I would like to make colloidal silver to kill mold. I think $\pu{100ppm}$ (weight) would be nice. I read a paper on it and in it they used citric acid + distilled water as fluid, two silver plates as electrodes, $\pu{300v}$ ($\pu{50Hz}$ Ac to reduce oxide buildup on electrodes), $\ce{N2H2}$ as reducing agent and a surfactant to encapsulate the uncharged $\ce{Ag}$ particles to prevent them from clotting and building aggregates.

I would like to make something similar. So I have a couple of questions:

About the electrolysis: Is there a problem with making the distance between the electrodes larger? In the paper, there was a distance of $\pu{2cm}$ between the electrodes. I'm planing on having perhaps $\pu{20-30 cm}$. What would be the impact?

Suggestion on alternative reducing agent for $\ce{Ag+}$ ions: Preferably something that is nontoxic and does not promote mold. I have heard of tannic acid and ammonia. Could it work? In the best of world, I would like a reducing agent that evaporate from the solution. It needs to work in a electrolysis environment of $\pu{240-300V}$ AC without contaminating the electrodes. Is there a way to calculate which reducing agent would work?

Why do they use citric acid in the water if it is to allow conductivity in the beginning? Could I omit it? If all the $\ce{Ag}$ particles are reduced to be without charge then the water will not conduct? I think I need it. Correct me if I'm wrong.

About surfactants: Could I use ordinary soap? But I do not think I need it since I'm not after a specific size and I think the $\pu{100ppm}$ solution is not saturated and hence the particles do not grow that much. Is there a way of measuring the $\pu{ppm}$ of uncharged species in a non lab environment or the best way is trough weighing the plates before and after electrolysis?

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    $\begingroup$ #1: You mention a paper you read. May you add the literature reference to your question? While the citric acid will protonate portions of the hydrazine, the relative amount mentioned in the paper's experimental section allows to infer the extent of this step. It will show the relative ratio of reducing agent vs. Ag used and set the procedure into context of other publications to achieve the task in mind, too. #2: Maybe starting from $\ce{AgNO3}$, $\ce{NaBH4}$ and ascorbic acid is much more convenient (and less toxic) than working with hydrazine hydrate; e.g. DOI 10.1007/s13204-014-0371-9. $\endgroup$ – Buttonwood Aug 25 '19 at 19:12
  • $\begingroup$ #1: The paper is "Synthesis of Ag nanoparticles using an electrolysis method and application to inkjet printing Jin Min Cheona, Jin Ha Leea, Yongsul Songb, Jongryoul Kima,∗" $\endgroup$ – Johannes Almborg Aug 25 '19 at 19:54
  • $\begingroup$ Thanks for the reply. The paper you are referring to they produce 24ppm solution of AA-Ag. Do you think I could get a stable solution of 75-100ppm? Normally without AA you can not get a stable solution at [Ag]colloidal = 1.72x10-4 M = 18.6 mg L-1 = 18,6ppm (DOI: 10.17628/ecb.2013.2.700-705). Would 75ppm AA-Ag have the same anti-mold properties as 75ppm colloidal Ag? $\endgroup$ – Johannes Almborg Aug 25 '19 at 21:41
  • $\begingroup$ I do not know the answer to this subsequent question. While I attribute the anti-mold properties to the colloïdal silver, the mere increase of the concentration of silver (ppm or mg/L) needn't be the complete story. By the very nature of colloïds I think the ratio of mass over exposed / accessible surface area of these little particles may play a significant role, too where again the ascorbic acid (as an stabilizer of the particles) may interfere, too. So in short: either someone already published this (-> library), or it needs to be determined experimentally (-> laboratory). $\endgroup$ – Buttonwood Aug 28 '19 at 19:29