There is a titration method commonly used in metal finishing for the
analysis of nitric acid etchants. This method does not use sodium
hydroxide. The problem with a sodium hydroxide titration is the
precipitation of the metal during the titration. A better method is to use
a ferrous ammonium sulfate - potassium dichromate titration method.
Here is the method I use for both aluminum and titanium etchants. It should
work nicely for your zinc etchant.
Available at any chemical supply house.
0.2 N or 0.25 N Ferrous Ammonium Sulfate
0.2 N or 0.25 N Potassium Dichromate
Concentrated Sulfuric Acid
Distilled or Deionized water
Volumetric Pipettes 1 Ml., 20 Ml., 25 Ml., 10 Ml
Burret 50 Ml.
2 - 100 Ml Volumetric Flasks
3 - 250 Ml Erlenmeyer Flasks
A Hot Plate
Pipette 1 milliliter of the etchant into a 100 milliliter Volumetric flask
and dilute to the mark with distilled or deionized water and mix well.
Pipette 20 milliliters of this solution into a second 100 milliliter
volumetric flask, dilute to the mark with distilled or deionized water and
Now pipette 10 milliliters of this solution into a 250 milliliter Erlenmeyer
flask. Pipette 25 milliliters of the Ferrous Ammonium Sulfate Solution into
the Erlenmeyer flask and add about 100 milliliters of water. Pipette 10
milliliters of distilled or deionized water into a second Erlenmeyer flask.
Pipette 25 milliliters of Ferrous Ammonium Sulfate into this Erlenmeyer
flask and add about 100 milliliters of distilled or deionized water. To
each Erlenmeyer flask add about 25 milliliters of concentrated Sulfuric
acid and mix well. Heat the flasks on a hot plate until the solution is a
light yellow color. Cool and add water to obtain a volume of about 150
milliliters of solution. Then cool to about 25 degrees Celsius. Add 4 to 5
drops of Ferrorin Indicator, mix well and titrate to a bright green color
with the Potassium Dichromate Solution. If I were you, I would also run a
pure sample of your etchant through this same procedure. I would at least
make a Nitric Acid Solution up of a known concentration to check my
(Normality of Potassium Dichromate)(Milliliters to titrate water Blank -
Milliliters to titrate etchant sample)
__________________________________________________ _________ = N Nitric Acid
6.45*N of Nitric Acid = % v/v Nitric Acid
63.02*N Nitric Acid = g/l Nitric Acid
I hope this is help full to you. If I can be of further assistance don't
hesitate to contact me. Sorry, I don't have my Metal Finishing Handbook
handy or I would give you the best Nitric Acid percentage range for a Zinc
Etchant. I would think a %0% to 60% v/v Nitric Acid should work.
I am sure you are aware that over time as the Zinc concentration becomes to
high the only way to restore performance is to decant a portion of the
solution and add fresh acid and water.
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