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Recently I started experimenting with making my own ink to be used in fountain pens.

I was wondering if hydrochloric acid in an iron-gall ink is used to prevent sediment ONLY by lowering the pH of the solution or if it also does something else that I don't know. The recipes found on Wikipedia call for varying amount of hydrochloric acid to be added [3-5cm^3 in 1000ml solution]; however I am quite sceptical of adding it to my ink because of its corrosive nature. I even spoke with one manufacturer of modern iron-gall inks and they said they do not use said acid in theirs; they also said it shouldn't really be a problem since its in such a small concentration. However I am still reluctant to use it honestly.

I've made couple of test batches with citric acid as a replacement, i added 2 and 5 grammes of it but after about half a day I noticed some sediment forming on the bottom of the container (which I assume is the iron). The ink works alright although I would like the ink to be more stable and predictable.

Here is the recipe I was following, however I cut the water to 500mls:

11.7 g tannic acid
3.8 g gallic acid C6H2(OH)3COOH
15 g iron(II) sulfate
3 cm3 hydrochloric acid (used to prevent sediment forming)
1 g carbolic acid (phenol) C6H5OH (biocide)
3.5 g china-blue aniline dye (water-soluble)
1000 cm3 distilled water

As you can probably see I am no chemist at all, I do not got much clue about it, but it would be lovely if you could tell me if and what I could replace the hydrochloric acid with in order to keep the solution under pH 3. I hope this makes sense.

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Most of replacements to keep the same very low pH would be more, not less corrosive.

As it is low pH, which is corrosive, not HCl presence. Replacements like organic acids would be more corrosive as there would be needed a higher molar amount of such an acid for that, taking more of a base to neutralize that.

Additionally, complexing abilities of most of organic acids would contribute to corrosion even more and could affect forming of iron-tannin complexes.

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