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Trouble with Concentration Calculations

Q. I am trying to recreate a recipe for an IN718 etching solution that I found online. The final concentrations listed are:
Ferric Chloride: 150 g/L
Hydrochloric Acid: 200 g/L
Nitric Acid: 10 g/L
Water: Balance

My first thought was to assume that my volume is 1L. From this I get that the mass of the three chemicals, other than water are: 150g, 200g, 10g, respectively.

The concentrations that I am starting with are:
37% Ferric Chloride (aq)
32% Hydrochloric Acid
67% Nitric Acid

I am confused how I can go from the mass fraction in my starting reagents, to the final concentration of g/L. Also, I am unsure how the balance of water comes into play?

R Bauer
- Seattle Washington
February 17, 2023

A. R,
Fortunately you don't even need to do any unit conversions here, this is just straightforward percentage math.

So, you want 150 g of Ferric Chloride from a 37% solution. 150 g/0.37 = 405.4 g so 405.4 g of stock solution contains your 150 g of chemical.

Similarly, for the HCl, 200 g/0.32 = 625 g, and for the HNO3, 10 g/0.67 = 14.93 g.

All of that adds up to 1045.33 g, so hopefully with the densities of these being above 1, that mix will be at or just under a liter, so you can add the remaining necessary water to the water that was already present from the stock solutions.

Take note, you are doing your acid stock solutions by mass, but the final solution is topped out with water by volume. In a lab setting where high precision is desired, a volumetric flask would be used for that final step.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
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McHenry, Illinois
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