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« Determination of Free Acidity as Hydrochloric Acid in Ferric Chloride Solution RDM-5003 | Main | Chemistry of the Cupric Chloride etch »

Determination of Free Acid in Ferric Etchants RDM-5001

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Determination of Free Acid in Ferric Etchants (RDM-5001)

Ferric Chloride is reduced with Sodium Thiosulfate to Ferrous, allowing titration of HCl without interference.

Equipment Required:

  • Balance, Top loading
  • Beaker 250 ml
  • Magnetic Stirrer Plate
  • Graduate 100 ml
  • Magnetic Stirrer Bar
  • Plastic Beaker 50 ml
  • Burette 50 ml
  • Disposable Pipettes

Reagents Required:

  • Copper Sulfate 10% solution (CuSO45H2O - 100 grams per liter)
  • Sodium Thiosulfate Anhydrous (Na2S2O3)
  • Sodium Hydroxide 0.100 N
  • Hydrochloric Acid 0.100 N
  • MO-XC Indicator mixture

(0.8 grams Xylene Cyanole FF;
0.6 grams Methyl Orange;
95 ml Methanol;
Deionized water to 1000 ml).


  1. Weigh 7.0 ± 0.5 grams Na2S2O3 into a 250 ml beaker.
  2. Add 100 ml of water, 1 ml of 10% CuSO45H2O, 1 ml Indicator mixture.
  3. Neutralize with 0.1N HCl from green to gray (do not record amount).
  4. Weigh ≈10.2 grams of etchant into 50 ml plastic beaker.
  5. Transfer weighted etchant to neutralized thiosulfate solution.
  6. Titrate with 0.1N NaOH to green If solution is already green, acid is essentially zero

Mls 0.1N NaOH x 3.45 = % Free HCl
weight of sample

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