum. Even the sparex poweder isn’t so pure
anyway, and you often get a little scum just from impurities in the
chemical anyway. Either way, it doesn’t hurt anything. If it offends
your sense of cleanliness, skim off whatever forms. Usually it’s
not much.
By the way, on the subject of pure water, if you actually need pure
water, like for a plating bath, then distilled isn’t what you need.
Distilling water only removes those mineral componants which don’t
boil, like the calcium carbonate, salt, and stuff like that. These
are what form crusty deposits, so for your steam iron, that’s great.
but impurities in the water that are at all volatile, like oils,
various organic compounds, and a raft of other such things, go through
a still just fine, and are still in the distilled water. Some of
these things will have a more serious effect on a plating bath than
the mineral impurities removed by distilling. So for these uses, what
you need is not distilled water, but what’s called deionized water, or
D.I. for short. This is usually produced by a filter arrangement,
similar to the reverse osmosis units used for home water filters, only
a bit more thorough. You can buy these units if you have a regular
need for D.I., starting for a couple hundred dollars for a small low
volume filter. Otherwise, commercial firms that use the stuff often
will sell it to you as well. Cost isn’t too different from that of
distilled in the grocery stores. And it’s a LOT purer.
The quality of purified water (D.I.) is usually stated by measuring
it’s electrical resistance. Any impurities dissolved in water are
ions that generally allow the water to carry a small current. Highly
pure water is a very good insulator. So the purity of D.I. is usually
stated by just stating it’s electrical resistance in megohms. The
actual measurement refers to a standard electrode size and distance
between the electrodes (don’t know what they are. maybe someone else
does?) but usually just the megohm value is given.
One other comment worth mentioning about “pure” water. It has a
distinct shelf life. One of the reasons water is so universally
useful to us, both in our life processes and in our professions and
formulations, is that it’s an exceptionally powerful solvent for a
very wide range of things. Normal tap water already has a great
number if things dissolved in it, and because this gives the water
some electrical conductivity, it is less effective as an agressive
solvent. Not so much that you cannot use it, but enough so that it
is’nt so agressively corrosive. Really pure deionized water, on the
other hand, is highly agressive as a solvent, and will quickly cause
corrosion in things like pipes, vessels, and containers. What that
means, among other things, is that even quite inert containers for
d.i. will over time contaminate the water. If you store the stuff for
a month, and then measure it’s purity again, you’ll find that often
the electrical resistivity has dropped by half in that time, just
sitting in a closed container. And the manufacturers of the filter
equipment that produces it have to be careful to use materials in the
filter that won’t be quickly corroded by the water… What fun…
HTH.
Peter Rowe